FISHERY BULLETIN: VOL. 71, NO. 1 



PROTEIN 



Biologically, the protein in solubles is consid- 

 ered inferior in quality because of its high con- 

 tent of gelatin, derived from fish collagen, which 

 is low in the sulfur amino acids and almost com- 

 pletely devoid of tryptophan. Therefore, the 

 essential amino acids content of the protein in 

 condensed fish solubles is not adequate as a sole 

 source of protein for the growth of chicks be- 

 cause tryptophan is deficient and the content of 

 the other essential amino acids in condensed fish 

 solubles is lower than that found in fish meal. 

 Nevertheless, the protein may serve very well 

 in a diet as a minor supplementary protein or in 

 a diet otherwise adequate in amino acids since 

 a 3% level of fish solubles contributes only 1% 

 protein to the total diet. 



AMINO ACIDS 



Table 4 lists the amino acid composition of 

 menhaden fish solubles published by Soares et al. 

 (1970) and shows that solubles do contain some 

 of the amino acids in reasonably high amount. 

 All amino acids except available lysine, trypto- 



phan, and cystine were analyzed by ion exchange 

 column chromatography. Available lysine was 

 determined by the method of Carpenter (1960). 

 Tryptophan was determined by the method of 

 Spies and Chambers (1948), and cystine was 

 analyzed by a microbiological assay. 



It is noteworthy that cystine averaged 1.05% 

 of protein, which is slightly higher than the 0.9% 

 of protein found in menhaden fish meal (Kifer 

 and Payne, 1968) . Glycine, an important amino 

 acid in poultry rations since it is necessary for 

 maximum growth of chicks, is the only other 

 essential amino acid found in fish solubles at 

 levels higher than in fish meal. As mentioned 

 earlier, tryptophan is the first limiting amino 

 acid based on chemical composition. Available 

 lysine averaged about 83% of total lysine which 

 is similar to the fish meal values that we have 

 obtained. The taurine content is also shown, 

 inasmuch as a recent report by Monson (1969) 

 indicates that this amino acid may be one of the 

 unknown growth factors. Table 4 also shows the 

 amino acid composition of sardine and herring 

 solubles. For the most part these data are simi- 

 lar to menhaden. However, some large differ- 

 ences exist in the histidine levels which are not 

 explainable. 



Table 4. — Amino acid content (% of protein) of various kinds of con- 

 densed fish solubles. 



1 Unknown origin. 



258 



