Differentiating mortality from delayed migration in subyearling fall Chinook salmon (Oncorhynchus tshawytscha)

Abstract

A total of 1154 acoustic-tagged subyearling fall Chinook salmon ( Oncorhynchus tshawytscha ) were monitored during their movement through Lower Monumental Reservoir, Snake River, Washington, USA. A release–recapture design was developed to partition their fates into migration, delayed migration (i.e., holdover or temporary residualization), and mortality using a series of standard detection arrays augmented with individual intrareach autonomous receivers. The standard detection arrays were used in conjunction with traditional release–recapture models to estimate the joint probabilities of migrating and surviving through the reservoir. Closed population estimators were used to estimate the abundance of tagged fish still alive in the river reaches and to differentiate mortality from delayed migration. Over the course of the study from 15 August to 14 November 2007, delayed migration rates increased and mortality rates generally declined. A minimum of 10.6% of the fish were estimated to have delayed migration in the reservoir during the study period.