20 MARCH 2013 • WORLD AQUACULTURE • WWW.WAS.ORG Larval rearing. Optimal survival rates are achieved when fertilized eggs are stocked in larval-rearing tanks at low densities of 0.3-0.5 larvae/mL. Newly hatched gastrula will develop into auricularia larvae and start feeding on the second day after hatching. It is general practise to culture a range of microalgal species to feed to developing larvae, although in Vietnam, requirements for live-feed production have been successfully reduced to the culture of a single species, Chaetoceros muelleri (Duy 2010, Mills et al. 2012). As larvae increase in size and complexity during the 10-d pelagic phase, the feeding ration is increased gradually from 20,000 cells/mL on day 2 to 40,000 cells/ mL on day 10. Settlement. Around ten days after hatching, larvae undergo complete metamorphosis, shrinking to half their size as they transform to doliolaria larvae (Fig. 4) that are ready to settle onto benthic substrates. Doliolaria larvae require both a hard substrate and a food source as a cue for settlement. Traditionally, doliolaria larvae are transferred to settlement tanks containing corrugated polycarbonate plates that have been conditioned or inoculated with benthic diatoms. An alternative technique was developed in Vietnam that is now widely used. Settlement plates are “painted” with Spirulina paste and introduced directly into larval rearing tanks (Duy 2010, Mills et al. 2012). After a few days, the doliolaria larvae transform into pentacula larvae when they start to develop characteristic tube feet and resemble adult sandfish. Newly settled juveniles are supplied with benthic diatoms or Chaetoceros muelleri. Nursery Production Once juveniles reach 10 mm they become endobenthic, meaning they are capable of burying into sediments. Consequently juveniles adopt the classic diurnal burial cycle, remaining buried in sediments for most of the day and emerging at night to feed. Formerly, juveniles were transferred to a first nursery of bare tanks, followed by a second nursery phase using sand as a substrate (Battaglene 1999, Agudo 2006). Currently practised techniques are more straightforward. In Madagascar, juveniles are transferred directly to outdoor nursery ponds containing sediment from seagrass areas. In Vietnam and the Philippines, juveniles are transferred to hapa nets constructed from 1-mm mosquito mesh installed in ponds, raceways or in protected inshore areas (Figs. 5 and 6). Hapa nets provide a low-cost option to the traditional second nursery in sand because juveniles require no additional feed, but more importantly, they overcome the main bottleneck to hatchery production, which is commonly a lack of space (Pitt and Duy, 2004). Furthermore, extension of the nursery phase into the pond or marine environment enables juveniles to acclimate to conditions in the wild, which may ultimately produce hardier juveniles at release (Gamboa et al. 2012, Robinson and Pascal 2012). Grow-out Technology The principal aim of the WorldFish Center program was to develop technology to produce sandfish juveniles for release into the wild, initially to enhance natural populations, and then, as recruitment failed to keep pace with exploitation, to restock overfished wild populations. Naturally research directions and practical applications of sandfish aquaculture have since diverged to cover a broader range of issues, including the development of sandfish aquaculture as an alternative livelihood for coastal communities, the bioremediation potential of sandfish in Integrated Multi-Trophic Aquaculture (IMTA) systems, and commercial aquaculture. Extensive to intensive techniques for sandfish grow-out technologies are currently practised. A range of alternative livelihood options are possible, as illustrated by case studies from around the world including sea ranching in Fiji and the Philippines, sea pen farming in Madagascar and pond culture in Vietnam. The choice and implementation of a particular development model are influenced by a wide range of social and economic factors that are characteristic of each geographic region. Sea Ranching in Fiji and the Philippines Sea ranching stems from the original objective of stock enhancement or restocking of overexploited wild populations through release of hatchery-reared sandfish into No-Take Zones (NTZs). The main approach involves the formation of a protected breeding population which, via larval export, supplies adjacent wild populations with new recruits. Based on modelling of individual movements, to protect 10,000 juveniles released into a 1-ha area as a nucleus breeding population for 10 years, the NTZ would need to be between 19 and 40 hectares (Purcell and Kirby 2006). For fishing communities, this represents a considerable area of fishing grounds to set aside as a NTZ, with little guarantee of tangible benefits in the future. Community sea ranching projects have a similar approach to NTZs but allow communities access to the ‘spillover’ area surrounding the core release area to undertake controlled harvesting. Sea ranching has been developed in regions where communities already have some involvement in marine resource management and a degree of ownership over nearshore areas. In Fiji, traditional marine tenure and control in the form of qoliqoli (traditional fishing-rights areas) provides a robust framework for community sea cucumber ranching projects (Hair 2012). In the Philippines, ranching projects center on co-management of FIGURE 4. Sandfish auricularia larvae metamorphosing into doliolaria larvae. Photo: G. Robinson.
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