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Lost fishing gear is omnipresent in the seas worldwide. Yet the fractions of fishing-related litter such as nets, ropes, lines and pots differ among the amount of plastic litter observed in the marine environment. In the Northeast Atlantic region, Pham et al. (2014) find between 25 and 30% of plastic litter items on the seafloor and near the surface originating from fisheries. In a recent review, Galgani et al. (2015) report up to 89% of seafloor litter in the Atlantic Ocean to originate from the fishing sector. On the surface of the Great Pacific Gyre, Lebreton et al. (2018) identified 46% of plastic items being composed of nets, ropes and lines. Increasing fractions of beach litter items are composed of fisheries plastic waste when progressing north into the Arctic regions of Europe, with as much as 80% of beach plastic litter originating from fisheries on beaches around Spitsbergen, including heavy fishing nets, ropes, and buoys/fenders (Bergmann et al., 2017).

Even in small numbers, abandoned, lost or discarded fishing gear (ALDFG, UNEP/FAO definition: Macfadyen et al., 2009), commonly called “ghost gear”, can cause substantial harm through entanglement and ingestion (Kühn and van Franeker, 2020, and references therein, Werner et al., 2016). Between 2,000 and 12,000 tonnes of fishing gear waste are estimated to enter the European seas each year (Sherrington et al., 2016). The amount entering the Baltic Sea alone is not known, although Predki et al. (2011) estimated between 150 and 450 tonnes entering the Baltic each year from the more extensive fishing effort in the early 2000s. Globally, fishing gear causes entanglement of both commercial and endangered species, and is frequently reported in the media for large cetaceans, e.g., in the Mediterranean where both entanglement in and ingestion of ropes and netting is observed (Fossi et al., 2018). In their recent review, Kühn and van Franeker (2020) find that at least 354 marine species are impacted by entanglement, with 27.4% of seabird species, 39.8% of marine mammal species (71% when seals are considered alone), and all 7 marine turtle species. Although no scientific study was identified for the Baltic Sea, entanglement in ALDFG might affect harbour porpoises, grey and common seals. Stranded whales are occasionally found to contain bundles of netting or ropes in their stomachs, which might have prevented natural feeding activity (Jacobsen et al., 2010). From our observations, entanglement of species in ALDFG in the Baltic Sea is rare in comparison to the impact of active fishing gear, because lost trawl netting made from nylon is bundled up on the seafloor. The dominant source of ALDFG lost in German Baltic waters according to participating fishers today are gillnets, which are considered one of the most hazardous forms of ALDFG (Gilman et al., 2021; Global Ghost Gear Initiative, 2021). Seabirds such as two cormorants and two long-tailed ducks were found in retrieved gillnets in two different locations ( Figures 1A, B ), where one of the cormorants became entangled within less than 6 days between the ghost net discovery and retrieval. Both gillnets had been overgrown with algae and contained fish skelletons as well as fresh fish and birds, suggesting they had been trapping fauna in the sea for several months.

On sensitive seafloor habitats, smothering degrades the ecosystem. While this has not been investigated in the Baltic Sea, severe disturbance of benthic communities and biogenic reefs are observed in the Mediterranean and Asian coastal seas (Moschino et al., 2019; Kim et al., 2020). Over centuries (Thompson et al., 2004), ALDFG slowly degrades into microplastic fibres. These microplastic fibres are contained in sediments and the water column (e.g., Koelmans et al., 2017) and may be ingested by filter feeders and bottom-dwelling fauna. Microplastic fibres and particles in the marine food web are found to affect the smallest zooplankton down to a depth of 7000m (Eurythenes plasticus, Weston et al., 2020) to the largest filter feeders including the large whales in the Mediterranean and the Gulf of Mexico (e.g., Fossi et al., 2012; Fossi et al., 2014a; Fossi et al., 2014b). How much ALDFG contributes to the density of marine mircoplastics is not known. It is paramount to remove ALDFG where possible to mitigate these long-term impacts on the marine ecosystems. In the European Union, the Marine Strategy Framework Directive (MSFD 2008/56/EC) requires Member States to mitigate the impact of plastic litter on the marine environment. Political measures are devised for the Baltic Sea in the HELCOM (Baltic Marine Environment Protection Commission, https://helcom.fi) pressure group on marine litter (https://helcom.fi/action-areas/marine-litterand-noise/marine-litter) and for the North Sea by the OSPAR Convention for the Protection of the Marine Environment of the North-East Atlantic (https://www.ospar.org, https://www.ospar.org/work-areas/eiha/marine-litter).

