Age‐0 Gizzard Shad Abundance is Reduced in the Presence of Macrophytes: Implications for Interactions with Bluegills

Abstract

AbstractThe abundance of gizzard shad Dorosoma cepedianum may be reduced in the presence of macrophytes, which may lessen the frequently observed negative interactions with coinhabiting bluegills Lepomis macrochirus. We used a series of experimental pond studies and a small‐lake data set to examine relationships among gizzard shad, bluegills, and macrophytes. Fall age‐0 gizzard shad abundance was lower in vegetated experimental ponds than in unvegetated ponds, but mean total length (TL) did not differ. Larval gizzard shad abundance did not differ between vegetated and unvegetated ponds, but the relative survival of age‐0 gizzard shad (ratio of fall juveniles to the total number of larvae collected) was higher in unvegetated ponds. In ponds with macrophyte coverage ranging from 0% to 100%, age‐0 gizzard shad abundance declined with increasing macrophyte coverage; however, age‐0 bluegill abundance was unrelated to macrophyte coverage. The abundance and size of age‐0 gizzard shad and age‐0 bluegills were unrelated in these ponds. By examining the small‐lake data, we found that the proportion of lakes containing gizzard shad decreased with increasing macrophyte coverage. The proportion of bluegill populations containing large adults (>203 mm TL) increased with macrophyte coverage for lakes with gizzard shad but not for lakes without gizzard shad. Although the actual mechanisms are not clear, we suggest that competitive interactions between gizzard shad and adult bluegills are density dependent and lessen with increasing macrophyte coverage because of decreasing gizzard shad abundance.