Ichthyoplankton (from Greek:

ἰχθύς The ichthys or ichthus (), from the Greek (, 1st cent. AD Koine Greek pronunciation: , "fish") is (in its modern rendition) a symbol consisting of two intersecting arcs, the ends of the right side extending beyond the meeting point so as to res ...

, , "fish"; and πλαγκτός, , "drifter") are the

eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

and larvae of fish. They are mostly found in the sunlit zone 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 ...

, less than 200 metres deep, which is sometimes called the

epipelagic 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 ...

or photic zone. Ichthyoplankton are planktonic, meaning they cannot swim effectively under their own power, but must drift with the ocean currents. Fish eggs cannot swim at all, and are unambiguously planktonic. Early stage larvae swim poorly, but later stage larvae swim better and cease to be planktonic as they grow into juveniles. Fish larvae are part 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 ...

that eat smaller plankton, while fish eggs carry their own food supply. Both eggs and larvae are themselves eaten by larger animals. Fish can produce high numbers of eggs which are often released into the open water column. Fish eggs typically have a diameter of about . The newly hatched young of oviparous fish are called

larva 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. ...

e. They are usually poorly formed, carry a large

yolk sac The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is ...

(for nourishment) and are very different in appearance from juvenile and adult specimens. The larval period in oviparous fish is relatively short (usually only several weeks), and larvae rapidly grow and change appearance and structure (a process termed metamorphosis) to become juveniles. During this transition larvae must switch from their yolk sac to feeding on

prey, a process which depends on typically inadequate zooplankton density, starving many larvae. Ichthyoplankton can be a useful indicator of the state and health of an

aquatic ecosystem An aquatic ecosystem is an ecosystem formed by surrounding a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms that are dependent on each other and on their environment. The t ...

. For instance, most late stage larvae in ichthyoplankton have usually been preyed on, so ichthyoplankton tends to be dominated by eggs and early stage larvae. This means that when fish, such as

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, are

spawning Spawn is the eggs and sperm released or deposited into water by aquatic animals. As a verb, ''to spawn'' refers to the process of releasing the eggs and sperm, and the act of both sexes is called spawning. Most aquatic animals, except for aquat ...

, ichthyoplankton samples can reflect their spawning output and provide an index of relative population size for the fish. Increases or decreases in the number of adult fish stocks can be detected more rapidly and sensitively by monitoring the ichthyoplankton associated with them, compared to monitoring the adults themselves. It is also usually easier and more cost effective to sample trends in egg and larva populations than to sample trends in adult fish populations.

History

Interest in plankton originated in Britain and Germany in the nineteenth century when researchers discovered there were

microorganism 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 in the sea, and that they could trap them with fine-mesh nets. They started describing these microorganisms and testing different net configurations. Ichthyoplankton research started in 1864 when the Norwegian government commissioned the

marine biologist Marine biology is the scientific study of the biology of marine life, organisms in the sea. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifi ...

G. O. Sars to investigate fisheries around the Norwegian coast. Sars found fish eggs, particularly cod eggs, drifting in the water. This established that fish eggs could be

pelagic 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 w ...

, living in the open water column like other plankton.
Google Translate
Around the beginning of the twentieth century, research interest in ichthyoplankton became more general when it emerged that, if ichthyoplankton was sampled quantitatively, then the samples could indicate the relative size or abundance of spawning fish stocks.

Sampling methods

File:PairoVET tow.jpg, PairoVET tow File:Bongo tow.jpg, Bongo tow File:THE "MARTIN KARLSEN," A CANADIAN SHIP, IS A FLOATING LABORATORY FROM WHICH SCIENTISTS ARE ANALYZING PHYSICAL... - NARA - 549596.tif, Retrieving a plankton sample Research vessels collect ichthyoplankton from the ocean using fine mesh nets. The vessels either tow the nets through the sea or pump sea water onboard and then pass it through the net.Ichthyoplankton sampling methods
Southwest Fisheries Science Center, '' NOAA''. Retrieved 11 July 2020.

