Evidence that channel catfish, Ictalurus punctatus (Rafinesque), mortality is not linked to ingestion of the hepatotoxin microcystin-LR

Catfish farms located in the south-eastern USA using brackish (3–5 g NaCl L⁻¹) well water experience sporadic fish kills sometimes with high mortality. An investigation of three catastrophic losses occurring in this region identified no involvement of infectious diseases or traditional water quality problems, including oxygen, ammonia or nitrite. The high mortality and time course of the problem was indicative of exposure to a toxin. Attempts by other workers to explain the cause of this unique syndrome (high chloride associated toxicosis of catfish, HCTC), suggested that the losses might be because of microcystin-producing blooms of Microcystis aeruginosa , but our investigations failed to support this conclusion . We found that (1) the liver histology of catfish experimentally exposed to pure microcystin-LR is very different from that of catfish sampled during outbreaks of HCTC; (2) measurements of microcystin-LR concentrations in the three cases were far lower than the concentration required to kill catfish by experimental immersion; (3) the HCTC toxin appears to have a short half-life, whereas microcystin-LR does not; (4) experimental gavage of catfish with massive amounts of microcystin-LR does not cause the acute mortality typical of HCTC; (5) outbreaks of HCTC appear to be associated with heavy blooms of Anacystis marina , a halophytic cyanobacteria, not with blooms of M. aeruginosa .