FEATURE ARTICLE
Preserving Salmon Biodiversity
The number of Pacific salmon has declined dramatically. But the loss of genetic diversity may be a bigger problem
Phillip Levin, Michael Schiewe
The Life Cycle
The life cycle of a typical salmon begins with females depositing eggs in nests, or redds, on the gravel bottoms of rivers and lakes. The young emerge from the redds and live in freshwater for periods ranging from a few days to several years. Then the juveniles undergo a physiological metamorphosis (called smoltification) and head toward the ocean. Once in the sea, the salmon often undertake extensive migrations of thousands of miles while they mature. After anywhere from a few months to a few years, adult salmon return—with high fidelity—to the river where they were born. There, they spawn and the cycle begins again.
In North America, there are five species of Pacific salmon, all in the genus Oncorhynchus: pink salmon (O. gorbuscha), chum (O. keta), sockeye (O. nerka), coho (O. kisutch) and chinook (O. tshawytscha). Most of these fish are anadromous—that is, they migrate to the sea and then return to freshwater to reproduce. They are also semelparous—they die after spawning once. Two species of trout in the Pacific Northwest are also members of the genus Oncorhynchus and thus are included among the salmonids: steelhead (O. mykiss) and sea-run cutthroat trout (O. clarki). Steelhead and cutthroat are also anadromous, but they do not necessarily die after spawning. (Fish that can spawn more than once are known as iteroparous.)
The true biological diversity of all the salmonids, however, is revealed only below the species level. Specialists have, for example, identified two kinds of chinook salmon—known as stream-type chinook and ocean-type chinook—that display significant differences in the age at which they move to the sea, in what patterns of ocean migration they follow and in when they spawn.
Stream-type chinook spend one or more years in freshwater before heading to sea; they also undertake extensive offshore voyages and return to their natal streams during the spring or summer, often holding in freshwater for several months before spawning. In contrast, ocean-type chinook move out very early in life, before they reach one year of age. But once these salmon reach open water, they do not travel far offshore. They usually spend their entire ocean residence on the continental shelf and return to their natal streams immediately before spawning.
Interestingly, the divisions among chinook salmon do not stop with these two groups. Within both categories there is tremendous variability in the timing of and age at spawning. For example, ocean-type chinook return to the Columbia River only during the summer and fall. Ocean-type chinook from the Sacramento River, however, return during every season of the year. Such differences motivated a formal classification of the salmonids in the lower 48 states that goes beyond the traditional Linnean system to define groups of populations as evolutionarily significant units, or ESUs. In all, between 50 and 55 salmonid ESUs inhabit the waterways of the American West.
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