Fin erosion in a feral population of goldfish, Carassius auratus (L.), exposed to bleached kraft mill effluent

. The prevalence of acute and healed fin erosion in goldfish, Carassius auratus (L.), from the Waikato River, New Zealand, peaked in the vicinity of a pulp and paper mill which utilized a kraft pulping process in association with chlorine bleaching, and decreased down the effluent gradient. About 49% of goldfish sampled from the Kopakorahi Stream. which is part of the effluent treatment system (site 1), displayed pathological signs of fin erosion, whereas about 9% of control fish from a reference site located upstream of the official effluent discharge point were similarly affected. Fin erosion was less prevalent in male than in female goldfish, and occurred with different frequencies on different fins, the pectoral (56%) and caudal fins (33%) being most affected. Both the pectoral and caudal fins also had significantly higher proportions of healed erosion rather than acute erosion. The prevalence of acute and healed fin erosion increased significantly with increasing fish length (age). Physical deformities, predominantly in the form of eroded or otherwise malformed opercula, were found in nearly 5% of fish caught at site 1, but were not observed at the reference site. Changes in processing and bleaching technology at the pulp and paper mill in question have since been carried out, and long‐term studies arc planned to monitor any subsequent improvements in fish health.