Copper Sulfate Pretreatment for Snail Control Reduces Channel Catfish Fry Survival.

Channel Catfish Ictalurus punctatus fry farmers are interested in treating ponds to prevent trematode infections. Control strategies rely on chemical treatments to reduce snail populations and break the trematode life cycle. On a commercial catfish fingerling operation, we evaluated water quality, zooplankton, phytoplankton, and fish production in catfish nursery ponds treated with 1.02‐mg/L Cu 21 d before fry stocking. These variables were also evaluated in controlled experimental ponds treated with 1.27‐mg/L Cu 21, 14, 7, or 1 d before stocking. In the on‐farm trial, copper sulfate treatment initially reduced phytoplankton and zooplankton populations, but communities recovered to adequate numbers at stocking. However, fry survival and revenue from ponds treated with copper sulfate were reduced relative to those from untreated ponds (survival: 17.7% versus 63.0%; revenue: $10,250 versus $27,467 per hectare). In the experimental pond study, free copper concentrations at fry stocking were 0.52 mg/L for the 1‐d interval between treatment and stocking, 0.10 mg/L for the 7‐d interval, 0.04 mg/L for the 14‐d interval, and 0.02 mg/L for the 21‐d interval. Similar results were observed in the experimental study, with lower survival in all treatment groups compared to controls; the lowest survival was observed in fry stocked 1 d after copper sulfate treatment. The cause of lower survival in copper‐treated ponds is unclear. With the exception of ponds stocked 1 d after copper treatment, lower survival was likely related to effects on environmental quality rather than direct copper toxicity. Regardless of the cause, until further work evaluates copper's environmental effects, copper sulfate should not be used in Channel Catfish nursery ponds during early developmental fry stages. [ABSTRACT FROM AUTHOR]