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 thousands of kilometres. Fish usually migrate to feed or to reproduce, but in other cases the reasons are unclear.
Migrations involve movements of the fish on a larger scale and duration than those arising during normal daily activities.Some particular types of migration are anadromous, in which adult fish live in the sea and migrate into fresh water to spawn, and catadromous, in which adult fish live in fresh water and migrate into salt water to spawn.
Marine forage fish often make large migrations between their spawning, feeding and nursery grounds. Movements are associated with ocean currents and with the availability of food in different areas at different times of year. The migratory movements may partly be linked to the fact that the fish cannot identify their own offspring and moving in this way prevents cannibalism. Some species have been described by the United Nations Convention on the Law of the Sea as highly migratory species. These are large pelagic fish that move in and out of the exclusive economic zones of different nations, and these are covered differently in the treaty from other fish.
Salmon and striped bass are well-known anadromous fish, and freshwater eels are catadromous fish that make large migrations. The bull shark is a euryhaline species that moves at will from fresh to salt water, and many marine fish make a diel vertical migration, rising to the surface to feed at night and sinking to lower layers of the ocean by day. Some fish such as tuna move to the north and south at different times of year following temperature gradients. The patterns of migration are of great interest to the fishing industry. Movements of fish in fresh water also occur; often the fish swim upriver to spawn, and these traditional movements are increasingly being disrupted by the building of dams.
As with various other aspects of fish life, zoologists have developed empirical classifications for fish migrations.Two terms in particular have been in long-standing wide usage:
Although these classifications were originated for fish, they are, in principle, applicable to any aquatic organism.
Forage fish often make great migrations between their spawning, feeding and nursery grounds. Schools of a particular stock usually travel in a triangle between these grounds. For example, one stock of herrings have their spawning ground in southern Norway, their feeding ground in Iceland, and their nursery ground in northern Norway. Wide triangular journeys such as these may be important because forage fish, when feeding, cannot distinguish their own offspring.
Capelin are a forage fish of the smelt family found in the Atlantic and Arctic oceans. In summer, they graze on dense swarms of plankton at the edge of the ice shelf. Larger capelin also eat krill and other crustaceans. The capelin move inshore in large schools to spawn and migrate in spring and summer to feed in plankton rich areas between Iceland, Greenland, and Jan Mayen. The migration is affected by ocean currents. Around Iceland maturing capelin make large northward feeding migrations in spring and summer. The return migration takes place in September to November. The spawning migration starts north of Iceland in December or January.
The diagram on the right shows the main spawning grounds and larval drift routes. Capelin on the way to feeding grounds is coloured green, capelin on the way back is blue, and the breeding grounds are red.
In a paper published in 2009, researchers from Iceland recount their application of an interacting particle model to the capelin stock around Iceland, successfully predicting the spawning migration route for 2008.
The term highly migratory species (HMS) has its origins in Article 64 of the United Nations Convention on the Law of the Sea (UNCLOS). The Convention does not provide an operational definition of the term, but in an annex (UNCLOS Annex 1) lists the species considered highly migratory by parties to the convention.The list includes: tuna and tuna-like species (albacore, bluefin, bigeye tuna, skipjack, yellowfin, blackfin, little tunny, southern bluefin and bullet), pomfret, marlin, sailfish, swordfish, saury and oceangoing sharks, dolphins and other cetaceans.
These high trophic level oceanodromous species 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 200 mile 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.
Highly migratory species can be compared with straddling stock and transboundary stock. Straddling stock range both within an EEZ as well as in the high seas. Transboundary stock range in the EEZs of at least two countries. A stock can be both transboundary and straddling.
It can be challenging to determine the population structure of highly migratory species using physical tagging. Traditional genetic markers such as short-range PCR products, microsatellites and SNP-arrays have struggled to identify population structure and distinguish fish stocks from separate ocean basins. However, population genomic research using RAD sequencing in yellowfin tunaand albacore has been able to distinguish populations from different ocean basins and reveal fine-scale population structure. Similar population genomics methods have also provided improved insight towards population structure in striped marlin.
