Effects of sediment enrichment with macroalgae, Sargassum spp., on the behavior, growth, and survival of juvenile sandfish, Holothuria scabra

To scale-up culture production of the sea cucumber Holothuria scabra using ocean grow-out systems, bottlenecks in enhancing the growth and survival of juveniles need to be addressed. As the selection of release sites is fraught with uncertainty, organic enrichment of sediment using dried Sargassum spp. to increase food abundance (i.e., microalgae and bacteria) may be a viable option to optimize ocean nursery systems. The effects of sediment enriched with dried Sargassum spp. on the diel burying behavior, growth, and survival of juvenile (>3 g) sandfish were investigated in the field for 60 days using sea pens (2.25 m2) with a stocking biomass of 66 g m−2, and in glass tanks (0.045 m2) with the same stocking biomass (∼3 g individual tank-1) in the laboratory for 30 days. Enrichment with Sargassum markedly changed sediment characteristics. Although diurnal burying behavior did not vary significantly between treatments, a greater percentage of juveniles in enriched pens and tanks remained buried during the daytime compared to unenriched pens. Without protection from predators in the wild, all juveniles from enriched pens survived compared to those from unenriched pens (85 ± 10%) after 7 days. After 30 days, absolute growth rates (AGR) of juveniles in enriched pens (0.51 ± 0.06 g day-1) and tanks (0.82 ± 0.27 g day-1) were significantly higher compared to unenriched pens (0.13 ± 0.04 g day-1) and tanks (0.49 ± 0.19 g day-1). Results showed that mean sediment chlorophyll-a content of ∼4 μg g-1 in sea pens enriched with Sargassum spp. could hasten the growth of ∼3 g juveniles to >20 g in 30 days. This study demonstrated that the organic enrichment of sediment is a viable option to optimize the growth of sandfish in an advanced ocean nursery system that may significantly improve survival prior to release for grow-out.