Riverine and early ocean migration and mortality patterns of juvenile steelhead trout (Oncorhynchus mykiss) from the Cheakamus River, British Columbia

High mortality (65-73%) occurred in the first month of the smolt migration in a population of wild steelhead trout. We used acoustic telemetry to monitor the downstream, estuarine, and early ocean migration of tagged smolts and estimate their mortality rates. After entering the Strait of Georgia most smolts migrated north through Johnstone and Queen Charlotte Straits rather than south through the Strait of Juan de Fuca. Of 51 smolts tagged in 2004 (49 in 2005), 36-38 (41-42) survived to leave freshwater and 14-19 (13-14) survived to leave the Strait of Georgia system. Mortality rates in separate segments of the migration were correlated with segment distances. An additional component of mobile sampling showed that few smolts died during the migration through Howe Sound. Migration rates averaged 0.7-0.9 body lengths per second (BL s(-1)) downstream and 1.0-2.6 BL s(-1) in ocean waters. Aggregated detection probabilities of 92-96% on lines of ocean receivers suggest that migration routes of small fishes can be quantified over several hundred kilometres, and survival rates can be estimated for even a modest number of tagged fish. Quantifying mortality patterns during the smolt migration could help to determine causes of low marine survival rates observed in recent years.