The influence of protein and energy density in commercial diets on growth, body composition, and nutrient retention of sunshine bass, Morone chrysops ♀ × Morone saxatilis ♂, reared at extreme temperature.

Three growth trials were conducted with juvenile sunshine bass reared at temperatures typical of winter or summer pond culture in the Southeastern USA. The trials were designed to determine if there was an advantage to feeding a commercial high-protein/high-fat diet during winter and a low-protein/high-fat diet during summer. In the first trial, two commercially extruded, practical diets (40% protein/10% lipid vs. 48/18) were fed to apparent satiation to fish held in variable cool water (8-20 C) or constant 26 C water for 14 wk. Temperatures in the cool water (8-20 C) tanks were chosen to simulate winter-spring conditions. In the second and third trials, factorial experiments were conducted in which four commercial diets (35/10, 35/15, 40/10, and 40/15) were fed to apparent satiation to fish held at 29 or 32 C for 4 wk to simulate near optimal versus extreme summer water temperatures. Survival was 100% in the first trial, 99% or more in second trial, and 90% or more in third trial. At 8 C, minimal feeding (0.3% of body weight/day) was observed and fish lost weight. Fish consumed feed daily and gained weight at 10 C or above. At 8-20 C, intake of the 48/18 diet was less than that of the 40/10 diet only when water temperature was above 15 C; however, gain was not different. At 26 C, fish consumed less of the 48/18 diet for greater gain than fish consuming the 40/10 diet. At 8-20 C, feed efficiencies increased with temperature and diet protein/lipid level. Visceral and whole-body fat tended to be diet-dependent but not temperature-dependent and averaged 4% higher in fish fed the 48/18 diet. Muscle ratio and whole-body protein retention were temperature-dependent but not diet-dependent. Energy retention was positively related to both temperature and diet nutrient density. At 29 and 32 C (summer culture trials), daily gain and final fish weight were positively related to protein but not lipid level in the diet. At 29 C, fish consumed less 35% protein diet than 40% protein diet regardless of dietary fat level, whereas consumption did not differ among diets at 32 C. Feed efficiencies were positively related to both dietary protein and lipid level at 29 and 32 C. The effects of diet nutrient density on fat versus muscle content and energy and protein retention differed at 29 and 32 C. Intraperitoneal fat (IPF) appeared unaffected by diet at 29 C, where muscle ratio was higher at the higher protein level (40%). At 32 C, IPF was positively related to dietary protein and fat, where muscle ratio was unaffected by diet. At 29 C, both energy and protein retention appeared unrelated to diet, whereas at 32 C energy retention was positively related to dietary fat level and protein retention was positively related to both protein and fat levels in the diet. In all trials, liver size (hepatosomatic index) was a sensitive indicator of culture temperature and dietary protein and fat levels. Livers from fish held in cool water were larger than those from fish held at 26 C, and fish fed diets of lower nutrient density had larger livers than fish fed diets of higher nutrient density, regardless of culture temperature. Hybrid striped bass showed remarkable adaptation to extremely high culture temperature and results suggest that judicious feeding of nutrient-dense diets when temperatures are above 15 C will improve production efficiency.