The role of natural and sexual selection in local adaptation of spawning behavior and morphology in sockeye salmon, Oncorhynchus nerka. Dissertation

Hamon, Troy

We studied three aspects of local adaptation in sockeye salmon populations in southwest Alaska The first study related morphology of fish at the time of the commercial fishery, which is size-selective, to morphology of spawning adults. We measured midorbital to hypural length (MOHP), body depth, girth, and snout length of migrating sockeye salmon as they entered their natal lake, tagged them, and measured them again after recapture on the spawning grounds. All measured dimensions of males changed; MOHP decreased, all other measurements increased. Females changed less than males. Changes in MOHP could not be explained by reallocation of resources. These changes in size have important consequences for the utility of these body measurements in a management context. The second study examined local adaptation in male spawning behavior. On three island beaches in Iliamna Lake, Alaska, large numbers of males gathered and spawned in artificial excavations that mimicked a female's nest, while apparently ignoring the control site. The number of males attracted was correlated positively with changes in the operational sex ratio. In contrast, on the mainland beach examined, no reaction to the artificial nests was apparent. The results are discussed in terms of mate searching behavior, the duration of the spawning period, and selection for males to use characteristics of their environment that provide information on availability of females. The third study measured selection on locally adaptive characters in male sockeye salmon. We developed an appropriate method and measured natural and sexual selection on body length, body depth, and time of entry to the breeding grounds. Sexual selection favored longer males that entered later, but did not select for body depth or snout length. Natural selection was variable among years, favoring deeper bodied and earlier arriving males in 1994 but shorter males with average arrival timing in 1995. These results differed from apparent patterns of selection by bear predation as inferred from carcass inspection. The primary explanation for differences in body depth among populations appears to be differences in sexual selection, while interpopulation variation in body length is likely due to balancing sexual and natural selection.