Heavy metal contamination of the sea cucumber Holothuria poli cultured in integrated multi-trophic aquaculture in a multi-use coastal area

The accumulation of heavy metals in the edible tissue of the white spot sea cucumber, Holothuria poli, revealed the transfer of metal contaminants to sea cucumbers when produced below fish cages in a Mediterranean port area. Sea cucumbers were cultured on the seafloor directly below a fish cage at 0 m, then at 10 m and at 25 m away from the cage, as part of an open-water integrated multi-trophic aquaculture (IMTA) system, and then at a reference site over 1 km from the fish farm, over a one-year period. At the end of the study, sea cucumbers and seafloor sediments were sampled from the IMTA sites near the fish cages, except at 0 m due to mass sea cucumber mortalities within the first month of the study, and again at the reference site. The concentrations of cadmium (Cd), copper (Cu), chromium (Cr), nickel (Ni), and zinc (Zn) were significantly higher in sediments near fish cages than the reference site. Localised enrichment from marine aquaculture could explain the significant concentration of metals in sediments below fish cages that are typically ascribed to their use in aquaculture. Arsenic (As), lead (Pb) and mercury (Hg), which are not associated with commercial fish diets, did not vary between sites. Concentrations of iron (Fe), which is available in commercial diets, were similar near fish cages and at the reference site. The body wall/muscle tissue of the sea cucumber. H. poli revealed high concentrations of the essential metals Fe and Zn near fish cages and in natural sediments at the reference site. H. poli can regulate these essential metals that characterised the edible tissue of the sea cucumbers. Non-essential metals like Hg and Cd had the lowest concentrations of all analysed metals in the sea cucumber tissue. However, the bioaccumulation of toxic metals, Hg and As, reveal the bioavailability of these contaminants in sediments and the propensity of bottom-dwelling sea cucumbers to bioconcentrate these metals, when cultured under a commercial fish cage in IMTA and elsewhere in natural sediments in this industrial environment. Holothuria poli did not exhibit bioaccumulation of Cu, Cr, Fe, Ni, Pb and Zn in its body wall/muscle tissue. The bioaccumulation of Hg and As reveal the need to account for the potential effects of farm-level variability throughout longer production cycles and bay-wide dynamics on sediment contamination and bioaccumulation in sea cucumbers until harvest. Site-specific dynamics in ports, whether natural or anthropogenic, can be expected to influence bioaccumulation of metal contaminants and therefore require long-term and fine resolution monitoring for better representation in open-water IMTA production.