Growth suppression and branchitis in trout exposed to hydrogen peroxide

This study was designed to investigate the effect of single short-duration bath-treatment with hydrogen peroxide (1000, 1250 or 1500 mg/l for 20 min), as currently used in treatment for sea lice, on the growth rate and gill morphology of rainbow trout. All three dose levels significantly reduced the specific growth rate of fish (22-36% reduction) during the first 3 weeks after treatment. A significant effect on growth rate (10-11% reduction) persisted over an additional 3 weeks for the two highest doses. Gill lesions occurred at all three dose levels and also at an additional peroxide concentration of 750 mg/l; these lesions were characterized by large foci of epithelial hyperplasia in which lamellar fusion, pillar cell necrosis, and pillar channel aneurysms had developed. The proportion of damaged filaments showed a significant positive linear relationship with dose during the first 2 weeks after treatment. A significant decline in number of lesions occurred at 1000-1500 mg/l over the 3-week sampling period. During healing, necrotic lamellae were replaced by means of a pillar channel regenerative process.