Acute toxicity of fine and coarse copper sulfate crystals to planorbid snails in experimental catfish ponds

Abstract

Bolbophorus damnificus , a digenetic trematode infecting ictalurid catfish, causes substantial economic losses in commercial aquaculture through a parasite‐induced anemia, leading to poor performance and increased susceptibility to secondary bacterial infections. Effective control depends on interrupting the parasite's life cycle by reducing populations of planorbid snails that serve as the first intermediate host. Copper sulfate pentahydrate (CSP) is a widely used molluscicide; however, traditional liquid applications are labor‐intensive, imprecise, and prone to dosing variability. This study evaluated the direct application of CSP crystals for improved snail control using a granular CSP precision‐application system designed to evenly distribute material along pond margins. Experimental ponds were treated with 1, 2, or 3 mg/L CSP using coarse (2–5 mm) and fine (30 mesh, 150 μm) crystals to assess effects on snail mortality, water quality, algal communities, and fish production. Calibration trials indicated coarse‐crystal applications averaged 7% above target concentrations, whereas fine‐crystal applications averaged 3.4% below target across all treatment rates. Application of coarse crystals produced relatively small increases in uncomplexed copper (Cu ²⁺ ) concentrations and negligible snail mortality compared to fine crystals, which significantly elevated Cu ²⁺ levels and achieved 70%–90% snail mortality. CSP applications also reduced cyanobacteria (Cyanophyta, dominated by Planktothrix spp.) densities by 93%–97% within 14 days and had no measurable effects on Chlorophyta. However, fish toxicosis occurred in some replicates, even at concentrations below calculated safety thresholds based on alkalinity, likely because of elevated water temperatures (30°C) and fish location within the treatment zone. These findings demonstrate application of fine CSP crystals effectively controls snail populations and suppresses cyanobacteria, but treatment safety is strongly influenced by environmental conditions affecting copper toxicity and fish distribution during application.