Impact of Copper Sulfate on Plankton in Channel Catfish Nursery Ponds

Many fish culturists are interested in applying copper sulfate pentahydrate (CSP) to channel catfish, Ictalurus punctatus, nursery ponds as a prophylactic treatment for trematode infection and proliferative gill disease by killing snails and Dero sp., respectively, before stocking fry. However, copper is an algaecide and may adversely affect phytoplankton and zooplankton populations. We evaluated the effects of prophylactic use of copper sulfate in catfish nursery ponds on water quality and phytoplankton and zooplankton populations. In 2006, treatments of 0 mg/L CSP, 3 mg/L CSP (0.77 mg/L Cu), and 6 mg/L CSP (1.54 mg/L Cu) were randomly assigned to 0.04-ha ponds. In 2007, only treatments of 0 and 3 mg/L CSP were randomly assigned to the 16 ponds. Ponds treated with CSP had significantly higher pH and significantly lower total ammonia concentrations. Treatment of both CSP rates increased total algal concentrations but reduced desirable zooplankton groups for catfish culture. CSP has been shown to be effective in reducing snail populations at the rate used in this study. CSP treatment also appears to be beneficial to the algal bloom, shifting the algal population to green algae and increasing total algal biomass within 1 wk after CSP treatment. Although zooplankton populations were adversely affected, populations of important zooplankton to catfish fry began rebounding 6–12 d after CSP treatment. Therefore, if CSP is used to treat catfish fry ponds of similar water composition used in this study, fry should not be stocked for about 2 wk after CSP application to allow time for the desirable zooplankton densities to begin increasing. Copper products have been used in aquaculture as algaecides for many years. Although copper is not considered to be selective, it has been used successfully to control the specific algae that cause off-flavors in catfish ponds (Tucker et al. 2001; Schrader et al. 2005). Application of copper sulfate pentahydrate (CSP) can also be effective in controlling snails in channel catfish, Ictalurus punctatus, production ponds. Snails, specifically the marsh rams-horn snail, Planorbella trivolvis, serve as intermediate hosts of the digenetic trematode Bolbophorus damnificus. This trematode has caused millions of dollars in economic losses and especially impacts fingerling catfish (Mitchell 2001). Reduction of snail populations is the most practical method of controlling this trematode. Copper is toxic to snails, and in laboratory studies, the 24-h LC50 of copper was 0.6 mg/L (Mischke et al. 2005). Shoreline treatments with CSP are effective in killing snails along the pondmargins (Mitchell 2002; Mitchell and Hobbs 2003) as is uniform application of CSP throughout the entire pond at a rate of 2.5–5.0 mg/L (0.64– 1.27 mg/L Cu) (Wise et al. 2006). In addition to snail control, copper may be useful in reducing Dero spp. populations. The oligochaete Dero digitata is an intermediate host to the parasite that causes proliferative gill disease in channel catfish. As with trematodes, reduction of the intermediate host is the most practical method of controlling the disease. In laboratory studies,Dero spp. appeared to be sensitive to copper exposure with the LC50 values decreasing from 7.6 mg/L at 24 h to 1.8 mg/L at 48 h (Mischke et al. 2001). 1 Corresponding author. JOURNAL OF THE WORLD AQUACULTURE SOCIETY Vol. 40, No. 1