Ecological and morphological shifts in Lake Michigan fishes: glimpses of the ghost of competition past

Synopsis Both historical patterns and recent evidence of resource partitioning and complementarity within the Lake Michigan fish community provide circumstantial evidence for interspecific competition. But competition is difficult to document in the field without controlled experimentation. In Lake Michigan, controlled experiments on competition within the fish community are nearly impossible, but we still need to understand the interactions among the dominant fishes. For this purpose, I have relied upon hypothesis-based field observation, natural experiments in the field and designed laboratory experiments to evaluate competitive interactions. Resource use patterns and trophic morphology of the bloater, Coregonus hoyi, a native cisco, from samples taken before alewife, Alosa pseudoharengus, became abundant (1960) were compared to more recent data (1979–80). After the alewife density increase, bloaters had significantly fewer and shorter gill rakers. This suggests a morphological shift toward greater benthic foraging efficiency in response to high abundances of an efficient pelagic planktivore, alewife. Resource use comparisons suggested that bloaters now shift from pelagic zooplanktivory to benthic habitats and diets at least two years earlier in their life history than they did before alewife became abundant. This evidence, albeit not experimental, provides strong support for the importance of competition in the structure of the current Lake Michigan fish community.In Lake Michigan, seasonal thermal habitat compression can pack fish into a narrow thermal zone across the lake bottom, leading to increased habitat overlap, reduced prey availability and fish diets containing fewer and smaller prey. Thermal habitat compression, which can occur intermittently through the season, may create competitive bottlenecks which help maintain the observed resource partitioning among these fishes.