FISHERY BULLETIN: VOL. 80. NO. 4 



ment of oxidative rancidity of the diets. However, 

 if fish are being hatchery cultured for release 

 into natural waters, high body lipid composition 

 may be beneficial as an energy source during 

 acclimation to natural food (Wedemeyer et al. 

 1980). Also, feeds that contain the minimal die- 

 tary lipid concentration required for maximal 

 protein sparing action for a particular species 

 are relatively cost-effective. 



Protein sparing action of various dietary lipid 

 concentrations has been examined for several 

 fish species. Channel catfish fingerlings (mean 

 initial weight = 1.25 g) cultured at 26.7° to 32.2°C 

 and fed 35% protein diets grew faster on 8% die- 

 tary lipid, either as beef tallow or corn oil, than 

 on 4% of either dietary lipid source (Dupree 

 1969a). Dietary lipid concentrations of 16% corn 

 oil or beef tallow reduced growth rate and pro- 

 tein deposition of channel catfish compared with 

 8% dietary lipid. Therefore, maximal protein 

 sparing action occurred at <16% of either die- 

 tary lipid source but >8% dietary lipid. In a sep- 

 arate study, larger channel catfish fingerlings 

 (initial mean weight = 7.0 to 7.5 g), cultured at 

 30°C and fed 25% protein diets containing either 

 5, 8, 12, 15, or 20% bleached menhaden oil, had 

 maximal weight gain and protein deposition 

 when fed 25% protein with 15% lipid (Dupree et 

 al. 1979). Channel catfish fingerlings (either 0.5 

 or 1.0 g initial mean weight in separate experi- 

 ments) reared at 28°C grew faster when fed 35% 

 protein plus 12% lipid, rather than 5% lipid com- 

 bined with 25 or 35% protein or 12% lipid com- 

 bined with 25% protein (Murray etal. 1977). Con- 

 versely, when channel catfish reared at 23°C 

 were fed 25 or 35% protein combined with 5 or 

 12% lipid, 5% lipid plus 25 or 35% protein was 

 sufficient for maximal weight gain and food con- 

 version, probably due to lower metabolic require- 

 ments. Channel catfish with initial and final 

 mean weights of 14 and 100 g, respectively, re- 

 quired 35% protein and 12% lipid, whereas sub- 

 adults weighing 114 to 500 g required 25% pro- 

 tein and 12% lipid when reared at 27°C (Page and 

 Andrews 1973). Protein-to-energy ratio require- 

 ments of channel catfish fingerlings (initial 

 mean weight = 7.0 g) as a function of protein 

 deposition were found to be 88 mg protein/kcal 

 between dietary energy concentrations of 275 to 

 341 kcal/100 g when reared at 26.7°C (Garling 

 and Wilson 1976). However, maximal weight 

 gain occurred in fish fed 32 to 36% protein. There- 

 fore, for the most cost-effective feed, the optimal 

 dietary protein concentration for channel catfish 



fingerlings reared at 26.7°C is between 24 and 

 32%, when considering weight gain, feed effi- 

 ciency, and protein deposition. Rainbow trout 

 fingerlings (initial mean weight = 4.8 g) reared 

 at 12.2°C over an 18-wk period had equally good 

 growth rates when fed 35% protein and 24% lipid 

 as individuals fed 44 or 53% protein, each com- 

 bined with either 8, 16, or 24% lipid. This indi- 

 cated a minimal dietary lipid concentration of 

 24% for maximal protein sparing action and an 

 optimal protein-to-energy ratio of 73 mg protein/ 

 kcal (Lee and Putnam 1973). Conversely, a study 

 evaluating three dietary protein concentrations 

 (33, 39, and 44%) combined with 22% lipid in each 

 of three forms (22% herring oil, 14.6% herring oil 

 plus 7.4% lard, or 11% herring oil plus 11% lard) 

 in a 3 X 3 factorial showed that better growth 

 was obtained in rainbow trout fingerlings (initial 

 mean weight = 5.4 g) fed 44% protein and 22% 

 lipid, regardless of the ratio of herring oil to lard 

 when reared at 1 1.5°C over a 14-wk period ( Yu et 

 al. 1977). Protein efficiency ratios and protein 

 retention values were similar, regardless of die- 

 tary protein concentration and lipid source com- 

 binations fed to rainbow trout fingerlings. The 

 contradictory results of these two studies in the 

 minimal dietary protein required (35% protein 

 plus 24% lipid versus 44% protein plus 22% lipid) 

 for maximal growth of rainbow trout fingerlings 

 can be partially explained by differences in die- 

 tary fiber concentrations. A low dietary fiber 

 concentration (6.5%) was incorporated in diets 

 containing 36, 44, or 53% protein plus 24% lipid, 

 whereas a high dietary fiber concentration 

 (22.4%) was incorporated in diets containing 36, 

 44, or 53% protein plus 8% lipid (Lee and Putnam 

 1973). However, in the study by Yu et al. (1977), 

 dietary fiber concentrations were held constant 

 at 11.2% in all diets. Therefore, variable dietary 

 fiber concentrations in the first study may have 

 differentially affected amino acid absorption 

 rates or feed utilization. Successive increases in 

 dietary lipid concentration of 7, 11, or 16% in 30% 

 protein diets or 9, 15, or 21% lipid in 40% protein 

 diets resulted in increased weight gain and im- 

 proved feed conversion of rainbow trout finger- 

 lings (initial mean weight =2g) within each 

 dietary protein concentration when reared at 

 11°C (Reinitz et al. 1978b). Minimal dietary pro- 

 tein and lipid concentration requirements were 

 not determined, since the highest dietary protein 

 (40%) plus lipid (21%) combination that was eval- 

 uated also produced maximal growth and opti- 

 mal feed conversion ratios for rainbow trout fin- 



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