Introduction
Atlantic bluefin tuna farming is associated with local organic enrichment of sediments, shifts in redox conditions and degradation of benthic communities, particularly beneath and around net-pen structures. Although environmental regulations increasingly require evidence of habitat recovery after farm cessation, the trajectory and timescale of benthic recovery under different hydrodynamic and depositional settings remain poorly resolved. This study aimed to characterise the spatial footprint of tuna farming and quantify benthic recovery using a combination of basic geochemical indicators and eDNA-based biotic indices at an active farm, two abandoned tuna farm sites and a reference site in the central Adriatic Sea.
Materials and Methods
One active tuna farm, two abandoned sites at different recovery stages (10 and 15 years after cage removal) and one reference location unaffected by aquaculture were sampled. At each site, nine stations were established along a 900 m radial transect, covering a gradient from directly beneath the former or current cages to off-farm reference areas beyond detectable organic enrichment. Surface sediments were analysed for total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP) and redox potential (Eh) as key indicators of organic loading and reduced conditions. In parallel, eDNA metabarcoding was applied to sediment samples to generate MOTU matrices, from which taxonomic richness and gAMBI-like biotic signals, a genetic derivation of the classical AMBI index, were derived. Spatial patterns in geochemical variables and eDNA-based indices were evaluated in relation to distance from cages and site type.
Results and Discussion
At the active farm, sediments beneath cages showed elevated TOC, TN and TP, strongly reduced Eh and benthic communities dominated by opportunistic polychaete taxa, including Capitella, Prionospio, Neanthes and Mediomastus, consistent with heavily impacted softbottom habitats. High gAMBI values confirmed severe ecological disturbance and demonstrated that the eDNAbased index effectively captured the footprint of intense organic enrichment. The 10year fallowing site showed partial recovery, with reduced TP and improved redox conditions but still elevated organic matter relative to the reference site. At the 15year site, TOC, TN, TP and Eh largely converged towards reference levels, and eDNAbased indices indicated advanced biological recovery approaching the undisturbed status of the control location. Across all sites, gAMBI derived from eDNA data closely tracked the gradient of organic enrichment and clearly separated active, recovering and reference conditions. The combined use of TOC, TN, TP, Eh and eDNA metabarcoding thus provides a practical, sensitive framework to map tuna farm impacts and quantify benthic recovery after production stops. The progression from strongly impacted sediments at the active farm to nearreference conditions at the longfallowed site highlights both the recovery potential of benthic habitats and the importance of sitespecific conditions and farm history in determining recovery timescales.
Acknowledgment
This research was fully funded by the Croatian Science Foundation under project HRZZIP2022107232 "Enhancing Environmental Performance of NetPen Marine Aquaculture"