Real-world complexities of nearshore integrated multi-trophic aquaculture in crowded coastal environments

The expansion of coastal aquaculture is challenged by a lack of coastal space and competition from maritime activities. In the Mediterranean Sea, commercial aquaculture in a multiple-use port in Marsaxlokk Bay, Malta, encounters complex natural and anthropogenic interactions. Intermittent water currents observed near the fish cages were not explained by atmospheric forcing but corresponded to vessel movements within the vicinity. At farm-level, cage management practices such as cohort dynamics and cage movements also influenced waste deposition around fish cages. The farm-scale model, Cage Aquaculture Particulate Output and Transport (CAPOT) was used to estimate waste distribution from the fish farm between October 2018 and July 2019 from the production of multiple species from different cohorts and cage arrangements. CAPOT then informed the placement of the sea cucumbers Holothuria (Roweothuria) poli, in an integrated multi-trophic aquaculture (IMTA) system. Between October 2018 and September 2019, H. poli was cultured directly below a fish cage at 0 m (E0), at 10 m (E10) and 25 m (E25) distances from a commercial fish cage, and at two reference sites (R1 and R2) over 800 m from the fish farm. Sea cucumbers grew better near the fish cages but mass mortalities were recorded at E0 within the first month of the study and poor survival rates were recorded at all sites. Still, their ability to recycle aquaculture organic wastes was validated using carbon (δ13C) and nitrogen (δ15N) isotope ratios and fatty acids in February, May and September of 2019. Isotopic analysis indicated that sea cucumbers at IMTA sites primarily relied on fish farm organic waste as their primary dietary source. This was complemented by a higher abundance of individual fatty acids, oleic (18:1n-9), linoleic (18:2n-6) and eicosenoic (20:1n-9) acids at E10 and E25, presumably linked to the terrestrial plant oil content of fish feeds, and marine-based fatty acids dominant in sea cucumber tissue at the reference sites. The full complexity of bay-wide processes and farm-level practices should be represented for effective management of wastes and maximised use of space in multiple-use environments. Detailed input data and finer scale modelling approaches are required to represent changes in waste deposition and food resources for sea cucumbers in IMTA. Additionally, the impact of temporal variation in food availability and quality in sediment, and the bioaccumulation of substances like mercury and arsenic that bioaccumulated in sea cucumbers under fish cages should be considered. Overall, a comprehensive representation of these factors is essential for effective waste management and sustainable aquaculture practices in multiple-use coastal areas.