Optimal dietary protein concentrations for largemouth bass (Micropterus salmoides) of different sizes (10-500 g)

Three feeding trials were conducted to determine the optimal dietary protein levels for largemouth bass (Micropterus salmoides) of different sizes. Six isolipidic (12%) diets with different protein levels (46%, 48%, 50%, 52%, 54% and 56%) were tested to feed three size groups of fish for 56 days (Trial 1 and Trial 2) or 84 days (Trial 3). The initial body weights were approximately 10 g in Trial 1, 100 g in Trial 2, and 200 g in Trial 3, respectively. The fish in each size class were randomly distributed into 18 cages (1 m x 1 m x 1 m) in six groups (three replicates) and fed by hand to apparent satiation two times per day. At the end of the feeding trials, the fish grew to an average size of 26-36 g, 276-320 g, and 434-526 g, respectively. The weight gain (WG) showed an increasing trend with increasing dietary protein level up to 52%, 50%, and 48%, respectively, and then plateaued in all the three trials, which indicated that the minimum dietary protein requirements for largemouth bass decrease with increasing fish size. The feed efficiency (FE) in the three trials showed a similar trend to growth performance. Protein efficiency ratio (PER) tended to decline when dietary protein levels exceeded 48%, 50%, and 48% in the three trails, respectively. Broken-line regression analysis showed that the optimal dietary protein levels based on SGR for largemouth bass of different sizes (initial size 10, 100, and 200 g) were 51.6%, 50.5%, and 47.8%, respectively.