OBSERVATIONS ON THE CHEMICAL-COMPOSITION OF SOME COMMERCIAL PRAWN FEEDS AND ASSOCIATED GROWTH-RESPONSES IN PENAEUS-MONODON

The chemical composition of five commercial and one natural diet (Pleb idonax sp.) were measured and food intake and digestibility determined in small (1.5 g initial weight) and large (32 g) Penaeus monodon, and interpreted in relation to growth rates over a 42-day period. There w as little variation in the crude protein content of the commercial die ts, averaging 43.6%. The calculated essential amino index was high for all diets (0.94-0.97). Substantial variation occurred in the crude li pid (4.94-9.02%), crude fibre (1.4-6.5%), ash (9.0-15.5%), nitrogen-fr ee extract (30-35%), as well as the calcium (1.3-3.3%) and phosphorus (1.2-2.0%) levels. Variation in n-3 fatty acid content was associated with different concentrations of 18:2n-6, 20:5n-3 and 22:6n-3. The nat ural diet was high in crude protein (75%), but low in crude fibre, NFE , calcium and phosphorus, and very low in the above indicated n-3 fatt y acids. Apparent digestibility was reduced (P< 0.05) in those diets c ontaining higher levels of crude fibre, and was very high for the natu ral diet (0.97). Food intake was not related to dry matter digestibili ty, but was lowest for the diet containing the lowest total quantity o f n-3 fatty acid. Intake of the natural diet was high initially but de clined to levels below that of the commercial diets as the experiment progressed (P<0.01). Growth rate of prawns fed the commercial diets wa s related (r2=0.92; P<0.01) to digestible dry matter intake in small b ut not in large prawns. Differences among diets with respect to growth of prawns were generally consistent in small and large prawns, althou gh small prawns appeared more sensitive to differences in fatty acid l evels in the diets. We conclude that differences among diets in food i ntake and prawn growth are associated with variation in the non-protei n fractions, and in particular the n-3 fatty acid, crude fibre and NFE content of the diets.