Outbreeding Depression after Two Generations of Hybridizing Southeast Alaska Coho Salmon Populations?

No fitness loss was detected in second‐generation hybrids, relative to parental controls, between three pairs of coho salmon Oncorhynchus kisutch populations from Southeast Alaska, in which groups were cultured in a common freshwater environment, released to sea together, and recovered together as adults. Divergence among the populations as measured by neutral molecular markers was significant. Marine survival did not differ among parental groups, F 1 groups, F 2 groups, and hybrid groups, between parental and hybrid groups, and between parental and F 2 groups. Marine survival of F 1 hybrids in the first generation of the experiment exceeded that of parental controls but did not differ among parental or F 1 groups, although the power of the latter tests was low. Tests for the loss of fitness in hybrids relative to parental controls based on fluctuating asymmetry (FA) yielded only one significant result among six tests, and in many instances hybrids exhibited less FA than parental controls. Body length differed among years, populations, sexes, and cross types (parental, F 1 hybrids, and F 2 hybrids); the differences among cross types were consistent with among‐population differences. In contrast, bilateral meristic counts exhibited little variability (coefficients of variation = 0.02–0.09). Meristic differences among years probably reflect the effects of the different environments experienced by the fish. The small variability in bilateral meristic characters suggests strong genetic canalization for these traits. Although no losses in fitness (as measured by marine survival and FA) were observed, the power of each of our tests was low and the among‐population differences were unique to this experiment.