Kao, Y-C., M.W. Rogers, D.B. Bunnell. 2017. Evaluating stocking efficacy in an ecosystem undergoing oligotrophication. Ecosystems.doi.org/10.1007/s10021-017-0173-5
Oligotrophication has negatively affected fisheries production in many freshwater ecosystems, and could conceivably reduce the effectiveness 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, 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 effects 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 provide the highest gains for these two species. Furthermore, simulated biomass of all recreational species increased with increasing TP, indicating the need for managers to consider how future oligotrophication will limit the carrying capacity of Lake Michigan to support the biomass of key salmonine fisheries.