Aspects of the biology and husbandry of portunid crabs relevant to aquaculture-based enhancement and fisheries management

J.R. Tweedley; T.I. Campbell; N.R. Loneragan; D. Johnstone

Summary This review summarises (i) the biology and behaviour of portunid crabs, focusing on the commercially and recreationally important species within the genera Callinectes, Portunus and Scylla, (ii) identifies previously successful aquaculture/restocking efforts for portunids and (iii) detail any successful aquaculture methods/techniques for these species. Portunids are highly fecund, fast growing, short-lived species, with high natural mortalities and whose populations are characterised by an abundance of immature individuals and fewer older, mature individuals. The dietary composition of these crabs, which are opportunistic, is influenced significantly by ontogeny, moult stage, habitat and season. Bivalves, gastropods, crustaceans and teleosts are the major prey categories consumed, with larger crabs consuming more mobile prey. Portunid crabs are more active during night than day, when many species bury. They have developed intricate mating and moulting behaviours that are influenced by water temperature and salinity. The females of many species undertake migrations associated with reproduction and the release of hatching eggs. Portunid fisheries occur globally, with the greatest capture rates occurring in the Indo-West Pacific. Global catches of portunid crabs are still increasing despite the relative stability in the catch rates of world finfish fisheries. High fishing pressure on many established portunid fisheries, exacerbated by environmentally driven fluctuations in recruitment, has led to several localised depletions. The management of portunid fisheries varies greatly between and within countries, but all employ input (i.e. licences, spatial and temporal closures) and/or output controls (size, sex and catch limits). Aquaculture-based enhancement has been suggested as a possible means is increasing stocks in depleted fisheries. A number of restocking and stock enhancement programs have been implemented around the world, including USA, Japan and South East Asia. These were all initiated in response to declines in stocks and, in each case, successful hatchery techniques were developed to enable small-scale production to occur. Releases were found to be most successful when the hatchery-reared individuals were released as crablets and not megalopae. Noting that far smaller numbers of individuals were produced due to cannibalism. Typically, release programs for brachyuran crabs appear to have been successful, albeit too different extents. For the most part, the evaluations of the effectiveness of these releases have been carried out at the pilot scale with only the culture of P. trituberculatus scaled up (~27-35 million juveniles per year). Species with fast growth rates, limited home ranges and a large economic value are likely to be the most appropriate species to culture and release. Any future release program for a portunid is likely to require extensive research and development, particularly to scale up production, and thus the biggest gains in this field to-date have come from increased knowledge rather than increased stocks. In terms of aquaculture, broodstock are typically collected from the wild using commercial fishers and to increase the success of the hatch, conditions in the culture environment should be similar to those in the waters the broodstock were collected from. If inseminated but not berried broodstock are collected, crabs require access to sandy substrate to spawn. Maintaining appropriate water quality is paramount and P. pelagicus larvae require relatively warm (25-30 °C) water at near-marine salinities (30-35 ppt) that is well oxygenated and of a slightly basic pH (8). The larvae require different feed throughout the transition from zoea to megalopa. Typically, zoea are fed a range of microalgae strains (which also enrich live feeds and provide a greenwater culture) and rotifers, with Artemia being provided once the larvae reach the later zoeal stages. At the crablet stage, diet in the hatchery comprises commercial shrimp/prawn feeds. Stocking density is important, with faster growth and greater survival occurring at lower densities. Cannibalism in portunids starts at the megalopa stages and lower stocking densities and the provision of greater substrate complexity reduce juvenile cannibalism. Portunid species have been shown to actively select more complex habitats, such as seagrass, macroalgae, pebbles and crushed shells over finer sand substrates and artificial habitats, such a nets, have also been successful in increasing survival.

Aspects of the biology and husbandry of portunid crabs relevant to aquaculture-based enhancement and fisheries management

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