MII.LIK1N: NUTRIENT REQUIREMENTS OF FISHES 



gerlings. A separate study demonstrated that 

 about 35% protein combined with either 23 or 

 27% dietary lipid provided better growth of rain- 

 bow trout fingerlings than 36% dietary protein 

 combined with 14% dietary lipid (Reinitz and 

 Hitzel 1980). However, the two high lipid concen- 

 tration diets did not produce better growth rates 

 of rainbow trout fingerlings than a 35% protein, 

 18% lipid diet. C. E. Smith et al. (1979) reported 

 that rainbow trout brood stock fed high energy, 

 high protein diets (16% lipid plus 48% protein or 

 17% lipid plus 49% protein) produced a greater 

 volume of larger eggs than did fish fed diets low 

 and intermediate in energy and protein (6% lipid 

 plus 36% protein or 9% lipid plus 42% protein). 

 However, considerable variation in protein (i.e., 

 amino acid profiles) and lipid (i.e., fatty acid 

 profiles) sources used between diets in this 

 study makes interpretation of the data difficult. 

 Striped bass fingerlings reared at20.5°C and fed 

 37, 47, or 57% protein with 7, 12, or 17% lipid in a 

 3X3 factorial design showed maximal protein 

 sparing action of lipid for growth when fed 12% 

 lipid combined with 47% dietary protein or 17% 

 lipid combined with 57% protein (Millikin in 

 press). Juvenile turbot, Scophthalmus maximus, 

 fed 35% protein combined with 3, 6, or 9% lipid 

 plus 9 or 18% carbohydrate attained best growth 

 and feed conversion when fed the diet containing 

 35% protein, 9% carbohydrate, and 9% lipid 

 (Adron et al. 1976). Possibly, additional dietary 

 lipid would have further spared protein for 

 growth rather than energy. Also, a diet contain- 

 ing 35% protein, 9% carbohydrate, and 9% lipid 

 produced similar growth and feed conversion of 

 turbot when compared with a diet containing 

 55% protein, 9% carbohydrate, and 3% lipid. Juve- 

 nile blue tilapia, Tilapia aurea, with initial and 

 final mean weights of 2.5 and 7.5 g, respectively, 

 require about 56% protein and 460 kcal/100 g 

 diet (123 mg protein/kcal) while fish >7.5 g re- 

 quired 34% protein and 320 kcal/100 g diet (109 

 mg protein/kcal) for maximum growth (Winfree 

 and Stickney 1981). 



Essential Fatty Acids 



Inclusion of either linolenic acid (18:3o>3) or 

 a more highly unsaturated fatty acid in the a>3 

 series in the diet of rainbow trout is essential. 

 Rainbow trout fingerlings fed diets containing 

 1% 18:3cu3 as a supplement to 7.8% corn oil dou- 

 bled their weight gain compared with individ- 

 uals fed 10% corn oil as the sole dietary lipid 



source (Lee et al. 1967). Castell et al. (1972a) de- 

 termined that 1% 18:3co3 or ethyl linolenate pro- 

 duced larger rainbow trout than 1% linoleic acid 

 (18:2oj6) or ethyl linoleate. Additionally, 1% 18: 

 3o»3 prevented essential fatty acid deficiency 

 symptoms (e.g., fin erosion, heart myopathy, 

 shock syndrome, and swollen, pale livers), while 

 as much as 5% 18:2a>6 did not cure such symptoms 

 (Castell et al. 1972b). Dietary linolenic acid's 

 essentiality and growth enhancing ability for 

 rainbow trout was confirmed in another study, 

 wherein 1% methyl linolenate plus 4% methyl 

 laurate (C 12:0) provided maximal growth and 

 prevention of fatty acid deficiency symptoms 

 compared with fish fed 5% methyl laurate (Wa- 

 tanabe et al. 1974). Rainbow trout fingerlings fed 

 either 0.5% 20:5a;3 or 0.5% 22:6o>3 had better 

 growth rates than individuals fed 0.5% 18:3a>3 in 

 diets containing 5% total lipid, thereby showing a 

 higher essential fatty acid efficiency of 20:5co3 

 and 22:6cu3 at low dietary concentrations of these 

 fatty acids (Takeuchi and Watanabe 1977). How- 

 ever, it is unknown whether evaluation of higher 

 dietary concentrations (e.g., 1 to 3%) of each of 

 these fatty acids would have produced similar 

 differences in essential fatty acid efficiencies in 

 terms of growth and feed efficiency of rainbow 

 trout fingerlings. Yu and Sinnhuber (1972) 

 found that 1% dietary 18:3cu3 or 1% docosahexae- 

 noic acid (22:6cw3) provided similar growth rates 

 in rainbow trout fingerlings. No higher dietary 

 concentrations of either fatty acid were studied, 

 nor were total dietary lipid concentrations >2% 

 (dry diet basis) examined. The essentiality of 18: 

 3o»3 for rainbow trout was further substantiated 

 in a 34-mo feeding study in which fingerlings fed 

 1% ethyl linolenate plus 5% ethyl laurate grew to 

 maturity and produced viable offspring which in 

 turn had normal growth rates (Yu et al. 1979). 

 Incorporation of 1.5% ethyl linoleate to a lipid 

 mix of 1% ethyl linolenate plus 3.5% ethyl laurate 

 did not confer any additional advantage in 

 growth rate, percent fertile eggs, or percent 

 viable fry for rainbow trout compared with indi- 

 viduals fed the 1% ethyl linolenate diet. 



Closely related species such as coho salmon 

 and rainbow trout have different quantitative 

 dietary requirements for 18:3a>3 administered as 

 the triacylglycerol, trilinolenin. Coho salmon fin- 

 gerlings required 1 to 2.5% trilinolenin for maxi- 

 mum growth and feed efficiency when fed die- 

 tary trilinolenin concentrations ranging from 

 to 5%. High dietary trilinolein concentrations 

 (>1%) in the presence of the optimal trilinolenin 



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