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Pelagic fish live in the
pelagic zone The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...
of ocean or lake waters—being neither close to the bottom nor near the shore—in contrast with
demersal fish Demersal fish, also known as groundfish, live and feed on or near the bottom of seas or lakes (the demersal zone).Walrond Carl . "Coastal fish - Fish of the open sea floor"Te Ara - the Encyclopedia of New Zealand. Updated 2 March 2009 They oc ...
that do live on or near the bottom, and
reef fish Coral reef fish are fish which live amongst or in close relation to coral reefs. Coral reefs form complex ecosystems with tremendous biodiversity. Among the myriad inhabitants, the fish stand out as colourful and interesting to watch. Hundreds ...
that are associated with
coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of Colony (biology), colonies of coral polyp (zoology), polyps held together by calcium carbonate. Most coral reefs are built from stony corals, wh ...
s. The marine pelagic environment is the largest aquatic habitat on Earth, occupying 1,370 million cubic kilometres (330 million cubic miles), and is the habitat for 11% of known
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
species. The
ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wor ...
s have a mean depth of . About 98% of the total water volume is below , and 75% is below . Moyle and Cech, p. 585 Marine pelagic fish can be divided into coastal (inshore) fish and oceanic (offshore) fish. Coastal pelagic fish inhabit the relatively shallow and sunlit waters above the
continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
, while oceanic pelagic fish inhabit the vast and deep waters beyond the continental shelf (even though they also may swim inshore). Pelagic fish range in size from small coastal
forage fish Forage fish, also called prey fish or bait fish, are small pelagic fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish feed near the base of the f ...
, such as
herring Herring are forage fish, mostly belonging to the family of Clupeidae. Herring often move in large schools around fishing banks and near the coast, found particularly in shallow, temperate waters of the North Pacific and North Atlantic Ocean ...
s and
sardine "Sardine" and "pilchard" are common names for various species of small, oily forage fish in the herring family Clupeidae. The term "sardine" was first used in English during the early 15th century, a folk etymology says it comes from the It ...
s, to large
apex predator An apex predator, also known as a top predator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the highest trophic lev ...
oceanic fishes, such as
bluefin tuna Bluefin tuna is a common name used to refer to several species of tuna A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, th ...
and oceanic
sharks Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorp ...
. They are usually agile swimmers with streamlined bodies, capable of sustained cruising on long-distance migrations. Many pelagic fish swim in
schools A school is an educational institution designed to provide learning spaces and learning environments for the teaching of students under the direction of teachers. Most countries have systems of formal education, which is sometimes compulsor ...
weighing hundreds of tonnes. Others, such as the large
ocean sunfish The ocean sunfish or common mola (''Mola mola'') is one of the largest bony fish in the world. It was misidentified as the heaviest bony fish, which was actually a different species, '' Mola alexandrini''. Adults typically weigh between . The sp ...
, are solitary . There are also freshwater pelagic fish in some of the larger lakes, such as the Lake Tanganyika sardine.


Epipelagic fish

Epipelagic fish inhabit the
epipelagic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
, the uppermost layer of the
water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
, ranging from
sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardis ...
down to . It is also referred to as the ''surface waters'' or the ''sunlit zone'', and includes the
photic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological pro ...
. The photic zone is defined as the surface waters down to the depth where the sunlight is attenuated to 1% of the surface value. This depth depends on how
turbid Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids c ...
the water is, but can extend to in clear water, coinciding with the epipelagic zone. The photic zone allows sufficient light for
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. ...
to photosynthesize. A vast habitat for most pelagic fish, the epipelagic zone is well lit so visual predators can use their eyesight, is usually well mixed and oxygenated from wave action, and can be a good habitat for
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
to grow. However, it is an almost featureless habitat. This lack of habitat variation results in a lack of
species diversity Species diversity is the number of different species that are represented in a given community (a dataset). The effective number of species refers to the number of equally abundant species needed to obtain the same mean proportional species abundan ...
, so the zone supports less than 2% of the world's known fish species. Much of the zone lacks nutrients for supporting fish, so epipelagic fish tend to be found in coastal water above the
continental shelves A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
, where land runoff can provide nutrients, or in those parts of the ocean where
upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted ...
moves nutrients into the area. Moyle and Cech, p. 571 Epipelagic fish can be divided broadly into small
forage fish Forage fish, also called prey fish or bait fish, are small pelagic fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish feed near the base of the f ...
and larger
predator fish Predatory fish are hypercarnivorous fish that actively prey upon other fish or aquatic animals, with examples including shark, billfish, barracuda, pike/ muskellunge, walleye, perch and salmon. Some omnivorous fish, such as the red-belli ...
that feed on them. Forage fish
school A school is an educational institution designed to provide learning spaces and learning environments for the teaching of students under the direction of teachers. Most countries have systems of formal education, which is sometimes co ...
and
filter feed Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...
on
plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...
. Most epipelagic fish have streamlined bodies capable of sustained cruising on migrations. In general, predatory and forage fish share the same morphological features. Predator fish are usually
fusiform Fusiform means having a spindle-like shape that is wide in the middle and tapers at both ends. It is similar to the lemon-shape, but often implies a focal broadening of a structure that continues from one or both ends, such as an aneurysm on a ...
with large mouths, smooth bodies, and deeply forked tails. Many use vision to prey on zooplankton or smaller fish, while others filter feed on plankton. Most epipelagic predator fish and their smaller prey fish are
countershaded Countershading, or Thayer's law, is a method of camouflage in which an animal's coloration is darker on the top or upper side and lighter on the underside of the body. This pattern is found in many species of mammals, reptiles, birds, fish, an ...
with silvery colours that reduce visibility by
scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
incoming light. The silvering is achieved with reflective
fish scale A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as ...
s that function as small mirrors. This may give an effect of transparency. At medium depths at sea, light comes from above, so a mirror that is oriented vertically makes animals such as fish invisible from the side. In the shallower epipelagic waters, the mirrors must reflect a mixture of wavelengths, and the fish accordingly, has crystal stacks with a range of different spacings. A further complication for fish with bodies that are rounded in cross-section is that the mirrors would be ineffective if laid flat on the skin, as they would fail to reflect horizontally. The overall mirror effect is achieved with many small reflectors, all oriented vertically. Although the number of species is limited, epipelagic fishes are abundant. What they lack in diversity they make up for in numbers. Forage fish occur in huge numbers, and large fish that prey on them often are sought after as premier food fish. As a group, epipelagic fishes form the most valuable
fisheries Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life; or more commonly, the site where such enterprise takes place ( a.k.a. fishing ground). Commercial fisheries include wild fisheries and fish farms, ...
in the world. Many forage fish are facultative predators that can pick individual
copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have ...
s or fish larvae out of the water column, and then change to filter feeding on
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. ...
when energetically, that gives better results. Filter feeding fish usually use long fine gill rakers to strain small organisms from the water column. Some of the largest epipelagic fishes, such as the
basking shark The basking shark (''Cetorhinus maximus'') is the second-largest living shark and fish, after the whale shark, and one of three plankton-eating shark species, along with the whale shark and megamouth shark. Adults typically reach in leng ...
and
whale shark The whale shark (''Rhincodon typus'') is a slow-moving, filter feeder, filter-feeding carpet shark and the largest known Extant taxon, extant fish species. The largest confirmed individual had a length of .McClain CR, Balk MA, Benfield MC, Bran ...
, are filter feeders, and so are some of the smallest, such as adult
sprat Sprat is the common name applied to a group of forage fish belonging to the genus '' Sprattus'' in the family Clupeidae. The term also is applied to a number of other small sprat-like forage fish ('' Clupeoides'', '' Clupeonella'', '' Corica'' ...
s and
anchovies An anchovy is a small, common forage fish of the family Engraulidae. Most species are found in marine waters, but several will enter brackish water, and some in South America are restricted to fresh water. More than 140 species are placed in 1 ...
. Moyle and Cech, p. 572 Ocean waters that are exceptionally clear contain little food. Areas of high productivity tend to be somewhat turbid from plankton blooms. These attract the filter feeding plankton eaters, which in turn attract the higher predators. Tuna fishing tends to be optimum when water turbidity, measured by the maximum depth a
secchi disc The Secchi disk (or Secchi disc), as created in 1865 by Angelo Secchi, is a plain white, circular disk in diameter used to measure water transparency or turbidity in bodies of water. The disc is mounted on a pole or line, and lowered slowly down ...
can be seen during a sunny day, is 15 to 35 metres.


