Spatial and Temporal Patterns of Upriver Migration by Sockeye Salmon Populations in the Wood River System, Bristol Bay, Alaska

Abstract

AbstractKnowledge of temporal segregation in migration timing among populations is critical for management of fisheries that exploit a complex of populations with an overlap in timing. We examined the potential for fishery selection on populations of sockeye salmon Oncorhynchus nerka in the Wood River system, Bristol Bay, Alaska, by determining the relationship between migration timing (hence, vulnerability to fishing) and the population of origin and breeding date. We assessed migration patterns among populations with a multiyear mark–recapture study testing the hypotheses that the timing of upriver migration varies systematically with respect to spawning habitat, lake of origin, and spawning population. The results revealed organization in migration timing through the Wood River among populations spawning in the streams, beaches, and rivers of the system. The primary source of variation in migration timing among populations was spawning habitat and, to a lesser extent, the lake of origin. Stream‐spawning populations migrated before river‐spawning populations, consistent with the earlier spawning in streams than in rivers. However, beach‐spawning populations were among the earliest to migrate, yet they spawned as late as river spawners. A likelihood model revealed that spawn timing was not tightly coupled with migration timing by populations through the Wood River, and there was broad overlap in migration timing among spawning populations. Results also revealed a link between migration timing and arrival on spawning grounds within a population that was intensively sampled, indicating that early upriver migrants also tended to enter the spawning grounds before later‐migrating individuals. Fisheries that are selective with respect to timing can have strong effects on the phenotypic and genotypic diversity of the populations under such pressure, and the phenomenon of temporally biased fisheries merits further investigation.