Some of the best-known anadromous fishes are the Pacific salmon species, such as Chinook (king), coho (silver), chum (dog), pink (humpback) and sockeye (red) salmon. These salmon hatch in small freshwater streams. From there they migrate to the sea to mature, living there for two to six years. When mature, the salmon return to the same streams where they were hatched to spawn. Salmon are capable of going hundreds of kilometers upriver, and humans must install fish ladders in dams to enable the salmon to get past. Other examples of anadromous fishes are sea trout, three-spined stickleback, sea lamprey andshad.
Several Pacific salmon (Chinook, coho and Steelhead) have been introduced into the US Great Lakes, and have become potamodromous, migrating between their natal waters to feeding grounds entirely within fresh water.
Remarkable catadromous migrations are made by freshwater eels. Examples are the American eel and the European eel which migrate huge distances from freshwater rivers to spawn in the Sargasso sea, and whose subsequent larvae can drift in currents for months and even years before returning to their natal rivers and streams as glass eels or elvers.
An example of a euryhaline species is the bull shark, which lives in Lake Nicaragua of Central America and the Zambezi River of Africa. Both these habitats are fresh water, yet bull sharks will also migrate to and from the ocean. Specifically, Lake Nicaragua bull sharks migrate to the Atlantic Ocean and Zambezi bull sharks migrate to the Indian Ocean.
Diel vertical migration is a common behavior; many marine species move to the surface at night to feed, then return to the depths during daytime.
A number of large marine fishes, such as the tuna, migrate north and south annually, following temperature variations in the ocean. These are of great importance to fisheries.
Freshwater (potamodromous) fish migrations are usually shorter, typically from lake to stream or vice versa, for spawning purposes. However, potamodromous migrations of the endangered Colorado pikeminnow of the Colorado River system can be extensive. Migrations to natal spawning grounds can easily be 100 km, with maximum distances of 300 km reported from radiotagging studies. Colorado pikeminnow migrations also display a high degree of homing and the fish may make upstream or downstream migrations to reach very specific spawning locations in whitewater canyons.
Sometimes fish can be dispersed by birds that eat fish eggs. They carry eggs in the digestive tracts and then deposit them in their faeces in a new place. The survival rate for fish eggs that have passed through a bird's digestive tract is low.
Since prehistoric times humans have exploited certain anadromous fishes during their migrations into freshwater streams, when they are more vulnerable to capture. Societies dating to the Millingstone Horizon are known which exploited the anadromous fishery of Morro Creekand other Pacific coast estuaries. In Nevada the Paiute tribe has harvested migrating Lahontan cutthroat trout along the Truckee River since prehistoric times. This fishing practice continues to current times, and the U.S. Environmental Protection Agency has supported research to assure the water quality in the Truckee can support suitable populations of the Lahontan cutthroat trout.
Because salmonids live an anadromous lifestyle, they encounter a larger range of viruses from both freshwater and marine ecosystems. Myxovirus resistance (Mx) proteins are part of a GTP-ase family that aid in viral immunity, and previously, rainbow trout ( Oncorhynchus mykiss ) have been shown to possess three different Mx genes to aid in viral defence in both environments. The number of Mx genes can differ among species of fish, with numbers ranging from 1-9 and some outliers like Gadiformes that have totally lost their Mx genes. A study was performed by Wang et al. (2019)to identify more potential Mx genes that resided in rainbow trout. An additional six Mx genes were identified in that study, now named Mx4-9. They also concluded that the trout Mx genes were “differentially expressed constitutively in tissues” and that this expression is increased during development. The Mx gene family is expressed at high levels in the blood and intestine during development, suggesting they are a key to immune defense for the growing fish. The idea that these genes play an important role in development against viruses suggests they are critical in the trout's success in an anadromous lifestyle.
Brackish water, also sometimes termed brack water, is water occurring in a natural environment having more salinity than freshwater, but not as much as seawater. It may result from mixing seawater with fresh water together, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root "brak". Certain human activities can produce brackish water, in particular civil engineering projects such as dikes and the flooding of coastal marshland to produce brackish water pools for freshwater prawn farming. Brackish water is also the primary waste product of the salinity gradient power process. Because brackish water is hostile to the growth of most terrestrial plant species, without appropriate management it is damaging to the environment.
