Dietary lysine requirement of large yellow croaker, Pseudosciaena crocea R.

A study was conducted to estimate the optimum requirement of dietary lysine for large yellow croaker in floating sea cages (1.0 × 1.0 × 1.5 m). Six isonitrogenous and isoenergetic practical diets were formulated to contain graded levels of lysine (1.27, 1.83, 2.41, 3.02, 3.60 and 4.22% dry matter) from dietary ingredients and lysine·HCl. Crystalline amino acid mixtures were supplemented to simulate the amino acid pattern of the whole-body protein of large yellow croaker except for lysine. Each diet was randomly assigned to triplicate groups of 180 juvenile fish (initial body weight, 1.23 ± 0.02 g). Fish were fed twice daily (0500 and 1700 h) to satiation for 10 weeks. During the experimental period, the water temperature fluctuated from 27 to 30 °C, salinity from 25 to 28‰ and dissolved oxygen was more than 7 mg l− 1. The results showed that specific growth rate (SGR), feed efficiency ratio (FER), protein efficiency ratio (PER) and protein retention (PR), significantly increased with increasing lysine level from 1.27 to 2.41% of diet (P < 0.05), and then leveled off. Apart from the reduced growth, no external pathological signs were observed in the fish fed diets limiting in lysine. The whole-body protein (15.6–17.7%), as well as whole-body lysine, increased significantly with increasing dietary lysine level (P < 0.05). The body lipid content (5.9–5.4%) showed an opposite trend to that of body protein, while moisture, ash and other essential amino acids showed no significant differences among the dietary treatments. Broken-line analysis based on SGR, FER and PER indicated the optimum requirements of large yellow croaker for dietary lysine were 2.48, 2.45 and 2.43% of diet (5.77, 5.70 and 5.65% of dietary protein), respectively.