A model of bluegill-largemouth bass interactions in relation to aquatic vegetation and its management

Dense, slow growing populations of bluegill sunfish are common in lakes where heavy vegetation prevents thinning of bluegill numbers by their primary predator, the largemouth bass. Since bass can effectively prey on bluegill only along the periphery of dense macrophyte beds, mowing vegetation to create additional edge may improve growth rates and size structure of both species. While this concept is straightforward, determining the optimal vegetation configuration for the two species is difficult because mechanisms such as feeding, competition, and mortality together determine the population response and may interact in complex ways. This paper describes a model that simulates the interactions of bluegill, largemouth bass and their invertebrate prey in the context of vegetation structure. The model is calibrated to data from a heavily vegetated temperate lake and is used to examine the fish response to additional edge created by mowing channels through plant beds. Bass growth rates and numbers increased after most simulated vegetation removals because their access to prey increased. Bluegill grew fastest when about 30% of the vegetation was cut, but responded negatively to mowing more than about half the plants. Bluegill responded to the manipulations more by changing growth rates, while bass responded more by increasing numbers. Moderate plant removals accomplished by mowing many narrow channels are most likely to simultaneously benefit both species. The model synthesizes current understanding of the relationship between macrophytes, bluegill and largemouth bass, and suggests which vegetation manipulations are most informative to test in large scale experiments.