Response of sunshine bass (Morone chrysops x M. saxatilis) to digestible protein/dietary lipid density and ration size at summer culture temperatures in the Southern United States

Temperature andammonia levels often increase dramatically in ponds during summer production of sunshine bass and summer temperatures are projected to increase in the Southern US. Extended periods of high ammonia result in fish stress, disease, mortality and significant loss of feeding days as producers attempt to reduce ammonia to manageable levels through reduced feeding or lower protein diets. A factorial feeding trial was conducted in temperature controlled tanks to investigatemain and interactive effects of three digestible protein (DP) levels (33, 40, 47%), two lipid levels (10, 18%) and two ration levels (full fed: satiation, restricted: 80% of satiation) on growth, body composition, nutrient and amino acid retention, and ammonia and phosphorus excretion in sunshine bass (mean weight:75 g) reared at elevated temperature (30.5±0.5 °C) . Diets were balanced on an available amino acid basis to the profile of hybrid striped bass muscle and supplemented with lysine and methionine at the equivalent of 330, 400, or 470 g/ kg of muscle protein. Each DP x lipid x ration treatmentwas fed to triplicate tanks of fish for 116 days. All measured responses were significantly altered bymain and interactive effects, but the patterns of interaction were similar among responses. Restricted feeding resulted in much lower final weights and weight gains but much higher ammonia excretion as a function of feed or N fed and body weight (BW), regardless of DP level. Lower dietary fat (10%) resulted in lower weight gains and poorer feed conversions aswell as higher ammonia excretion (per g N fed/kg BW) regardless of DP or ration level. Weight gain (475%) and final fish weight (434 g) were highest at 47% DP/18% dietary lipid, but feed conversion, protein, energy and amino acid retention efficiencies were markedly poorer in the 47% DP diets regardless of lipid level due to hyperphagia in fish fed this diet. The 40/18 diet consistently outperformed the 33/18 diet in better growth and lower ammonia excretion as a function ofN fed/ BW, and nearly equaled the growth attained by fish fed 47/18 diet. Increasing ration to satiation at elevated temperature resulted inmuch greater improvements in weight gain, final weight and ammonia excretion of fish fed the 40/18 diet, as opposed to those fed the 33/18 diet. Amino acid retentions were nearly equal between the 33/18 and 40/18 diets and restricting feed to 80% of satiation slightly improved feed conversions and protein and amino acid retentions. Consistent lipid x DP interactions generally indicated that the differences among responses to DP level seen at 18% dietary lipid significantly decreased or disappeared at 10% dietary lipid. Phosphorus excretion was low and not significantly altered bymain effects. Results suggest that a producer desiring to reduce pond ammoniawith the least compromise to production efficiency would be better served by feeding a 40% DP/ 18% lipid diet at a reduced level instead of switching to a lower protein diet.