Stocking density is an aquaculture chronic stressor that can directly suppress the growth potential of a cultured animal. The sea cucumber Stichopus cf. horrens is a target species in the Indo-Pacific region whose culture production is emergent. Its cryptic and nocturnal nature indicate that shelters may be of importance in the rearing of early-stage juveniles, particularly under stressful conditions including stocking density.
Field and laboratory experiments were conducted to investigate the effects of stocking density on the behavior, growth, and survival of S. cf. horrens juveniles. In the field, juveniles were reared at 25 ind. tray-1, 50 ind. tray-1, and 100 ind. tray-1, with and without shelters for 30 days. The growth and survival of the juveniles were monitored biweekly. The diel activity, location, and aggregation behavior of the juveniles at three stocking densities (i.e., 13 ind. tray-1, 26 ind. tray-1, and 52 ind. tray-1) were also investigated in the laboratory. In both of these experiments, the equivalent stocking biomass was 4 g tray-1, 8 g tray-1, and 16 g tray-1 for the low, medium, and high-density treatments, respectively.
The results revealed that growth in length of S. cf. horrens juveniles is more significantly affected by stocking density than shelter. High-density conditions lead to significantly lower growth rates and greater size variation compared to low-density treatments, suggesting crowding stress. The occurrence of high-density aggregations at high stocking densities may be the causal mechanism of crowding stress that lead to growth suppression. The presence of shelters resulted to higher biomass increment than the unsheltered treatments. However, it was only at low density shelter treatments where a significant difference was found, indicating that the benefits of shelter may have diminished in the medium and high density treatments. Survival rate was also significantly influenced by shelter denoting that its presence may have conferred benefits.
No distinct differences were found in the diel activity of S. cf. horrens juveniles at different stocking densities. The juveniles were mostly inactive during daytime between 06:00-18:00h, and increased activity from dusk to nighttime. When inactive, a strong preference to shelter and aggregate towards the PVC undersides was observed across density treatments. This aggregation behavior may be a reflex response related to the cryptic nature of this species. Moreover, the probability encounter when shelter-seeking, which increases at high stocking densities, may have influenced aggregation densities. The results of this study highlight the importance of determining the appropriate stocking level and shelter complexity that could reduce crowding stress and optimize the growth performance and survival of S. cf. horrens juveniles.