WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2017 37 supplementation with ractopamine can change metabolic responses and carcass traits of market-size tambaqui. Tissue Deposition in Tambaqui The first trial determined the rate of tissue deposition in tambaqui (e.g. adipose, muscle and visceral tissues). Briefly, 1000 tambaqui of 19 g average weight were stocked into a 27-m3 net cage on a commercial fish farm (Pôr do Sol farm, Gouvelândia-GOBrazil, geographic coordinates 18º39’30.5”S and 50º10’58.8”W) (Fig. 5). Fish were fed a commercial feed (Guabi, Nutrição Animal) for eight months (from May to December). A sample (10 percent) of fish were weighed monthly, and feeding rate and pellet size were adjusted according to guidance provided by the feed manufacturer. At each sampling, ten fish were killed and carcass traits and chemical composition were measured. To determine visceral fat index, hepatosomatic index and visceral adiposity, viscera, liver and celomatic fat were removed and weighed. At the end of the trial, data were plotted against fish weight and the growth curve was determined and the nutrient deposition trend line was fitted to determine the proper time to include ractopamine in fish diets (Fig. 6a). A Gompertz exponential model provided the best fit to body weight data. Tissue growth curves showed great variation within the population, however adipose and muscle tissues tended to show the same exponential growth as observed for wet weight (Fig. 6b). The best time to incorporate ractopamine occurred when the rate of increase in fat deposition was greater than that of muscle deposition. In this trial, the greatest change in visceral fat increase occurred between 116 and 176 days. Dietary Supplementation with Ractopamine One hundred and thirty-five market-size tambaqui (1.00 ± 0.04 kg) were randomly assigned to 15 500-L aquaria connected to a recirculating system. This density was selected to maintain approximately the same stocking density as in the first trial. Treatments were randomly assigned to aquaria according to a completely randomized design with five treatments and three replicates. Treatments consisted of five dietary ractopamine levels (0, 2.5, 5, 10, and 20 mg/kg) reached by including commercially available ractopamine HCl (Ractosuin®, OuroFino Saúde Animal Ltda.) into diets. Proximate composition of the diets was similar for all nutrients (Table 1). Fish were fed ad libitum two times a day. After 30 days, fish were bulk weighed and three fish per tank were anesthetized with eugenol, bled by caudal puncture and the liver removed for determination of serum metabolites (cholesterol, triacylglycerol, albumin, total serum protein and globulin), hepatic ALT (alanine aminotransferase) and AST (aspartate aminotransferase) activities. Carcasses were ground and used for determination of carcass traits. Growth parameters evaluated were daily weight gain (DWG), feed intake (FI), feed conversion rate (FCR) and protein efficiency ratio (PER) while for carcass traits, the dress-out percentage (DP), fillet percentage (F), hepatosomatic index (HSI) and visceral fat index (VFI) were evaluated. Data were analyzed with ANOVA and when differences between treatment means were significant (p<0.05), regression or SNK multiple range test were used to compare the results for growth performance and carcass traits, while Duncan’s multiple range test was used to compare the results for serum metabolites and hepatic enzymes. Dietary ractopamine supplementation did not affect growth parameters and protein efficiency ratio of tambaqui (Table 2). On the other hand, ractopamine supplementation at 20 mg/kg reduced the visceral fat index by 13.4 percent (Figure 7a, Table 3) compared to the control group. Similarly, the HIS was inversely related to dietary ractopamine supplementation, reaching a plateau at 3.4 mg/ kg (Figure 7b). Other carcass traits were not affected by ractopamine supplementation (Table 3). Generally, there was no pronounced effect of ractopamine supplementation on serum metabolites (Fig. 8). A significant decrease of plasma triacylglycerol concentration (Figure. 8a) was observed in fish fed diets containing 5 mg/kg ractopamine. There was a limited effect of ractopamine administration on hepatic ALT and AST activity (Fig. 9). A trend of increased ALT and AST activity was observed on fish fed 5 mg/kg ractopamine, however this was not significant. Modest Effects of Ractopamine Supplementation As expected, ractopamine supplementation did not affect growth performance or feed utilization of tambaqui. A minor and not statistically significant effect on some carcass traits was observed, mainly on fish fed diets supplemented with ractopamine at the highest rate (20 mg/kg). Generally, most of the studies on the effect of ractopamine supplementation on fish growth performance showed inconsistent results. For instance, ractopamine supplementation does not affect growth and feed utilization of rainbow trout, blue catfish, Hungarian carp and pacu (Bicudo et al. 2012, Devens et al. 2012, Lortie et al. 2004, Salem et al. 2006, Vandenberg et al. 1998, (CONTINUED ON PAGE 38) TABLE 2. Growth performance of tambaqui fed ractopamine-supplemented diets for 30 days (DWG=daily weight gain, FI=feed intake, FCR=feed conversion ratio, PER=protein efficiency ratio). Ractopamine (ppm) DWG (g) FI (g) FCR PER 0.0 5.0±0.4a 16.2±4.4a 1.03±0.23a 0.91±0.20a 2.5 4.7±0.4a 16.5±2.0a 1.13±0.16a 0.79±0.11a 5.0 4.2±0.4a 14.6±3.0a 1.10±0.13a 0.79±0.09a 10.0 4.4±0.6a 14.5±1.0a 1.08±0.22a 0.83±0.16a 20.0 4.4±1.0a 16.3±1.3a 1.23±0.25a 0.17±0.16a Means within column followed by a different superscript were significantly different by SNK multiple range test (p < 0.05).
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