Ecological effects of chemicals used in pond culture of catfish and percid fishes

One of the major concerns of expanding aquaculture industry is the extensive use of chemicals. For balanced and sustainable aquaculture, it is imperative to determine the optimal rate of application of chemicals and the possible effects on pond water quality, non-target species and overall fish production. To this end, we performed a series of experiments to evaluate the efficacy and effects of fertilizers, herbicides and algaecides used in percid and catfish ponds of the State Fish Hatcheries of Ohio.To determine whether lowering of phosphorus inorganic fertilization rate will increase juvenile percid production as well as lower the risk of proliferation of inedible algal species, we conducted field experiments comparing the original phosphorus fertilization rate of 30 mug P/L with lowered fertilization rates of 20 mug P/L and 10 mug P/L and a constant inorganic nitrogen fertilization rate of 600 mug N/L. Our results showed that lowering of phosphorus fertilization rate from 30 to 10 mug P/L neither affected the larval saugeye production (in terms of survival, yield, and growth) nor the maintenance of adequate zooplankton forage base throughout the culture duration. The phytoplankton biomass and species composition remained similar among treatments the inoculum-weighted dominance of Cyanobacteria prevented proliferation of edible algal species in the initial weeks of fish culture. Lowering the phosphorus fertilization rate in percid ponds may reduce the potential risks of elevated pH, unionized ammonia, low dissolved oxygen concentration, macrophyte infestation and the discharge of nutrient-rich effluent into surrounding waters. Based on the results of this study, a juvenile percid pond fertilization protocol with lowered phosphorus fertilization rate of 10 mug P/L has been implemented in State Fish Hatcheries of Ohio.To examine whether the use of herbicides and algaecides is justified in catfish ponds, we conducted field experiments to investigate the efficacy of commonly used herbicide-fluridone and algaecide-copper sulfate applications at controlling plants and algae and their possible effects on water quality, seasonal dynamics and species composition of phytoplankton and zooplankton, and overall catfish production. Our results showed that the combined application of fluridone and copper sulfate resulted in substantially less macrophyte biomass than did the fluridone-alone treatment. Fluridone and copper treatments elicited different responses within the phytoplankton community. Copper treatments reduced Cyanophyta biomass, whereas biomass of Chlorophyta and Chrysophyta was increased. Fluridone treatments reduced total phytoplankton biomass as well as Cyanophyta biomass and influenced the response of Chlorophyta and Chrysophyta to copper, demonstrating some algaecidal potential. The phytoplankton community composition shifted towards species tolerant to copper in the treated ponds which in turn affected zooplankton community composition along with direct toxic effects of copper on sensitive zooplankton species. Copper treatments significantly reduced Cladocera biomass, whereas Copepoda biomass was significantly higher in copper-treated ponds than in controls. Catfish survival and yield were not significantly different among treatments. As an aquaculture pond can be considered as a model freshwater ecosystem, this study elucidated underlying effects and responses of natural aquatic communities to environmentally realistic concentrations of fluridone and copper sulfate, yielding meaningful results both for aquaculture ponds as well as other freshwater ecosystems.To assess the extent and effects of carry-over copper in double-cropped ponds on juvenile percid culture, we conducted field monitoring of copper in the sediment and pond waters of double-cropped ponds at Senecaville State Fish Hatchery, where consistently low percid yield and low biomass of zooplankton forage base for planktivorous percids have been observed. To complement our field data, we conducted a laboratory experiment to assess the effect of copper concentrations encountered in Senecaville waters on the reproductive output of a resident cladoceran species (Daphnia parvula). We found relatively higher copper concentrations in the pond sediments from repeated applications of copper sulfate during catfish culture in summer, which remobilized into the water column during percid culture in spring. The reproductive output of D. parvula was significantly affected by dissolved copper concentrations as low as 6.3 and 9.9 mug/L. Further manipulative experiments need to be conducted to establish the direct and indirect effects of low concentrations of copper on plankton biomass, species composition and percid production.