The European eel is a species of eel, a snake-like, catadromous fish. They are normally around 60–80 cm (2.0–2.6 ft) and rarely reach more than 1 m, but can reach a length of up to 1.5 m in exceptional cases.
The brown trout is a European species of salmonid fish that has been widely introduced into suitable environments globally. It includes purely freshwater populations, referred to as the riverine ecotype, Salmo trutta morpha fario, a lacustrine ecotype, S. trutta morpha lacustris, also called the lake trout, and anadromous forms known as the sea trout, S. trutta morpha trutta. The latter migrates to the oceans for much of its life and returns to fresh water only to spawn. Sea trout in Ireland and Britain have many regional names: sewin in Wales, finnock in Scotland, peal in the West Country, mort in North West England, and white trout in Ireland.
The capelin or caplin is a small forage fish of the smelt family found in the North Atlantic, North Pacific, and Arctic oceans. In summer, it grazes on dense swarms of plankton at the edge of the ice shelf. Larger capelin also eat a great deal of krill and other crustaceans. Among others, whales, seals, Atlantic cod, Atlantic mackerel, squid, and seabirds prey on capelin, in particular during the spawning season while the capelin migrate south. Capelin spawn on sand and gravel bottoms or sandy beaches at the age of two to six years. When spawning on beaches, capelin have an extremely high post-spawning mortality rate which, for males, is close to 100%. Males reach 20 cm (8 in) in length, while females are up to 25.2 cm (10 in) long. They are olive-colored dorsally, shading to silver on sides. Males have a translucent ridge on both sides of their bodies. The ventral aspects of the males iridesce reddish at the time of spawn.
The salmon run is the time when salmon, which have migrated from the ocean, swim to the upper reaches of rivers where they spawn on gravel beds. After spawning, all Pacific salmon and most Atlantic salmon die, and the salmon life cycle starts over again. The annual run can be a major event for grizzly bears, bald eagles and sport fishermen. Most salmon species migrate during the fall.
The Atlantic salmon is a species of ray-finned fish in the family Salmonidae. It is the 3rd largest of the Salmonidae and can grow up to a meter in length. Siberian Taimen, and Pacific Chinook Salmon can be larger, with Taimen reportedly able to grow to 6 ft long. It is found in the northern Atlantic Ocean and in rivers that flow into this ocean. Most populations of this fish species are anadromous, hatching in streams and rivers but moving out to sea as they grow where they mature, after which the adult fish seasonally move upstream again to spawn.
The sockeye salmon, also called red salmon, kokanee salmon, or blueback salmon, is an anadromous species of salmon found in the Northern Pacific Ocean and rivers discharging into it. This species is a Pacific salmon that is primarily red in hue during spawning. They can grow up to 84 cm in length and weigh 2.3 to 7 kg (5–15 lb). Juveniles remain in freshwater until they are ready to migrate to the ocean, over distances of up to 1,600 km (1,000 mi). Their diet consists primarily of zooplankton. Sockeye salmon are semelparous, dying after they spawn. Some populations, referred to as kokanee, do not migrate to the ocean and live their entire lives in freshwater.
The Arctic char or Arctic charr is a cold-water fish in the family Salmonidae, native to alpine lakes and arctic and subarctic coastal waters. Its distribution is Circumpolar North. It spawns in fresh water and populations can be lacustrine, riverine, or anadromous, where they return from the ocean to their fresh water birth rivers to spawn. No other freshwater fish is found as far north; it is, for instance, the only fish species in Lake Hazen on Ellesmere Island in the Canadian Arctic. It is one of the rarest fish species in Britain and Ireland, found mainly in deep, cold, glacial lakes, and is at risk from acidification. In other parts of its range, such as the Nordic countries, it is much more common, and is fished extensively. In Siberia, it is known as golets and it has been introduced in lakes where it sometimes threatens less hardy endemic species, such as the small-mouth char and the long-finned char in Elgygytgyn Lake.
Sea trout is the common name usually applied to anadromous forms of brown trout, and is often referred to as Salmo trutta morpha trutta. Other names for anadromous brown trout are sewin (Wales), peel or peal, mort, finnock (Scotland), white trout (Ireland) and salmon trout (culinary). The term sea trout is also used to describe other anadromous salmonids—coho salmon, brook trout, Arctic char, cutthroat trout and Dolly Varden. Even some non-salmonid species are also commonly known as sea trout—Northern pikeminnow and members of the weakfish family (Cynoscion).
