ASSESSING THE FEASBILITY OF DIETARY SOYBEAN MEAL REPLACEMENT FOR FISHMEAL TO THE SWIMMING CRAB, Portunus pelagicus, JUVENILES  

The use of fishmeal is increasingly becoming uneconomical and unsustainable, which has prompted increasing research on identifying suitable alternatives. Among these, soybean meal (SBM) is the most successful due to a well-balanced amino acid profile and relatively low amounts of anti-nutritional factors (ANF). Information is lacking, however, on the feasibility of replacing fishmeal with SBM in the diets of the blue swimmer crab Portunus pelagicus.  The aim of this study was to feed P. pelagicus with increasing amounts of SBM, at the expense of fishmeal, and assess their survival, growth, hemolymph superoxide dismutase (SOD) activity, whole-body crude protein and cholesterol as well as hepatopancreatic trypsin activity, histopathology and glycogen reserves.

Six isonitrogenous diets were formulated with increasing SBM at 0, 20, 40, 60, 80 or 100% based on a total protein while equal amounts of fish oil and soybean oil were maintained. Each treatment had 30 individually cultured replicate crabs. After six consecutive molts, three-day post molt crabs were used for final sampling.

Results showed the best growth was obtained for crabs fed 20% SBM, which was significantly higher than all other treatments and is in agreement with other studies on Eriocheir sinensis. The cause for this requires further investigations but did lead to more hepatopancreatic glycogen and a better overall condition despite significantly decreased trypsin activity. Crab growth in the 60% SBM treatment significantly decreased compared to the control treatment (0% SBM), as well as less hepatopancreatic glycogen. Structural integrity of crabs at 100% SBM became compromised at 100% SBM, along with substantially lower survival and growth, which may be due to excessive dietary ANF, although whole-body cholesterol, hemolymph SOD, crude protein and moisture were unaffected by the diets.

Dietary SBM can replace fishmeal up to 40% without reducing the growth of P. pelagicus juveniles, but 20% SBM is recommended for optimal growth, which can reduce feeding costs and improve portunid crab farming sustainability.