Factors Contributing to Apparent Overwinter Survival of Coho Salmon in Three Adjacent Tributaries in Southwest Washington

A life cycle monitoring program in three adjacent tributaries of the lower Columbia River was initiated in 2005 in order to understand salmonid survival across freshwater life stages. In this study, we examine factors contributing to apparent overwinter survival of Coho Salmon. Abundance was estimated at the end of the summer rearing period (summer parr) and at the spring outmigration (spring smolts). Overwinter survival differed among watersheds and years and was negatively correlated with summer parr abundance suggesting that availability of overwinter rearing habitat may limit the number of spring smolts. Outmigrant characteristics were investigated using Passive Integrated Transponder tags implanted in juvenile Coho Salmon at the end of the summer rearing period. An antenna array detected consistent downstream movements in the fall suggesting that a portion of juveniles may overwinter outside the natal watershed. Fish detected as either fall or spring migrants were larger than those not detected. Coho tagged in the lower mainstem were more likely to be fall migrants whereas those tagged in the tributaries and headwaters were more likely to be spring migrants. Our results demonstrate that growth and location during the summer rearing period are important factors contributing to successful outmigration from these watersheds.