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 because of diet or reproductive needs, although in some cases the reason for migration remains unknown.
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Classifications can be either fundamental (like biological classification that rests on a phylogenetic basis), or are merely heuristic typologies (as here, about migrations) to assist communication about complex issues. Classifications are judged according to their fundamental accuracy, whether they are convenient or not. Typologies in contrast are essentially arbitrary and their effectiveness is to be judged solely by the problems they solve or create. Secor and Kerr (2009)[1] for example show several typologies that encapsulate various aspects of fish life history.
Migration is a word used in multiple senses. It is important to distinguish "true" migration, i.e. a life-history-structured or at least patterned activity such as seen in anadromous species like salmons, from mere movement or wandering as may happen, say, with euryhaline species that easily move between fresh and salt water but not necessarily or with regularity. "Vertical migration", for example, the phenomenon of plankton and fishes regularly changing their depth throughout the 24h day, is a special usage unlike migrations of (e.g.) salmons that range over distances in migrations that may cover river, lake, and sea, or the great migrations of game through Africa's Serengeti.
Anadromous and catadromous are words that have been commonly used for centuries. They are slightly more narrowly used in the following classification of [truly] migrating fish by Myers 1949[2]:
The "-ous" endings are for the adjectival form of the terms; nouns are obtained by replacing that ending with "-y", e.g. anadromy.
The terms anadromous and catadromous were of long standing (and similar but not identical usage); the other terms were coined by Myers. Myers was hesitant about introducing new terms however, saying:
Myers' term diadromous has proved useful as an inclusive term. But his prediction was otherwise accurate: anadromous and catadromous, are indeed the main widely understood terms. Secor and Kerr (2009)[1] found only one paper that used oceanodromous, 18 using potamodromous, and 122 using amphidromous compared to 985 times for anadromous, 143 times for diadromous, 71 times for catadromous. Obviously the amount of work done on a group is one source of this variation, but another source is the perceived utility and standing of the term itself.
Amphidromy is rarely understood by the wider fish biology audience, or alternatively "There seems to be some reluctance to use the term amphidromy"[4]. Myers' definition's ambiguity ("or vice versa") and teleology ("for the purpose of") is one part of that problem[3]. Its meaning/usage has been adjusted (but two such adjustments were found zero times by Secor and Kerr (2009)[1]), and recently it has been stated as differing from anadromy simply because return to fresh water occurs at a juvenile or immature stage; other differences have been claimed (such as where most growth occurs) but they are necessary consequences of the stage at return and therefore are not independent. Some authors have therefore eschewed amphidromy in favour of the more widely understood terms: either anadromy, with or without a remark that return to rivers occurs at an earlier stage (e.g. "juvenile-return anadromy"[3] ), or diadromy which discards information that would be conveyed by anadromy with or without remark.
It has to be borne in mind that any typological system has heuristic value as a convenience for description, and need not reflect phylogeny: each category (term) may include representatives of many distantly related taxa, each of which may well have close relatives that are in another category (term) or that are not migratory and thus fall completely outside the typology. These terms are therefore of limited safety when used alone to screen data to be considered for an analysis, or to decide which species should be read about to explore a phenomenon or possible comparison.
The limitations of typologies were clearly stated by Myers himself[2]. Each may be useful in one context, for one purpose, but not another. I.e. they may categorise along different axes (see Secor and Kerr (2009)[1]). For example, Myers in the same year devised another interesting set of terms, also directed at fishes, but on the basis of their salt-tolerance[5].
As a footnote it is interesting to note that George S. Myers had previously been one of the degree supervisors for Porfirio Manacop, whose ground-breaking Master's work (in 1941, later published 1953[6]) on a Sicyopterus species overturned the prevailing notion that it was catadromous. It is likely that Myers was impressed by this, and used Sicydium as the "type" genus for amphidromous.
And, as a contrary footnote showing how science fails: regrettably, Manacop's work[6] seems to have been as locally unpopular as it was innovative, because over 20 years later the group he worked on was still being referred to by a colleague in the Philippines who would certainly have known of his work[7], incorrectly and without evidence, as catadromous.
And although these systems were originated for fishes, they are in principle applicable to any 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.
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.[8] 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 ocean going sharks, dolphins and other cetaceans.
These high trophic 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.[9]
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.[10]
Some of the best-known anadromous fish are the six species of Pacific salmon, which are Chinook (King), Coho (Silver), Sockeye (Red), Chum (Dog), Pink (Humpback), and Cherry. The 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, and shad.
The most remarkable catadromous fishes are freshwater eels of genus Anguilla, whose larvae drift from swawning grounds in the Sargasso sea, sometimes for months or years, before entering freshwater river and streams as glass eels or elvers (see eel life history).
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 fish migrations are usually shorter, typically from lake to stream or vice versa, for spawning purposes. However, potamodromous migrations of Colorado pikeminnow of the Colorado River system can be extensive. Migrations to natal spawning grounds easily be 100 km, with maximum distances of 300 km reported from radiotagging studies.[11]
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 Creek[12] and 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.
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.[13]
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