Fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

go through various life stages between

fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

and adulthood. The life of fish start as

spawn Spawn or spawning may refer to: * Spawning, the eggs and sperm of aquatic animals Arts, entertainment and media * Spawn (character), a fictional character in the comic series of the same name and in the associated franchise ** ''Spawn: Armageddon' ...

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the ...

s which hatch into immotile larvae. These larval

hatchling In oviparous biology, a hatchling is a newly hatched fish, amphibian, reptile, or bird. A group of mammals called monotremes lay eggs, and their young are hatchlings as well. Fish Fish hatchlings generally do not receive parental care, similar t ...

s are not yet capable of feeding themselves and carry a

yolk sac The yolk sac is a membranous wikt:sac, 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' ...

which provides stored nutrition. Before the yolk sac completely disappears, the young fish must mature enough to be able to

forage Forage is a plant material (mainly plant leaves and stems) eaten by grazing livestock. Historically, the term ''forage'' has meant only plants eaten by the animals directly as pasture, crop residue, or immature cereal crops, but it is also used m ...

independently. When they have developed to the point where they are capable of feeding by themselves, the fish are called fry. When, in addition, they have developed scales and working

fin A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. F ...

s, the transition to a juvenile fish is complete and it is called a fingerling, so called as they are typically about the size of human fingers. The juvenile stage lasts until the fish is fully grown,

sexually mature Sexual maturity is the capability of an organism to reproduce. In humans, it is related to both puberty and adulthood. ''Puberty'' is the biological process of sexual maturation, while ''adulthood'', the condition of being socially recognized as ...

and interacting with other adult fish.

Growth stages

Ichthyoplankton ''(planktonic or drifting fish)'' are the

s and

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

e of fish. They are usually found in the sunlit zone of the

water column The (oceanic) water column is a concept used in oceanography to describe the physical (temperature, salinity, light penetration) and chemical ( pH, dissolved oxygen, nutrient salts) characteristics of seawater at different depths for a defined ...

, less than 200 metres deep, sometimes called the

epipelagic The photic zone (or 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 ...

photic zone The photic zone (or 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 ...

. Ichthyoplankton are

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

ic, meaning they cannot swim effectively under their own power, but must drift with 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 heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...

that eat smaller plankton, while fish eggs carry their own food supply. Both eggs and larvae are themselves eaten by larger animals.What are Ichthyoplankton?
Southwest Fisheries Science Center, NOAA. Modified 3 September 2007. Retrieved 22 July 2011.Moser HG and Watson W (2006
"Ichthyoplankton"
Pages 269–319. In: Allen LG, Pondella DJ and Horn MH, ''Ecology of marine fishes: California and adjacent waters'' University of California Press. . According to Kendall et al. 1984Kendall 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. there are three main developmental stages of fish: * Egg stage: From

ing to hatching. This stage is named so, instead of being called an

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

nic stage, because there are aspects, such as those to do with the egg envelope, that are not just embryonic aspects. * Larval stage: From the eggs hatching till to when fin rays are present and the growth of protective scales has started (squamation). A key event is when the

notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...

associated with the tail fin on the

ventral Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

side of the spinal cord develops and becomes flexible. A transitional stage known as the ''sac larval'' stage, lasts from hatching to the complete resorption of the

. * Juvenile stage: Starts when the morphological transformation or

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and different ...

from larva to juvenile is complete, that is, when the larva develops the features of a functional fish. These features are that all the fin rays are present and that scale growth is under way. The stage completes when the juvenile becomes adult, that is, when it becomes

or starts interacting with other adults. This article is about the larval and juvenile stage. * Hatchling – refers to a recently hatched fish larva that is still too immature to achieve

motility Motility is the ability of an organism to move independently using metabolism, metabolic energy. This biological concept encompasses movement at various levels, from whole organisms to cells and subcellular components. Motility is observed in ...

