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Capture-Based Aquaculture of Mud Crabs (Scylla Spp.)

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Capture-Based Aquaculture of Mud Crabs (Scylla Spp.) 255 Capture-based aquaculture of mud crabs (Scylla spp.) Colin Shelley YH & CC Shelley Pty. Ltd Brisbane, Australia E-mail: [email protected] Shelley, C. 2008. Capture-based aquaculture of mud crabs (Scylla spp.). In A. Lovatelli and P.F. Holthus (eds). Capture-based aquaculture. Global overview. FAO Fisheries Technical Paper. No. 508. Rome, FAO. pp. 255–269. SUMMARY There is limited understanding of wild mud crab resources and how best to manage them in many countries, particularly where fisheries management resources and enforcement capabilities are limited. The growth of mud crab aquaculture is likely to lead to changes to the ecological, socioeconomic and livelihoods currently associated with mud crab fisheries. This paper provides an overview of the issues, needs, opportunities and risks in trying to maintain sustainable mud crab fisheries, whilst supporting the ecologically sustainable development of mud crab aquaculture. The uncontrolled fishing of juvenile crabs for farming in some countries has led to recruitment overfishing, even though mud crabs are very fecund and have extended spawning seasons over much of their range. Conserving of mud crabs primary habitat, mangrove forests, is critical to supporting their populations, as is the regular monitoring of stocks to guard against their over-fishing. Environmentally sustainable farming of mud crabs in mangrove pens is seen as an important tool in both conserving mangrove forests, and expanding farm production areas. Significant growth of mud crab aquaculture is only going to occur from hatchery sourced seed-stock, as wild populations are at either at their limit or over-fished in many countries. Such growth will also be dependent on the development of formulated diets to reduce mud crab farming’s current dependence on trash-fish, a resource which is already under pressure from other types of aquaculture. During the transition from an industry dependent on wild mud crab seed-stock and wild feed resources, to hatchery produced seed-stock and formulated fields there will be changes to the current supply chains, and employment opportunities. Consideration needs to be given to programmes to assist fishers of both wild mud crab seed-stock and trash-fish (and associated middlemen) as the farming of mud crabs moves to a more industrial scale as is currently taking place in China, as both groups are amongst the poorest in many coastal communities. DESCRIPTION OF MUD CRABS AND THEIR USE IN AQUACULTURE Species There are four species of mud (or mangrove) crabs in the genus Scylla, S. serrata, S. olivacea, S. tranquebarica and S. paramamosain (Keenan, 1999b; Keenan, Davie 256 Capture-based aquaculture: global overview and Mann, 1998), all of which FIGURE 1 Juvenile mud crab support capture fisheries and aquaculture. In most countries where mud crabs are fished or farmed, they are an important source of income from both export and local sales, and are utilized by recreational fishers. Life cycle All mud crabs commonly display 6 larval stages; 5 zoeal stages, followed by a megalops larval stage which precedes the first crab stage (Figure 1). Mud crabs typically undergo 14–16 moults prior to reaching their maximum size. Reported daily weight gain COURTESY COURTESY OF D. MANN for mud crabs varies from 1–4 g per day and varies with species, and sex, with males reportedly growing faster than females (Trino, Millamena and Keenan, 1999b; Christensen, Macintosh and Phuong, 2004). All mud crabs can mature within their first year of life, with S. paramamosain maturing at a size of 102 mm carapace width at around 160 days from settlement (Le Vay, Ut and Walton, 2006; Le Vay, Ut and Walton, 2007), whilst S. serrata have reportedly grown to 750 g within 145 days and shown signs of maturity at day 147 (Field, 2006). They are highly fecund with individual females carrying over 3 million eggs. Apart from spawning migrations where females may travel considerable distances offshore most crabs appear to move little within their local habitat, which is typically mangrove forest (Hill, 1975; Hill, 1976; Le Vay, Ut and Walton, 2006; Le Vay, Ut and Walton, 2007). Mud crabs of different sizes occupy different niches within mangrove forests and the adjacent sub- tidal zone (Walton et al., 2006). Habitat Mud crabs are a common component of the fauna of mangrove forests, usually burrowing in mud or sandy-muds. They have a diverse diet and are omnivorous in nature, feeding on a wide range of animal and plant resources (Hill, 1976). Geographical distribution The distribution of mud crabs extends from South Africa, along the southern coasts of middle-eastern countries, across the Indian Ocean and northerly to the southern tip of Japan, east as far as Micronesia and south to the