AbstractPiscivorous fish can exert considerable influence on the structure and function of aquatic ecosystems. However, current U.S. Environmental Protection Agency guidelines for aquatic meso‐cosm tests for pesticide registration assume that mesocosms with a volume of<1,000 m3are too small to support a piscivorous fish population. We tested this assumption between early April and mid‐December by stocking ten 0.04‐ha, 345‐m3, earthen pond mesocosms with 30 adult bluegill and two of the 10 with 0, 10, 20, 40, or 80 fingerling largemouth bass. As the stocking rate of bass increased, the average size and condition factor of bass and the number of recovered juvenile bluegill decreased, whereas the average length and mass of juvenile bluegill increased. The total biomass of juvenile bluegill in each pond was not affected by bass stocking rate. Adult bluegill were not affected by bass stocking rate, except for a slight increase in total biomass in the 80‐bass ponds. Bass stocking rate had marginal impacts on the crustacean zooplankton and no significant effect on emergent insects. The survival of reasonable numbers of juvenile bluegill over a wide range of bass densities indicates that 0.04‐ha mesocosms can be feasibly stocked with piscivorous fish. The key is probably sufficient macrophyte abundance to provide cover for the juveni
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