Floating objects

Epipelagic fish are fascinated by floating objects. They aggregate in considerable numbers around objects such as drifting flotsam, rafts, jellyfish, and floating seaweed. The objects appear to provide a "visual stimulus in an optical void". Floating objects may offer refuge for
juvenile fish Fish go through various life stages between fertilization and adulthood. The life of a fish start as spawned eggs which hatch into immotile larvae. These larval hatchlings are not yet capable of feeding themselves and carry a yolk sac whic ...
from predators. An abundance of drifting seaweed or jellyfish can result in significant increases in the survival rates of some juvenile species. Many coastal juveniles use seaweed for the shelter and the food that is available from invertebrates and other fish associated with it. Drifting seaweed, particularly the pelagic ''
Sargassum ''Sargassum'' is a genus of brown (class Phaeophyceae) macroalgae ( seaweed) in the order Fucales. Numerous species are distributed throughout the temperate and tropical oceans of the world, where they generally inhabit shallow water and coral ...
'', provide a niche habitat with its own shelter and food, and even supports its own unique fauna, such as the
sargassum fish The sargassum fish, anglerfish, or frog fish (''Histrio histrio'') is a frogfish of the family (biology), family Antennariidae, Monotypic taxon, the only species in its genus. It lives among ''Sargassum'' seaweed which floats in subtropical ocea ...
. One study, off Florida, found 54 species from 23 families living in flotsam from ''Sargassum'' mats. Jellyfish also are used by juvenile fish for shelter and food, even though jellyfish can prey on small fish. Moyle and Cech, p. 576 Mobile oceanic species such as
tuna A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max length: ...
can be captured by travelling long distances in large
fishing vessel A fishing vessel is a boat or ship used to catch fish in the sea, or on a lake or river. Many different kinds of vessels are used in commercial, artisanal and recreational fishing. The total number of fishing vessels in the world in 2016 was ...
s. A simpler alternative is to leverage off the fascination fish have with floating objects. When fishermen use such objects, they are called fish aggregating devices (FADs). FADs are anchored rafts or objects of any type, floating on the surface or just below it. Fishermen in the Pacific and Indian oceans set up floating FADs, assembled from all sorts of debris, around tropical islands, and then use
purse seine Seine fishing (or seine-haul fishing; ) is a method of fishing that employs a surrounding net, called a seine, that hangs vertically in the water with its bottom edge held down by weights and its top edge buoyed by floats. Seine nets can be de ...
s to capture the fish attracted to them. A study using
sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances (ranging), communicate with or detect objects on o ...
in French Polynesia, found large shoals of juvenile
bigeye tuna The bigeye tuna (''Thunnus obesus'') is a species of true tuna of the genus '' Thunnus'', belonging to the wider mackerel family Scombridae. In Hawaiian, it is one of two species known as ahi, the other being the yellowfin tuna. Bigeye tuna are ...
and
yellowfin tuna The yellowfin tuna (''Thunnus albacares'') is a species of tuna found in pelagic waters of tropical and subtropical oceans worldwide. Yellowfin is often marketed as ahi, from the Hawaiian , a name also used there for the closely related bigeye ...
aggregated closest to the devices, 10 to 50 m. Farther out, 50 to 150 m, was a less dense group of larger yellowfin and albacore tuna. Yet farther out, to 500 m, was a dispersed group of various large adult tuna. The distribution and density of these groups was variable and overlapped. The FADs also were used by other fish, and the aggregations dispersed when it was dark. Larger fish, even predator fish such as the
great barracuda ''Sphyraena barracuda'', commonly known as the great barracuda, is a species of barracuda: large, predatory ray-finned fish found in subtropical oceans around the world. Distribution and habitat The great barracuda is present in tropical to warm ...
, often attract a retinue of small fish that accompany them in a strategically safe way. Skindivers who remain for long periods in the water also often attract a retinue of fish, with smaller fishes coming in close and larger fishes observing from a greater distance.
Marine turtle Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhead ...
s, functioning as a mobile shelter for small fish, can be impaled accidentally by a swordfish trying to catch the fish.


Coastal fish

Coastal fish (also called neritic or inshore fish) inhabit the waters near the
coast The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...
and above the
continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
. Since the continental shelf is usually less than 200 metres deep, it follows that coastal fish that are not demersal fish, are usually epipelagic fish, inhabiting the sunlit epipelagic zone. Coastal epipelagic fish are among the most abundant in the world. They include forage fish as well as the predator fish that feed on them. Forage fish thrive in those inshore waters where high productivity results from the upwelling and shoreline run off of nutrients. Some are partial residents that spawn in streams, estuaries, and bays, but most complete their life cycle in the zone.


