Dietary threonine requirement of juvenile largemouth bass, Micropterus salmoides

Threonine is an essential amino acid in fish nutrition, but no information is available on the dietary threonine requirement in largemouth bass, Micropterus salmoides. Thus, 360 fish (19.0 +/- 0.1 g) were randomly divided into six experimental groups (20 fish per cage; three replicates), and offered diets supplemented with crystalline Lthreonine at 0 g kg-1 (control), 2.5 g kg-1 (Thr-2.5), 5.0 g kg-1 (Thr-5.0), 7.5 g kg-1 (Thr-7.5), 10.0 g kg-1 (Thr10.0) and 12.5 g kg- 1 (Thr-12.5) for eight weeks to determine the optimum threonine requirement of largemouth bass. The results showed that dietary threonine significantly (P < 0.05) influenced final body weight, weight gain, specific growth rate, feed conversion ratio, protein efficiency ratio, protein retention efficiency and lipid retention efficiency of largemouth bass. Weight gain and specific growth rate increased firstly, then decreased with the increasing threonine supplementation. Thr-5.0 group had the highest weight gain (267%) and the lowest FCR (0.76) among all groups. Body indices such as hepatosomatic index, viscerosomatic index and condition factor were not affected (P > 0.05) by any dietary treatments. Whole-body crude protein and lipid were lower (P < 0.05) in fish fed threonine-supplemented diets than the control group, although those contents were higher in muscle. Monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) in fish muscle were also significantly (P < 0.05) modulated by threonine supplementation. Overall, threonine had a positive impact on most of the above mentioned parameters of largemouth bass, and the optimum level of supplemental threonine that maximized fish growth and feed utilization was 5.0 g kg- 1 (Thr-5.0), corresponding to 14.3 g kg- 1 dietary threonine.