Salish Sea Ecosystem Conference

Telemetry tracking of salmon smolt migrations through the Salish Sea: examining behaviour, survival and causes of mortality

The Salish Sea is an important migratory corridor and feeding area for juvenile salmonids yet little is known regarding movement behaviours and survival through this region. We used acoustic telemetry tracking in sockeye and steelhead smolts to: 1) identify marine travel rates and routes, 2) estimate migration survival rates, and 3) identify factors contributing to mortality. Sockeye smolts travelled at 10 km / day in their clear natal river, 200 km / day in the turbid Fraser River mainstem, and 10-15 km / day in the marine environment. Steelhead smolts travelled approximately 2-3 times faster in the same marine segments, potentially owing to their larger size. Both sockeye and steelhead seemed to prefer migrating through western or central routes en-route to the open ocean, rather than more easterly routes when transiting the Discovery Islands chain – and survival to the open ocean, at least for steelhead, was higher if travelling through the most western route. Survival was poorest through the earliest migratory segments for both species and both exhibited less than 10% survival by the final acoustic receiver station near northern Vancouver Island. Several factors were associated with mortality including smolt age, size, tag burden, and fish health as assessed by biomarkers and pathogens. Pathogen-laden sockeye were far more likely to be predated upon (found in the stomachs of bull trout). Sockeye smolts could increase their probability of survival by out-migrating in high densities – survival probability doubled with ~ 2-fold increase in migrating density. This evidence of depensatory mortality indicates that salmonid smolts may survive poorly in years of low marine abundance.