Extended freshwater rearing of juvenile Coho salmon (Oncorhynchus kisutch) in Northern California streams

Thesis (M.S.)--Humboldt State University, Natural Resources: Fisheries, 2007Extended rearing (rearing for more than one summer) of juvenile coho salmon was documented in five of six study streams in northern California and for four consecutive years in three study streams during 2000 through 2003. The numbers of older juvenile coho salmon varied greatly by year and by stream. The proportion of a cohort that exhibited extended rearing ranged from 0% to almost 30% among streams. Within one study stream, the proportion of a cohort that reared an additional year varied from 2% to as much as 30% over three consecutive cohorts. Initial year class strength, which was measured by density, varied substantially among years, as did average size of juvenile coho salmon during summer and fall. Neither of these variables showed clear relationships with subsequent extended rearing. Peak winter streamflow appeared to best explain the amount of extended rearing. The highest densities of age 1+ juvenile coho salmon and the largest proportions of cohorts that exhibited extended rearing were observed during the summer following the winter with the mildest streamflows. Results from this study suggest that extended freshwater residence may be an important component of the life history of coho salmon in northern California and indicate that winter streamflows may have a strong effect on the numbers of individuals that rear in freshwater for an additional year.California Cooperative Fish Research Unit\ud National Marine Fisheries Servic