A study of feral carp, Cyprinus carpio L, exposed to Microcystis aeruginosa at Lake Mokoan, Australia, and possible implications for fish health

Feral carp, Cyprinus carpio L., were collected at bimonthly intervals for 12 months from Lake Wellington, where there was no evidence of significant growth of Microcystis aeruginosa. Various blood parameters of these carp were measured to ascertain the normal range values for feral carp in this region. A similar study was conducted at Lake Mokoan, where M. aeruginosa formed a visible scum that contained detectable levels of microcystins for 4 consecutive months. Carp were taken from Lake Mokoan each month, and the histology and blood biochemistry of the fish were examined for indications of microcystin toxicity. During February, March and April, 66% of the carp from Lake Mokoan had livers with widespread hepatocyte atrophy, and 37% of the fish had gills with pinpoint necrosis, epithelial ballooning, folded lamellar tips and exfoliation of the lamellar epithelium. Compared to carp from Lake Wellington, the serum aspartate aminotransferase activity and the serum bilirubin concentrations of carp from Lake Mokoan were elevated during January, February, March and April. The data were consistent with impaired hepatocyte function in some carp from Lake Mokoan. Serum sodium and chloride concentrations in carp from Lake Mokoan were lower than those for carp at Lake Wellington from December to April, indicating that most carp from Lake Mokoan had difficulty maintaining cation-anion homeostasis during this period. Nevertheless, carp at Lake Mokoan always appeared healthy, and no mortalities were reported.