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 prophylact ic formalin treatments on the gill structure of salmonids. In trial 1, which involved Atlantic salmon, Salmo salar L., approaching smoltific ation in a commercial facility, fish were treated with either 167 or 2 50 mg l(-1) formalin for 90 min every 2 weeks for 12 weeks. Formalin-t reated salmon had slight, but not significant, increases in the freque ncy of lamellar fusion, numbers of lamellar mucous cells, and numbers of an endemic gill ciliate, Trichophyra piscium, after 6 and 12 weeks of treatment. In trial 2, which involved juvenile rainbow trout, Oncor hynchus mykiss (Walbaum), fish were treated with 200 mg l(-1) formalin for 60 min twice weekly for 12 weeks. Significant effects were limite d to an increase in the numbers of mucous cells present on gill lamell ae. In both trials, there was no evidence of lamellar oedema or necros is of lamellar epithelial cells.