In this pilot project, WWF Germany works alongside fisherfolk to mitigate the impacts of lost fishing gear on the Baltic Sea ecosystem. For fisherfolk, gear loss is an economic burden as well as a hazard to fishing grounds. Entanglement in gear lost during previous fishing sets multiplies this hazard, retrieval operations are costly, and catch in ALDFG is lost for commercial use (Brown and Macfadyen, 2007; Mouat et al., 2010; Newman et al., 2015; GESAMP, 2021 and references therein). As an inward sea, any litter entering the Baltic has no escape route. In the 1960-70s, the so-called “cod boom” led to an extensive trawler fishing fleet with a peak of 103 high-sees trawlers in Eastern Germany alone (http://www.rostocker-hochseefischerei.de/schiffe/schiffe.php). GPS positions of wrecks and other obstacles were not available at the time, and conflict between different fisheries can be assumed more common, and – with fishers still used to natural fibre materials - discarding of end-of-life nets before returning to port was not yet considered a problematic practice for the marine environment. Most of the 24 tonnes of ALDFG retrieved during this pilot project were historic netting recovered in the vicinity of Sassnitz harbour, which was one of the largest fishing ports of Eastern Germany. During a similar pilot project in 2015, WWF Poland retrieved 270 tonnes of trawl netting from offshore fishing grounds in Polish waters (WWF Poland, private communication). As ALDFG is one of the most harmful plastic litter for flora, fauna and habitats (Werner et al., 2016), WWF engaged in the development of a methodology that can lead to political implementation of lost gear mitigation measures through state authorities.

Globally, other initiatives such as the Global Ghost Gear Initiative (https://www.ghostgear.org), ghostdiving (https://www.ghostdiving.org), and many smaller, private organisations collect lost fishing gear from sensitive seafloor habitats worldwilde. One of the longest projects is carried out by the Northwest Straits Foundation in Puget Sound, USA, where since 2002, more than 5.800 nets and 6.000 crab pots were removed (https://nwstraitsfoundation.org/derelict-gear). This project utilises sonar technology developed by Fenn Enterprises since more than 25 years, which led to the collaboration for the method development in the German Baltic Sea detailed below. The longest-standing government-led project is organised by the Norwegian Fisheries Directorat since the mid 1980s, where fisherfolk are involved in the retrievals of deep-set gillnets and lobster pots in Norwegian fjords to conserve both the sensitive rocky habitats and the fishing grounds (https://www.fiskeridir.no/English/Fisheries/Marine-litter/Retrieval-of-lost-fishing-gear). In the Baltic Sea, the most consolidated initiative devising lost fishing gear mitigation measures so far was the MARELITT Baltic EU INTERREG project (2016-2019) with partners from four countries, Estonia, Germany, Poland and Sweden, in which WWF Germany was the partner on the German side (https://marelittbaltic.eu).

Since 2014, WWF Germany has developed a methodology to search for, retrieve and find a waste-management solution for lost fishing gear from the Baltic Sea. The pilot project was enabled by private-sector partnerships, the European Union Baltic Sea INTERREG programme, the German Federal Environment Agency, and other organisations (see Sec. 8 for details). From the beginning, WWF Germany was in close exchange with federal and state authorities to ensure a solution that can lead to longterm implementation. The project had several foci: 1) to ensure that mitigation activities reduce harm to the marine environment, 2) to engage local divers in the reporting of lost gear and encourage fisherfolk to participate in retrieval actions, and 3) to establish a method that can be used by state authorities for long-term mitigation of the impacts of ALDFG in the marine environment.

WWF Germany has developed environmentally sensitive methods to search for, retrieve and waste manage ALDFG from the Baltic Sea. Being too small to develop lunar tides, the Baltic Sea provides the ideal testing ground with diving times exclusively depending on water depth. In tidally dominated seas such as the North Sea, divers are limited to a narrow time window during the turning points of the tides to avoid the drag from tidal currents. As an inland sea only connected to the North Sea through the narrow straits of the Skagerrak and Kategatt, any pollution entering the Baltic is unlikely to escape into the North Sea or the wider Atlantic.

From the beginning of the project, fisherfolk, in particular trawlers, were employed for WWF retrieval activities and for search trials. Since March 2021, a pilot project is carried out with the support of the Environmental Ministry of Mecklenburg-Western Pomerania (MV). In this project, the key element is to employ some of the remaining fisherfolk to carry out search and retrieval activities at sea.

