Combined culture of Trochus niloticus and giant clams (Tridacnidae): benefits for restocking and farming

Abstract

The potential benefits of using giant clam farming systems to rear trochus for restocking were investigated in Solomon Islands. Trochus with a mean size of 5.61 mm±0.06 SE maximum basal diameter (MBD) reared in concrete tanks used to produce juvenile giant clams attained a mean size of 28.28 mm MBD±0.25 SE after 22 weeks and grew approximately twice as fast as trochus raised in fibre-glass tanks fitted with poly-carbonate plates. Trochus from the concrete tanks were then transferred to the small (0.36 m −2) sea cages used to grow-out Tridacna derasa for the marine aquarium trade. Mortality in the sea cages was negligible and the trochus grew from a mean size of 30.52 mm MBD±0.04 to 46.33 mm±0.39 SE in 18 weeks. Stocking density and husbandry (to remove algae and sediment) had a significant effect on the growth of trochus in sea cages: animals stocked at a density of five individuals per cage grew significantly faster than those stocked at 10 per cage and growth was significantly greater in cages where the larger species of algae were removed regularly. Trochus did not appear to have any deleterious effects on the growth and survival of T. derasa. On the contrary, there was some evidence that growth and survival of the giant clams were improved at the highest stocking density of trochus. Trochus grazed on a range of filamentous and turfing algae within the sea cages but were ineffective at removing larger species of algae compared to husbandry procedures. Trochus harvested from sea cages were released at seven coral reef flat sites, with two levels of shelter. Mean survival at all sites after 4 weeks was at least 76%±4.6% SE and was not affected significantly by the amount of shelter. There were no apparent morphological weaknesses, or behavioural deficits, in the cultured trochus compared to wild individuals. In fact, the shells of cultured animals were significantly heavier than those of wild trochus. Rearing trochus in combination with giant clams produces individuals that are fit for survival in the wild at reduced cost, and paves the way for giant clam farmers to restock trochus by releasing them on reefs under their customary marine tenure, or in marine protected areas.