Culturing the sea cucumber Holothuria poli in open-water integrated multi-trophic aquaculture at a coastal Mediterranean fish farm

The survival and growth of the sea cucumber Holothuria poli were assessed during a 12-month field study when cultured at a commercial fish farm in Malta as part of an integrated multi-trophic aquaculture (IMTA) system. Sea cucumbers were cultured directly below a fish cage at 0 m, E0, at 10 m (E10) and 25 m (E25) distances from the cage and at two reference sites (R1 and R2) located over 800 m away from the fish farm. Mass mortalities were recorded at E0 within the first month of the study due to smothering by settled wastes. All individuals died at one of the reference sites, R1, by the end of the study. After deducting missing sea cucumbers, survival rates at E10 (23%) and E25 (33%) from the fish cage were similar to the remaining reference site (R2) (27%). Stocking density and physical disturbances to the sea cucumber cage setup were also probable cause for the low survival rates. The relative weight gain (RWG) and specific growth rates (SGR) of H. poli varied significantly between sites close to the fish farm and the reference site. The SGR of H. poli at E10 (0.18 ±0.02% day-1) and E25 (0.20 ±0.01% day-1) was positive over the whole study period while no average growth was recorded at the reference site (-0.04 ±0.07% day-1) over the same period. Differences in RWG and SGR were recorded throughout the study. The overall growth observed in H. poli by January was followed by a drop in growth rate across all sites and an increase in SGR at E25 in July. Slower growth rates were observed as water temperature approached 15 oC. The results indicated that the sediments near the commercial fish cage provided an enriched source of food that supported significantly better growth in H. poli. This suggests that H. poli in IMTA might have the potential to uptake organic farm waste and increase aquaculture production, albeit with important considerations for setup design and stocking density