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Abstract
Feasibility of utilizing fish community biomanipulation as an algal management technique in Nebraska farm ponds was examined in two small, eutrophic impoundments located in the Lower Platte South Natural Resources District. Biomanipulations consisted of the removal of fish from the ponds by rotenone treatment, and the subsequent stocking of either black crappe (Pomoxis nigromaculatus) or nothern pike (Esox lucius) over a period of three years. Unreplicated treatments were staggered in time between the ponds to allow one pond to be used as a "natural control" in comparison against the other.
In the absence of fish, the zooplankton communities were dominated by large crustacean grazers (i.e. Daphnia pulex, D. ambiqua, Diaptomus sicilis, Simocephalus serratulus, Ceriodaphnia quadragula) which maintained phytoplankton biomass and numbers at low levels and led to excellent water clarity, reaching the pond's bottom. The addition of zooplanktivorous YOY crappie dramatically reduced the large crustacean grazer populations which were replaced by rotifers and the smaller nauplii and copepodid stages of Tropocyclops prasinius and Diacyclops bicuspidatus. Phytoplankton abundance and biomass increased and water clarity was reduced following this shift in the zooplankton community.
Stocking of the piscivorous pike resulted in large reductions in the number of planktivorous crappie present (average decrease of 78.9% in both ponds) by the end of the study. However, the shift in zooplankton composition and size structure back towards a community dominated by large-sized crustacean grazers was slow and insufficient to allow for grazer control of phytoplankton biomass during the growing season. Water clarity improved somewhat but remained low. The slow decline in predation pressure of zooplanktivores was, at least in part, a result of high levels of inorganic turbidity which reduced planktivore visibility to piscivores and habitat structure which may act as refuge for planktivores.
