Temporal and Spatial Trends in the Abundance of Coho Salmon Smolts from Western North America

Abstract

Coast-wide trends in the catch of adult Pacific salmon (Oncorhynchus spp.) are often attributed to large-scale variation in marine survival caused by changing ocean conditions; however, the roles of the freshwater stages have not usually been assessed. I used a compilation of published coho salmon Oncorhynchus kisutch data to determine whether the spatial scale of common variation in smolt abundance is sufficient for the freshwater segment of the life cycle to contribute to regional trends in adult coho salmon abundance. I found positive covariation in annual smolt estimates for streams that were less than 20–30 km apart, but smolt abundances were, on average, uncorrelated for streams that were more separated by a greater distance. The proportion of variation of interannual fluctuations in coho salmon smolt abundance that was common to neighboring streams was small. Thus, it appears that large-scale influences on smolt production are likely diluted by the effects of watershed-specific landscape and biotic factors. Because variation in smolt production is not spatially coherent, the freshwater part of the life history is not likely to contribute to regional variations in adult abundance, unless spawner abundances are very low.