Relatively high mortality rates are common in farmed largemouth bass (LMB) . These mortalities are associated with chronic anemia and pale necrotic livers. C arbohydrate sensitivity has been the suspected problem as c ereal grains are added to feed formulations at >15% to produce floating feed. However alternative explanations should also be considered. Mycotoxins are natural poisons produced by fungi that contaminate cereal grain products prior to harvest and/or during post-harvest storage. Aflatoxin B1 (AFB1 ) is a mycotoxin produced by Aspergillus sp. fungi and is one of the most toxic naturally occurring substances . Many of the symptoms of liver disease in LMB are similar to those produced mycotoxicosis in some fish species . To evaluate the susceptibility of the LMB to mycotoxins and determine if some of the health issues in commercial LMB production might be attributable to mycotoxins, researchers at Kentucky State University conducted a trial where increasing concentrations of AFB1 were fed to LMB .
The diet concentrations were 0 (control) , 0.25, 2.5, 5 and 10 mg AFB1/kg diet . Basal diets contained 40% fish meal, 25% soybean meal, 21% wheat flour and 10% fish oil. Juvenile LMB (3.6g) were randomly stocked at a rate of 1 5 fish per aquarium into 20 110-L acrylic aquaria . Each of the five diets were randomly assigned to four replicate aquaria. Fish were fed once daily to apparent satiation for 9 weeks. At harvest, all fish from each aquarium were individually weighed and measured (total length). Livers from three fish from each aquarium were weighted and preserved in 10% formalin for histological analysis.
Fish fed the highest concentration of 10 mg AFB1/kg diet exhibited a poor feed response and higher (P<0.05) feed conversion ratios (3.3) than fish fed the other diets, which were similar, averaging 1.2 overall. Survival was reduced in fish fed the 10 mg AFB1/kg diet (68%) compared to fish fed the other diets, which were similar and averaged 98%. Average weights were similar for fish fed the control diet (26.5g) and the 0.25 mg AFB1/kg diet (24.5g). Average weights showed a significant incremental reduction as the AFB1 concentrations increased from 2.5 (15.2g), to 5.0 (9.7g) and then 10 (5.5g) mg AFB1/kg diet . Hepato-Somatic Index (HSI) was significantly higher in fish fed the control diet (2.2) than for fish fed diets containing AFB1 which were similar and averaged 1.1 overall. Low HSI values can indicate toxicity , liver disease , or septicemia . Th ough fish fed the lowest concentration of 0.25 mg AFB1/kg diet exhibited similar growth to the control diet, H SI values may indicate underlying mycotoxicosis suggesting that LMB may be very sensitive to AFB1.