, and therefore not yet capable of active feeding. A hatchling still possesses a

upon which it depends for nutrition, and are thus also known as a sac fry. * Fry – refers to a more developed hatchling whose yolk sac has almost disappeared, and its

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ (anatomy), organ in bony fish that functions to modulate buoyancy, and thus allowing the fish to stay at desired water depth without having to maintain lift ...

is functional to the point where the fish can move around and perform limited

foraging Foraging is searching for wild food resources. It affects an animal's fitness because it plays an important role in an animal's ability to survive and reproduce. Foraging theory is a branch of behavioral ecology that studies the foraging behavi ...

to nourish itself.Guo Z, Xie Y, Zhang X, Wang Y, Zhang D and Sugiyama S (2008) tp://ftp.fao.org/docrep/fao/010//i0111e/i0111e00.pdf ''Review of fishery information and data collection systems in China''Page 38. FAO Fisheries and Aquaculture, Circular 1029. FAO, Rome. . At this stage, the fish usually filter-feeds on

s as it is still too small and slow to venture away from covers without being consumed by

predator Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

s. * Fingerling – refers to a fish that has reached the stage where the

s can be extended and protective scales have covered the body. At this stage, the fish is typically about the size of a human finger, hence the name. Once reaching this stage, the fish can be considered a juvenile, and is usually active enough to move around a large area. The feeding diet also changes from planktons to

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s and

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

, occasionally other fish. The fish also starts to morphologically resemble

adult An adult is an animal that has reached full growth. The biological definition of the word means an animal reaching sexual maturity and thus capable of reproduction. In the human context, the term ''adult'' has meanings associated with social an ...

fish gradually (though still smaller), although it is not yet achieved sexual maturity.

Juvenile salmon

Fry and fingerling are

generic term Trademark distinctiveness is an important concept in the law governing trademarks and service marks. A trademark may be eligible for registration, or registrable, if it performs the essential trademark function, and has distinctive character. Re ...

s that can be applied to the juveniles of most fish species, but some groups of fishes have juvenile development stages particular to the group. This section details the stages and the particular names used for juvenile salmon. * ''Sac fry'' or ''alevin'' – The life cycle of salmon begins and usually also ends in the backwaters of streams and rivers. These are their

spawning Spawn is the Egg cell, eggs and Spermatozoa, sperm released or deposited into water by aquatic animals. As a verb, ''to spawn'' refers to the process of freely releasing eggs and sperm into a body of water (fresh or marine); the physical act is ...

grounds otherwise known as redds, where salmon eggs are deposited for among the

gravel Gravel () is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentation, sedimentary and erosion, erosive geological processes; it is also produced in large quantities commercially as crushed stone. Gr ...

s of

stream bed A streambed or stream bed is the bottom of a stream or river and is confined within a Stream channel, channel or the Bank (geography), banks of the waterway. Usually, the bed does not contain terrestrial (land) vegetation and instead supports d ...

s. The salmon spawning grounds are also the salmon nurseries, providing a more protected environment than the ocean usually offers. After 2 to 6 months the eggs hatch into tiny salmon

e called ''sac fry'' or ''alevin''. These

s have a

containing the remainder of the

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 bec ...

and they stay hidden in the gravels for a few more days, as they are still largely immobile and rely on the remaining nutrients stored in the sac for survival. * ''Fry'' – When the larvae develop further, their yolk sac gets depleted. Once the sac is almost completely gone the larvae must begin

for food by themselves, so they leave the protection of the gravel bed and start using their now-stronger

tail The tail is the elongated section at the rear end of a bilaterian animal's body; in general, the term refers to a distinct, flexible appendage extending backwards from the midline of the torso. In vertebrate animals that evolution, evolved to los ...

s to swim around feeding on

. At this point these mobile alevins become ''fries''. * ''Parr'' – When a fry has grown to roughly the size of a human finger, it develops protective scales and sufficiently strong

s, and is thus colloquially known as a ''fingerling''. Fingerlings will start moving to a more

carnivorous A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose nutrition and energy requirements are met by consumption of animal tissues (mainly mu ...

diet and quickly gain body mass, and at the end of the summer they develop into juvenile salmon called ''parr''. Parr feed on small

s and are camouflaged with a pattern of spots and vertical bars. They remain in this stage for up to three years.Bley 1988Lindberg 2011 Some older male parr are even already able to fertilize adult females' eggs in the spawning season, without having passed through the subsequent stages of development, and compete to do so with much larger

anadromous Fish migration is mass relocation by fish from one area or body of water to another. Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousa ...