* There are many types of plankton tows: :*

Neuston Neuston, also known as pleuston, are organisms that live at the surface of the ocean or an estuary, or at the surface of a lake, river or pond. Neuston can live on top of the water surface or may be attached to the underside of the water surface. ...

net tows are often made at or just below the surface using a nylon mesh net fitted to a rectangular frame :* The PairoVET tow, used for collecting fish eggs, drops a net about 70 metres into the sea from a stationary research vessel and then drags it back to the vessel. :* Ring net tows involve a nylon mesh net fitted to a circular frame. These have largely been replaced by bongo nets, which provide duplicate samples with their dual-net design. :* The bongo tow drags nets shaped like bongo drums from a moving vessel. The net is often lowered to about 200 metres and then allowed to rise to the surface as it is towed. In this way, a sample can be collected across the whole photic zone where most ichthyoplankton is found. :*

MOCNESS MOCNESS is the acronym for ''Multiple Opening/Closing Net and Environmental Sensing System'' and is a net system for plankton in the ocean. The system is towed behind a research vessel with a speed of up to 2.5 knots and consists of five to twenty n ...

tows and Tucker trawls utilize multiple nets that are mechanically opened and closed at discrete depths in order to provide insights into the vertical distribution of the plankton :* The manta trawl tows a net from a moving vessel along the surface of the water, collecting larvae, such as

grunion Grunion are two fish species of the genus ''Leuresthes'': the California grunion, ''L. tenuis'', and the Gulf grunion, ''L. sardinas''. They are sardine-sized teleost fishes of the New World silverside family Atherinopsidae, found only off ...

, mahi-mahi, and

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 ...

which live at the surface. :After the tow the plankton is flushed with a hose to the cod end (bottom) of the net for collection. The sample is then placed in preservative fluid prior to being sorted and identified in a laboratory. * Plankton pumps: Another method of collecting ichthyoplankton is to use a Continuous Underway Fish Egg Sampler (see illustration). Water from a depth of about three metres is pumped onto the vessel and filtered with a net. This method can be used while the vessel is underway.

Developmental stages

Ichthyoplankton researchers generally use the terminology and development stages introduced in 1984 by Kendall and others. This consists of three main developmental stages and two transitional stages.Kendall Jr AW, Ahlstrom EH and Moser HG (1984
"Early life history stages of fishes and their characters"
''American Society of Ichthyologists and Herpetologists'', Special publication 1: 11–22.

Skin ionocytes

Ionocyte An ionocyte (formerly called a chloride cell) is a mitochondrion-rich cell within ionoregulatory organs of animals, such as teleost fish gill, insect Malpighian tubules, crustacean gills, antennal glands and maxillary glands, and copepod Crusalis ...

s (formerly known as mitochondrion-rich cells or as chloride cells) are responsible for maintaining optimal osmotic, ionic, and acid-base levels within the fish. Ionocytes are typically found within adult gills. However, embryonic and larval fishes often lack or have underdeveloped gills. Instead, ionocytes are found along the skin, yolk sac, and fins of the larva. As growth progresses and the gill becomes more developed, ionocytes can be found on the gill arch and gill filament. In larval fishes, the number, size, and density of ionocytes can be quantified as a relative ionocyte area, which has been proposed as a proxy for osmotic, ionic, and/or acid-base capacity of the organism. Ionocytes are also known to be plastic. Ionocyte's apical openings can widen during periods of high activity, and new ionocytes can develop along the gill lamellae during periods of environmental stress. Due to the abundant presence of Na⁺/K⁺-ATPase in the basolateral membrane, ionocytes can often be located using immunohistochemistry.

Survival

Recruitment of fish is regulated by larval fish survival. Survival is regulated by prey abundance, predation, and

hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is call ...

. Fish eggs and larvae are eaten by many marine organisms.Bax NJ (1998
"The significance and prediction of predation in marine fisheries"
''ICES Journal of Marine Science'', 55: 997–1030. For example, they may be fed upon by

marine invertebrate Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some hav ...

s, such as copepods, arrow worms,

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 umbrell ...

, amphipods,

marine snail A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class G ...

s and

krill Krill are small crustaceans of the order Euphausiacea, and are found in all the world's oceans. The name "krill" comes from the Norwegian word ', meaning "small fry of fish", which is also often attributed to species of fish. Krill are consi ...

