Seasonal and ontogenetic variation in Largemouth Bass diets across a gradient of Gizzard Shad abundance in Kansas small impoundments

Objective We sought to quantify Largemouth Bass Micropterus nigricans diets in small impoundments and investigate how seasonal and ontogenetic factors interact across a gradient of Gizzard Shad Dorosoma cepedianum relative abundances to influence prey selection. Methods We collected stomach contents from 3,500 Largemouth Bass across 12 small Kansas impoundments (<166 ha) during June, August, and October 2017 through 2020. We used generalized linear mixed-effects models to quantify the influence of Largemouth Bass size, season, and Gizzard Shad relative abundance on the frequency of coarse-scale (fish, crayfish, aquatic insects, and zooplankton) and fine-scale (Centrarchidae, Gizzard Shad) prey taxa in Largemouth Bass diets. Results Gizzard Shad relative abundance significantly altered diet composition in addition to notable effects of season and Largemouth Bass size. Increasing Gizzard Shad abundance was associated with reduced consumption of alternative littoral prey (Lepomis spp. and crayfish) and increased aquatic insect consumption, particularly in individuals <150 mm. While consumption of Gizzard Shad increased with Gizzard Shad abundance, its occurrence in diets remained low (<10%) and was seasonally restricted to October. Conversely, consumption of Centrarchidae declined significantly as Gizzard Shad abundance increased. Conclusion Despite their low occurrence in diets, Gizzard Shad appeared to alter food-web structure by disrupting consistent littoral energy pathways and diverting resources into a dynamic pelagic base that was largely unavailable to Largemouth Bass. The negative association between Gizzard Shad abundance and Centrarchid consumption suggests that small impoundments with high densities of Gizzard Shad may exhibit reduced trophic efficiency, as energy is sequestered in gape-protected Gizzard Shad biomass rather than being transferred to Largemouth Bass. Further research integrating growth and bioenergetic data is warranted to determine how energy is most successfully transferred to Largemouth Bass, specifically comparing net benefits of a transient autumn pulse of Gizzard Shad compared with a consistent supply of littoral forage.