Cathy Hair

The use of wild-caught juveniles in coastal aquaculture and its application to coral reef fishes

Worldwide, there are many substantial coastal aquaculture and stock enhancement operations based on collection of wild juveniles. These include: growout of shrimp (Penaeidae), milkfish (Chanos chanos), eels (Anguilla spp.), yellowtail (Seriola quinqueradiata), southern bluefin tuna (Thunnus maccoyii), edible oysters (Ostreidae) and mussels (Mytilidae); stock enhancement of scallops (Pectinidae); and the culture of pearls in farmed blacklip pearl oysters (Pinctada margaritifera). The growout of wild puerulus larvae of spiny lobsters (Palinuridae) is also developing rapidly. The advantages of using wild-caught juveniles for aquaculture are: 1) low costs of obtaining animals for stocking as compared with hatchery production, 2) availability of individuals fit for growout in the sea, 3) no risks of “genetic pollution” from deliberate or accidental releases, 4) reduced likelihood of transferring diseases, and 5) a broader range of economic benefits, including opportunities for coastal dwellers in developing countries to sell stock to larger enterprises. In addition, responsible capture and culture of wild juveniles can improve overall fisheries productivity for target species by circumventing the high rates of natural mortality associated with settlement of postlarvae from the plankton. Careful management of this process is needed, however, to ensure that replenishment of the stock, and fisheries targeting adults, are not affected. Where large numbers of postlarvae are taken, or where aquaculture is based on larger juveniles, these goals can be met by returning a proportion of the cultured juveniles to the wild, or through the transfer of fishing effort from adults to juveniles. The disadvantages of using wild juveniles for aquaculture are: 1) that the number of animals available for growout can be limited and variable, 2) there is no scope for increased productivity through selective breeding, and 3) potential effects on the ecosystem stemming from mortality of bycatch and removal of prey from the foodchain. On balance, the advantages outweigh the disadvantages and new applications for the use of wild-caught juveniles are under investigation. In particular, there has been interest in using aquaculture to supply the trade in ornamental and live food fish from coral reefs to overcome problems resulting from overfishing of adults and the use of destructive fishing techniques. However, it is technically difficult and expensive to propagate postlarvae of many coral reef fishes so cost-effective hatchery production of juveniles for aquaculture is likely to develoop only for a minority of target species. As an alternative, the feasibility of harvesting pre-settlement coral reef fishes from the plankton in numbers that do not affect the replenishment of natural populations, and rearing them for a short period before sale to the ornamental trade or as juveniles for growout for the live fish market, is being assessed. Two sampling techniques, light traps and crest nets, have proved suitable for the capture of live pre-settlement fishes and substantial progress has now been made in applying these methods to the development of artisanal fisheries for ornamental species. Although the capture and culture of postlarvae is unlikely to meet the demand for all tropical marine fish required by the ornamental trade, it has the potential to create important niche markets, e.g. for eco-labelled specimens, and provide sustainable economic benefits from coral reef resources for coastal villagers.

The Capture and Culture of Post-Larval Fish and Invertebrates for the Marine Ornamental Trade

Governments, non-government organizations, and other stakeholders are striving to develop practices, policies, and vehicles to make the tropical marine ornamental trade sustainable. Small-scale fisheries based on post-larval capture and culture (PCC) promise to contribute to this goal by (1) removing the risk of damaging corals (inherent in harvesting adults of target species established on reefs) by collecting post-larvae with light traps, nets, and purpose-built temporary shelters as they settle from the plankton to the substrate; and (2) translating the high mortality of post-larvae at settlement into high rates of survival in culture. Possible concerns about overfishing of post-larvae, harvesting the juveniles after they have run the gauntlet of predation at settlement, and the large proportion of bycatch can be eliminated or greatly alleviated by restricting the size and quantity of fishing gear, designing it to retain bycatch alive, and releasing bycatch at times and places that minimize predation. However, special caution is needed when PCC is used at small, isolated islands with self-replenishing populations. Although PCC is environmentally friendly, its contribution to the ornamental trade is expected to be limited. Large variation in the abundance and species composition of settling post-larvae among years, the logistics and costs of operating labor-intensive operations in remote locations, and competition with responsible enterprises harvesting wild adults or producing ornamentals in hatcheries are expected to constrain the viability and market share of dedicated PCC enterprises. PCC is expected to have the greatest uptake by part-time artisanal fishers in developing countries with infrastructure for exporting marine ornamentals. Such fishers are more immune to temporal variation in the supply of post-larvae—they can engage in PCC when valuable post-larvae are abundant and switch to other sources of income when they are scarce. Livelihood opportunities for smallholders could be enhanced through promotion of the environmental benefits of PCC among hobbyists maintaining marine ornamentals.