Exploring optimal foraging by largemouth bass (Micropterus salmoides) from three experimental lakes

The largemouth bass (Micropterus salmoides) is a keystone species in many north temperate lakes (CARPENTER et al. 1987, MITTELBACH et al. 1995) and can impact upon food web structures and primary productivity. Its foraging versatility (ANDERSON 1984, COLGAN err al. 1986, Hooson & Kartell 1987, ÜLSON 1996) and efficiency have caused prey-species extirpation (CARPENTER et al. 1987, MITTELBACH et al. 1995), and have influenced recent applications of bass foraging models (SCHINDLER et al. 1997). The Energy Maximization Premise (EMP) of Optimal Foraging Theory (OFT) (KAMIL et al. 1987, HUGHES 1990) predicts thar prey-specific consumption should follow a ranked order correlated with bioenergeric cost-effectiveness (WERNER 1979). Predators choose not to consume low-ranking prey types when highly-ranked prey types are available, resulting in decreased diet breadth (number of significant prey types). When highly-ranked prey types become scarce, the predator must include low-ranking prey types, causing an increase in diet breadth. We analyzed relationships among diet breadth, density, and bioenergerics of largemouth bass from three small northern Michigan (USA) experimental lakes. The bioenergerics approach involves the model parameter P, the proportion of maximum daily consumption required to maintain predator mass balance consistent with growth, diet, and temperature (KITCHELL et al. 1977). The modeling strategy is based on the laws of thermodynamics (K!TCHELL et al. 1974). Our population-level analyses of diet breadth and P of largemouth bass from one unmanipularted and two food web-manipulated lakes over a multi-year duration is of value in creating a broader understanding of bass foraging behavior in relation to population density and prey availability. We tested four principal hypotheses under the general tenet of demonstrating OFT by largemouth bass: (l) diet breadth decreases as population density decreases; P increases (2) as population decreases, (3) as breadth decreases, and ( 4) as the proportion of high-quality prey consumed increases. Assumptions for the existence of primarily in prey selection have undergone serious debate (KAMIL et al. 1987). Our objective is not to support or refuse tehets of OFT, but to apply patterns of bass foraging behavior in relation to OFT. Any observation in our analyses that does not include optimal shifts in diet breadth and/or predicted corresponding variations in the magnitude of P should provide support for alternate foraging hypotheses.