Influence of blueback herring, Alosa aestivalis, on zooplankton in a southeastern US reservoir

Grove L, Stell EG, Grove LJW, Wright RA, DeVries DR. 2022. Influence of blueback herring, Alosa aestivalis, on zooplankton in a southeastern US reservoir. Lake Reserv Manage. XX:XXX-XXX. Forage fishes like blueback herring (Alosa aestivalis) have been widely introduced, sometimes without careful consideration of potential ecological consequences. We compared biotic and abiotic factors before and after blueback herring introduction into Lewis Smith Lake, Alabama (United States), and tested for diet overlap among planktivorous species to quantify their potential ecological influences. Abiotic and biotic factors varied among regions of the lake, consistent with differences in agricultural practices and nutrient input within each region. Secchi depth increased and zooplankton density decreased, while chlorophyll a remained unchanged relative to before blueback herring introduction. Juvenile and adult blueback herring and threadfin shad (Dorosoma petenense) consumed the same zooplankton taxa; however, blueback herring consumed significantly larger individuals and significantly greater numbers of zooplankton than threadfin shad. Blueback herring also selectively consumed larger zooplankton than the average size in the reservoir population, while threadfin shad did not. Threadfin shad and blueback herring positively selected Bosmina and cyclopoid copepods. The taxonomic overlap and size selectivity in prey choice suggest that if blueback herring reduce larger zooplankton, threadfin shad and blueback herring diets are likely to increasingly overlap. Because of their differences in feeding strategies (i.e., related to zooplankton size differences) and habitat preferences, current evidence for potential competitive interactions between these planktivores appears limited; however, longer term concerns are that blueback herring could eventually reduce larger zooplankton to the extent where they would increasingly compete with threadfin shad as the most abundant forage fish in Lewis Smith Lake.