Morphometric and histochemical assessment of the branchial tissue response of rainbow trout, Oncorhynchus mykiss (Walbaum), associated with chloramine-T treatment

12 tanks of juvenile rainbow trout of mean weight 98 g were treated twice weekly for 11 weeks with a one-hour static bath of chloramine-T (10 mg/litre). 16 fish from each tank were killed for examination. The morphometric indices of gill damage and the histochemical characteristics of gill mucous cells of fish from treated and paired untreated tanks were compared. The use of chloramine-T evoked a slight increase in mean lamellar width, but it did not induce lamellar oedema, lamellar fusion, tissue infiltration, epithelial hyperplasia, chloride cell metaplasia or thrombosis of pillar channels in treated fish. Treatment caused a trend towards an increased number of mucous cells on lamellae, associated with a significant shift from neutral mucin to acid mucin production based on histochemical characteristics. It is concluded that the mechanism by which chloramine-T treatment inhibits growth and feed conversion does not involve gill damage..