Stock composition and coastal migration characteristics of coho salmon in fisheries of Norton Sound, Alaska

Abstract

Knowledge of the dynamic interactions between directed fishing effort and exploitable stocks of Pacific salmon Oncorhynchus spp. is needed to balance harvest opportunities against diversity conservation in mixed stock fisheries. This is often accomplished by relating stock proportions in commercial landings with spatial and temporal strata of harvests, but catch partitioning is hindered in fisheries targeting local stocks that do not display enough discernable variation for classification using modern tools (e.g., genetic stock identification). This study investigates the composition and migratory behavior of Coho Salmon O. kisutch stocks in the Norton Sound district of Alaska by mimicking local fisheries to capture and tag 578 fish in the contiguous Shaktoolik and Unalakleet subdistricts over two seasons. A network of acoustic receivers monitored coastal movement and freshwater entry, and 341 individuals last detected in spawning areas were assigned to a stock of origin based on assumed natal site fidelity. In Chapter 1, models using spatiotemporal and demographic variables were fit to commercial harvest data in project years and estimated that 32.9% of Shaktoolik subdistrict catch was Shaktoolik stock, while 86.7% of landings were Unalakleet stock in Unalakleet subdistrict. In Chapter 2, significant differences in migratory characteristics were identified among stocks within the study area, with local stocks swimming slower and further offshore relative to fish bound for spawning streams outside the study area, and residency time within coastal fishery boundaries decreased for all stocks as the season progressed. Detection histories were used to estimate parameters for multistate movement models which indicated that Unalakleet stock members are more likely than other observed stocks to explore coastal waters outside their natal subdistrict before moving into freshwater. Results of this study confirm that coastal salmon fisheries have access to a mixture of stocks whose unique migratory characteristics might be leveraged for stock identification and diversity conservation.