Estimating Cohort Survival through Tributaries for Salmonid Populations with Variable Ages at Migration

Abstract

AbstractSurvival of juvenile anadromous Pacific salmonids from their earliest age of seaward movement (“out‐migration”) through the tributaries that connect their rearing grounds to larger‐order rivers (“cohort survival”) is an important yet often unmonitored factor in the complex life history of these species. Populations with variable age at out‐migration (e.g., steelhead Oncorhynchus mykiss) or multiple juvenile rearing strategies (e.g., Chinook Salmon O. tshawytscha) raise particular challenges in survival monitoring. A multiple‐state release–recapture model is presented to estimate cohort survival for salmonid species with variable age at out‐migration and demonstrated in two case studies. Annual releases of fish tagged with passive integrated transponder (PIT) tags from the same brood year (cohort) and their detections by instream PIT‐tag detection arrays in tributaries and at hydroelectric dams are used to estimate survival through tributaries and the age composition of juvenile migrants reaching downstream sites. The 2010 cohort of Chinook Salmon from the Chiwawa River in Washington State had estimated survival of 0.59 to the middle Wenatchee River array in Tumwater Dam Reservoir (river distance of 30 km) and 0.12 to McNary Dam (364 km) on the Columbia River. The age composition shifted from 60.1% subyearlings at the Wenatchee River array to 100% yearlings at McNary Dam, reflecting the use of the Wenatchee River as a nonnatal rearing area, whereas McNary Dam is in the migratory corridor. The 2010 cohort of steelhead from the Twisp River in Washington State had estimated survival from the yearling stage of 0.30 to Rocky Reach Dam (146 km from the Twisp River mouth) on the Columbia River and 0.17 to McNary Dam (439 km). The low survival to the Columbia River resulted from both mortality and adoption of a resident life history and reflects the diverse population structure in the Twisp River.Received February 4, 2015; accepted June 11, 2015