Short-Term Pond Plankton Impacts of Threadfin Shad, Silver Carp and Hybrids of Silver and Bighead Carp in Outdoor Plankton Mesocosms

Threadfin shad is an important forage fish, with potential diet overlap with invasive silver carp. Fish filtration impacts, including that of the exotics’ hybrids, were compared to unstocked controls in outdoor 500-l tanks at the UAPB Aquaculture Research Station in 1997. Unfed silver carp (100g) and threadfin shad (15g) were each stocked in triplicate at 1.0 g/l, and hybrid juveniles (25g) at 0.1 g/l, immediately following filling with eutrophic pond water dominated by cyanobacteria. After 48 h, water column samples were examined for plankton abundances and chlorophyll a (pheophytin acorrected).

Shad and silver carp significantly reduced numbers of cyanobacteria. Chlorophyll a greatly increased with shad and slightly decreased with silver carp. No change was found with hybrids.

Shad and silver carp were similar in reduction of copepods (-93% and -92%) and rotifers (-77% and -75%), respectively. However, they differed in reduction of cladocerans (-77% and -95%). Nauplii increased with silver carp (+388%), but were reduced with shad (-96%). The large increase in pheophytin a with carp, but no increase with shad, may be related.

Food competition with threadfin shad, as well as with fish larvae, may be expected from silver carp. Hybrid carp appear less effective planktivores than parents.