Search method: In 2018, WWF Germany adapted the sonar search technology for ALDFG developed by Fenn Enterprises and successfully applied by the Northwest Straits Foundation since more than 25 years in the Puget Sound (https://fennenterprises.com/projects). Towing a Marine Sonics ArcExplorer sonar fish with a transponder frequency of 600kHz as low as 5m above the seafloor at a speed of 3-4 knots, the obtained sonar spatial resolution of a few centimetres is sufficient to detect gillnet lines as thin as 1cm and other lines from trawl netting, as well as fish traps ( Figure 2 ). This frequency is outside the hearing range of marine mammels. The swath width of 100m allows us to cover a much larger area in a few hours than could be searched by diving teams. Within the state pilot project, gillnet vessels are employed for sonar excursions in coastal fishing areas. The knowledge of present-day and historic loss areas of local fisherfolk is essential for defining sonar search areas.

Verification: Positions are visually identified during post-processing data analysis and need to be verified by divers. ALDFG suspect GPS positions are published in the WWF Ghostdiver App for verification ( Figure 1C ). “WWF Ghostdiver” also provides a communication platform to warn recreational divers from the risks of retrieving ALDFG from the seafloor.

Retrievals: Trawl netting is found in the Baltic Sea in large bundles weighing 1-4 tonnes each. Retrievals need to be carried out with fishing or working vessels hosting strong winches or cranes with 2-4 tonnes capacity ( Figures 1D, H ). Gillnets and traps are removed from shallow coastal waters (depth < 30m) with scientific divers. In contrast to recreational diving organsiations, which carry out the bulk of ghost gear retrievals worldwide on a volunteer basis, WWF retrievals are carried out with professional diving teams and not with recreational divers for efficiency and health risk minimisation. Since the beginning of the state pilot project in Mecklenburg-Western Pomerania, small 8-12m gillnetting vessels are involved in the retrieval of gillnet fragments ( Figures 1G ). For retrievals of trawl netting, 17m trawlers carry out the lifting of the netting from the seafloor ( Figures 1E, F ). In the first project year, five fishing companies were engaged for search and retrieval activities activities at sea, which is increasing in each of the two following project years.

Waste management: Recycling was found not to be viable for ALDFG retrieved from the seafloor in the Baltic Sea (Stolte and Schneider, 2018, MARELITT Baltic). Heavy contamination with organic matter, sediments, hazardous lead from sink lines and mixed plastics are prohibiting material recovery. Dismantling and cleaning are cost-, labour- and energy-intensive processes which might cause damage to machinery (Stolte and Schneider, 2018). In Germany, incineration is the only pathway for mixed plastic waste, after lead lines and metals are extracted manually for metal recycling.

A map of all transects covered by the sonar survey is shown in Figure 3 , with detailed results given in Table 1 . A total of 326 suspect positions were identified in the German coastal state of Schleswig-Holstein (SH), of which 93 were verified until February 2022, and while 40 were ALDFG (success rate 43%), 53 were other objects or active nets (false suspect rate 57%, Table 1 ). In Mecklenburg-Western Pomerania, near Rügen Island, 83 of 223 sonar positions were verified with 54 ALDFG retrieved before December 2021. In a testbed area ( Figure 4 ) near Neustadt, SH, after 3 days of sonar charting, 22 of 49 sonar positions verified during 5 diving days were confirmed as ALDFG, a success rate of 45%. All 22 nets could be retrieved within 9 recovery days with scientific divers. When all sonar data verified in both states so far are considered, the total success rate is 52%. The sonar success rate has to be compared to the blind search approach commonly used by fisherfolk after loosing a net, where a search hook is dragged over the seafloor in the area where the presumed loss occured. In the case of a trawl net, this can be hundreds of metres from the actual snagging point. In addition, nets lost or discarded decades ago cannot be located in blind searches unless vast areas of seafloor are covered with ground-touching gear, which is ecologically not warranted (Sahlin and Tjensvoll, 2018). Diver searches, on the other hand, focus on wrecks or on submarine structures. The sonar technology fills the gap to cover extended areas and re-locate lost gear in regions where divers are not active. Hence, a success rate of more than 50% is an excellent result for this approach. The 100m sonar swath with a total area of 4425 ha covered in Schleswig-Holstein in coastal fishing areas and 1395 ha in Mecklenburg-Western Pomerania in 45 days at sea, implies that an average area of at least 130 ha per day could be searched. This is a lower limit as no area estimation is available for the 7 days in 2018.

