Development of diets for soft-shell mangrove crabs (Scylla spp)

Abstract

The objective of this research was to develop practical formulated diets for soft-shell mangrove crab farming. Although the large-scale farming of soft-shell crab has been going for more than 20 years, little research has been conducted regarding formulated feeds. Presently, there is an urgent need to replace “trash fish” with manufactured diets. In the first study, we evaluated three compounded feeds for their acceptance. A total of 600 crabs were placed into individual boxes with 150-boxed crabs stocked into one PVC pontoon raft all in the same pond. Crabs were fed 3% of their body weight split between two feedings a day over a 45-day feeding trial. The growth of crabs fed the trash fish control was not significantly different from the high fishmeal content diet, both near 0.80 g/day. The growth rate when fed a high soy content diet was significantly lower, 0.71g/day. However, the soy-based diet had the highest survival rate (98%) versus crab feed trash fish (94%) and high fishmeal diets (91%). The results of the present study demonstrate that mangrove crabs will accept formulated feeds using either simple laboratory process or commercial extrusion. Hence, the development of specific diets for crabs is technically possible. To address pellet stability and durability issues we modified the high soy basal diets and evaluated the efficacy of various binders both under laboratory pelleting conditions as well as using a laboratory extruder. We hypothesize a binder would provide much longer stability in the water and the pellets would not fall apart as quickly even with aggressive handling by the crab. Two different binders: methylcellulose (CMC) and methylhydroxypropyl cellulose (MHPC) were used at inclusion levels of 0%, 0.15%, 0.3%, 0.6%, 0.9%, 1.2%, and 2.4% and pellet stability was tested at one, two and three hours. Results showed that MHPC was not significantly different at reducing dry matter leaching than CMC, except at the highest inclusion level of 2.4%. The level of binder in the diet did not seem to have a significant effect within the range tested, except for MHPC at 2.4% when all the times were included. In the third study, a series floating extruded basal diets were formulated to contain 40% protein and 10% lipid. The basal diet was modified to produce three additional diets each using 1% of a binder (Sodium alginate, methylhdroxypropyl cellulose and methylcellulose). A total of 600 wild caught crabs, placed into individual boxes with 150-boxes randomly assigned to each treatment. Crabs were fed 3% of body weight and fed once a day in a 45-day feeding trial. The result showed that diet-using binder methylhdroxypropyl cellouse MHPC produced crabs with better survival and weight gain than sodium alginate SA or methylcellulose CMC. However, molting took longer for the crab fed with reference diet (35 days) as compare to other treatment groups less than 34 days. We also observed that crabs struggled to grasp and hold the floating feed with their claws indicating that a floating feed is inappropriate. In my final experiment, we compared traditional trash fish to three (sinking, slow sinking and floating), extruded diets to consider the preferred physical form of formulated feed by mangrove crabs. Diet were randomly assigned to the crabs (150 per treatment) across one raft. Crabs were fed 3% of body weight once a day in a 45-day feeding trial. The result showed that there is no significant difference in average weight gain or survival between crabs fed sinking pellets compared to trash fish in term of weight gain. However, crabs fed with slow slinking pellet have the shortest average number of days to molting (29 days) and crabs fed with sinking diet have the longest average period to molting of 38 days. These experiments appear to be the first on-farm research that focused on formulated diets for soft-shell mangrove crabs. The results of these trials will contribute to the industry by demonstrating that formulated diets could be used for soft-shell crab farming and reduce the demand for “trash” fish.