Dietary protein requirement of juvenile spotted knifejaw Oplegnathus punctatus

Abstract

A dietary strategy was conducted with graded crude protein (CP) levels (various from 27.08% to 58.10%) in six semi-purified diets to investigate the appropriate dietary CP supplement for juvenile spotted knifejaw (Oplegnathus punctatus). Experimental diets were respectively offered to the juvenile fish with three replicates (18 individuals each replicate, 19.91 g fish-1) for 8-week. After the feeding trial, the weight gain (WG) was improved by increment of dietary CP levels, but deteriorated slightly when dietary CP over 45.87%. The daily feed intake (DFI) elevated as dietary CP decreasing. Besides, the lowest protein efficiency ratio (PER) and feed efficiency (FE) were observed in 27.08% CP treatment. However, the PER and energy retention (ER) were not significantly different among 32.97%, 39.71% and 45.87% CP groups. In the current study, fish fed diet with 58.10% CP obtained slightly lower PER and nitrogen retention (NR), but relatively higher lipid retention (LR) value compared to those fed a diet with 45.87% CP. And the daily nitrogen intake (DNI) in the fish fed on the low CP levels (27.08 to 39.71%) were significantly lower than that in 45.87% CP treatment. The various dietary treatments did not differ in the visceral index (VSI), hepatosomatic index (HSI) and serum glucose, but fish in the lowest CP group had a decreased intraperitoneal fat ratio (IPF). The protein content in the whole body and dorsal muscle of the fish significantly elevated with dietary protein increasing. The aspartate transaminase (AST) in serum increased with increasing dietary CP from 27.08%-52.65 and then decreased. The regression analysis in second order polynomial and two slope broken-line mode based on NR and WG indicated that the optimal protein supplement for juvenile O. punctatus is 42.92%-46.44%. The present study also indicated that O. punctatus had poor ability to utilization dietary carbohydrate.