Selection on Morphology of Spawning Wild Sockeye Salmon by a Gill-Net Fishery

Human activities can cause artificial selection in wild animals. To examine the effects of gill-net selectivity on locally differentiated populations of sockeye salmon Oncorhynchus nerka in Bristol Bay, Alaska, we completed a three-part study: (1) We showed differentiation in the body form of mature sockeye salmon spawning in beach and stream habitats that were separated by less than 300 m. (2) Because gill-net selection acts directly on the girth of immature sockeye salmon, we correlated girth at capture with the morphological characters distinguishing locally differentiated populations on the spawning grounds. By tagging individual fish and measuring them both when immature and when mature, we found morphology at maturity to be highly correlated with girth during immaturity. (3) Using selection regimes from the fishery catch and escapement data for 1994, we examined the effects of gill-net selectivity on populations of mature adults. We showed that although populations of mixed ocean age-classes may be subject to disruptive selection, single age-class populations are more likely to experience directional selection. The effect of this selection depends on cumulative selection pressures, which probably include natural and sexual selection on this trait. Even so, gill-net selection can be a strong selective force, resulting in significant additional selection on body size and shape within populations.