Optimal release strategies for restocking and stock enhancement of the tropical sea cucumber, sandfish (Holothuria scabra)

• Australian Institute of Marine Science, Australia
• Ministry of Agriculture and Fisheries, Solomon Islands

Sea cucumbers, processed into 'beche-de-mer', are a valuable source of income for many coastal communities in the Pacific Islands and the developing nations of South and Southeast Asia. The sea cucumbers are easy to harvest, process and store. Wholesale prices for first grade beche-de-mer of around US$50 per kg have resulted in severe over-fishing in many tropical developing countries. It is now apparent that depleted stocks of sea cucumbers can take decades to recover. The release of juvenile sea cucumbers produced in hatcheries is seen as the most expeditious way to rebuild stocks. Eventually, the release of cultured juvenile sea cucumbers also has the potential to increase production beyond historical levels.

ACIAR has funded research at the WorldFish Center in the Solomon Islands to progressively develop, assess and transfer the technology for propagating and releasing tropical sea cucumbers. During the first stage of the project it was established that sandfish (Holothuria scabra) is the species most suited to restocking. They can be reared en masse in hatcheries on micro-algal diets, and juveniles can be grown in simple, land-based nursery systems with minimal input of food.

This project is the second stage of the partnership, which aims to identify the strategies for optimising survival of released juveniles, by developing cost-effective methods to ensure that a large proportion of cultured, juvenile sandfish released into the wild survive to adulthood.

Over a period of four years, ICLARM will scale-up the production of sandfish at the Coastal Aquaculture Centre in the Solomon Islands and conduct large-scale field experiments involving the release of tens of thousands of juvenile sandfish in the Western Province of the country.

Together, these experiments will identify how best to transport juveniles, and determine the size at release, release habitat, release season and stocking density that results in the best trade-off between production costs and survival.

Towards the end of the project, a pilot-scale mass-release of juveniles will be made to set the stage for evaluating the effects of restocking technology in rebuilding overfished populations of sandfish in the Western Province of Solomon Islands.

The results of the project will be disseminated to key personnel in Pacific Islands and developing countries in South and Southeast Asia through specialised workshops, and to a broader audience through technical and scientific publications. A strategy to continue the development of aquaculture in the Pacific being undertaken through the Secretariat of the Pacific Community will also be used to disseminate the main outputs from the research.

The project follows from the previous ACIAR-funded research conducted by the WorldFish Center (then ICLARM) in Solomon Islands, which developed methods for culturing the valuable sea cucumber, 'sandfish' (Holothuria scabra). Following conflict in Solomon Islands the project relocated to New Caledonia, partnered by the Secretariat of the Pacific Community (SPC). Higher costs to operate the project were met through substantial funding from the three Provinces of New Caledonia, the French Government and the ATSE Crawford Fund. Additional support was gained from internships of students from two French universities, INTECHMER and CREUFOP.

After constructing a simple 'field' hatchery (10 x 15 m) at the Saint-Vincent Aquaculture Station, tens of thousands of juvenile sandfish were produced from 2003 to 2005. Following the pioneering work by WorldFish Center in Vietnam, experiments were conducted to improve methods for growing sandfish juveniles in mesh enclosures in earthen ponds rather than in tanks. The approach occurs in two steps: firstly in 'hapas' (670 m mesh), then in 'bag-nets' (1 mm Tentex mesh). From a length of at least 5 mm, sandfish can be transferred from the hatchery to hapas in ponds. No feeding is required in hapas but growth rates were higher when units of 'fake seagrass', made from shadecloth, were includeda method that will speed up the time taken to grow juveniles to larger sizes for release in the wild. At 0.5 g in weight juveniles are transferred, at lower density, to bag-nets.
Feeding and added sediments were not necessary in ponds with high natural productivity; otherwise, feeding with ground shrimp pellets was best. Reasonable survival in bag nets placed in the sea within sheltered bays suggested that opportunities exist for rearing sandfish at hatcheries without earthen ponds. But poor growth implied that research would be needed for managing bag nets at sea.

A study with IFREMER scientists showed that sandfish and blue shrimp (Litopenaeus stylirostrus) could be co-cultured as juveniles. However, further experiments quashed the promise of commercial co-culture since larger shrimp at commercial densities killed sandfish juveniles. Sandfish can grow well in ponds directly on the sediment, but are difficult to collect for release into the wild. Land-based grow-out is possibleestimated growth of juveniles of 1 g to market size of 500-1000 g in 1.5 yearsbut economic feasibility will depend on the costs for ponds, labour and seawater pumpage.

WorldFish collaborated with scientists at the Australian Institute of Marine Science to determine the genetic structure of sandfish stocks through allozyme electrophoresis analyses. Restricted gene flow of sandfish meta-populations directed a responsible approach of translocating hatchery-produced juveniles close to the natal sites of the broodstock. A further study, incorporating mtDNA sequencing, resolved the species distinction of a putative subspecies, the 'golden sandfish' Holothuria scabra var. versicolor, and showed that it can hybridise naturally with sandfish.

Sandfish juveniles are hardy for transport at high densities for up to 24 hours. Hatchery-based simulations affirmed that this is best done at cool (26oC), stable, temperature in buffered seawater. Collaboration with a Belgian scientist led to a published protocol for examining sea cucumbers for disease and health prior to release.

In a significant breakthrough, two fluorochrome markers, tetracycline and calcein, were effective in 'tagging' sandfish juveniles for at least 1 year. Experiments then pinpointed the optimal immersion conditions to fluorescently mark dermal spicules of juveniles. The marking is cheap and only 2-5 mm of outer body wall of an animal is needed for mark verification with an epifluorescence microscope. Marked sandfish released into the wild can now be distinguished from wild counterparts.

Small-scale field experiments, of 1-5 weeks duration using 1-m2 sea pens, examined optimal release conditions. Several key habitat characteristics were identified for maximising the survival of released juveniles: water depth of 0.5-2.5 m, moderately high seagrass cover, and moderate penetrability of sediments. Restocking programs can be flexible about the release time, since post-release survival differed little among groups released at sunrise, mid-day or sunset. Size does matter; juveniles smaller than 1 g were at risk of complete mortality. Acclimation of juveniles in mesh enclosures showed promise to overcome the initial shock of transport and release into the sea.

Two large-scale experiments were then conducted, in which marked juveniles were released into 500-m2 sea pens. In the first, around 20% of the initial 2000 juveniles released at one site survived after 12 months and most reached the size of first sexual maturity. However, growth at the second site was slower and most animals died after 6 months. The second experiment used 9000 juveniles, 12 sea pens among four sites, and tested the effects of release density and juvenile size. Again, survival varied widely among sites, pointing that stock restoration programs should employ multiple sites.

The experiments give encouraging expectations of survival up to 25% after 12 months. Sandfish juveniles should be released into No-Take Zones (NTZs) to preserve them as nucleus breeding populations for restocking fisheries. An Individual-Based Movement model, incorporating field data on sandfish movement and growth, showed that NTZs would need to be 19-40 ha to protect the released animals over a 10 year period.

WorldFish conducted broad-scale underwater surveys of sea cucumbers as a commitment to the Provincial governments. Sandfish populations in the Northern and Southern Provinces appeared heavily exploited, with few sites having plentiful breeding populations. In the Loyalty Islands Province, stocks of commercially-important reef-dwelling species were spatially variable but high densities of black teatfish (Holothuria whitmaei) still exist. WorldFish provided management advice to the fishery departments and plans to assess reef-dwelling sea cucumbers in the Northern and Southern Provinces in a forthcoming project.