WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2015 39 experimental diet and terminal samples of body tissues of fish were analyzed at the UNCW Center for Marine Science. Crude protein was determined by the Kjeldahl method. Crude lipid, ash, and moisture contents in diets were analyzed by standard methods (AOAC 2000). Fish whole-body moisture content was determined by freeze drying. After 10 months of feeding, final proximate composition was similar between black sea bass fed the high fishmeal-based (control) diet 1 and the high soybean meal-based diet 3 (Table 3). Final moisture was lower than initial moisture and final lipid content was greater than initial content (Table 3). After 10 months (Trial 1) or 7 months (Trial 2) of feeding on the high fishmeal-based (control) diet and the high soybean meal-based diet, final proximate composition of southern flounder fed the two diets was similar (Table 4). Final proximate composition was also similar to initial composition. Fatty Acid Composition of Diets and Fish Fillets Fatty acid composition of diets and fish fillets was determined after extraction of total lipids into chloroform: methanol (Folch et al. 1957). Fatty acid analysis was performed on a HP-6890 Gas Chromatograph using a 25 meter × 0.25 µm HP-5 capillary column with FID detection (Department of Chemistry and Biochemistry, UNCW). In general, concentrations of eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and n-3 polyunsaturated acids (PUFAs) of diet 1 were greater than those of diet 2 and diet 3 (Table 5), reflecting the greater proportion of fishmeal in diet 1. The ratio of DHA to EPA in all diets (0.68-0.91) was within the range of 0.5-1.0 required by marine finfish (Sargent et al. 2002), suggesting sufficient DHA and EPA was provided in all diets. In general, the fatty acid composition of fish fillets reflected the composition of diets (Table 5). For black sea bass, EPA, DHA, total n-3 PUFA and the ratio of n-3/n-6 PUFA were greater in fish fed diet 1 than in fish fed diet 3. These differences reflect the higher levels of fishmeal in diet 1. At harvest, concentrations of total n-6 PUFA and total monounsaturated fatty acids (MUFA) were greater in fillets of black sea bass fed diet 3 than in fish fed diet 1, reflecting the higher levels of n-6 PUFAs and MUFAs in diet 3. In black sea bass, EPA, DHA and n-3 PUFA levels in fillets were much higher than in diets, reflecting the higher lipid levels found in whole bodies of black sea bass. For southern flounder, EPA, DHA, total n-3 PUFAs and the ratio of n-3/n-6 PUFA were similar in fish fed the two diets, reflecting the comparable fatty acid profiles of these diets (Table 5). There were clear differences in the fatty acid profiles of southern flounder and black sea bass fillets (Table 5), with generally greater concentrations of total saturated fatty acids (SFA), MUFA, n-3 and n-6 PUFA levels in black sea bass than in southern flounder, irrespective of diet. This was related to the much higher total whole body fat levels in black sea bass than in southern flounder, even when fish were fed diets with similar protein and lipid levels, irrespective of the level of substitution of fishmeal with soybean meal in diets. For both species, the differences in fatty acid profile of fish fed any of the test diets were minimal and not expected to affect taste or consumer acceptance, but organoleptic testing is needed to confirm this. Conclusions The results of feeding trials with black sea bass and southern flounder validate short-term laboratory studies and further demonstrate that black sea bass and southern flounder can be raised on high soybean meal-based diets under commercial-scale rearing conditions and over practical time periods from advanced juvenile to market size without a diminution of performance, compared to fish fed a high fishmeal-based control diet. The substitution of 30 percent (diet 2) and 50 percent (diet 3) soybean meal for fishmeal reduced feed costs from US$1696/t (diet 1) to US$1432/t (diet 2) and US$1256/t (diet 3) (i.e., price reductions of 14.7 and 26.1 percent, respectively) assuming current prices for menhaden fishmeal (US$ 1540/t) and solvent-extracted soybean meal (US$369/t) in September 2015. Reducing the cost of feeds is critical to reducing the cost of finfish production and increasing the probability of obtaining a positive return on investment. (CONTINUED ON PAGE 40) FIGURE 6. Growth of southern flounder fed a high fishmeal-based (control) diet 1 or a high soybean meal-based diet 2 in 16-m3 recirculating brackish water (8 - 10 g/L) tanks over 10 months at Blue Ocean Farms (Trial 1). Each tank was stocked with 650 hatchery-reared fish. Plotted points represent means of 150 fish. FIGURE 7. Growth of southern flounder fed a high fish meal-based (control) diet 1 and a high soybean meal-based diet 2 in 16-m3 recirculating brackish water (8-10 g/L) tanks (N = 2 per treatment) over 7 months at Blue Ocean Farms (Trial 2). Each tank was stocked with 455 hatchery-reared fish. Plotted points represent means of 150 fish.
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