EFFECTS OF DIETARY SULFUR AMINO ACID DEFICIENCY ON JUVENILE SOUTHERN FLOUNDER Paralichthys lethostigma

Replacement of fish meal as the major source of protein in fish diets is essential to sustain and encourage the growth of the aquaculture industry.  Common fish meal replacements are often deficient in ≥ 1 indispensable amino acid (AA).  Methionine and cysteine are sulfur-containing indispensable AAs. Sulfur AA deficiency has been known to cause severely reduced growth in fish as well as cataracts in some species such as Arctic charr (Salvelinus alpinus) and yellowtail (Seriola quinqueradiata).  The southern flounder (Paralichthys lethostigma) is a commercially and recreationally important species in the Gulf of Mexico.  In recent years, stock enhancement programs have been implemented in order to replenish and support the wild population of southern flounder in the Gulf.  This is a preliminary study to establish the essentiality of sulfur-containing AAs to the southern flounder.

A deficient and a sufficient diet were prepared according to total sulfur AA requirements of other cultured flatfish species.  The diets were designed to contain 45% crude protein (from 15% by weight of red drum muscle as an intact protein and crystalline AAs), 12% lipid, and 4.16 kcal/g digestible energy.  The total amount of sulfur AAs provided by 15% red drum muscle was 0.7% of dry diet.  The deficient diet contained 0.7% total sulfur AAs and the sufficient diet contained 2.3% total sulfur AAs.  The experimental diets were kept isonitrogenous by adjusting the inclusion of an aspartate/glycine premix.  Juvenile southern flounder (2.05 ± 0.34 g) were stocked in 38-L aquaria at a density of 10 fish/tank.

There was a large difference in total weight gain percentage between fish fed the sulfur AA deficient and sufficient diets (Table 1).  Survival also was greater among fish fed the diet with a sufficient amount of sulfur AAs.  Fish fed the deficient diet did not feed as aggressively and performed less evasive maneuvers when netted than those fed the sufficient diet.  Significant differences in % whole-body moisture (p < 0.0451), % whole-body lipid (dry) (p < 0.0005), feed efficiency (p < 0.0001), protein efficiency (p < 0.0001), and condition factor (p < 0.0032) were observed between fish fed the two diets; however, there were no significant differences in % body protein (dry), viscerosomatic index, or hepatosomatic index.  Results of this experiment confirmed that sulfur AA deficiency in southern flounder was manifested by reduced weight gain and survival although cataracts were not detected.