The contribution of naturally occurring food items to the diet of 'Cherax destructor' when fed formulated diets of differing protein levels

Abstract

To improve growth rates of 'Cherax' spp. raised for human consumption, considerable research has been conducted to establish nutritional requirements for the formulation of a food pellet. The problem with much of this research is that feed requirements between laboratory raised animals and those under pond culture conditions often differ. The difference is thought to be due to the consumption of naturally occurring food sources in addition to formulated pellets. Our study utilised stable isotope techniques to investigate the contribution of naturally occurring food items to the growth of 'Cherax destructor' fed formulated diets differing in composition and raised in two different environments created in artificial ponds. Environment 1 contained no natural food items, whilst environment 2 recreated the naturally occurring food items contained within a commercial growout pond. Dietary treatments 1 and 2 were isoenergetic (12.50 MJ/kg) pellets designed specifically for crayfish, differing only in protein level (30% and 19% crude protein (CP) respectively), dietary treatment 3 was a commercially available pellet used to feed silver perch (40.5% CP, 13.5 MJ/kg) and dietary treatment 4 received no formulated pellets and had access only to naturally occurring food items. Despite the differences between diets and environments, there was no significant difference between weight, abdomen length and abdomen width in any of the fed treatments in either of the environments. Although unfed treatments were significantly smaller in all growth characteristics than fed treatments, those in the natural environment were larger than those in the clean environment. Orbital carapace length (OCL) did not differ between any of the diets or environments. Survival of animals was significantly higher in environment 1 than in environment 2. The cause for this difference was a result of the death of the majority of animals in a single tub. Analysis of C and N stable isotope signatures revealed that animals in environment 1 consumed naturally occurring food items. Of the natural food items available, zooplankton had the largest contribution to the growth of tissue. Microphytobenthos (MPB) also had a significant contribution to animals fed crayfish diet 1. However, despite the contribution of these items to growth, no growth advantage was conferred when pelleted feed was also provided. These results demonstrate that 'C. destructor' consume naturally occurring food items even when provided with a formulated pellet and that suitable growth can be achieved with diets containing CP levels as low as 19%. We recommend testing of even lower protein levels in formulated diets as these may have no negative impact on growth, whilst further reducing the cost of feeds.