Fisheries restoration potential for a large lake ecosystem: using ecosystem models to examine dynamic relationships between walleye, cormorant, and perch
Andrea M McGregor, University of Alberta, Department of Renewable Resources; Alberta Environment and Sustainable Resource Development
Christopher L Davis, Ontario Ministry of Natural Resources
Carl J Walters, UBC Fisheries Centre
Lee Foote, University of Alberta, Department of Renewable Resources
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Increased population sizes of Double-crested Cormorants (Phalacrocorax auritus)
and small-bodied (<15 cm total length) yellow perch (Perca flavescens)
have occurred at Lac la Biche, Alberta, Canada, since fisheries collapsed the walleye (Sander vitreus)
population. A walleye restoration program was introduced in 2005, but uncertainty around the ecosystem’s response to management made it difficult to evaluate program success. This study used 40 variations of Ecopath with Ecosim models representing ecosystem conditions over 200 years to test the potential for multiple attractors, i.e., possible ecosytem states, in a large lake ecosystem. Results suggest that alternate stable states, defined by walleye-dominated and cormorant-dominated equilibriums, existed in historical models (1800, 1900), whereas contemporary models (1965, 2005) had a single cormorant-dominated attractor. Alternate stable states were triggered by smaller perturbations in 1900 than in 1800, and model responses were more intense in 1900, suggesting a decline in system resilience between model periods. Total prey biomass consumed by walleye was up to four times greater than the biomass consumed by cormorants in historical models, but dropped to 10% of cormorant consumption in 2005 models. Differential size-selection pressures of cormorants and walleye on yellow perch provided strong feedback that stabilized each state. These results provide important theoretical support for alternate stable states as well as practical insights for restoration of large lake ecosystems affected by human induced overharvest of top-level fish predators.
alternate stable states; cormorant; ecosystem modeling; Lac la Biche; restoration; walleye; yellow perch
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