An 8-week feeding trial was conducted to evaluate the effects of dietary aflatoxin B 1 (AFB 1 ) on growth performance, haematological parameters and histological changes in juvenile Pacific white shrimp, Litopenaeus vannamei. Six practical diets (455 g kg -1 protein, 78 g kg -1 lipid) with different levels of AFB 1 (0, 25, 50, 100, 500, 1000 lg kg À1 ) were formulated. Each diet was fed to triplicate groups of shrimps (initial weight: 0.52 g). The results showed that shrimp fed with control diet (0 lg kg À1 AFB 1 ) had significant higher weight gain (WG) and specific growth rate (SGR) than other groups. However, there were no significant differences in feed efficiency (FE) or hepatosomatic index (HSI) among all groups. Compared to the control diet, AFB 1 supplementation significantly changed the activities of shrimp serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), total antioxidant capacity (T-AOC) and glutathione S-transferase (GST) and the content of cholesterol (CHO). Histological damages were identified in the hepatopancreas of shrimp when dietary AFB 1 level was over 107.6 lg kg À1 . Based on this study, it was concluded that the AFB 1 level in Pacific white shrimp diet should be <38.1 lg kg À1 .

An 8-week feeding trial was conducted to determine the effect of dietary L-methionine supplementation on growth performance, serum immune and antioxidative responses of juvenile Nile tilapia Oreochromis niloticus. Six iso-nitrogenous (282 g kg À1 crude protein) diets were formulated to contain graded levels of methionine (0.32%, 0.54%, 0.75%, 0.93%, 1.14% and 1.33% of dry weight) at a constant dietary cysteine level of 0.83 g kg À1 . Each diet was randomly assigned to triplicate groups of 25 juvenile fish (2.3 AE 0.0 g), which were fed three times daily (8:30, 12:30 and 17:00 hours). The results showed that growth performance and feed utilization significantly improved when dietary methionine levels increased (P < 0.05). Using quadratic regression analysis of weight gain against dietary methionine levels indicated that, the optimal dietary methionine requirement for maximum growth of juvenile tilapia was 9.12 g kg À1 of the dry diet in the presence of 0.83 g kg À1 cystine. In addition, maximum C4 content and lysozyme activity were observed in fish fed 7.50 g kg À1 methionine diet; maximum C3 content and superoxide dismutase activity were obtained in fish fed 9.30 g kg À1 dietary methionine level. While there was no significant difference in serum glutathione peroxidase activity among all methionine supplemented treatments (P > 0.05). Methionine supplementation decreased malondialdehyde content in serum significantly (P < 0.05) when compared with the control diet, while there was no significant difference among supplemented treatments. These data suggested that L-methionine affected antioxidant status and promoted serum immune response in juvenile Nile tilapia, and at non-stressed status, the requirement of dietary methionine in maintaining normal immunity and physiology is lower than that for maximum growth.