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Two experiments were conducted to determine whether the undigested high molecular fraction (HMF) of soybean protein reduces growth of yellowtail through impairment of lipid digestion and absorption. Soybean meal (SBM), soy protein isolate (SPI), digested SPI (DSPI), and digested and purified SPI (DPSPI) were used as experimental diets. In experiment 1 (exp. 1), yellowtail were fed 4 diets with aforementioned soybean products for 3 weeks. In experiment 2 (exp. 2), the fish were fed soybean protein or fish meal (FM) diets for 8 weeks. Growth performance of DPSPI-fed fish was better than that of fish fed other soybean proteins, and comparable to that of fish fed with FM. Plasma and tissue lipid levels of DPSPI-fed fish tended to be higher than those of fish fed other soybean proteins. Significantly lower intestinal lipid levels and higher bile acid levels were found in DPSPI-fed fish than in fish fed with SBM, SPI, and DSPI. These findings indicate that one of the factors contributing to growth depression in yellowtail fed with SBMbased diets is the negative effect of undigested HMF of soybean protein on bile acid levels and lipid digestion.

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Taurine is often added to artificial fish diets to compensate for a reduction in fish meal (FM). However, the taurine content of FM-based diets is typically lower than in diets consisting of raw fish, even in diets where FM is the only protein source. We evaluated the effects of dietary taurine in FM-based diets on epidermal thickness and scale detachability in red sea bream Pagrus major. We compared the effect of diets containing 0% (control), 0.3% (Tau-0.3%), 0.6% (Tau-0.6%) and 1.0% (Tau-1.0%) taurine. Red sea bream (average body weight, 39 g) were fed these diets for 7 weeks. Taurine supplementation had no effect on growth, feed intake, feeding efficiency, or survival. However, the epidermal thickness was higher in fish in the Tau-0.6% and Tau-1.0% groups than in the control and Tau-0.3% groups. Similarly, scale loss was significantly higher in the control group than in the Tau-0.6% and Tau-1.0% groups. Our results suggest that supplementation with >0.6% taurine (1.0% in diet) improves skin condition.