.Bailey, K. M., and Houde, E. D. (1989
"Predation on eggs and larvae of marine fishes and the recruitment problem"
''Advances in Marine Biology'', 25: 1–83. Because they are so abundant, marine invertebrates inflict high overall mortality rates. Adult fish also prey on fish eggs and larvae. For example,

haddock The haddock (''Melanogrammus aeglefinus'') is a saltwater ray-finned fish from the family Gadidae, the true cods. It is the only species in the monotypic genus ''Melanogrammus''. It is found in the North Atlantic Ocean and associated seas wher ...

were observed satiating themselves with herring eggs back in 1922. Another study found cod in a herring spawning area with 20,000 herring eggs in their stomachs, and concluded that they could prey on half of the total egg production. Fish also cannibalise their own eggs. For example, separate studies found northern anchovy ( ''Engraulis mordax'') were responsible for 28% of the mortality in their own egg population,Santander, H.; Alheit, J.; MacCall, A.D.; Alamo, A. (1983) tp://ftp.fao.org/docrep/fao/005/x6851b/x6851b21.pdf Egg mortality of the Peruvian anchovy (''Engraulis ringens'') caused by cannibalism and predation by sardines (''Sardinops sagax'')''FAO Fisheries Report'', 291(2-3): 443–453. Rome. while

Peruvian anchoveta The Peruvian anchoveta (''Engraulis ringens'') is a species of fish of the anchovy family, Engraulidae, from the Southeast Pacific Ocean. It has yielded greater catches than any other single wild fish species in the world, with annual harvests ...

were responsible for 10% and South African anchovy ( ''Engraulis encrasicolus'') 70%. The most effective predators are about ten times as long as the larvae they prey on. This is true regardless of whether the predator is a crustacean, a jellyfish, or a fish.

Dispersal

Fish larvae develop first an ability to swim up and down the

for short distances. Later they develop an ability to swim horizontally for much longer distances. These swimming developments affect their dispersal.Cowen RK, CB Paris and A Srinivasan (2006) "Scaling of connectivity in marine populations". ''Science'', 311 (5760): 522–527.
PDF
/ref> In 2010, a group of scientists reported that fish larvae can drift on ocean currents and reseed fish stocks at a distant location. This finding demonstrates, for the first time, what scientists have long suspected but have never proven, that fish populations can be connected to distant populations through the process of larval drift.Christie MR, Tissot BN, Albins MA, Beets3 JP, Jia Y, Ortiz DL, Thompson SE, Hixon MA (2010
Larval Connectivity in an Effective Network of Marine Protected Areas
''PLoS ONE'', 5(12) The fish they chose to investigate was the

yellow tang The yellow tang (''Zebrasoma flavescens'') is a saltwater fish species of the family Acanthuridae. It is one of the most popular marine aquarium fish. It is bright yellow in color, and it lives in reefs. The yellow tang spawn around a full m ...

, because when a larva of this fish find a suitable reef it stays in the general area for the rest of its life. Thus, it is only as drifting larvae that the fish can migrate significant distances from where they are born. The tropical yellow tang is much sought after by the aquarium trade. By the late 1990s, their stocks were collapsing, so in an attempt to save them nine

marine protected areas Marine protected areas (MPA) are protected areas of seas, oceans, estuaries or in the US, the Great Lakes. These marine areas can come in many forms ranging from wildlife refuges to research facilities. MPAs restrict human activity for a conserv ...

(MPAs) were established off the coast of Hawaii. Now, through the process of larval drift, fish from the MPAs are establishing themselves in different locations, and the fishery is recovering. "We've clearly shown that fish larvae that were spawned inside marine reserves can drift with currents and replenish fished areas long distances away," said one of the authors, the

Mark Hixon. "This is a direct observation, not just a model, that successful marine reserves can sustain fisheries beyond their borders."Drifting Fish Larvae Allow Marine Reserves to Rebuild Fisheries
''ScienceDaily '', 26 December 2010.

Gallery

File:C. l. maraenaeggs01.jpg, ''

Coregonus maraena ''Coregonus maraena'', referred to in English as the maraene, maraena whitefish or the whitefish, is a whitefish of the family Salmonidae that occurs in the Baltic Sea basin - in the sea itself and the inflowing rivers, and in several lakes as ...

