Importance of Gizzard Shad in the Diet of Largemouth Bass in Lake Shelbyville, Illinois

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Abstract
Changes in food habits of largemouth bass Micropterus salmoides were compared with seasonal and annual changes in size composition of a gizzard shad population Dorosoma cepedianum to better understand the temporal dynamics of this species as prey for largemouth bass. Stomachs of 5,283 largemouth bass were examined during spring, summer, and fall from 1978 to 1981. Gizzard shad was the most important species in the diet of age‐I and older largemouth bass in all years. However, the age and size composition, as well as the percentage of the volume contributed by this species to the diet, varied substantially among years and seasonally within each year. Variation of gizzard shad in the diet of yearling and older largemouth bass reflected changes in availability caused by high winter mortality of young‐of‐the‐year gizzard shad in some years as well as by annual variation in the summer growth of this age group. Seasonal gaps in the availability of important size classes of gizzard shad reduced its value as a forage species. Fish contributed a major portion of the diet of young‐of‐the‐year largemouth bass in all years but gizzard shad were important only in 1981. Age‐0 gizzard shad grew more slowly in 1981, a larger fraction remained vulnerable to predation, and more were eaten by age‐0 largemouth bass than in any other year.

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