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Mortality of migrating Pacific salmon smolts in southern British Columbia, Canada Melnychuk, Michael Colin

Abstract

Determination of causes for recent declines in marine survival rates of many Pacific salmon populations has been hindered by lack of information on where and when mortality predominantly occurs. Acoustic telemetry was used to monitor movements of tagged outmigrating juvenile salmon, and mark-recapture models were used to estimate survival. Several methods were developed for estimating detection probabilities (p) of tagged fish: models for combining multiple populations with partially-overlapping migration routes allowed for sharing of information among populations to compensate for small sample sizes; models for estimating p using only local information at detection stations reliably predicted mark-recapture p at large sample sizes; estimated p during mobile transect surveys attenuated from transect lines and depended on boat speed; variation among tags in acoustic strength resulted in minor heterogeneity in p; and p estimates derived from test tag passes were used to estimate freshwater residualization rates. Migration patterns varied among species, with steelhead and sockeye salmon quickly leaving Georgia Strait, and coho and Chinook salmon instead likely residing in the inshore Strait during summer months, thereby confounding interpretation of early ocean mortality. High mortality often occurred before ocean entry. Variation in survival during the downstream and inshore coastal migration was largely attributed to size-selective mortality against smaller fish and variation in migration distance or travel time. Some components of mortality during the downstream and inshore migration were independent of distance travelled, suggesting high mortality periods soon after release in freshwater and soon after ocean entry. Compared to wild conspecifics, hatchery-reared steelhead suffered high mortality immediately after release, and this survival difference was maintained through the inshore migration. In an experiment to test the effect of solar ultraviolet radiation during freshwater rearing on smolt mortality, no survival differences were observed between UV-shaded and UV-exposed treatment groups of coho or sockeye salmon. Overall, instantaneous mortality rates (time-based or distance-based) were highest soon after release, declined during the inshore migration, and were lowest for the remainder of smolt-to-adult ocean life, supporting hypotheses of high mortality early in the migration but not ruling out hypotheses of high mortality during the first marine winter.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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