SEA RANCHING AND RESTOCKING SANDFISH IN NIGERIA
1.1 BACKGROUND TO THE STUDY
Holothuria scabra, or the sandfish, is a species of sea cucumber in the family Holothuriidae. It was placed in the subgenus Metriatyla by Rowe in 1969 and is the type species of the subgenus. Sandfish are harvested and processed and eaten in several coastal communities all over the world. Sea cucumbers are marine invertebrates and are closely related to sea urchins and starfish. All these groups tend to have radial symmetry and have a water vascular system that operates by hydrostatic pressure, enabling them to move around by use of many suckers known as tube feet. Sea cucumbers are usually leathery, gherkin-shaped animals with a cluster of feeding tentacles at one end surrounding the mouth. The sandfish is greyish-black on the upper side with dark-coloured wrinkles but paler on the underside. It grows up to four centimetres long, is broader than it is high and has a pliable skin. It is covered by calcareous spicules in the form of tablets and buttons. In many areas the fisheries have declined over the years because of over fishing, so ranching, aquaculture and hatchery rearing are being attempted (FAO, 2005).
Sandfish stocks are being increased in some areas by being bred in tanks at hatcheries. Spawning is induced by temperature shock and the fertilized eggs incubated for a day. The auricularian larvae that hatch from the eggs are fed on microalgae for nine days after which time they develop into doliolarian larvae. These are fed on diatoms and after five days settle as pentactular larvae onto diatom-coated plates. Juveniles are grown on in tanks and may reach one to two centimetres in three months. They are then moved to larger ponds for a few months before being released into suitable habitats for ranching or restocking (Adams, 2008). Sea ranching is a culture method whereby juvenile animals, generally produced in hatcheries but could also be wild-caught, are introduced into the natural environment and allowed to grow without containment structures. The environment provides the animals with everything they need and no additional feed is required (Adams, 2008). Sea ranching of sandfish where hatchery-produced juveniles are placed on the sea bed and allowed to grow to marketable size, is being trialed in the West African sub-region.
Juvenile sandfish are produced in hatcheries. sea cucumber ranching is only viable if the culture area is closed to commercial trawl fishing while the animals grow and if the animals remain in the specified culture area until harvest. Sandfish are normally harvested by hand either by diving or hand harvesting at low tide in shallow water. Aquaculture, in a variety of forms, is needed to redress the problem of low yields from inshore invertebrate fisheries including the sandfish (Bell, 1999). Aquaculture can be used to increase productivity in a way that the cultured juveniles can be released into the wild to restore stocks to levels at which they provide substantial, sustainable yields. This process is known as “restocking”. This phenomenon occurs when the natural supply of juveniles fails to reach the carrying capacity of the habitat, even when there are great numbers of breeding adults. Other important features of the sea ranching and restocking sandfish in Nigeria are that it causes little damage to the environment, and does not conflict with the subsistence food requirements of villagers (Dalzell, Adams and Polunin, 1996).
The method being developed for the restocking of sandfish involves the propagation of juveniles in hatcheries for release in the wild. Other methods for restocking sandfish have been proposed, including collection, rearing and transplantation of postlarvae; induced fission; and translocation of adults. However, these methods are not suitable for the large-scale restocking of sandfish. Collection of wild juveniles is not practical because they are difficult to find, even though the settlement habitat of postlarvae has now been identified (Mercier, Battaglene and Hamel, 2000; Hamel et al., 2001). In addition, their abundance is likely to be limited as a result of overfishing, and methods for collecting them at settlement would still involve the use of aquaculture facilities for grow-out. Induced fission is not considered to be an option as it only works for a few generally lower-value species of sandfish and is a slow and labour-intensive way to increase numbers. Finally, the translocation of adults would only be effective where their propagules are retained, and at best would benefit some localities at the expense of others. The methods that have been developed for the propagation of juvenile sandfish can be adopted in Nigeria.