UBC Theses and Dissertations
Variation in pink shrimp populations off the west coast of Vancouver Island : oceanographic and trophic interactions Martell, Steven James Dean
This thesis examines the historical dynamics of pink shrimp populations off the west coast of Vancouver Island. Fisheries for pink shrimp in this area started in 1973 and stocks collapsed on two occasions. Shrimp populations continued to decline in the absence of fishing. Using a single species stock assessment model, I reconstructed time series on shrimp abundance, recruitment, and natural mortality in two statistical areas to investigate environmental and trophic interactions that would explain historical variability in recruitment and natural mortality. In the southern statistical area, there was evidence of recruitment over-fishing, and in both statistical areas juvenile survival rate was density dependent. There was no evidence that northern flowing coastal current advected shrimp larvae from southern fishing grounds to northern fishing grounds. Incorporating oceanographic indices as environmental correlates did not explain much of the residual variation in the stock-recruitment relationships. Members of the family Gadidae primarily consume shrimp, and I examined if variation in estimated shrimp mortality was correlated with Pacific cod and Pacific hake abundance. No significant negative correlations were found between shrimp abundance arid predator abundance; however, the Pacific hake time series was incomplete. Contrary to other studies, shrimp abundance and Pacific cod abundance were positively correlated, as well as shrimp recruitment and juvenile Pacific cod abundance. To examine pink shrimp dynamics from an ecosystem perspective I constructed a simplified ecosystem model that represented the main trophic interactions and commercial fisheries off the west coast of Vancouver Island. A time series of relative primary production off the west coast of Vancouver Island was estimated by fitting the ecosystem model to time series data,. Including trophic interactions and variation in primary production explained an additional 29% and 15% of the residual sum of squares for pink shrimp biomass and landings, respectively. Predation mortality by Pacific cod on pink shrimp declines in the mid 1980's and late 1990's due to depleted Pacific cod stocks, however, total mortality for pink shrimp increases due to increased shrimp fishing. The west coast Vancouver Island ecosystem is sensitive to variation in primary production, and estimated primary production shifts downward around 1977.
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