The Anguillidae are a family of ray-finned fish that contains the freshwater eels. Eighteen of the 19 extant species and six subspecies in this family are in the genus Anguilla. They are elongated fish with snake-like bodies, their long dorsal, caudal and anal fins forming a continuous fringe. They are catadromous fish, spending their adult lives in fresh water, but migrating to the ocean to spawn. Eels are an important food fish and some species are now farm-raised, but not bred in captivity. Many populations in the wild are now threatened, and Seafood Watch recommend consumers avoid eating anguillid eels.
Pelagic fish live in the pelagic zone of ocean or lake waters – being neither close to the bottom nor near the shore – in contrast with demersal fish that do live on or near the bottom, and reef fish that are associated with coral reefs.
The Arctic lamprey, also known as the Japanese river lamprey or Japanese lampern, is a species of lamprey, a jawless fish in the order Petromyzontiformes. It inhabits coastal freshwater habitat types in the Arctic. Some populations are anadromous, spending part of their lives in the ocean. It is the most common and widespread lamprey in the Arctic region.
The New Zealand smelt, also known as the New Zealand common smelt or New Zealand cucumber fish, is a smelt of the family Retropinnidae, found only in New Zealand at shallow depths in estuaries and rivers. Their length is between 8 and 13 cm.
The Japanese eel is a species of anguillid eel found in Japan, Korea, China, and Vietnam, as well as the northern Philippines. Like all the eels of the genus Anguilla and the family Anguillidae, it is catadromous, meaning it spawns in the sea, but lives parts of its life in fresh water. The spawning area of this species is in the North Equatorial Current in the western North Pacific to the west of the Mariana Islands. The larvae are called leptocephali and are carried westward by the North Equatorial Current and then northward by the Kuroshio Current to East Asia, where they live in rivers, lakes, and estuaries. The Japanese eel is an important food fish in East Asia, where it is raised in aquaculture ponds in most countries in the region. In Japan, where they are called unagi, they are an important part of the food culture, with many restaurants serving grilled eel, which is called kabayaki.
Freshwater fish are those that spend some or all of their lives in fresh water, such as rivers and lakes, with a salinity of less than 1.05%. These environments differ from marine conditions in many ways, the most obvious being the difference in levels of salinity. To survive fresh water, the fish need a range of physiological adaptations.
Animal migration is the relatively long-distance movement of individual animals, usually on a seasonal basis. It is the most common form of migration in ecology. It is found in all major animal groups, including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans. The trigger for the migration may be local climate, local availability of food, the season of the year or for mating reasons.
The coastal cutthroat trout, also known as the sea-run cutthroat trout, blue-back trout or harvest trout, is one of the several subspecies of cutthroat trout found in Western North America. The coastal cutthroat trout occurs in four distinct forms. A semi-anadromous or sea-run form is the most well known. Freshwater forms occur in both large and small rivers and streams and lake environments. The native range of the coastal cutthroat trout extends south from the southern coastline of the Kenai Peninsula in Alaska to the Eel River in Northern California. Coastal cutthroat trout are resident in tributary streams and rivers of the Pacific basin and are rarely found more than 100 miles (160 km) from the ocean.
The twait shad or twaite shad is a species of fish in the family Clupeidae. It is found in the eastern Atlantic Ocean and the Mediterranean Sea and is an anadromous fish which lives in the sea but migrates into fresh water to spawn. In appearance it resembles an Atlantic herring but has a row of six to ten distinctive spots on its silvery flanks. They become mature when three or more years old and migrate to estuaries, later swimming up rivers to spawn. Populations of this fish have declined due to overfishing, pollution and habitat destruction. Conservation of this species is covered by Appendix III of the Bern Convention and Appendix II and V of the European Community Habitats Directive.
This is a glossary of terms used in fisheries, fisheries management and fisheries science.
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 food chain on plankton, often by filter feeding. They include particularly fishes of the family Clupeidae, but also other small fish, including halfbeaks, silversides, smelt such as capelin and goldband fusiliers.
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