Is derelict fishing gear impacting the biodiversity of the Northern Adriatic Sea? An answer from unique biogenic reefs

Countless submerged rocky formations are scattered in the marine coastal area of the Veneto region (Northern Adriatic Sea, Italy). They are known to be suitable substrates for supporting the development of highly complex fish and zoobenthic communities. However, these areas are threatened by local fishing, due to their peculiar morphology and topography which promote gear entanglement. With the aims of quantifying the impacts of abandoned, lost or otherwise discarded fishing gear (ALDFG) on these rocky habitats and monitoring the biodiversity recovery after ALDFG removal, acoustic and diving surveys were performed to map 15 selected rocky outcrops and one artificial reef (for a total surface area of 21 km2). The recovery capability of benthic and fish communities was evaluated by non-destructive techniques in 5 outcrops periodically sampled throughout 18 months. The data collected (number of individuals, species, taxa, and coverage per each species and taxon) were used to calculate three biodiversity indices (species richness, Margalef and Shannon indices) and to assess community recovery. Mapping activities verified the occurrence of 347 ALDFG distributed unevenly over half of the investigated rocky outcrops. The following cleaning operations led to the removal of 362 items, for a total dry weight of 514 kg. The study on biodiversity recovery showed that the complexity of benthic communities gradually improved. In particular, the various biodiversity indices evaluated at the end of the monitoring period generally showed values comparable, or even higher, than those recorded in the reference sites. An increase in biodiversity and species abundance was also observed for fish community; in particular, cryptic species were able to occupy cracks and fractures of rocks previously obstructed by the derelict fishing gear.