"Bythotrephes" competes with several fish, including panfish and perch, for prey. It has been suggested that very small larval fish are not able to feed on the spiny water flea itself because of the barbs the flea possesses, but it is readily consumed by several fish species once the fish are larger. The spiny water flea's diet consists mostly of Daphnia zooplankton, leading to competition with small and baby fish, and also with native water flea species. Daphnia zooplankton populations have declined in recent years, though there is no conclusive evidence as to the cause.