Survival and growth of juvenile fluted giant clams, Tridacna squamosa, in large-scale grow-out trials in the Solomon Islands

Two large-scale experiments were conducted with juvenile Tridacna squamosa at coastal village sites in the Solomon Islands to provide robust estimates of survival and growth during grow-out. Juveniles were reared to ~ 24 mm shell length (10–11 months) in land-based tanks and then transferred to subtidal sites near villages for grow-out for 8 months. During the first experiment, which started in 1993, survival varied greatly among sites, ranging from 7% to 83%. Exposure (‘fetch area’) and geographic location had a significant influence on survival, indicating that adverse weather conditions affected mortality rates. Inserts made of 5-mm galvanized mesh (‘settlement rings’) fitted to the inside perimeter of grow-out cages significantly reduced loss of clams from cages after transfer to villages. Growth varied from 2.3 to 8.6 mm month −1 and was significantly, positively correlated with the number of clams surviving in the cages. During the second experiment, which commenced in 1994, survival was greater and less variable, ranging from 42% to 83%. Average growth also improved, ranging from 4.0 to 7.2 mm month −1. Growth was positively correlated with water flow, and negatively correlated with Secchi disc visibility and the number of predatory ranellid gastropods found in the grow-out cages. A weak negative relationship between growth and survival occurred during the second experiment. Modifications to aquaculture protocols in 1994 were evaluated in a series of concurrent experiments conducted at the village sites. Grooves in the bases of cages, support of clams using pieces of coral, and retention of ‘settlement rings’ for 4 instead of 2 months had no effect on survival and growth of the clams. However, survival of juveniles preconditioned in the ocean was significantly higher than those reared exclusively in tanks. On the basis of these two experiments, the mean time required to grow T. squamosa from 24 to 50 mm shell length, a size suitable for sale to the aquarium market, was only 5 to 7 months. Our results reveal that village-based farming can be economically feasible in the Solomon Islands, provided reliable hatchery facilities are available.