Oceanic fish

Oceanic fish (also called open ocean or offshore fish) live in the waters that are not above the continental shelf. Oceanic fish can be contrasted with coastal fish, who do live above the continental shelf. However, the two types are not mutually exclusive, since there are no firm boundaries between coastal and ocean regions, and many epipelagic fish move between coastal and oceanic waters, particularly in different stages in their life cycle. Oceanic epipelagic fish can be true residents, partial residents, or accidental residents. True residents live their entire life in the open ocean. Only a few species are true residents, such as
tuna A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max length: ...
,
billfish The term billfish refers to a group of saltwater predatory fish characterised by prominent pointed bills (rostra), and by their large size; some are longer than . Extant billfish include sailfish and marlin, which make up the family Istioph ...
,
flying fish The Exocoetidae are a family of marine fish in the order Beloniformes class Actinopterygii, known colloquially as flying fish or flying cod. About 64 species are grouped in seven to nine genera. While they cannot fly in the same way a bird d ...
, sauries, pilotfish,
remora The remora (), sometimes called suckerfish, is any of a family (Echeneidae) of ray-finned fish in the order Carangiformes. Depending on species, they grow to long. Their distinctive first dorsal fins take the form of a modified oval, sucker-li ...
s,
dolphinfish ''Coryphaena'' is a genus of marine ray-finned fishes known as the dolphinfishes, and is currently the only known genus in the family Coryphaenidae. The generic name is from Greek κορυφή (''koryphē'', "crown, top") and -αινα (-''ain ...
, ocean sharks, and
ocean sunfish The ocean sunfish or common mola (''Mola mola'') is one of the largest bony fish in the world. It was misidentified as the heaviest bony fish, which was actually a different species, '' Mola alexandrini''. Adults typically weigh between . The sp ...
. Most of these species migrate back and forth across open oceans, rarely venturing over continental shelves. Some true residents associate with drifting jellyfish or seaweeds. Partial residents occur in three groups: species that live in the zone only when they are juveniles (drifting with jellyfish and seaweeds); species that live in the zone only when they are adults (salmon, flying fish, dolphin, and whale sharks); and deep water species that make nightly migrations up into the surface waters (such as the lanternfish). Accidental residents occur occasionally when adults and juveniles of species from other environments are carried accidentally into the zone by currents. File:Sunfish.jpg, The huge
ocean sunfish The ocean sunfish or common mola (''Mola mola'') is one of the largest bony fish in the world. It was misidentified as the heaviest bony fish, which was actually a different species, '' Mola alexandrini''. Adults typically weigh between . The sp ...
, a true resident of the ocean epipelagic zone, sometimes drifts with the current, eating
jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbre ...
. File:Whale shark Georgia aquarium.jpg, The giant
whale shark The whale shark (''Rhincodon typus'') is a slow-moving, filter feeder, filter-feeding carpet shark and the largest known Extant taxon, extant fish species. The largest confirmed individual had a length of .McClain CR, Balk MA, Benfield MC, Bran ...
, another resident of the ocean epipelagic zone, filter feeds on
plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...
, and periodically dives deep into the mesopelagic zone. File:Protomyctophum subparallelum (no common name).gif, Lanternfish are partial residents of the ocean epipelagic zone During the day they hide in deep waters, but at night they migrate up to surface waters to feed.


Deep water fish

In the deep ocean, the waters extend far below the epipelagic zone and support very different types of pelagic fishes adapted to living in these deeper zones. In deep water,
marine snow In the deep ocean, marine snow (also known as "ocean dandruff") is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to ...
is a continuous shower of mostly organic
detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts comm ...
falling from the upper layers of the water column. Its origin lies in activities within the productive
photic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological pro ...
. Marine snow includes dead or dying
plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...
,
protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...
s (
diatom A diatom ( Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising se ...
s), fecal matter, sand, soot, and other inorganic dust. The "snowflakes" grow over time and may reach several centimetres in diameter, travelling for weeks before reaching the ocean floor. However, most organic components of marine snow are consumed by
microbe A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s,
zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
, and other filter feeding animals within the first 1,000 metres of their journey, that is, within the epipelagic zone. In this way marine snow can be considered the foundation of deep-sea
mesopelagic The mesopelagic zone (Greek μέσον, middle), also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins at ...
and
benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
s: As sunlight cannot reach them, deep-sea organisms rely heavily on marine snow as an energy source. Some deep-sea pelagic groups, such as the lanternfish, ridgehead, marine hatchetfish, and lightfish families are sometimes termed ''pseudoceanic'' because, rather than having an even distribution in open water, they occur in significantly higher abundances around structural oases, notably
seamount A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise a ...
s, and over
continental slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...
s. The phenomenon is explained by the likewise abundance of prey species that also are attracted to the structures. The fish in the different pelagic and deep water benthic zones are physically structured, and behave, in ways that differ markedly from each other. Groups of coexisting species within each zone all seem to operate in similar ways, such as the small mesopelagic vertically migrating plankton-feeders, the bathypelagic
anglerfish The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence ...
es, and the deep water benthic
rattail Grenadiers or rattails are generally large, brown to black gadiform marine fish of the subfamily Macrourinae, the largest subfamily of the family Macrouridae. Found at great depths from the Arctic to Antarctic, members of this subfamily are amon ...
s. Moyle and Cech, p. 591 Ray finned species, with spiny fins, are rare among deep sea fishes, which suggests that deep sea fish are ancient and so well adapted to their environment that invasions by more modern fishes have been unsuccessful. The few ray fins that do exist are mainly in the
Beryciformes The Beryciformes are a poorly-understood order of carnivorous ray-finned fishes consisting of 7 families, 30 genera, and 161 species. They feed on small fish and invertebrates. Beyond this, little is known about the biology of most member specie ...
and
Lampriformes Lampriformes is an order of ray-finned fish. Members are collectively called lamprids (which is more properly used for the Lampridae) or lampriforms, and unite such open-ocean and partially deep-sea Teleostei as the crestfishes, oarfish, opah ...
, which also are ancient forms. Most deep sea pelagic fishes belong to their own orders, suggesting a long evolution in deep sea environments. In contrast, deep water benthic species are in orders that include many related shallow water fishes. Moyle and Cech, p. 586 Many species move daily between zones in vertical migrations. In the following table, they are listed in the middle or deeper zone where they regularly are found.


