Fish migration
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 thousands of kilometres. Such migrations are usually done for better feeding or to reproduce, but in other cases the reasons are unclear.
Fish migrations involve movements of schools of fish on a scale and duration larger than those arising during normal daily activities.[1] 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.[2]
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.[3]
As with various other aspects of fish life, zoologists have developed empirical classifications for fish migrations.[5] The first two following terms have been in long-standing wide usage while others are of more recent coinage.
George S. Myers coined the following terms in a 1949 journal article.
Although these classifications were originated for fish, they are, in principle, applicable to any aquatic organism.
List of diadromous orders and families, and the number of known species:[9][11]
Historic exploitation[edit]
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[25] 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.
Myxovirus genes[edit]
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) had 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 to 9 and some outliers like Gadiformes that have totally lost their Mx genes. A study was performed by Wang et al. (2019)[26] 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.
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