Evaluation of strategies for intermediate culture of Trochus niloticus (Gastropoda) in sea cages for restocking

The marine topshell Trochus niloticus (‘trochus’) has been overfished throughout its distribution. Intermediate culture in reef-based cages may be an effective approach for providing sub-adults for restocking. In Vanuatu, 15–30 mm (basal shell width; BSW) tagged juveniles were grown over a 9-month period at varied stocking densities inside steel reef, plastic reef and plastic floating cages; n=12 for each caging type. Shell sizes, mortality and escapement were recorded at 3-month intervals. Repeated measures ANCOVA showed that shell growth was inversely dependent on stocking density and differed significantly among caging types. Growth rates decreased significantly through time in all treatments. Reef-based cages were more problematic than floating cages, but yielded faster shell growth of trochus, averaging 2.6 mm month⁻¹. Stocking densities up to 30 juveniles m⁻² are recommended. In all caging types, mortality was relatively low (8–21%) but escapement was high, indicating that cages should be purpose-built for trochus culture. The findings show that high numbers of small juvenile trochus can be cultured in cages to 40–50 mm BSW in 9 months then released onto reefs with expected high survival to broodstock size. Intermediate culture in sea cages thus provides an effective, intermediate step for restocking.