Mesopelagic fish

Below the epipelagic zone, conditions change rapidly. Between 200 metres and approximately 1000 metres, light continues to fade until there is almost none. Temperatures fall through a
thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...
to temperatures between and . This is the twilight or
mesopelagic The mesopelagic zone (Greek μέσον, middle), also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins at ...
zone. Pressure continues to increase, at the rate of one atmosphere every 10 metres, while nutrient concentrations fall, along with dissolved oxygen and the rate at which the water circulates. Sonar operators, using the sonar technology developed during World War II, were puzzled by what appeared to be a false sea floor 300–500 metres deep at day, and less deep at night. This turned out to be due to millions of marine organisms, most particularly small mesopelagic fish, with swimbladders that reflected the sonar. These organisms migrate up into shallower water at dusk to feed on plankton. The layer is deeper when the moon is out, and may become shallower when clouds pass over the moon. This phenomenon has come to be known as the deep scattering layer.Ryan
"Deep-sea creatures: The mesopelagic zone"
''Te Ara – the Encyclopedia of New Zealand''. Updated 21 September 2007.
Most mesopelagic fish make daily vertical migrations, moving at night into the epipelagic zone, often following similar migrations of zooplankton, and returning to the depths for safety during the day. These vertical migrations often occur over a large vertical distances, and are undertaken with the assistance of a
swimbladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...
. The swimbladder is inflated when the fish wants to move up, and, given the high pressures in the mesopelagic zone, this requires significant energy. As the fish ascends, the pressure in the swimbladder must adjust to prevent it from bursting. When the fish wants to return to the depths, the swimbladder is deflated. Some mesopelagic fishes make daily migrations through the
thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...
, where the temperature changes between 10 and 20 °C, thus displaying considerable tolerances for temperature change. Moyle and Cech, p. 590 These fish have muscular bodies, ossified bones, scales, well developed gills and central nervous systems, and large hearts and kidneys. Mesopelagic plankton feeders have small mouths with fine gill rakers, while the piscivores have larger mouths and coarser gill rakers. The vertically migratory fish have
swimbladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...
s. Mesopelagic fish are adapted for an active life under low light conditions. Most of them are visual predators with large eyes. Some of the deeper water fish have tubular eyes with big lenses and only
rod cell Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in pe ...
s that look upward. These give binocular vision and great sensitivity to small light signals. This adaptation gives improved terminal vision at the expense of lateral vision, and allows the predator to pick out
squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fittin ...
,
cuttlefish Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of ...
, and smaller fish that are silhouetted against the gloom above them. Mesopelagic fish usually lack defensive spines, and use colour to
camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the b ...
them from other fish.
Ambush predator Ambush predators or sit-and-wait predators are carnivorous animals that capture or trap prey via stealth, luring or by (typically instinctive) strategies utilizing an element of surprise. Unlike pursuit predators, who chase to capture prey ...
s are dark, black or red. Since the longer, red, wavelengths of light do not reach the deep sea, red effectively functions the same as black. Migratory forms use
countershaded Countershading, or Thayer's law, is a method of camouflage in which an animal's coloration is darker on the top or upper side and lighter on the underside of the body. This pattern is found in many species of mammals, reptiles, birds, fish, an ...
silvery colours. On their bellies, they often display
photophore A photophore is a glandular organ that appears as luminous spots on various marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye; equipped with lenses, shutters, color filters and reflectors, ...
s producing low grade light. For a predator from below, looking upward, this
bioluminescence Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...
camouflages the silhouette of the fish. However, some of these predators have yellow lenses that filter the (red deficient) ambient light, leaving the bioluminescence visible. File:Dmawsoni Head shot.jpg, The
Antarctic toothfish The Antarctic toothfish (''Dissostichus mawsoni'') is a large, black or brown fish found in very cold (subzero) waters of the Southern Ocean near Antarctica. It is the largest fish in the Southern Ocean, feeding on shrimp and smaller fish, and pr ...
have large, upward looking eyes, adapted to detecting the silhouettes of prey fish. File:Opisthoproctus soleatus.png, The
Barreleye Barreleyes, also known as spook fish (a name also applied to several species of chimaera), are small deep-sea argentiniform fish comprising the family Opisthoproctidae found in tropical-to-temperate waters of the Atlantic, Pacific, and I ...
has barrel-shaped, tubular eyes that generally are directed upward, but may be swivelled forward. Image:Malacosteus niger.jpg, The
stoplight loosejaw The stoplight loosejaws are small, deep-sea dragonfishes of the genus ''Malacosteus'', classified either within the subfamily Malacosteinae of the family Stomiidae, or in the separate family Malacosteidae. They are found worldwide, outside of t ...
has a
lower jaw In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...
one-quarter as long as its body. The jaw has no floor and is attached only by a hinge and a modified tongue bone. Large fang-like teeth in the front are followed by many small barbed teeth. Image:Malacosteus.JPG, The
stoplight loosejaw The stoplight loosejaws are small, deep-sea dragonfishes of the genus ''Malacosteus'', classified either within the subfamily Malacosteinae of the family Stomiidae, or in the separate family Malacosteidae. They are found worldwide, outside of t ...
is also one of the few fishes that produce red
bioluminescence Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...
. As most of their prey cannot perceive red light, this allows it to hunt with an essentially invisible beam of light.
The
brownsnout spookfish The brownsnout spookfish or brown-snout spookfish (''Dolichopteryx longipes'') is a species of barreleye in the family Opisthoproctidae. It and the glasshead barreleye fish are the only vertebrates known to employ a mirror, in addition to a len ...
is a species of
barreleye Barreleyes, also known as spook fish (a name also applied to several species of chimaera), are small deep-sea argentiniform fish comprising the family Opisthoproctidae found in tropical-to-temperate waters of the Atlantic, Pacific, and I ...
and is the only vertebrate known to employ a mirror, as opposed to a lens, to focus an image in its eyes.Smith, L. (8 January 2009)
"Fish with four eyes can see through the deep sea gloom"
''Times Online''. Times Newspapers Ltd. Retrieved 14 March 2009.
Sampling via deep
trawling Trawling is a method of fishing that involves pulling a fishing net through the water behind one or more boats. The net used for trawling is called a trawl. This principle requires netting bags which are towed through water to catch different spec ...
indicates that lanternfish account for as much as 65% of all deep sea fish
biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bio ...
. Indeed, lanternfish are among the most widely distributed, populous, and diverse of all
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
s, playing an important
ecological Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
role as prey for larger organisms. The estimated global biomass of lanternfish is 550–660 million
tonne The tonne ( or ; symbol: t) is a unit of mass equal to 1000  kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...
s, several times the entire world fisheries catch. Lanternfish also account for much of the biomass responsible for the deep scattering layer of the world's oceans.
Sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances (ranging), communicate with or detect objects on o ...
reflects off the millions of lanternfish
swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...
s, giving the appearance of a false bottom.
Bigeye tuna The bigeye tuna (''Thunnus obesus'') is a species of true tuna of the genus '' Thunnus'', belonging to the wider mackerel family Scombridae. In Hawaiian, it is one of two species known as ahi, the other being the yellowfin tuna. Bigeye tuna are ...
are an epipelagic/mesopelagic species that is carnivorous, eating other fish. Satellite tagging has shown that bigeye tuna often spend prolonged periods cruising deep below the surface during the daytime, sometimes making dives as deep as . These movements are thought to be in response to the vertical migrations of prey organisms in the deep scattering layer. File:Longnoselancetfish.jpg,
Longnose lancetfish ''Alepisaurus ferox'', the long snouted lancetfish, longnose lancetfish, or cannibal fish, is a species of lancetfish found in the ocean depths down to 1,830 m (6,000 ft). This species grows to in total length and a weight of . It is ...
. Lancetfish are ambush predators that frequent the mesopelagic. They are among the largest mesopelagic fishes (up to 2 metres). Moyle and Cech, p. 336 File:gigantura chuni.png, The
telescopefish Telescopefish are small, deep-sea aulopiform fish comprising the small family Giganturidae. The two known species are within the genus ''Gigantura''. Though rarely captured, they are found in cold, deep tropical to subtropical waters worldwide ...
has large, forward-pointing telescoping eyes with large lenses. File:Daggertooth.PNG, The
daggertooth The daggertooths (genus ''Anotopterus'') are a genus of marine mesopelagic fish in the order Aulopiformes, the sole genus of the family Anotopteridae. They are found in oceans worldwide, but prefer cooler waters. Description Daggertooths are not ...
slashes other mesopelagic fish when it bites them with its dagger-like teeth. File:Thobe u0.gif,
Bigeye tuna The bigeye tuna (''Thunnus obesus'') is a species of true tuna of the genus '' Thunnus'', belonging to the wider mackerel family Scombridae. In Hawaiian, it is one of two species known as ahi, the other being the yellowfin tuna. Bigeye tuna are ...
cruise the epipelagic zone at night and the mesopelagic zone during the day. File:Lestidiops affinis (1).jpg, A collection of mesopelagic forage fishes trawled from the Gulf of Mexico that includes
Myctophids Lanternfishes (or myctophids, from the Greek μυκτήρ ''myktḗr'', "nose" and ''ophis'', "serpent") are small mesopelagic fish of the large family Myctophidae. One of two families in the order Myctophiformes, the Myctophidae are represente ...
, larval anglerfishes, bristlemouths, and a
barracudina Barracudinas are any member of the marine mesopelagic fish family Paralepididae: 50 or so extant species are found almost worldwide in deep waters. Several genera are known only from fossils dating back to the Ypresian epoch. The generic name ...


