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A six‐week feeding trial was conducted to determine the effect of butyric acid (BA) inclusion in diet on performance of Barramundi (Lates calcarifer). Fish (12.0 ± 0.2 g) were fed with four experimental diets contained 0.0 (control), 2.5, 5.0 and 10.0 g BA/kg in triplicate. Fish fed on 5 and 10 g BA/kg diets had higher growth and total alkaline protease and lipase activities than the other treatments. Fillet protein content in BA‐supplemented groups was higher than the control. Liver catalase activity was highest in 2.5 g BA/kg group compared with the other groups. The values of liver superoxide dismutase activity and serum total protein concentration in 5 and 10 g BA/kg treatments were higher than those in the control (p < .05). Moreover, the highest and the lowest levels of serum lysozyme activity were noticed in the 10 g BA/kg and the control groups, respectively (p < .05). Also, the highest respiratory burst activity was observed in fish fed 5 g BA/kg feed. Supplementing diet with 2.5 or 5 g BA/kg resulted in higher serum haemolytic activity in fish compared with the other groups. The number of red and white blood cells increased in fish fed BA‐supplemented diets compared with the control (p < .05). In conclusion, the results of this study showed inclusion of BA at 5 g/kg diet in L. calcarifer juveniles improved growth and upgraded the general health condition by enhancing fish antioxidant enzyme activities and haemato‐immunological responses.

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Passive immunization of rainbow trout, Oncorhynchus mykiss {Walbaum), was carried out to determine the persistence of SLXin-Streptococcus sp. antibodies (ASA) raised in sheep, rabbits or rainbow irout. The protection afForded by passive immunization was compared with the protection obtained from active immunization by immersion in or intraperitoneal {i.p.) injection with formalin-killed cells. Assessments were undertaken concurrently for up to 3 months post-immunization (PI) to evaluate the practical potential of passive immunization. Passively administered sheep and rabbit antibodies were detected in fish sera by enzyme-linked immunosorbenr assay for more than 60 days after i.p. injection. Fish responded immunologically to these antibodies and the highest humoral responses ro sheep and rabbit ASA occurred at 2 monrhs PI. The relative per cent survival (RPS) of rainbow trout challenged with virulent Streptococcus sp. after an i.p. injection (0-1 ml 100 g"' fish body weight) of sheep, rabbit or fish ASA was: 88-8, 50 and 0% after 1 month; 33-3, 6-8 and 6-8% afi:er 2 months; and 13 3, 0 and 6-6% after 3 months PI, respectively. Fish immunized actively had an RPS of 88-8 and 11-1% after 1 month, 38-1 and 4-7% after 2 months, and 36 and 0% after 3 months PI for the i.p. injection and immersion routes, respectively.

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