Timing of adult migration and stock structure for sockeye salmon in Bear Lake, Alaska

Sockeye salmon Oncorhynchus nerka returning to the Bear Lake system on the Alaska Peninsula exhibit a bimodal pattern of adult spawning migration timing produced by early and late migrants. We examined, by field observation of tagged adults, the relationship between adult migration date, breeding site, and breeding date for sockeye salmon returning to Bear Lake to assess stock structure within the system and determine whether distinct populations were segregated as they passed through the commercial fishery. Observed tag recoveries documented that (1) early and late migrants breed in different locations, (2) early migrants breed earlier in the reproductive season than late migrants (i.e., the chronological order of spawning followed the adult migration order), and (3) the adult migration has three major temporal components rather than two. Bear Lake's sockeye salmon population complex is composed of at least seven distinct spawning populations that display substantial temporal separation as they enter freshwater. The low level of abundance at the middle of the season is misleading; it is predominated by a single, discrete population rather than being merely a low point between two peaks. Given the brief (14-km), low-gradient migration from the Bering Sea to Bear Lake, the populations are probably segregated as they pass through the commercial fishery and thus the temporal distribution of fishery openings has the potential for disproportionate exploitation and reduction of representation by some genotypes.