The effects of the repartioning agents or ß-agonists, isoproterenol, salbutamol, fenoterol and clenbuterol on body composition and growth of the African catfish Clarias gariepinus (Burchell, 1822) were studied. Some of these products have already been successfully tried out by several research groups in cattle, sheep, poultry and swine to partition nutrients away from fat deposition towards protein accretion. In fish however, and more so, in the African catfish, this is the first time that the effects of these ß-agonists have been tested on body composition and that some positive effects were obtained. Uptil now in fish, only the effects of growth hormone, anabolic steroids and thyroid hormones - all of which mobilize lipids from lipid stores to provide energy while sparing dietary protein - have been tested by other research groups. The effects of these ß-agonists were tested during 10 weeks on two size categories of African catfish - a group of smaller fish weighing 185 ± 18 g. and bigger fish weighing 266 ± 26 g. Emphasis was laid on muscle protein and fat content. The products seem to have acted differently on these two size categories of fish. In the group of smaller fish, only isoproterenol reduced fat levels significantly (p < 0.05) compared to the control fish after 17 and 80 days. A significant increase (p < 0.05) of protein content due to all the products compared to the control was seen after 17 days. However, isoproterenol still gave significantly higher protein levels on days 61 and 80 (p < 0.05) compared to the control. The specific growth rate due to all the ß-agonists tested in the smaller fish was significantly higher than the control (p < 0.05). Food conversion ratio and efficiency though not significantly different from the control was ameliorated. In the bigger fish, no statistically significant difference (p < 0.05) was seen on protein and fat levels as well as on growth rate and feed utilisation throughout the experimental period. It is known from the farm animal industry, that these products act rapidly: with time, their effect wanes. We can only hypothesize that this could have possibly happened on the first 17 days on protein accretion and then with time their effect waned in the smaller fish. The relatively more consistent significant increase in protein and decrease in fat in the muscle of smaller catfish due to isoproterenol as opposed to the rest of the other products is possibly due to its greater potency at a similar dosage than the other products. Since Isoproterenol is a mixed ß-agonist, while clenbuterol, salbutamol and fenoterol are pure ß2-agonists, we can only propose a hypothesis that in our case ß1 receptors are predominant in catfish muscle. This makes the effect of isoproterenol more pronounced on body protein and fat. These products are known to have an influence on the endocrine system of cattle, poultry and swine. The improvement in growth of the smaller fish could be due to the influence on the endocrine system by these products. How they affect the endocrine system especially the pituitary is not yet known in the African catfish and should be an issue of further research. Age related differences could possibly explain the overall lack of effect of these products in the bigger fish. C. gariepinus has relatively low fat in the muscle. It might not be easy to see the influence of these products in the muscle tissue fat. Probably in fish with a higher fat content, the effects of these products could provide more easily detectable results. Once the use of these products is approved in the farm animal industry, then they can be tried out in aquaculture and more so in fish species with a great culture potential such as the African catfish. These products need to be tested more closely in catfish to help identify the nature of receptors in the effective dosages as well as their direct or indirect influence on the endocrine system. This would enhance the |