Bathypelagic fish

Below the mesopelagic zone it is pitch dark. This is the midnight or
bathypelagic zone The bathypelagic zone or bathyal zone (from Greek βαθύς (bathýs), deep) is the part of the open ocean that extends from a depth of below the ocean surface. It lies between the mesopelagic above, and the abyssopelagic below. The bathypelagi ...
, extending from 1000 m to the bottom deep water
benthic zone The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...
. If the water is exceptionally deep, the pelagic zone below sometimes is called the lower midnight or abyssopelagic zone. Conditions are somewhat uniform throughout these zones, the darkness is complete, the pressure is crushing, and temperatures, nutrients, and dissolved oxygen levels are all low. Bathypelagic fish have special
adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
s to cope with these conditions – they have slow
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ...
s and unspecialized diets, being willing to eat anything that comes along. They prefer to sit and wait for food rather than waste energy searching for it. The behaviour of bathypelagic fish can be contrasted with the behaviour of mesopelagic fish. Mesopelagic are often highly mobile, whereas bathypelagic fish are almost all lie-in-wait predators, normally expending little energy in movement. Moyle and Cech, p. 594 The dominant bathypelagic fishes are small bristlemouth and
anglerfish The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence ...
; fangtooth,
viperfish A viperfish is any species of marine fish in the genus ''Chauliodus''. Viperfishes are mostly found in the mesopelagic zone and are characterized by long, needle-like teeth and hinged lower jaws. A typical viperfish grows to lengths of . Viperf ...
,
daggertooth The daggertooths (genus ''Anotopterus'') are a genus of marine mesopelagic fish in the order Aulopiformes, the sole genus of the family Anotopteridae. They are found in oceans worldwide, but prefer cooler waters. Description Daggertooths are not ...
, and
barracudina Barracudinas are any member of the marine mesopelagic fish family Paralepididae: 50 or so extant species are found almost worldwide in deep waters. Several genera are known only from fossils dating back to the Ypresian epoch. The generic name ...
are also common. These fishes are small, many about long, and not many longer than . They spend most of their time waiting patiently in the water column for prey to appear or to be lured by their phosphors. What little energy is available in the bathypelagic zone filters from above in the form of detritus, faecal material, and the occasional invertebrate or mesopelagic fish. About 20% of the food that has its origins in the epipelagic zone falls down to the mesopelagic zone, but only about 5% filters down to the bathypelagic zone.Ryan
"Deep-sea creatures: The bathypelagic zone"
''Te Ara – the Encyclopedia of New Zealand''. Updated 21 September 2007.
Bathypelagic fish are sedentary, adapted to outputting minimum energy in a habitat with very little food or available energy, not even sunlight, only bioluminescence. Their bodies are elongated with weak, watery muscles and
skeletal A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
structures. Since so much of the fish is water, they are not compressed by the great pressures at these depths. They often have extensible, hinged
jaws Jaws or Jaw may refer to: Anatomy * Jaw, an opposable articulated structure at the entrance of the mouth ** Mandible, the lower jaw Arts, entertainment, and media * Jaws (James Bond), a character in ''The Spy Who Loved Me'' and ''Moonraker'' * ...
with recurved teeth. They are slimy, without scales. The central nervous system is confined to the lateral line and olfactory systems, the eyes are small and may not function, and
gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
s, kidneys and hearts, and
swimbladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...
s are small or missing. Moyle and Cech, p. 587 These are the same features found in fish
larvae A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. T ...
, which suggests that during their evolution, bathypelagic fish have acquired these features through
neoteny Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compare ...
. As with larvae, these features allow the fish to remain suspended in the water with little expenditure of energy. Despite their ferocious appearance, these beasts of the deep are mostly miniature fish with weak muscles, and are too small to represent any threat to humans. The swimbladders of deep sea fish are either absent or scarcely operational, and bathypelagic fish do not normally undertake vertical migrations. Filling bladders at such great pressures incurs huge energy costs. Some deep sea fishes have swimbladders that function while they are young and inhabit the upper epipelagic zone, but they wither or fill with fat when the fish move down to their adult habitat. The most important sensory systems are usually the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in th ...
, which responds to sound, and the
lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...
, which responds to changes in water pressure. The
olfactory The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, ...
system also can be important for males who find females by smell. Bathypelagic fish are black, or sometimes red, with few
photophore A photophore is a glandular organ that appears as luminous spots on various marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye; equipped with lenses, shutters, color filters and reflectors, ...
s. When photophores are used, it is usually to entice prey or attract a mate. Because food is so scarce, bathypelagic predators are not selective in their feeding habits, but grab whatever comes close enough. They accomplish this by having a large mouth with sharp teeth for grabbing large prey and overlapping gill rakers that prevent small prey that have been swallowed from escaping. It is not easy finding a mate in this zone. Some species depend on
bioluminescence Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...
. Others are
hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have ...
s, which doubles their chances of producing both eggs and sperm when an encounter occurs. The female anglerfish releases
pheromone A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
s to attract tiny males. When a male finds her, he bites onto her and never lets go. When a male of the anglerfish species '' Haplophryne mollis'' bites into the skin of a female, he release an
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
that digests the skin of his mouth and her body, fusing the pair to the point where the two circulatory systems join up. The male then atrophies into nothing more than a pair of
gonads A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...
. This extreme sexual dimorphism ensures that, when the female is ready to spawn, she has a mate immediately available. Many animal forms other than fish live in the bathypelagic zone, such as squid, large whales, octopuses, sponges, brachiopods, sea stars, and echinoids, but this zone is difficult for fish to live in. File:Eurypharynx pelecanoides.jpg, The gulper eel uses its mouth like a net by opening its large mouth and swimming at its prey. It has a luminescent organ at the tip of its tail to attract prey. Image:Chiasmodon niger.jpg, The black swallower, with its distensible stomach, is notable for its ability to swallow, whole, bony fishes ten times its mass. Image:Hamol u0.gif, Female '' Haplophryne mollis'' anglerfish trailing attached males that have atrophied into a pair of
gonads A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...
, for use when the female is ready to spawn. File:Anoplogaster cornuta 2.jpg, The widespread fangtooth has the largest teeth of any fish, proportionate to body size. Despite their ferocious appearance, bathypelagic fish are usually weakly muscled and too small to represent any threat to humans. File:Messina Straits Chauliodus sloani.jpg, The Sloane's viperfish can make nightly migrations from bathypelagic depths to near surface waters.


