Changes in the Salmonine Community of Lake Michigan and Their Implications for Predator–Prey Balance

Abstract

AbstractWe combined statistical stock assessment methods with bioenergetic calculations to assess historical changes in abundance and consumptive demand of the hatchery‐supported salmonine community in Lake Michigan, with the goal of providing information needed to examine the lake's predator–prey balance. Especially for Chinook Salmon Oncorhynchus tshawytscha, the most dominant salmonine predator in the lake, our analysis revealed density‐dependent changes in growth, survival, production levels, consumptive demand, and fishery characteristics, suggesting that increased salmonine abundance possibly had substantial impacts on prey abundance that led to predators being food limited. Indeed, the estimated changes in the salmonine community were consistent with historical changes in prey abundances that were previously documented for Lake Michigan. Specifically, higher salmonine abundance and consumption were estimated for the early 1980s, during which time Alewife Alosa pseudoharengus abundance experienced a marked decline, leading to a Chinook Salmon mass mortality event in 1987. Similarly, increased salmonine abundance and consumption were estimated for the years since the early 2000s, and the Alewife population in Lake Michigan has been driven to historically low levels during these years. Increased salmonine abundance estimates in recent years were attributable to improved survival rates and natural reproduction of Chinook Salmon. Although past revisions to stocking rates may have been reasonable measures taken to stabilize the predator–prey system, our analysis suggests that recent reductions in stocking have not been sufficient to reduce predatory pressure on the Alewife population; however, they may have ameliorated potential effects of increased natural reproduction of Chinook Salmon. Along with a complementary assessment of the production dynamics of key prey species, our retrospective assessment of the dynamics of the Lake Michigan predator community and their consumptive demands can provide the basis for making future fishery management decisions from an ecosystem perspective.