Bioenergetic Values of Shads and Sunfishes as Prey for Largemouth Bass

We conducted laboratory and field investigations to detennine the relative value of shads (Dorosoma spp.) and sunfishes (Lepomis spp.) as prey for largemouth bass (Micropterus salmoides). In the laboratory, we examined the energy content of gizzard shad (D. cepedianum) and bluegill (L. macrochirus) and digestibility by largemouth bass. Although the ratios of proximate components in the 2 prey species differed, total energy contents as calories per gram were equal. The rates at which gizzard shad and bluegill were digested were similar at feeding rates <1.5%-2.0% of largemouth bass body weight, but gizzard shad were digested increasingly faster than bluegill at progressively higher feeding rates: maximum rate of digestion occurred at about 5% of the largemouth bass body weight for gizzard shad, and about 3% for bluegill. We examined the food of largemouth bass of the 1984 year class during 1984 and 1985 in 2 reservoirs with different densities of shads and sunfishes, and related the food to vital statistics of the largemouth bass populations. Largemouth bass ate more fish (mainly shads), grew faster, and had lower mortality in a reservoir where shads and sunfishes were about equally available than in a reservoir where sunfishes were the most readily available and most important forage. We attributed these differences to differences in diet. Vital statistics of the population with a shad-dominated diet were superior because the largemouth bass were able to obtain larger rations, digest food and empty the stomach at a faster rate, and possibly locate and capture prey more easily.

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