Demersal fish

Demersal fish live on or near the bottom of the sea.Walrond
Carl . "Coastal fish – Fish of the open sea floor"
Te Ara – the Encyclopedia of New Zealand. Updated 2 March 2009
Demersal fish are found by the seafloor in coastal areas on the
continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
, and in the open ocean they are found along the outer continental margin on the continental slope and the continental rise. They are not generally found at abyssopelagic or hadopelagic depths or on the abyssal plain. They occupy a range of seafloors consisting of mud, sand, gravel, or rocks. In deep waters, the fishes of the demersal zone are active and relatively abundant, compared to fishes of the
bathypelagic zone The bathypelagic zone or bathyal zone (from Greek βαθύς (bathýs), deep) is the part of the open ocean that extends from a depth of below the ocean surface. It lies between the mesopelagic above, and the abyssopelagic below. The bathypelagi ...
. Rattails and Ophidiidae, brotulas are common, and other well-established families are eels, eelpouts, hagfishes, greeneyes, Pancake batfish, batfishes, and lumpfishes. The bodies of deep water
benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...
fishes are muscular with well developed organs. In this way they are closer to mesopelagic fishes than bathopelagic fishes. In other ways, they are more variable. Photophores are usually absent, eyes and
swimbladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...
s range from absent to well developed. They vary in size, with larger species greater than one metre not uncommon. Deep sea benthic fish are usually long and narrow. Many are eels or shaped like eels. This may be because long bodies have long
lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...
s. Lateral lines detect low-frequency sounds, and some benthic fishes appear to have muscles that drum such sounds to attract mates. Smell is also important, as indicated by the rapidity with which benthic fish find traps baited with bait fish. The main diet of deep sea benthic fish is invertebrates of the deep sea benthos and carrion. Smell, touch, and lateral line sensitivities seem to be the main sensory devices for locating these. Moyle and Cech, p. 588 Deep sea benthic fish can be divided into strictly benthic fish and benthopelagic fish. Usually, strictly benthic fish are negatively buoyant, while benthopelagic fish are neutrally buoyant. Strictly benthic fish stay in constant contact with the bottom. They either lie-and-wait as ambush predators or move actively over the bottom in search for food.


Benthopelagic fish

Benthopelagic fish inhabit the water just above the bottom, feeding on benthos and benthopelagic
zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
. Most dermersal fish are benthopelagic. They can be divided into flabby or robust body types. Flabby benthopelagic fishes are like bathopelagic fishes, they have a reduced body mass, and low metabolic rates, expending minimal energy as they lie and wait to Ambush predator, ambush prey. An example of a flabby fish is the cusk-eel ''Acanthonus armatus'', a predator with a huge head and a body that is 90% water. This fish has the largest ears (otoliths) and the smallest brain in relation to its body size of all known vertebrates. Robust benthopelagic fish are muscular swimmers that actively cruise the bottom searching for prey. They may live around features, such as
seamount A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise a ...
s, which have strong currents. Examples are the orange roughy and Patagonian toothfish. Because these fish were once abundant, and because their robust bodies are good to eat, these fish have been harvested commercially.


Benthic fish

Benthic fish are not pelagic fish, but they are discussed here briefly, by way of completeness and contrast. Some fishes do not fit into the above classification. For example, the family of nearly blind spiderfishes, common and widely distributed, feed on benthopelagic zooplankton. Yet they are strictly benthic fish, since they stay in contact with the bottom. Their fins have long rays they use to "stand" on the bottom while they face the current and grab zooplankton as it passes by. The deepest-living fish known, the strictly benthic ''Abyssobrotula galatheae'', eel-like and blind, feeds on benthic invertebrates. File:Pacific hagfish Myxine.jpg, Pacific hagfish resting on bottom. Hagfish coat themselves and any dead fish they find with noxious slime, making them inedible to other species. File:Bathypterois grallator.jpg, The tripodfish (''Bathypterois grallator''), a species of spiderfish, uses its fin extensions to "stand" on the bottom. File:Taeniura meyeni reef.jpg, The blotched fantail ray feeds on bottom-dwelling fish, bivalves, crabs, and shrimps. At great depths, food scarcity and extreme pressure works to limit the survivability of fish. The deepest point of the ocean is about . Bathypelagic fishes are not normally found below . The greatest depth recorded for a benthic fish is . It may be that extreme pressures interfere with essential enzyme functions. Benthic fishes are more diverse and are likely to be found on the
continental slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...
, where there is habitat diversity and often, food supplies. Approximately 40% of the ocean floor consists of abyssal plains, but these flat, featureless regions are covered with Pelagic sediments, sediment and largely devoid of benthic life (benthos). Deep sea benthic fishes are more likely to associate with canyons or rock outcroppings among the plains, where invertebrate communities are established. Undersea mountains (
seamount A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise a ...
s) can intercept deep sea currents and cause productive upwellings that support benthic fish. Undersea mountain ranges may separate underwater regions into different ecosystems.


