Branchial lesions associated with intermittent formalin bath treatment of Atlantic salmon, Salmo salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum)

Two trials were conducted to assess the effects of repeated prophylactic formalin treatments on the gill structure of salmonids. In trial 1, which involved Atlantic salmon, approaching smoltification in a commercial facility, fish were treated with either 167 or 250 mg/litre formalin for 90 min every 2 weeks for 12 weeks. Formalin-treated salmon had slight, but not significant, increases in the frequency of lamellar fusion, numbers of lamellar mucous cells, and numbers of an endemic gill ciliate, Trichophrya piscium, after 6 and 12 weeks of treatment. In trial 2, which involved juvenile rainbow trout, fish were treated with 200 mg/litre formalin for 60 min twice weekly for 12 weeks. Significant effects were limited to an increase in the numbers of mucous cells present on gill lamellae. In both trials, there was no evidence of lamellar oedema or necrosis of lamellar epithelial cells..