Improving culture techniques for village based farming of giant clams (Tridacnidae)

Eight experiments aimed at improving methods for the village-based farming of giant clams were conducted in the Solomon Islands, The experiments focus ed on either improving the fitness of seed clams delivered to village farme rs, assessing whether differential growth rates of seed clams in nursery ta nks persisted during grow-out at farms, or testing the effects of alteratio ns to the design of grow-out cages on the growth and survival of clams. We found that Tridacna squamosa (Lamarck) 'seed' transferred from land-based n ursery tanks to a floating ocean nursery (FON) for approximate to 3 months at the end of the nursery phase were significantly larger than seed reared only in land-based nursery tanks, Similarly, T, maxima (Roding) placed in a FON for 2-5 months generally grew at a significantly greater rate than tan k-reared 'seed'. However, the use of FONs did not improve survival. There w ere no consistent differences in the growth and survival of fast- and slow- growing seed of T. derasa (Roding) at village sites when slow-growing seed were retained in the nursery until reaching a larger size. The survival of T. maxima was enhanced significantly by placing an insert of smaller mesh ( a 'settlement ring') in grow-out cages for the first 2 months after deliver y of seed to farmers. The settlement ring retained clams in cages until the y found a suitable place to attach their byssal threads. Attempts to remove the sediment which impedes the attachment of T, maxima to the base of grow -out cages by perforating the substrate did not improve survival: the perfo rated substrate resulted in poor attachment of clams and harboured predator s (Cymatium spp.). The survival of T. crocea (Lamarck) was not improved by 'softening' the concrete base of grow-out cages to simulate dead coral rock and to encourage the clams to burrow in the substrate. The survival of T, crocea in grow-out cages was enhanced significantly by enclosing the cages in fine mesh after the delivery of the seed clams to prevent predation and disturbance by juvenile wrasse, Thalassoma spp. The experiments indicate th at the critical stage for village farming of giant clams is during the init ial weeks following distribution of seed. Further research is needed to imp rove the survival of T. crocea and T. maxima during this phase.