Evaluating Stocking Efficacy in an Ecosystem Undergoing Oligotrophication
Oligotrophication has negatively affected fisheries production in many freshwater ecosystems and could conceivably reduce the efficacy of stockings used to enhance fisheries. In Lake Michigan, offshore oligotrophication has occurred since the 1970s, owing to reductions in total phosphorus (TP) inputs and nearshore sequestration of TP by nonindigenous dreissenid mussels. We evaluated simultaneous effects of stock enhancement and oligotrophication on salmonine species (Chinook salmon Oncorhynchus tshawytscha, lake trout Salvelinus namaycush, and steelhead O. mykiss) that support valuable recreational fisheries. We employed a novel application of an Ecopath with Ecosim model by conducting a full factorial simulation experiment. Our design included multiple levels of salmonine stocking, consumption by invasive quagga mussels (Dreissena bugensis), and TP that were informed by manager interests. Under all levels of TP and quagga mussel consumption, our results showed that stock enhancement could still increase salmonine biomass, but positive responses were stronger for lake trout and steelhead than Chinook salmon. Simulations showed that quagga mussel consumption has deleterious effects on pelagic-oriented prey fishes and Chinook salmon, which feed almost exclusively on the pelagic-oriented alewife (Alosa pseudoharengus). In summary, results from our simulation experiment suggested that lake trout and steelhead are better suited to the current ecosystem than Chinook salmon, and therefore, stock enhancement provides the highest gains for these two species. Furthermore, simulated biomass of all recreational salmonine species increased with increasing TP, indicating the need for managers to consider how potential future oligotrophication will limit the carrying capacity of salmonine biomass in Lake Michigan.
Keywordsstock enhancement Lake Michigan Ecopath with Ecosim recreational fisheries salmonines quagga mussels total phosphorus eutrophication
We thank the Great Lakes Fisheries Commission’s Lake Michigan Technical Committee, U.S. EPA’s Great Lakes National Program Office, Michigan Department of Natural Resources, Mark Rowe, Iyob Tsehaye, and David Warner for sharing data. We also thank Richard Barbiero, Brian Breidert, David Caroffino, Randall Claramunt, Euan Reavie, and Catherine Riseng for data compilation. For developing simulation scenarios in this study, we thank fishery managers, water quality managers, and researchers that responded to our online survey, which was designed and launched with assistance of Yanni Liu. Ashley Baldridge Elgin provided most recent updates on quagga mussel biomass across the Great Lakes. Charles Madenjian provided useful information for model development and comments for an earlier draft of this manuscript. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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