Pelagic fisheries


Forage fish

Small pelagic fish are usually
forage fish Forage fish, also called prey fish or bait fish, are small pelagic fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish feed near the base of the f ...
that are hunted by larger pelagic fish and other predators. Forage fish Filter feeder, filter feed on
plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...
and are usually less than long. They often stay together in
schools A school is an educational institution designed to provide learning spaces and learning environments for the teaching of students under the direction of teachers. Most countries have systems of formal education, which is sometimes compulsor ...
and may Fish migration, migrate large distances between spawning grounds and feeding grounds. They are found particularly in
upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted ...
regions around the northeast Atlantic, off the coast of Japan, and off the west coasts of Africa and the Americas. Forage fish are generally short-lived, and their Fish stocks, stocks fluctuate markedly over the years.Checkley D, Alheit J and Oozeki Y (2009) ''Climate Change and Small Pelagic Fish'', Cambridge University Press. . Herring are found in the North Sea and the North Atlantic at depths to . Important herring fisheries have existed in these areas for centuries. Herring of different sizes and growth rates belong to different populations, each of which have their own migration routes. When spawning, a female produces from 20,000 to 50,000 eggs. After spawning, the herrings are depleted of fat, and migrate back to feeding grounds rich in plankton.Pelagic species
Pelagic Freezer-trawler Association. Retrieved 22 July 2009.
Around Iceland, three separate populations of herring were fished traditionally. These stocks collapsed in the late 1960s, although two have since recovered. After the collapse, Iceland turned to capelin, which now account for about half of Iceland's total catch. Blue whiting are found in the open ocean and above the
continental slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...
at depths between 100 and 1000 meters . They follow vertical migrations of the
zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
they feed on to the bottom during daytime and to the surface at night time. Traditional fisheries for
anchovies An anchovy is a small, common forage fish of the family Engraulidae. Most species are found in marine waters, but several will enter brackish water, and some in South America are restricted to fresh water. More than 140 species are placed in 1 ...
and
sardine "Sardine" and "pilchard" are common names for various species of small, oily forage fish in the herring family Clupeidae. The term "sardine" was first used in English during the early 15th century, a folk etymology says it comes from the It ...
s also have operated in the Pacific, the Mediterranean, and the southeast Atlantic.#Bone, Bone and Moore, p. 443 The world annual catch of forage fish in recent years has been approximately 22 million tonnes, or one quarter of the world's total catch. File:Pacific sardine002.jpg, These Shoaling and schooling, schooling Pacific sardines are
forage fish Forage fish, also called prey fish or bait fish, are small pelagic fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish feed near the base of the f ...
. File:Herringramkils.jpg, Herrings Forage fish#Hunting copepods, ram feeding on
copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have ...
s File:Mallotus villosus.gif, Capelin File:Anchovy closeup.jpg, Anchovy, Anchovies File:Enrin u0.png, Peruvian anchoveta


Predator fish

Medium size pelagic fishes include trevally, barracuda,
flying fish The Exocoetidae are a family of marine fish in the order Beloniformes class Actinopterygii, known colloquially as flying fish or flying cod. About 64 species are grouped in seven to nine genera. While they cannot fly in the same way a bird d ...
, bonito, mahi mahi, and coastal mackerel. Many of these fish hunt forage fish, but are in turn, hunted by yet larger pelagic fish. Nearly all fish are predator fish to some measure, and apart from the top predators, the distinction between predator fish and prey or forage fish, is somewhat artificial. Around Europe there are three populations of coastal mackerel. One population migrates to the North Sea, another stays in the waters of the Irish Sea, and the third population migrates southward along the west coast of Scotland and Ireland. The cruise speed of the mackerel is an impressive 10 kilometres per hour.Mackerel
Institute of Marine Research. Retrieved 23 July 2009.
Many large pelagic fish are oceanic nomadic species that undertake long offshore migrations. They feed on small pelagic forage fish, as well as medium-sized pelagic fish. At times, they follow their schooling prey, and many species form schools themselves. Examples of larger pelagic fish are
tuna A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max length: ...
,
billfish The term billfish refers to a group of saltwater predatory fish characterised by prominent pointed bills (rostra), and by their large size; some are longer than . Extant billfish include sailfish and marlin, which make up the family Istioph ...
, king mackerel, sharks, and large Ray-finned fish, rays. Tuna in particular are of major importance to commercial fisheries. Although tuna migrate across oceans, trying to find them there is not the usual approach. Tuna tend to congregate in areas where food is abundant, along the boundaries of currents, around islands, near seamounts, and in some areas of upwelling along continental slopes. Tuna are captured by several methods: Seiner, purse seine vessels enclose an entire surface school with special nets, Fishing vessel#Line vessels, pole and line vessels that use poles baited with other smaller pelagic fish as baitfish, and rafts called fish aggregating devices are set up, because tuna, as well as some other pelagic fish, tend to congregate under floating objects. Other large pelagic fish are premier game fish, particularly marlin and swordfish. File:Yellowfin tuna nurp.jpg, Yellowfin tuna are being fished as a replacement for the now largely depleted Southern bluefin tuna. File:Brama brama.jpg, Atlantic pomfret File:Xiphias gladius1.jpg, Swordfish File:Sccav u0.gif, alt=King mackerels cruise on long migrations at 10 kilometres per hour, King mackerels cruise on long migrations at 10 kilometres per hour.


Productivity

Upwelling occurs both along coastlines and in midocean when a collision of deep ocean currents brings cold water that is rich in nutrients to the surface. These upwellings support blooms of phytoplankton, which in turn, produce zooplankton and support many of the world's main fisheries. If the upwelling fails, then fisheries in the area fail. Moyle and Cech, pp. 574–575 In the 1960s the Peruvian anchoveta fishery was the world's largest fishery. The anchoveta population was greatly reduced during the 1972 El Niño event, when warm water drifted over the cold Humboldt Current, as part of a 50-year cycle, lowering the depth of the
thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...
. The upwelling stopped and
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. ...
production plummeted, as did the anchoveta population, and millions of seabirds, dependent on the anchoveta, died. Since the mid-1980s, the upwelling has resumed, and the Peruvian anchoveta catch levels have returned to the 1960s levels. Off Japan, the collision of the Oyashio Current with the Kuroshio Current produces nutrient-rich upwellings. Cyclic changes in these currents resulted in a decline in the
sardine "Sardine" and "pilchard" are common names for various species of small, oily forage fish in the herring family Clupeidae. The term "sardine" was first used in English during the early 15th century, a folk etymology says it comes from the It ...
''sardinops melanosticta'' populations. Fisheries catches fell from 5 million tonnes in 1988 to 280 thousand tonnes in 1998. As a further consequence, Pacific bluefin tuna stopped moving into the region to feed. Ocean currents can shape how fish are distributed, both concentrating and dispersing them. Adjacent ocean currents can define distinct, if shifting, boundaries. These boundaries can even be visible, but usually their presence is marked by rapid changes in salinity, temperature, and turbidity. For example, in the Asian northern Pacific, albacore are confined between two current systems. The northern boundary is determined by the cold North Pacific Current and the southern boundary is determined by the North Equatorial Current. To complicate things, their distribution is further modified within the area defined by the two current systems by another current, the Kuroshio Current, whose flows fluctuate seasonally. Epipelagic fish often spawn (biology), spawn in an area where the eggs and larvae drift downstream into suitable feeding areas, and eventually, drift into adult feeding areas. Islands and Fishing bank, banks can interact with currents and upwellings in a manner that results in areas of high ocean productivity. Large eddies can form downcurrent or downwind from islands, concentrating plankton. Banks and reefs can intercept deep currents that upwell. * Scombrids


