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
How Important is Biodiversity?
Although the value of genetic diversity is often taken as a truism by conservation biologists, for some species the loss of variability does not necessarily increase the likelihood of extinction. The biological diversity seen in the northern elephant seal, for example, is very low; yet there is no evidence that this animal is endangered because of it.
Faced with certain changes to salmon biodiversity, fisheries biologists must determine whether or not salmonids are fundamentally like elephant seals. The answer depends in large part on two factors: first, the extent to which salmon have adapted to their local environments and second, the speed with which salmon adapt (or readapt) to their surroundings.
Because much of the diversity within and among Pacific salmon has at least some genetic component and because there is little gene flow among these populations, one expects to see some local differences in homing ability, disease resistance and response to stream flow, for example. The failure of most attempts to transplant stocks to a new habitat also suggests that salmonids have evolved specializations suited to particular local environments.
Nevertheless, the possibility remains that some highly variable traits do not reflect genetic adaptations. This hypothesis receives far less attention than adaptationist theories, yet evolutionary biologists acknowledge that populations of any species may diverge randomly as a result of genetic drift over generations. To evaluate local adaptation (or lack thereof) among salmon, one needs some idea of how much drift has taken place. A recent survey of sockeye salmon sheds some light on this question.
Jay Hensleigh and Andrew Hendry of the University of Washington explored the response of sockeye to the direction of the current. This species is particularly sensitive to flow because after emerging from the gravel river bottom, young sockeye must move to lakes where they grow. Fry born in outlet streams must migrate upstream to get to the lake; fry born in inlet streams must travel downstream. This response is genetically determined and is usually under strong selection pressure, because fish that migrate in the wrong direction will die. Remarkably, however, some sockeye spawn on the beaches of lakes. Because these fish do not need to travel upstream or downstream, there is no selective pressure for this behavior.
Hensleigh and Hendry tested the response to stream flow in two sockeye populations: one from an inlet stream (genetically programmed to migrate downstream) and one from a lake, which had been established 13 generations previously by salmon from an inlet stream. Using laboratory raceways, the two researchers found that both groups migrated downstream. Surprisingly, though, fry from the lake showed a greater tendency to migrate downstream than the inlet population did. Presumably, this result reflects genetic drift. Or it may be that natural selection indeed operated—but for an entirely different trait that was by happenstance linked to the gene controlling downstream migration. In any event, this study shows that salmon may possess an array of traits that do not necessarily reflect the selective pressures of their local environment.
Yet even if the majority of these traits do reflect local adaptations, the long-term persistence of salmon will not be hampered by the loss of some genetic diversity if the fish can evolve rapidly enough. Just how quickly can a new trait arise? By again examining these same two populations of sockeye, Hendry, in a paper published last year, suggested that reproductive isolation and evolutionary divergence can happen in as little as 13 generations. Specifically, among sockeye, the size of the male body is sexually selected (females almost always mate with the larger males) and reaches a maximum among beach-spawning populations, because shallow water limits the size of stream-spawning males. After only 13 generations, males of beach-spawning sockeye had significantly larger bodies than males from the parent stream-spawning population—these lake dwellers had evolved to reflect their new environment in just decades.
These findings remain controversial, but regardless of whether they prove to be in error, Hendry and coworkers have raised the specter that the conservation of a wide spectrum of observable traits is not necessarily of paramount concern—a somewhat surprising outcome. How then should resource managers charged with saving salmon respond?
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