Lake Michigan’s balancing act
Boom and bust: Nature is full of examples of what happens when the balance between predators and prey gets out of whack. But when that balance is between two nonnative species in the vastly altered ecosystem of the Great Lakes, puzzling patterns may emerge. In a recent paper in Transactions of the American Fisheries Society, researchers at Michigan State University and the Michigan Department of Natural Resources attempt to explain the ups and downs of populations of Chinook Salmon and their primary prey, the Alewife, in Lake Michigan. When Lake Michigan’s previous top predator, the Lake Trout, was diminished after Sea Lamprey invaded, Alewife populations exploded. Chinook Salmon, among other species, were then stocked to eat the Alewives and ever since, fisheries managers have attempted to maintain a delicate balance between the populations of the two species while also working to restore native species. Using vast amounts of data and information on stocking rates, the researchers modeled factors such as fishing harvest, growth rates, and natural reproduction to determine salmon and trout abundance over the past few decades. They found that the Chinook population is tightly linked to the Alewife population while other Lake Michigan predators have a more varied diet. Improved Chinook survival rates and natural reproduction have offset recent reductions in stocking, and Alewife populations remain very low. The authors suggest that determining the appropriate stocking rate for Chinook must take into account the lake’s production capacity for prey and the consumption rates of all of the major predators in the lake. Changes in the Salmonine Community of Lake Michigan and Their Implications for Predator-Prey Balance, by Iyob Tsehaye, Michael L. Jones, Travis O. Brenden, James R. Bence, and Randall M. Claramunt. Transactions of the American Fisheries Society 143:420-437.