'' eggs about one month after fertilization File:Ragfish egg.jpg,

Ragfish The ragfish (''Icosteus aenigmaticus'') is a ray-finned fish of the northern Pacific Ocean; although a perciform, its skeleton is mostly cartilage, and the larvae have pelvic fins that disappear as they mature. It is the sole member of the family ...

egg File:Salmoneggskils.jpg, Salmon eggs in different stages of development. File:Frai de poissons rouges anim.gif, Male

goldfish The goldfish (''Carassius auratus'') is a freshwater fish in the family Cyprinidae of order Cypriniformes. It is commonly kept as a pet in indoor aquariums, and is one of the most popular aquarium fish. Goldfish released into the wild have bec ...

encourage a spawning female and discharge sperm to externally fertilize her eggs File:Goldfishfry.JPG, Within days, the vulnerable goldfish eggs hatch into

e, and rapidly develop into fry File:Pacific cod larvae.jpg,

Pacific cod The Pacific cod (''Gadus macrocephalus)'' is a species of ray-finned fish in the family Gadidae. It is a bottom-dwelling fish found in the northern Pacific Ocean, mainly on the continental shelf and upper slopes, to depths of about . It can gro ...

larva File:Walleye larva (8740460659).jpg,

Walleye The walleye (''Sander vitreus'', synonym ''Stizostedion vitreum''), also called the yellow pike or yellow pickerel, is a freshwater perciform fish native to most of Canada and to the Northern United States. It is a North American close relat ...

larva File:Larval stage of bluefin tuna.jpg,

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 ...

larva File:Common sturgeon larva.jpg,

Common sturgeon The European sea sturgeon (''Acipenser sturio''), also known as the Atlantic sturgeon or common sturgeon, is a species of sturgeon native to Europe. It was formerly abundant, being found in coastal habitats all over Europe. It is anadromous and b ...

larva File:Clupeaharenguskils2.jpg, Atlantic herring eggs, with a newly hatched larva File:Clupealarvamatchkils.jpg, Freshly hatched herring larva in a drop of water compared to a match head. File:Clupeaharenguslarvaeinsitukils.jpg, Early stage herring larvae imaged ''in situ'' with

yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example ...

remains File:Molalavdj.jpg, A 2.7mm long larva of the ocean sunfish, Mola mola, File:FMIB 47039 Ostracion hoops.jpeg, Boxfish larva

Notes

References

* Ahlstrom, Elbert H. and Moser, H. Geoffrey (1976
"Eggs and larvae of fishes and their role in systematic investigations in fisheries"
''Revue des Travaux de l'Institut des Pêches Maritimes'', 40(3-4): 379–398. * Balon, Eugene K. (1990
"Epigenesis of an epigeneticist: the development of some alternative concepts on the early ontogeny and evolution of fishes
''Guelph Ichthyology Reviews'', 1: 1–48. * Blaber, Stephen J. M. (2000
''Tropical estuarine fishes: ecology, exploitation and conservation''
John Wiley and Sons, Page 153–156. . * Browman, Howard I. and Skiftesvik, Anne Berit (2003
The Big Fish Bang: Proceedings of the 26th Annual Larval Fish Conference
Institute of Marine Research. . * Finn, Roderick Nigel and Kapoor, B. G. (2008
''Fish larval physiology''
Science Publishers.. * Cowan, J.H., Jr. and R.F. Shaw (2002
"Recruitment"
Chap. 4. pp. 88–111. In: L.A. Fuiman and R.G. Werner (eds.) ''Fishery Science: The unique contributions of early life stages'', John Wiley and Sons. .+ * Chambers RC and Trippel EA (1997
''Early life history and recruitment in fish population''
Springer. . * Houde ED (2010
"Fish Larvae"
Page 286–295. In: JH Steele, SA Thorpe and KK Turekian, ''Marine Biology'', Academic Press. . * Kendall Jr., Arthur W. (2011
''Identification of Eggs and Larvae of Marine Fishes''
東海大学出版会, 2011. . * Miller, Bruce S. and Kendall, Arthur W. (2009
''Early life history of marine fishes''
University of California Press. . * Miller TJ (2002
"Assemblages, Communities, and Species Interactions"
Pages 183–205. In: Lee A. Fuiman and Robert G. Werner, ''Fishery science: the unique contributions of early life stages'', John Wiley and Sons. .
Ichthyoplankton Information System
Alaska Fisheries Center, ''NOAA''.
Early life history section
''American Fisheries Society''.
Larval Fish Laboratory
''Colorado State University''.
''Recent advances in the study of fish eggs and larvae''
''Sci. Mar.'', 70S2: 2006.
Guides and keys to larval and early juvenile fishes
Warner College of Natural Resources, ''Colorado State University''.

External links

Ichthyoplankton Survey Methodology
Presentation by Yoshinobu Konishi, ''SEAFDEC-MFRDMD''.
Salmon Eggs Hatching at the Seymour Hatchery
''Youtube'' video. {{diversity of fish Aquatic ecology Biological oceanography Fish reproduction Ichthyology Planktology