Effects of angling and life history processes on bluegill size structure: Insights from an individual-based model

Bluegill Lepomis macrochirus is a common sport fish for anglers, yet populations often suffer from stunting. We examined the effects of life history shifts and fishing mortality on bluegill size structure and on the rate of recovery from exploitation. We constructed an individual-based model that simulated size-dependent age at maturation, differential growth of mature fish, and harvest of individuals larger than 150 mm. We used this model to separate the direct effects of harvest, the direct effects of life history process, and their interactive effects on bluegill size structure. We also evaluated the rate at which bluegill size structure recovered to pre-exploitation levels after eliminating harvest. The direct effect of harvest had the largest effect on bluegill size structure, reducing the mean size of harvestable fish by as much as four times (as compared to results associated with simulations with only life history effects). There was no evidence for an interaction between these two effects. However, model simulations indicated that recovery rates of exploited populations were two to three times greater with life history processes than without them. These results indicate that the most important variable structuring bluegill populations is angling mortality. Long recovery times of large bluegills in the population indicate that changes in regulations designed to reduce harvest may take years to achieve the desired results.