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Juvenile grouper (Epinephelus malabaricus) were fed seven experimental diets, one control diet and one reference diet for 12 wk to determine the dietary requirement of grouper for docosahexaenoic (DHA) and eicosapentaenoic (EPA) acids. Each of the seven diets contained 1 g/100 g DHA and EPA in various combinations and 9 g/100 g tristearin. The control diet contained 1 g/100 g trilinolenin and trilinolein (3:1, wt/wt), and no supplemental EPA or DHA. The reference diet contained only natural oils from a mixture of cod liver oil, linseed oil and safflower oil at a ratio of 2:1:1 (wt/wt/wt). Significant differences (P < 0.05) in growth were observed among the dietary treatments but not in survival rate or relative liver weight. Only the diet with the highest DHA/EPA ratio (3:1) promoted significantly greater growth than the control diet. Purified EPA and DHA did not perform better in promoting growth than did the impure EPA and DHA oils. Enhanced growth was observed when the dietary DHA/EPA ratio was greater than 1, indicating that DHA was superior to EPA in promoting fish growth. Neutral lipid (NL) was the predominant lipid fraction (>70%) in both liver and muscle. Tissue NL/polar lipid did not differ among groups except the reference diet group that had a higher ratio (P < 0.05). DHA and EPA levels in the grouper tissues, especially muscle, were highly reflective of dietary levels of DHA and EPA, indicating that direct incorporation was likely. In addition, the 20:1(n-9), concentration in NL fractions seems to be an appropriate indicator of dietary essential fatty acid deficiency in grouper.

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During larviculture of giant grouper, Epinephelus lanceolatus , intracohort cannibalism usually results in mass mortality. Thus, in the present study, we attempted to develop a cannibalprey length relationship from morphometric measurements for giant grouper fry and to verify it with pairwise predation experiments. Based on measurements of morphometric characteristics (i.e. mouth width, body depth, and total length), a model of prey length (mm) to cannibal length was constructed: TL prey = 0.83 TL cannibal -2.48. According to the equation, approximately 30% is a threshold in total length differences to use for grading giant grouper fry. The threshold was supported by our pairwise experiments using 136 pairs; we found that 33 out of 36 cannibalism incidents occurred only when the prey was of equal or smaller size than that predicted by the equation. In aquaculture practice of this species, we thus suggest that when the length of larger fry exceeded that of smaller fry by more than 30%, the potential cannibals should be removed.