Invasive species are a prevalent threat to communities and ecoystems, acting to alter primary productivity and nutrient pathways in native ecosystems, affecting condition of native species. Invasive, planktivorous Asian Carp (Hypophthalmichthys spp.) may alter food webs by altering native zooplankton communities and competing with native, planktivorous and detritivorous gizzard shad (Dorosoma cepedianum). Asian carp may also increase phytoplankton densities through top-down regulation of zooplankton and increased detrital quality. Additionally, as gizzard shad can switch from zooplankton to detritus, Asian carp may actually have a neutral effect on this native species. We conducted a mesocosm experiment in 0.10-acre ponds to assess food web interactions among plankton, gizzard shad and silver carp (H. Molitrix), with three treatments (n=3): gizzard shad only, gizzard shad and silver carp together, and a no-fish control. Ponds were sampled weekly for zooplankton density, chlorophyll a concentration, and water NH4, NO3, and PO4 concentrations. Guts from each species were collected to assess dietary overlap.
Results/Conclusions
Silver carp did not affect gizzard shad growth or survival. Detritus was abundant in gizzard shad gut contents in the presence and absence of silver carp, indicating a lack of competition. Zooplankton density and chlorophyll a concentrations declined in the presence of silver carp and were significantly greater than in the presence of gizzard shad only or in control ponds. This suggests both zooplanktivory and herbivory by silver carp, rather than a trophic cascade. Water column NH4, NO3, and PO4 concentrations did not differ across treatments, which suggests that neither silver carp nor gizzard shad excretion and egestion altered nutrient availability in the water column. However, silver carp contributions to the detrital nutrient pool, which have not yet been assessed, could be substantial, thereby shedding light on interactions between silver carp and gizzard shad, given the high degree of detritivory by gizzard shad. Nevertheless, this study shows that invasive Asian carp may not negatively impact native gizzard shad.