By Dr. Dave Beatty
11/30/21
Among the estimated 28,000 extant species of bony fishes (teleosts) there are five groups with each having a distinct pattern of migratory behavior during the life cycle.
All eastern Pacific Ocean salmon (Chinook, coho, chum, pink, and sockeye) plus steelhead trout and sea-run cutthroat trout, genus Oncorhynchus, have a life history pattern of the diadromous type meaning there are two distinct ecological components to the life cycle, fresh water and salt water. The salmonid species spawn in freshwater habitats (rivers, creeks, and for sockeye also along lake shores with appropriate size gravel and upwelling water) to produce juveniles which migrate to the ocean as smolts for growth and the eventual migration as adults back to the natal body (home stream) of fresh water for spawning. This type of diadromy is the anadromous type meaning spawning in fresh water and major growth in the ocean. The catadromous fishes represent another type of diadromy in which spawning occurs in the ocean and major growth occurs in freshwater habitats. An example of catadromy is in the eel species of the genus Anguilla found in the eastern and western Atlantic Ocean and the western Pacific ocean and adjacent coastal streams. A third type of diadromy is the amphidromous fishes that spawn in estuaries/fresh water and whose larvae drift downstream to salt water to feed and grow for short periods before the juveniles return to estuaries/fresh water for growth and subsequent spawning. This group does not have extensive migrations in salt water or fresh water when compared to the other two groups.
The potamodromous and oceanodromous fishes represent two other groups that migrate during the life cycle. The fishes of the former group live entirely in fresh water and spawn upstream then migrate downstream as juveniles but remain in fresh water to grow before migrating back upstream to spawn. The fishes of the latter group live entirely in salt water for spawning and the larvae/juveniles drift on ocean currents to grow before migrating back to the natal site to spawn.
There are more species of anadromous fishes, e.g., Pacific salmon, steelhead, sea-run cutthroat, Atlantic salmon, brown trout, bull char, Dolly Varden, arctic char, smelt, shad, striped bass, and sturgeon, than of the catadromous fishes essentially limited to true eels. Of the estimated 28,000 extant species of bony fish on earth, only about 170 (0.61%) are truly diadromous and of these about 100 (0.59%) are anadromous species, about 40 (0.23%) are catadromous and about 30 (0.18%) are amphidromous.
Among diadromous fishes, anadromy is more common at higher latitudes whereas catadromy is more common at lower latitudes. Generally, the oceans at higher latitudes are more productive, thus greater food for growth, than is fresh water. This is demonstrated by the higher growth rates in the anadromous salmonids.
During their life cycle, diadromous fishes by the nature of the two distinct environments are euryhaline meaning they are physiologically adapted to living in fresh water with its low salt content and in marine water with its much higher salt content. Fishes that reside strictly in fresh water or the ocean during their life cycle are stenohaline, thereby they only tolerate a narrow range of the salinities characteristic of their environment.
A central feature of the evolution of the anadromous life cycle in salmon is the limits of fresh waters to provide the nutrition and space for growth when compared to the ocean even though there are metabolic costs (energy expenditure) to undergo the long-distance migrations between the two types of habitat. Furthermore, there is the protection afforded to fertilized salmonid eggs which hatch into alevins within the spawning gravel compared to strictly marine fishes that generally produce much greater numbers of eggs which are not so protected during early development.
Thus the salmons’ anadromy provides the spawning part of the life cycle with an investment that maximizes fertilized egg survival. Compared to a strictly marine or freshwater fish, a female salmon produces relatively fewer, but larger eggs with sufficient yolk for the early development (larvae, alevin or sac fry after hatching) to the free-swimming stage of larger fry that emerge from the spawning gravel into open fresh water where they must feed.
A central feature of the anadromy of Pacific salmon is death (semelparity) after spawning whereas steelhead and sea-run cutthroat can survive (iteroparity), return to the ocean as kelts and return to spawn in a subsequent year. Salmonids have a high metabolic rate when there is sufficient food to grow fast, however they do not live long. A general feature of our five species of Pacific salmon is their progeny become smolts and migrate to the ocean for growth before returning as adults to spawn.
Juvenile Pacific salmon do not become freshwater residents. The exception is the kokanee (same species as the sockeye) ecotype that completes its life cycle entirely in fresh water (streams and lakes) and does not migrate.
Steelhead and resident rainbow can have complex results with respect to whether their progeny become either steelhead or resident rainbow. The mating of two steelhead may produce some progeny that never become smolts and remain in the stream as resident rainbow. The mating of two resident rainbow may produce some progeny that become smolts and migrate to the ocean as anadromous steelhead. Moreover, a steelhead may mate with a resident rainbow producing progeny of which some become steelhead and others become resident rainbow. There is plasticity in the steelhead/rainbow reproductive life history that is not fully understood and is expressed under certain circumstances of the species genetic diversity and the environment. The steelhead/rainbow trout life history complexity occurs in coastal populations of O. mykiss irideus.