An analysis of the effect of diet and genotype on protein and energy utilization by the black tiger shrimp, Penaeus monodon - why do genetically selected shrimp grow faster?

Selected (G8) and wild-type (W) genotypes of black tiger shrimp (Penaeus monodon) juveniles (initial weight G8 = 9.14 +/- 0.36g per animal and W=8.44 +/- 0.10g per animal) were fed either of two diet types in a clear-water tank trial to examine the effects of diet type and genetics on growth and feed utilization parameters. Animals were fed twice daily at one of the five ration levels from starvation to apparent satiety. All uneaten feed was accounted for and moults removed. Starved animals were measured after 3weeks; those fed were measured at both three and 6weeks. Diet type varied by protein content, raw material choice and the presence [high-specification diet (HSD)] or absence [low-specification diet (LSD)] of bioactive substances. At the end of the study, faecal samples were also collected to determine the digestible protein and energy content of each diet by each genotype. Whole animal protein and energy content were also assessed from samples from the initial populations and those from each tank. Growth after 6weeks of those animals fed to satiety showed that the G8 animals fed the HSD diet had grown at a rate of 2.56gweek1, significantly faster than any other treatment. Those G8 animals fed the LSD diet (1.81gweek1) had grown significantly faster than the W animals fed the HSD diet (1.25gweek1), while those W animals fed the LSD diet (0.61gweek1) grew the slowest. Using the data from the varying ration levels, we were able to define that the growth gains of the G8 animals were achieved not only by a greater appetite, but also through lower maintenance energy costs (29 versus 57 kJ kg0.8 day1) and a more efficient energy conversion (19.5% versus 11.6% when fed the HSD diet). Use of a low-specification diet with the G8 and W shrimps limited their growth and impaired their potential as demonstrated by a curvilinear response of growth to intake. By comparison, those shrimp fed the HSD diet had a relatively linear growth response to intake.