Growth, feed utilization, body composition and swimming performance of giant croaker, Nibea japonica Temminck and Schlegel, fed at different dietary protein and lipid levels

A 50-day feeding trial was conducted to examine the effects of dietary protein and lipid levels on growth, feed utilization, body composition and swimming performance of giant croaker, Nibea japonica. Fish (initial body weight 44.6g ind(-1)) were fed ten test diets which were formulated at 5 crude protein levels (360, 400, 440, 480 and 520g kg(-1)) and 2 crude lipid levels (90 and 150g kg(-1)). In addition, a raw fish diet (fillet of small yellow croaker) served as the reference. The weight gain (WG) increased, whereas the feed intake (FI) and feed conversion ratio (FCR) decreased, with increasing dietary protein level from 360 to 520g kg(-1). At the same dietary protein level, no significant difference was found in the WG between fish fed the diets containing 90 or 150g kg(-1) crude lipid. Fish fed the diet containing 480g kg(-1) crude protein and 90g kg(-1) crude lipid exhibited higher WG, nitrogen retention efficiency (NRE) and energy retention efficiency (ERE) but lower nitrogen wastes output (TNW). At the end of the feeding trial, the hepatosomatic index (HSI) and viscerosomatic index (VSI) decreased, whereas the body protein content increased, with increase in dietary protein level. The body lipid content was higher in fish fed at the 150g kg(-1) lipid level than in fish fed at the 90g kg(-1) lipid level. No significant difference was found in the maximum sustained swimming speed (MSS) between fish fed at different dietary protein and lipid levels. The WG, NRE, ERE and condition factor (CF) were higher, whereas the FI, FCR, HSI, VSI and TNW were lower, in fish fed the raw fish diet than in fish fed the diet containing 480g kg(-1) crude protein and 90g kg(-1) crude lipid. No significant difference was detected in the MSS between fish fed the raw fish diet and diet containing 480g kg(-1) crude protein and 90g kg(-1) crude lipid. The results of this study suggest that the suitable dietary crude protein and crude lipid levels are 480g kg(-1) and 90g kg(-1) for giant croaker reared in net pens.