Factors Affecting Migration Timing, Growth, and Survival of Juvenile Coho Salmon in Two Coastal Washington Watersheds

Abstract

Recent improvements in tagging technology allow for the examination of the migration of individual fish, the detection of previously unidentified life histories, and the detailed examination of factors affecting growth, migration, and survival. Using passive integrated transponder tags and instream readers installed near tidewater, we examined the migration, growth, and survival of 18,642 juvenile coho salmon Oncorhynchus kisutch in two small western Washington rivers from 2005 to 2009. In most years, more than 50% of the juvenile coho salmon from a given brood year migrated to sea between 1 October and 31 December (fall migrants). These fall migrants were significantly smaller at tagging than fish that migrated between 1 January and 30 June (spring migrants) but were similar in size to fish that were never detected after tagging and assumed to have died. Annual coho salmon survival estimates from tagging to out-migration ranged from 31% to 40% for fall and spring migrants combined but from 5% to 15% for spring migrants only. The best fitting regression models indicated that survival differed by river and year and was negatively correlated with tagging location (river kilometer) and positively correlated with fish length: larger fish and those tagged lower in the watershed were more likely to survive. The number of days juvenile coho salmon spent in freshwater before migrating to sea was positively correlated with tagging location, fish length (mm), and habitat depth (m) and negatively with density (coho salmon/m²). Our results suggest that fall or early winter migration is a common life history for juvenile coho salmon that is driven in part by fish size and location in the watershed. The exclusion of fall migrants may lead to underestimates of the total number of migrants and parr-to-smolt survival.

Received March 12, 2011; accepted January 9, 2012