AFSC/ABL: Genetic data for juvenile chum salmon samples collected in the eastern Bering Sea on the U.S. BASIS cruises during 2003-2007.

Chum salmon (Oncorhynchus keta) are an important natural resource in western Alaska for subsistence, commercial and cultural reasons. Declines in chum salmon returns in some western Alaska drainages over the last couple of decades have prompted regulatory changes and bolstered research on this species in this region. Since 2002, juvenile chum salmon have been collected as part of the annual U.S. Bering-Aleutian Salmon International Survey (BASIS) during the late summer/fall season in the eastern Bering Sea. From the 2003-2007 collections, nearly 5,000 juvenile chum salmon samples were genetically analyzed. With the available microsatellite baseline, regional stock estimates were produced from mixed-stock analyses. The proportions of juvenile chum salmon from four western Alaska regionsNorton Sound, lower Yukon (summer-run), upper Yukon (fall-run), and Kuskokwim/northeastern Bristol Baywere remarkably similar across years during early marine residence, especially given the latitudinal shifts from year-to-year in the distribution across the eastern Bering Sea shelf of this highly migratory species. Most of the juvenile chum salmon were from the Yukon River, which has two life-history types, an earlier and typically more abundant summer run, and a later fall run. The Kuskokwim/northeastern Bristol Bay contribution within the study area (lat. 58-63N) was negligible, indicating that these stocks do not migrate northward during their first summer. The Norton Sound group contribution varied annually, but in general, increased with latitude. These results support a migration model whereby western Alaska juvenile chum salmon, after leaving freshwater, head primarily west and south across the eastern Bering Sea shelf. A relative abundance index was developed from the proportions of the two life-history types in the Yukon River of juvenile chum salmon in the survey area. In all five years of collections, the summer-run contribution was higher than the fall-run contribution in the juvenile chum salmon samples. The proportions of the two life-history types in the juvenile chum salmon collected at sea were compared with those in the Yukon River adult returns. A correlation was found between the juveniles and subsequent adult returns. This suggests that it is during the period of freshwater and early marine residence that the cohort strength of Yukon River summer- and fall-run chum salmon is determined.