Tracking wild sockeye salmon smolts to the ocean reveals distinct regions of nocturnal movement and high mortality

Abstract

Abstract. Few estimates of migration rates or descriptions of behavior or survival exist for wild populations of out-migrating Pacific salmon smolts from natal freshwater rearing areas to the ocean. Using acoustic transmitters and fixed receiver arrays across four years (2010-2013), we tracked the migration of >1850 wild sockeye salmon (Oncorhynchus nerka) smolts from Chilko Lake, British Columbia, to the coastal Pacific Ocean (>1000 km distance). Cumulative survival to the ocean ranged 3-10% among years, although this may be slightly underestimated due to technical limitations at the final receiver array. Distinct spatial patterns in both behavior and survival were observed through all years. In small, clear, upper-river reaches, downstream migration largely occurred at night at speeds up to 50 km/d and coincided with poor survival. Among years, only 57-78% of smolts survived the first 80 km. Parallel laboratory experiments revealed excellent short-term survival and unhindered swimming performance of dummy-tagged smolts, suggesting that predators rather than tagging effects were responsible for the initial high mortality of acoustic-tagged smolts. Migration speeds increased in the Fraser River mainstem (~220 km/d in some years), diel movement patterns ceased, and smolt survival generally exceeded 90% in this segment. Marine movement rates and survival were variable across years, with among-year segment-specific survival being the most variable and lowest (19-61%) during the final (and longest, 240 km) marine migration segment. Osmoregulatory preparedness was not expected to influence marine survival, as smolts could maintain normal levels of plasma chloride when experimentally exposed to saltwater (30 ppt) immediately upon commencing their migration from Chilko Lake. Transportation of smolts downstream generally increased survival to the farthest marine array. The act of tagging may have affected smolts in the marine environment in some years as dummy-tagged fish had poorer survival than control fish when transitioned to saltwater in laboratory-based experiments. Current fisheries models for forecasting the number of adult sockeye returning to spawn have been inaccurate in recent years and generally do not incorporate juvenile or smolt survival information. Our results highlight significant potential for early migration conditions to influence adult recruitment.

Predator swamping reduces predation risk during nocturnal migration of juvenile salmon in a high‐mortality landscape

“…Survival of smolts was poor (68%) in the first 13·5 km of the Chilko River even though migration rates were rapid and smolt duration in the system was generally <48 h. These results are consistent with previous telemetry research in the system and confirm this landscape to be high risk for migrant sockeye salmon smolts, as survival is nearly 100% once smolts reach the large, fast‐flowing and turbid Fraser River (Clark et al . ). We are confident that poor survival is not a result of holding and/or tagging effects.…”

“…Survival of dummy‐tagged smolts was 100%, similar to the 96–100% survival of held dummy‐tagged smolts 2010–2012 (Clark et al . ). In addition, Clark et al .…”

“…The detection probability ( p ) for this further site was fixed at 0·97, which is the mean value determined by similar CJS models constructed for 4 years of broad‐scale telemetry in the system on smolts (Clark et al . ).…”

“…Outmigrant survival in the small and clear Chilko River is consistently poor across years relative to the larger and turbid Fraser River (Clark et al . ). Nocturnal‐timed migrations by smolts and the presence of a multiple avian predators (mergansers, Bonaparte gulls, loons, etc.)…”

Evidence of Olfactory Imprinting at an Early Life Stage in Pink Salmon (Oncorhynchus gorbuscha)

“…This should not bias the core results as the two parent sources are nearly identical in their freshwater imprinting timing and duration. Based on acoustic tagging results, juvenile sockeye salmon migrate down the Fraser River at an average speed of 150 km per day34, which is the approximate speed of the Fraser River in spring. Assuming pink salmon also travel at the speed of the river, juvenile Seton River pink salmon, which are spawned further up the Fraser River than juvenile Weaver Creek pink salmon, would only take ~1.5 days longer to migrate to the ocean than Weaver Creek juveniles.…”