Assessing the potential for low cost formulated diets for mud crab aquaculture in Australia, Indonesia and Vietnam

• Gondol Research Institute for Mariculture, Indonesia
• Queensland Department of Primary Industries and Fisheries, Australia
• Research Institute for Aquaculture No. 3, Vietnam

Global demand for mud crabs has risen over the past decade, led by expanding wealthier markets such as those in Hong Kong, Singapore and elsewhere in Asia. This demand has largely been met by exploitation of wild stocks, causing many to go into decline. Current trends in these fisheries suggest this exploitation is unsustainable. This situation continues to be exacerbated by rising demand for seafood.
Mud crabs (Scylla species) are widely distributed across the Indo-Pacific region, mainly in coastal and estuarine areas, making them ideal for fishing. This does also make them highly suitable for aquaculture, providing some barriers to production can be overcome. Past ACIAR-supported research has developed laboratory-scale technologies for hatching crabs from larvae, a first step in aquaculture development. Large-scale hatchery production is now under way in Vietnam where a leading centre for crab aquaculture has been established, along with others in Indonesia, the Philippines and Australia.
Until diets suitable for crab grow-out can be formulated, based on meeting their nutritional needs, further advances will be limited. Most aquaculture of crabs uses 'trash-fish' collected from marine inshore areas or mussel meat from intertidal areas. This can damage these environments and not all feed is likely to be consumed, fouling hatchery ponds. Growing exploitation of trash-fish is also leading to declining numbers, threatening the viability of aquaculture. A cost-effective replacement diet is needed to ensure the benefits gained to date are not lost.

The aim of this project was to:
evaluate potential for formulated feeds to replace trash fish
determine critical nutritional requirements and evaluate key ingredients
determine the protein/energy requirements of mud crabs during grow-out phases
formulate and evaluate improved diets.

Experimental diets formulated for use in mud crab aquaculture in the partner countries were tested for their ability to promote growth under laboratory conditions, as a preliminary step to developing artificial feeds optimised for mud crab aquaculture. This project sought to develop diets that are:
cheaper than the current diets used in intensive or semi-intensive mud crab aquaculture (i.e. prawn feeds in Australia and trash fish in Indonesia and Vietnam)
able to provide similar or superior production performance to currently available crustacean feeds
less likely to impact on water quality than current mud crab diets
based on food grade resources which can accommodate the projected growth of the mud crab industry in Australia and the partner countries
made without terrestrial animal protein sources that may transmit diseases such as BSE.
The researchers first assessed existing formulated diets. They determined critical nutritional requirements and evaluated key ingredients, identifying digestible plant-based ingredients suitable for mud crab feeds by in vivo digestibility trials with large crabs. They then focused on the potential of low-cost, plant-based ingredients for use in mud crab feeds in partner countries. Nutrient requirements of mud crabs were determined by growth trials with small crabs using a range of diets.
The improved diet formulations developed were tested under laboratory conditions, and the results assisted the development of formulated diets and pellet production. Finally the work came together in a workshop to evaluate results and identify R&D priorities for development of practical diets.

This project built on earlier progress by aquaculture research teams in Australia (Bribie Island Aquaculture Research Centre; BIARC), Indonesia (Gondol Research Institute for Mariculture: GRIM) and Vietnam (Research Institute for Aquaculture No. 3; RIA3). An improved dietary formulation based on a lobster diet was developed in collaboration with nutrition experts Dr Kevin Williams and Dr David Smith (CSIRO Marine Research, Cleveland, Australia). Feeding trials were conducted at BIARC using this improved formulation and resulted in crab growth rates that were approximately 90% of those obtained using a fresh diet (fish, squid and mussel). Small-scale feasibility studies conducted in earth ponds at RIA3 also revealed that mud crabs fed artificial diets could grow at rates that were equivalent to those fed a traditional diet of trash fish.
Nutrient digestibility coefficients were obtained for a broad range of animal- and plant-based ingredients that were equivalent (or superior) to those obtained for fishmeal, which is traditionally the major source of protein in formulated aquaculture diets. A significant finding was that defatted soybean meal was readily digested when incorporated into diets for local mud crab species in Australia, Indonesia or Vietnam. On the basis of its' high digestibility, the research team recommends that soybean meal be further investigated for its potential to replace fishmeal in formulated mud crab diets.
In laboratory-based culture systems, juvenile crabs demonstrated best growth performance when fed artificial diets containing 45-55% crude protein and 9-15% lipid. This is significantly less protein and more lipid, respectively, than contained in many artificial prawn diets used to support mud crab aquaculture. Such findings are encouraging and suggest it may be possible to: 1) reduce diet cost by lowering the amount of expensive, marine animal-based protein (i.e. fishmeal) required for mud crab diets; 2) increase the energy density of mud crab diets by incorporating higher levels of lipid.
Despite these encouraging results, growth rates of individual crabs in laboratory-based culture systems used for the current project were less than those typically reported for mud crabs grown communally in earth ponds. As a consequence, the research team suggests that alternatives to laboratory-based culture systems be developed for future studies that attempt to optimise nutrient profiles in mud crab diets.