UBC Theses and Dissertations
Harbour seals, transgenic coho salmon and euphausiids : food dynamics in the Strait of Georgia, British Columbia Li, Lingbo
This thesis examines top-down control and a policy of predator culling, the possible impacts of invasive species, and the bottom-up effects of zooplankton on the fish populations in the Strait of Georgia (SoG), British Columbia. In summary, my study highlights indirect interactions and strongly suggests ecosystem-based management in the SoG. For top-down control, I examined interactions between harbour seals and fisheries using Ecopath with Ecosim modelling. Harbour seals feed on herring, hake, and many other commercial fish. Many fish populations in this region have declined in recent decades, while harbour seals increased exponentially after a ban of hunting in 1970, until they reached their carrying capacity in the 1990s. However, model results indicated that a cull of harbour seals may not increase total fisheries catch in the SoG because increased hake would eat more herring. With seals absent, the SoG ecosystem may be dominated by hake. The Ecopath model was then modified to investigate the ecological impacts of invasive species with altered physiology — growth-hormone transgenic (GH) coho. GH coho have the potential to greatly increase the yield of fish farms, but could cause ecological harm should they ever invade natural systems. My model scenarios showed that GH coho may impact the whole ecosystem largely through indirect interactions. Many functional groups were impacted depending on GH coho diet. However, functional groups were more strongly impacted when a bottom-up effect was introduced by changing ocean conditions. Sensitivity analyses showed that the predictions were robust to uncertainty in model parameters although predator-prey vulnerabilities were more sensitive than Ecopath parameters. I assessed the bottom-up effects by analyzing nocturnal zooplankton samples collected from the top 20m of the water column between 1990 and 2007. An abrupt step-like decline occurred in community composition in 1998/1999, especially in euphausiids and copepods. Local environmental factors had low coherence and changes in the SoG zooplankton communities correlated more with large-scale climate forcing than with local factors. The decline in zooplankton communities may be an important factor in the lack of recovery of predatory fish, such as coho salmon and lingcod, even with fishery closures in the 1990s.
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