The sandfish sea cucumber H. scabra is a candidate species for aquaculture. Despite decades of research efforts, the production of tropical sea cucumber is still hampered by the availability of seed stock for ranching and stock enhancement efforts. Increasing the stocking density in sandfish nursery systems results in slow growth and small sizes. Severe effects of density dependent growth have resulted in low productivity nursery systems compared to commercialised aquaculture species, requiring extensive nursery and growout systems. Management of stocking density at an early life stage (settlement) is a way to achieve rapid growth. Optimising stocking density to achieve fast growing healthy seedstock whilst maximising the number of juveniles produced in nursery systems will be key to the success of tropical sea cucumber aquaculture operations. The study reported here was designed to develop our understanding of appropriate pre settlement stocking rates in a plate type nursery system to increase the number of juveniles produced without compromising growth.
The number of juvenile H. scabra produced in a plate tank nursery system was assessed in a nursery stocking density trial. Fibreglass nursery tanks (6m x 1m x 0.45m) each containing 120, vertically oriented acrylic sheets (0.6m x 0.3m) as settlement substrate were supplied with 1µm filtered flow through seawater under ambient conditions. Fourteen-day old doliolaria stage larvae were stocked at three different stocking rates, A = 587 larvae m-2, B = 352 larvae m-2 and C = 252 larvae m-2.
After 90 days the number of juveniles produced per square metre in the A stocking density treatment group (61 ± 5.6m-2) was significantly greater than in the other B and C stocking rate treatments, 42 ± 3.5m-2 and 32 ± 2.7m-2, respectively. Growth and recovery rate (% larvae recovered as juveniles) were not significantly reduced in the high stocking group.
The trial yielded fewer than expected numbers of juveniles, however growth was rapid. Some tanks in all treatment groups were impacted by poor productivity resulting in very small stock and low biomass in those tanks.
Determining the best combination of substrate preconditioning, stocking rate and management practices to balance the trade-off between stocking rate and productivity will be vital to prevent growth being compromised to a point that impacts survival or success in sea ranching or stock enhancement operations.