east coast of Australia. Scylla serrata is the most widely distributed species, whilst Indonesia appears to be the centre of diversity for the genus, where all four species of Scylla are found. Capture fishery The mud crab is a targeted species for harvest across its range. Techniques vary from catching by hand to the use of fishing gear including tangle nets, baited traps and lift nets. Fishery trends for the last decade are detailed in Table 1. However it should be noted that figures for Sri Lanka and Australia were missing from the FAO database and not included here, so that these figures represent an underestimate of the production of Capture-based aquaculture of mud crabs (Scylla spp.) 257 TABLE 1 Capture of Scylla serrata in tonnes Country 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Indonesia 7 980 7 342 8 298 8 161 8 707 8 774 11 753 11 240 14 802 20 129 18 750 Philippines 4 835 4 258 1 133 1 124 1 211 1 247 1 604 1 692 1 663 1 466 1 432 Taiwan PC 1 339 935 180 215 269 299 230 337 375 9 717 Thailand 5 776 4 243 4 031 3 732 5 736 6 921 5 417 3 823 1 259 2 859 1 865 Fiji Islands 234 208 290 270 281 250 268 180 82 324 280 Source: FAO–FIGIS Scylla serrata. Whilst Indonesia has shown an increasing catch, FIGURE 2 all other major producers have Live crabs on sale in Viet Nam shown either a decreasing or static catch. Harvest products Juvenile crabs or crablets are actively harvested throughout Southeast Asia for use as seed- stock for crab farms. Sizes harvested vary from a few centimetres across the carapace to just under harvest size for sale direct to market. Crabs of close to, or at a marketable size are caught for a range of activities. Crabs which have recently moulted and have not fully grown to fill their new shells are commonly referred to as “empty” crabs. Such crabs may be put into fattening pens, ponds or enclosures and fed until they are “full” and ready for market. COURTESY OF C. SHELLEY Other crabs of varying sizes will be caught and put into soft shell shedding facilities. Such crabs are commonly placed in individual containers and monitored until they moult. On moulting the crabs will either be chilled and put on ice, or frozen for the soft shell crab market, where all parts of the crab can be consumed as the shell has not been allowed to harden after moulting. Finally, hard-shell crabs of a marketable size are collected, secured to ensure traders and customers are not injured by their powerful claws and sold; most commonly in the live form (Figure 2). The size of crabs marketed varies with species. In the Philippines S. serrata is most commonly harvested at weights over 500 g, whilst for S. olivacea and S. tranquebarica the weight is usually over 350 g. Whilst mud crabs are usually a targeted species, they may be caught occasionally in various nets which are targeting other mangrove or reef species and are caught as they move across their habitat. Only a very small part of the mud crab harvested is bycatch of other fisheries. Mud crabs adapt very well to a farmed environment. With their omnivorous diet they will eat a wide range of feeds, from trash fish through to pelletised aquaculture feeds. There are a number of problems encountered by collectors and farmers, involved with using wild harvested crabs in various farming systems. Stock will often consist of a wide variety of sizes, and as mud crabs have a tendency to cannibalism, larger specimens will often predate on smaller crabs, causing significant mortalities amongst farm stock. 258 Capture-based aquaculture: global overview Life cycle status Mud crabs born in captivity have been successfully mated with both wild and other captive stock so that some organizations and companies now use domesticated stock. Almost all hard shell, mature females collected from the wild will have been impregnated and will spawn if held under appropriate conditions. Each mature female will usually be able to spawn 2 or 3 batches of larvae when held under satisfactory conditions following a single copulation. The use of farm produced seed is now becoming common in Viet Nam and China in particular. In some countries, such as the Philippines, there has been caution in the use of hatchery produced stock to date (Shelley, 2004a). Farmers have reported a range of concerns with crablets produced in hatcheries; will they be as robust as wild stock, will they grow as fast, will they be more prone to disease, and which is the better value for money – wild or farm produced stock? In some countries where mud crab fisheries are actively managed e.g. Australia, crablets or under-size crabs cannot be legally harvested. Farming techniques Considerable efforts have been made over the last few decades to develop effective technology for mud crab aquaculture (Brick, 1974; Angell, 1992; Heasman and Fielder, 1983; Keenan and Blackshaw, 1999a; Anon., 2001; Anon., 2005; Shelley et al., In Press; Wang et al., 2005).
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