adult males returning from the sea. * ''Smolt'' – As they approach the time when they are ready to migrate out to the sea, the parr lose their camouflage bars and undergo a process of physiological changes called smoltification - this allows them to survive a shift from freshwater to saltwater environments. At this point these young salmon are called ''smolt''. Smolt spend time in the

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

waters of the

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...

while their body chemistry adjusts (

osmoregulation Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration ...

) to the higher salt levels they will encounter in the ocean.''Atlantic Salmon Trust'' 2011 Smolt also grow silvery scales with

countershading Countershading, or Thayer's law, is a method of camouflage in which animal coloration, 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, reptile ...

, which visually confuse ocean predators. * ''Post-smolt'' – When they have matured sufficiently in late spring and are about 15 to 20 centimetres long, the smolt swim out of the rivers and into the sea. There they spend their first year as ''post-smolt''. Post-smolt form

schools A school is the educational institution (and, in the case of in-person learning, the building) designed to provide learning environments for the teaching of students, usually under the direction of teachers. Most countries have systems of ...

with other post-smolt and set off to find deep-sea feeding grounds. They then spend up to four more years as adult ocean salmon while their full swimming and reproductive capacity develops. File:Salmo salar eggs.jpg, Salmon eggs. The growing larvae can be seen through the transparent egg envelope. The black spots are the eyes. File:Salmonlarvakils 2.jpg, Salmon egg hatching into a ''sac fry''. In a few days, the sac fry will absorb the yolk sac and become a salmon fry File:Atlantic salmon redd.jpg, Sac fry remain in the gravel habitat of their redd (nest) while their yolk sac, or "lunch box" is depleted ''(click to enlarge)'' File:SalmonoidsBergeau.jpg, The juvenile salmon, ''parr'', grow up in the relatively protected natal river File:Coho.jpg, The parr lose their camouflage bars and become ''smolt'' as they become ready for the transition to the ocean File:Salmo salar GLERL 1.jpg, Salmon enter the ocean as ''post-smolt'' and mature into adult salmon. They gain most of their weight in the ocean

Protection from predators

Juvenile fish need protection from predators. Juvenile species, as with small species in general, can achieve some safety in numbers by schooling together.Bone Q and Moore RH (2008
''Biology of Fishes''
pp. 418–422, Taylor & Francis Group. Juvenile coastal fish are drawn to turbid shallow waters and to

mangrove A mangrove is a shrub or tree that grows mainly in coastal saline water, saline or brackish water. Mangroves grow in an equatorial climate, typically along coastlines and tidal rivers. They have particular adaptations to take in extra oxygen a ...

structures, where they have better protection from predators. As the fish grow, their foraging ability increases and their vulnerability to predators decreases, and they tend to shift from mangroves to

mudflat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal ...

s. In the open sea juvenile species often aggregate around floating objects such as jellyfish and '' Sargassum '' seaweed. This can significantly increase their survival rates.

As human food

Juvenile fish are marketed as food. * ''

Whitebait Whitebait is a collective term for the immature fry of fish, typically between long. Such young fish often travel together in schools along coasts, and move into estuaries and sometimes up rivers where they can be easily caught using fine-mes ...

'' is a marketing term for the fry of fish, typically between 25 and 50 millimetres long. Such juvenile fish often travel together in

along the coast, and move into

and sometimes up rivers where they can be easily caught with fine meshed

fishing net A fishing net or fish net is a net (device), net used for fishing. Fishing nets work by serving as an improvised fish trap, and some are indeed rigged as traps (e.g. #Fyke nets, fyke nets). They are usually wide open when deployed (e.g. by cast ...

