Morphometric model and laboratory analysis of intracohort cannibalism in giant grouper Epinephelus lanceolatus fry

During larviculture of giant grouper, Epinephelus lanceolatus, intracohort cannibalism usually results in mass mortality. Thus, in the present study, we attempted to develop a cannibal-prey 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: TLprey=0.83 TLcannibal-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.