Variation in size, condition, and abundance of juvenile chum salmon (Oncorhynchus keta) in relation to marine factors in Southeast Alaska

Abstract

Understanding how environmental conditions influence productivity of Pacific salmon (Oncorhynchus spp.) requires identifying relationships between ocean-driven processes and early marine growth and survival. We studied size, condition, and abundance of juvenile chum salmon (O. keta) captured in July in Icy Strait, Alaska over 17 years (1997−2013). We investigated relationships between these juvenile chum salmon characteristics and a suite of marine variables expressed at basin-wide, regional, and local scales using correlation and multivariate analyses. Juvenile chum salmon tended to be longer in years with warmer sea-surface temperatures in spring and summer. Average energy density was higher in years when variables such as local winds and mixed-layer depth indicated enhanced water-column mixing in June and July. Marine factors at the regional scale, specifically freshwater discharge and upwelling, were associated with the abundance of juvenile chum salmon. Catch per unit effort of juvenile chum salmon was higher in years with greater freshwater discharge and more intense downwelling in the prior winter, suggestive of mechanisms increasing recruitment of juvenile chum salmon through enhanced primary productivity and possibly reduced predation risk. Overall, our study supported previous work suggesting that a strong Aleutian Low indicates good conditions for salmon recruitment in the Gulf of Alaska, although we found that local conditions explained more variation in juvenile chum salmon responses than did basin-scale climate indicators. Furthermore, our work revealed a compelling role for freshwater discharge in driving ecological mechanisms that influence the growth and survival of juvenile salmon in the nearshore marine waters of Southeast Alaska.