s. ''Whitebaiting'' is the activity of catching whitebait. Whitebait are tender and edible, and can be regarded as a delicacy. The entire fish is eaten including head, fins and gut. Some species make better eating than others, and the particular species that are marketed as "whitebait" varies in different parts of the world. As whitebait consists of fry of many important food species (such as

herring Herring are various species of forage fish, belonging to the Order (biology), order Clupeiformes. Herring often move in large Shoaling and schooling, schools around fishing banks and near the coast, found particularly in shallow, temperate wate ...

sprat Sprat is the common name applied to a group of forage fish belonging to the genus ''Sprattus'' in the Family (biology), family Clupeidae. The term also is applied to a number of other small sprat-like forage fish (''Clupeoides'', ''Clupeonella ...

sardine Sardine and pilchard are common names for various species of small, oily forage fish in the herring suborder Clupeoidei. The term "sardine" was first used in English during the early 15th century; a somewhat dubious etymology says it com ...

mackerel Mackerel is a common name applied to a number of different species of pelagic fish, mostly from the family Scombridae. They are found in both temperate and tropical seas, mostly living along the coast or offshore in the oceanic environment. ...

bass Bass or Basses may refer to: Fish * Bass (fish), various saltwater and freshwater species Wood * Bass or basswood, the wood of the tilia americana tree Music * Bass (sound), describing low-frequency sound or one of several instruments in th ...

and many others) it is not an ecologically viable foodstuff and in several countries strict controls on harvesting exist. * ''Glass eels'' are young

eels Eels are ray-finned fish belonging to the order (biology), order Anguilliformes (), which consists of eight suborders, 20 Family (biology), families, 164 genus, genera, and about 1000 species. Eels undergo considerable development from the earl ...

, the stage in eel life history when eels first arrive in rivers and swim upstream from the sea in which they hatched. Because the eel cannot be farmed, eels have instead been caught from the wild as juveniles and reared in captivity for human consumption, reducing the wild population further. Glass eels become ''Elvers'' after they swim to the open ocean. Traditionally, fishermen consumed elvers as a cheap dish, but environmental changes have greatly reduced eel populations and the European eel is now

critically endangered An IUCN Red List critically endangered (CR or sometimes CE) species is one that has been categorized by the International Union for Conservation of Nature as facing an extremely high risk of extinction in the wild. As of December 2023, of t ...

. Like whitebait, elvers are now considered a delicacy and are priced at up to 1000

euro The euro (currency symbol, symbol: euro sign, €; ISO 4217, currency code: EUR) is the official currency of 20 of the Member state of the European Union, member states of the European Union. This group of states is officially known as the ...

per kilogram. A small serving of Spanish ''angulas'', for example, can cost the equivalent of US$100, and other species which can be purchased cheaply are prepared and eaten as "''angulas''" instead. Glass eels are regularly smuggled out of Europe having been harvested illegally for Asian and Russian consumers; smugglers can earn millions of

pounds sterling Sterling (Currency symbol, symbol: Pound sign, £; ISO 4217, currency code: GBP) is the currency of the United Kingdom and nine of its associated territories. The pound is the main unit of account, unit of sterling, and the word ''Pound (cu ...

. Elvers reach a higher price in China than does

beluga caviar Beluga caviar is caviar consisting of the roe (or eggs) of the beluga sturgeon (''Huso huso''). The fish is found primarily in the Caspian Sea, which is bordered by Iran, Azerbaijan, Kazakhstan, Russia, and Turkmenistan. It can also be found in ...

. The

Marine Conservation Society The Marine Conservation Society is a UK-based charitable organisation working with businesses, governments and communities to clean and protect oceans. Founded in 1978 as the Underwater Conservation Society, the group claims to be working towards ...

advises against buying European eels.

Notes

References

* ''Atlantic Salmon Trust'
Salmon Facts
Retrieved 15 December 2011. * Bley, Patrick W and Moring, John R (1988
Freshwater and Ocean Survival of Atlantic Salmon and Steelhead: A Synopsis"
''US Fish and Wildlife Service''. * Lindberg, Dan-Erik (2011
Atlantic salmon (Salmo salar) migration behavior and preferences in smolts, spawners and kelts
Introductory Research Essay, ''Swedish University of Agricultural Sciences''. {{Authority control Aquatic ecology Fish reproduction Ichthyology