Efficacy of formalin in reducing the levels of peritrichous ciliates on cultured marine shrimp

Abstract. Tests were performed to measure the efficacy of formalin in reducing peritrichous ciliates on the surface of cultured marine shrimp. The tests were conducted at concentrations above and below those being used in shrimp culture and were designed to define the concentration providing the most effective rapid reduction of ciliates. Penaeus stylirostris Stimpson,‘preconditioned’ in a single large tank, were divided equally (after diagnosis of significant ciliate epifaunation) into ten 1000 litre fibreglass tanks; each received 235 shrimp of 10–15 g average weight. These tanks were divided into five treatments: 0, 12·5, 25, 50 and 100 ppm; two replicates per treatment. Treatments were conducted under static conditions for 4 h. Ciliate levels were measured on day 0 (pretreatment) and days 1, 2 and 7 post‐treatment. Formalin at 25, 50 and 100 ppm was demonstrated to be effective in reducing ciliates, while 12·5 ppm did not differ from the control (0 ppm). In addition, the higher the level of formalin used (within the effective range), the more pronounced was the initial ciliate reduction and the longer was the protection from further ciliate epifaunation.