How will fisheries managers respond?
The Great Lakes have seen successive invasions by non-native species that alter the ecosystem, including quagga mussels that filter the water and remove nutrients. At least partly as a result of the invasive mussels, Lake Michigan is becoming less hospitable to Chinook salmon, according to a new study led by scientists with the U.S. Geological Survey and Michigan State University.
The scientists concluded that stocking could help sustain a population of Chinook salmon, but that the lake’s ecosystem is now more conducive to stocking lake trout and steelhead salmon. These two species can switch from eating alewife, which are in decline, to bottom-dwelling round goby, another newly established invasive prey fish that feeds on quagga mussels.
“Findings from our study can help managers determine the most viable ways to enhance valuable recreational fisheries in Lake Michigan, especially when the open waters of the lake are declining in productivity,” said Yu-Chun Kao, an MSU post-doctoral scientist and the lead author of the report.
Managers have reduced Lake Michigan stocking levels of Chinook salmon at least three times over the past decades in response to declining prey fish and the natural reproduction of Chinook salmon.
The scientists modeled Lake Michigan’s food web dynamics under 288 scenarios that accounted for various levels of stocking and nutrients, as well as the effects of invasive mussels. These scenarios were developed based on responses to a survey from fishery managers, water-quality managers and researchers.
“Interestingly, reducing stocking by 50 percent in the model resulted in long-term Chinook population numbers that were similar to the numbers when stocking was not reduced,” said David “Bo” Bunnell, a USGS co-author on the study.
Lake Michigan’s open-water food web has changed significantly since the 1970s, becoming less productive as a result of decreased nutrients such as phosphorus, a process called oligotrophication. Nutrients help sustain phytoplankton and zooplankton, the tiny aquatic plants and animals at the base of the food web that support other aquatic life, including prey fishes. In Lake Michigan, oligotrophication occurred partly as a result of invasive mussels, which filter phytoplankton from the water column.
Declines in prey fishes such as alewife were likely due, in part, to decreases in plankton. Because the Chinook salmon diet consists of over 90 percent alewife, the new study predicts a smaller Chinook salmon population if nutrients remain low and invasive mussels remain abundant.