Highly migratory species

Epipelagic fish generally move long distances between feeding and spawning areas, or as a response to changes in the ocean. Large ocean predators, such as salmon and tuna, can migrate thousands of kilometres, crossing oceans. Moyle and Cech, p. 578 In a 2001 study, the movements of Atlantic bluefin tuna from an area off North Carolina were studied with the help of special popup tags. When attached to a tuna, these tags monitored the movements of the tuna for about a year, then detached and floated to the surface where they transmitted their information to a satellite. The study found that the tuna had four different migration patterns. One group confined itself to the western Atlantic for a year. Another group also stayed mainly in the western Atlantic, but migrated to the Gulf of Mexico for spawning. A third group moved across the Atlantic Ocean and back again. The fourth group crossed to the eastern Atlantic and then moved into the Mediterranean Sea for spawning. The study indicates that, while there is some differentiation by spawning areas, there is essentially only one population of Atlantic bluefin tuna, intermixing groups that between them, use all of the north Atlantic Ocean, the Gulf of Mexico, and the Mediterranean Sea. The term highly migratory species (HMS) is a legal term that has its origins in Article 64 of the United Nations Convention on the Law of the Sea (UNCLOS). The highly migratory species include:
tuna A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max length: ...
and tuna-like species (Albacore tuna, albacore, Atlantic bluefin,
bigeye tuna The bigeye tuna (''Thunnus obesus'') is a species of true tuna of the genus '' Thunnus'', belonging to the wider mackerel family Scombridae. In Hawaiian, it is one of two species known as ahi, the other being the yellowfin tuna. Bigeye tuna are ...
, Skipjack tuna, skipjack, Yellowfin tuna, yellowfin, Blackfin tuna, blackfin, Euthynnus alletteratus, little tunny, Pacific bluefin tuna, Pacific bluefin, Southern bluefin tuna, southern bluefin and Auxis rochei, bullet), Bramidae, pomfret, marlin, sailfish, swordfish, saury and oceangoing sharks, as well as mammals such as dolphins, and other cetaceans. Essentially, highly migratory species coincide with the larger of the "large pelagic fish", discussed in the previous section, if cetaceans are added and some commercially unimportant fish, such as the Molidae, sunfish, are excluded. These are high trophic level species that undertake migrations of significant, but variable distances across oceans for feeding, often on forage fish, or reproduction, and also have wide geographic distributions. Thus, these species are found both inside the exclusive economic zones and in the high seas outside these zones. They are pelagic species, which means they mostly live in the open ocean and do not live near the sea floor, although they may spend part of their life cycle in nearshore waters.


Capture production

According to the Food and Agriculture Organization (FAO), the Fishing by country, world harvest in 2005 consisted of 93.2 million
tonne The tonne ( or ; symbol: t) is a unit of mass equal to 1000  kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...
s captured by commercial fishing in wild fisheries. Of this total, about 45% were pelagic fish. The following table shows the world capture production in
tonne The tonne ( or ; symbol: t) is a unit of mass equal to 1000  kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...
s.


Threatened species

In 2009, the International Union for Conservation of Nature (IUCN) produced the first IUCN Red List, red list for threatened oceanic sharks and rays. They claim that approximately one third of open ocean sharks and rays are under Threatened species, threat of extinction.Third of open ocean sharks threatened with extinction
IUCN. 25 June 2009.
There are 64 species of oceanic sharks and rays on the list, including Hammerhead shark, hammerheads, giant Mobula, devil rays, and porbeagle.Fishing puts a third of all oceanic shark species at risk of extinction
''guardian.co.uk'', 26 June 2009.
Oceanic sharks are incidental catch, captured incidentally by swordfish and tuna high seas fisheries. In the past there were few markets for sharks, which were regarded as worthless bycatch. Now sharks are being increasingly targeted to supply emerging Asian markets, particularly for Shark finning, shark fins, which are used in shark fin soup. The northwest Atlantic Ocean shark populations are estimated to have declined by 50% since the early 1970s. Oceanic sharks are vulnerable because they do not produce many young, and the young can take decades to mature. File:Sphyrnalewini.jpg, The scalloped hammerhead is classified as endangered. File:Oceanic Whitetip Shark.png, alt=The oceanic whitetip shark has declined by 99% in the Gulf of Mexico, The oceanic whitetip shark has declined by 99% in the Gulf of Mexico. File:Atlantic mobula lisbon.jpg, The devil fish, a large ray, is threatened. File:Lamna nasus.jpg, The Porbeagle, porbeagle shark is threatened. In parts of the world the scalloped hammerhead shark has declined by 99% since the late 1970s. Its status on the red list is that it is globally endangered, meaning it is near extinction.


See also

* Deep sea * Deep sea fish * Demersal fish * Freshwater fish * Nekton * Ocean Tracking Network * Oily fish * Pacific Ocean Shelf Tracking Project * Tagging of Pacific Predators


References

Notes Bibliography * *


Further reading

* Collette, BB (2010
"Reproduction and development in epipelagic fishes"
In: Kathleen S Cole, ''Reproduction and Sexuality in Marine Fishes: Patterns and Processes'', pp. 21–64, University of California Press. . * Freon, Pierre (1998) ''Dynamics of Pelagic Fish Distribution and Behaviour: Effects on Fisheries and Stock Assessment'', Wiley-Blackwell. . * * * Pepperell J (2011
''Fishes of the Open Ocean: A Natural History and Illustrated Guide''
University of New South Wales Press, . * Salvanesa AGV and Kristoffersen J
"Mesopelagic Fishes"
In: ''Encyclopedia of Ocean Sciences'', pp. 1711–1717.

''PhysOrg.com'', 26 March 2009.
One fish, two fish: New MIT sensor improves fish counts
''PhysOrg.com'', 2 February 2006.


External links


Glowing life in an underwater world
TED video from Edith Widder
The Open Ocean
''MarineBio.org''. MarineBio.org. Updated 28 August 2011. TED video from Edith Widder
The Open Ocean
''MarineBio.org''. MarineBio.org. Updated 28 August 2011.
Pelagic Advisory Council
of the European Commission {{DEFAULTSORT:Pelagic Fish Ichthyology Fishing industry