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

https://doi.org/10.1016/j.scitotenv.2019.01.363Get rights and content

Highlights

  • Presence and impact of derelict fishing gear were studied in North Adriatic outcrops.

  • The occurrence of derelict fishing gear was evidenced in half of the monitored areas.

  • A total of 362 items were removed by scuba divers.

  • The complexity of macro-benthic community gradually improved in the cleared areas.

  • Cryptic fish species occupied cracks previously obstructed by derelict fishing gear.

Abstract

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.

Introduction

Abandoned, lost or otherwise discarded fishing gear (ALDFG) is recognized as a problem of global concern due to the expansion of fishing effort and grounds and the introduction of non-biodegradable fishing gear, made primarily of plastics (Macfadyen et al., 2009). ALDFG has serious ecological impacts to the marine environment as well as to the socio-economic context (Gilman, 2015), endangering the health and structure of marine habitats, covering large portion of substrates and benthic communities, and depriving the seabed of surfaces ideal for survival and reproduction. Ghost fishing, occurring when ALDFG continues to indiscriminately catch organisms, causes losses not only to commercial fish stock but also to non-target species, including endangered and protected species such as turtles, seals and cetaceans (Donohue et al., 2001; Good et al., 2010; Stelfox et al., 2016; Wilcox et al., 2015). It is estimated that, in some fisheries, ghost fishing removes up to 30% of commercial species catches, consequently reducing economic opportunities for the marine capture sector (Gilman et al., 2016). Ghost nets and gear may be dragged by sea currents for long distances, thus contributing to the transport of potential alien species, which are a major threat to the biodiversity of marine ecosystems (Gilman, 2015).

At global level, an estimated 640,000 tons of fishing gear is lost, abandoned or discarded in our oceans each year (Macfadyen et al., 2009), but their amount can greatly vary among regional areas depending on the importance of fishing activities at local level and on specific hydrological and geomorphological conditions (Pham et al., 2014).

Fishing represents an important economic resource for the Veneto Region (North Adriatic, Italy). The high productivity and richness in fishery resources makes the Northern Adriatic Sea the most exploited area of the Mediterranean Sea (Mazzoldi et al., 2014). The fleet, according to the 2017 EU Fleet Register, was composed of 662 vessels provided with a range of fishing devices, both active and passive, such as bottom trawl, mid-water trawl, hydraulic dredges, gill and trammel nets, and traps (Veneto Agricoltura, 2017; Pranovi et al., 2015). Moreover, the region hosts a number of mussel farms, accounting for nearly the 33% of the national production (Prioli, pers. comm.).

The seabed of the North-Western Adriatic Sea is predominantly sandy and muddy, sometimes interrupted by solid substrates creating areas abounding in ecological gradients and microenvironments which increase the biodiversity of the benthic community and fish fauna. The so-called tegnùe, a dialectical term meaning “holding back” as fishermen had noticed that their nets remained often entangled and lost within these structures, are submerged rocky outcrops of partial biogenic origin located along the coastal area stretching from the mouth of the river Brenta to the Grado lagoon. Their surface area varies greatly, from few m2 to some km2, raising up from the sea-bottom few decimetres up to few meters (Stefanon and Boldrin, 1979; Mizzan, 1995). The localization of these rocky outcrops in eutrophic and shallow waters promotes the growth and expansion of animal and plant assemblages rich in species and biomass, supported also by the vertical gradient determined by the elevation from the sandy bottom, and by the abundance of suspended organic matter and sediments as food sources (ARPAV, 2010). The ecological, aesthetic and economic value of these environments makes them unique and extraordinary, but also extremely threatened by human activities (Tonin, 2018). The attractive effect of these rocky outcrops combined to the high productivity of the Northern Adriatic basin leads to a very significant inflow of fish species, which are often of commercial interest (Tosi et al., 2017). The richness in species of these areas attracts a large number of professional and recreational fishermen, as well as divers who are particularly interested in the rich local biodiversity.

The general aim of this research was to assess presence and impacts of ghost nets and other lost or discarded fishing gear on the hard substrate ecosystem. In particular, a step-by-step procedure was set up and applied to: 1) identify the most probably impacted outcrops by the presence of ALDFG; 2) verify the presence and quantify ALDFG through mapping activities in the identified outcrops; 3) remove the ALDFG following the most appropriate operative procedures; 4) ex-post assess of the recovery capability of benthic and fish communities in areas freed from the physical impact of gear.

Section snippets

Identification of the most affected rocky outcrops and mapping of abandoned and lost fishing gear

The study was carried out in a marine area covering about 500 km2 off the Veneto coast (Northern Adriatic Sea, Italy; see Fig. S1). A step-by-step approach was applied to identify the most probably impacted areas by the presence of ALDFG. In this phase, not only rocky outcrops but also two underwater artificial hard substrates were taken into account as possible impacted areas. The Veneto coastal area is characterized by various artificial structures, which, similarly to natural outcrops, have

Identification of the most affected rocky outcrops, ALDFG mapping and removing

The survey performed in collaboration with Veneto fishing cooperatives and divers and through the detailed analysis of the busiest routes of fishing boats, allowed to identify 49 outcrops/artificial reefs for a total area of 30 km2, showing probability of ALDFG occurrence ranging from “medium” to “very high”. Sixteen among these were selected as the most probably impacted areas, i.e. 15 rocky outcrops and one artificial reef (covering a total submerged area of about 21 km2, Table 1). Their

Conclusions

The present study shows that natural as well as artificial hard substrates located along the Veneto coastline are strongly affected by the presence of derelict fishing gear. According to the results of the mapping and removing activities (i.e. about 350 pieces of gear and other fishing waste in 8 natural rocky outcrops and in the artificial reef) and taking into account that at least 3000 distinct rocky outcrops are estimated to outcrop in the Gulf of Venice (Gordini et al., 2010), it can be

Acknowledgments

Blue Box data were courtesy of the Venice Port Authority through the General Command of Port Authorities, in particular Captain P. Preziosi. We also wish to thank the Scuba Diving unit of the Italian National Police, Venice team, for their support in all the underwater activities, and Dr. Mauro Bastianini for providing temperature data from the Acqua Alta oceanographic platform.

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