Abstract
The use of detritivores under sea farms is a promising avenue to mitigate the benthic impacts of marine fish farms. Sea cucumbers are interesting candidates for integrated multi-trophic aquaculture (IMTA) due to their prevalence in the marine environment, their diversified diet and their economic value. Yet limited information is available regarding their capacities to be stocked and reared underneath aquaculture cages and the associated effects on their survival, growth rate and body composition. This study focused on Holothuria tubulosa, a Mediterranean sea cucumber species candidate for rearing in the vicinity to marine fish cages. We investigated its potential for co-culture on the seabed more or less influenced by marine fish cages. The farm's waste footprint was predicted using a dispersion model (NewDEPOMOD) to estimate the farm's influence along a transect where we also sampled sediment at four distances from the cages (0 m, 25 m, 100 m from the cages, plus a reference site at 150 m). Organic composition of the sediment was analysed (TOC, TON, TOP, OM, stable isotope signature) and linked to the results from the dispersion model. Based on the model simulation, the maximum flux of matter reached almost 17 kg solids.m−2.year−1 below the cages, and gradually decreased with distance from the cages. An isotopic gradient was also found in the sediments according to the distance from the farm, with an enrichment in δN15 and a depletion in δC13 with increasing proximity to the farm. In parallel we investigated the response of adult sea cucumbers placed at varying distances from the fish cages for a period of one month, measuring their proximate composition, isotopic concentration, and fatty acid and protein composition. We found that despite good survival, growth was null over the experiment. While the isotope signature of the sea cucumbers was significantly affected by distance from the cage, this did not follow the pattern found in sediment. There was a clear difference in fatty acid composition between sites, with sea cucumbers closer to the cages having lower levels of short-chain fatty acids. The protein content was also lower in sea cucumbers reared right below the cages. These results suggest that while adult H. tubulosa can survive the environmental conditions below marine aquaculture cages, they do not nutritionally benefit from fish waste over short periods in the stocking conditions we tested. Their use in IMTA requires further investigation to find optimal stocking conditions.