Effects of dietary soybean and fish-protein ratios on growth and body composition of red drum ( Sciaenops ocellatus) fed isonitrogenous diets

Abstract

An 8-week growth trial was conducted with red drum ( Sciaenops ocellatus) to determine the amount of soybean protein that could be substituted for fish protein in formulated diets without reducing growth. In the laboratory, juvenile red drum (7.4 ± 0.1 g) were fed 34%-crude-protein diets in which dietary protein was supplied by soybean (soy) meal, menhaden fish meal or an isonitrogenous mixture of soybean and fish meal calculated to provide graded levels of each protein source. Dietary protein was provided as: (1) 100% soy protein; (2) 100% soy protein with a methionine supplement (calculated to approximate the methionine content of menhaden fish meal); (3) 75% soy protein: 25% fish protein; (4) 50% soy protein: 50% fish protein, (5) 25% soy protein: 75% fish protein; and (6) 100% fish protein. Diets containing only soy protein or soy protein plus a methionine supplement were poorly consumed by red drum. Approximately half of the fish that received these diets died during the first 4 weeks of the growth trial and the remainder lost weight. The 100%-soy and 100%-soy + methionine diets were therefore considered unacceptable for practical use and both treatments were terminated after 4 weeks. Among the remaining treatment groups, red drum fed diets containing only fish protein (100 FP), or soy-protein: fish-protein (SP: FP) ratios of 1: 3 (25 SP: 75 FP) and 1: 1 (50 SP: 50 FP) had higher ( P < 0.01) growth rates than fish fed a 3: 1 (75 SP:25 FP) soy-protein: fish-protein ratio. Feed efficiency ratio was highest ( P < 0.01) in fish fed a 1: 1 mixture of soy and fish protein. However, feed consumption was lower ( P < 0.01) in red drum fed a 1: 1 soy-protein: fish-protein ratio than in those fed 3: 1 and 1: 3 SP: FP ratios, or 100% fish protein. Although consumption of the 1: 1 SP: FP diet was lower than that of the 1: 3 SP: FP and 100% fish-protein diets, weight gain of fish fed the 1: 1 SP: FP ratio did not differ ( P > 0.05) from that of fish fed the diets with higher levels of fish protein. This indicated a superior cost: benefit ratio for the 1:1 soy-protein: fish-protein mixture in a 34%-crudeprotein diet for fingerling red drum.