In comparison, scientific divers can search a circumference around a single, expected lost gear GPS position, or carry out a scooter search along a strip. In a circular area around a single point, a one-hour scientific dive covers a radius of approximately 30m and a search area of 2827m². Rounding this to 3000m², a day search with a rotating scientific diving team of four divers and six dives yields an area coverage of 18.000m², less than 2 hectares or just 1.5% of the average daily sonar area. During a scooter search, divers cover extended swathes. Assuming a strip length of 1km and visibility of 3m as an upper limit in the Baltic summer months implying a strip width of 6m, a single dive might cover 4 strips or an area of 24.000m². If six dives can be achieved, a total area of 14.4 ha can be covered in a single day during scooter searches. or just 11% of the average search area of 130 ha/day with the sonar, rendering the sonar charting followed by exactly positioned verification dives the most efficient search methodology. Sonar searches require a smaller team of 2-3 crew, compared to 4-5 members in a scientific diving team, and cover substantially more area, yielding economic benefits. The sonar is operated for 4-5 hours during a typical sonar cruise, implying an efficiency of 4.5h/130 ha or 2 minutes per hectar compared to 25 min/ha for 6 diving hours with scooters. Charter cost depends on vessel type, small diving or gillnetting vessels operate at lower charter and fuel costs than working or larger fishing vessels. Assuming a smaller vessel cost of at most 1000 Euros/130 ha results in a cost efficiency of 8 Euros per hectar. This cost has to be compared to a full scientific diving team with a maximum area coverage of 14.4 ha/day with scooters with at least 3 divers and a skipper, where professional costs depend on country and region and in Germany, are typically between 2000 and 4000 Euros/day or at least 140 Euros per hectar. The sonar search turns out to be 12 times more efficient in time and 17 times more efficient in cost than the diver search, in addition to the larger area covered by sonar charting. Even with a success rate of 50%, the chances are much higher to detect lost gear in fishing areas where exact loss positions are not known.

The ArcExplorer sonar and examples of ghost gear detection images are shown in Figure 2 . The efficiency of the WWF methodology allowed 94 ALDFG to be retrieved from the Baltic seafloor during consolidated retrieval campaigns on accurate GPS positions. The 54 collected ALDFG in front of Rügen Island were comprised mainly of trawl netting, mixed with other forms of fisheries and marine litter, including metals, anchors, cables, tires, and gillnets ( Figures 1D, F, H ). While the focus of the state pilot project lies on retrieval with 9-17m class fishing vessels ( Figures 1E-G ), working vessels with heavy lifting cranes have to be employed for large trawl nets ( Figure 1H ). The total wet weight collected during the pilot project from 2014 to 2021 added to at least 24 tonnes. In Schleswig-Holstein, sonar searches were focussing on the coastal fisheries areas operating predominantly gillnets and traps. These smaller and lower-weight items were retrieved from the seafloor by scientific diving teams. While total weight estimates are not available for ALDFG collected in this coastal state, more than 2000m of gillnet fragments could be retrieved.

The methodology to search for and retrieve lost fishing gear from the Baltic Sea was developed to present a concept to state authorities, such as the Federal Environment Agency and the ministries of the German coastal states. In the European Union, all member states are required to achieve “good environmental status” under the Marine Strategy Framework Directive (MSFD 2008/56/EC). Cleaning actions to improve seafloor habitats and remove plastic litter as a longterm hazard to marine species are explicit measures implemented in Germany under the MSFD (BMUB, 2016, Annex 1, p. 22). The Baltic Sea, in particular, is a marine environment under severe multiple stressors: temperature inrease as a direct consequence of climate change invokes oxygen-depleted zones potentially affecting fish nursery grounds (see Meier et al., 2022 for an in-depth review), enhanced by severe eutrophication from intensive agriculture causing algae blooms (Löptien and Dietze, 2022; Meier et al., 2022), contamination with toxins from ammunitions and other historic contaminants from at-sea disposal (Vanninen et al., 2020). The high density of shipping routes adds to the pressure on the ecosystem along with decades of intensive fishing without ecological consideration. Relieving the seafloor from lost fishing gear is a comparably low-cost measure to improve seafloor habitats without negative impact for any of the economic sectors, while providing benefits for the fishing sector. Incorporating the fishing sector for mitigation measures has the added benefit that fisheries contribute to the mitigation of longterm negative impacts caused by this industry. In Mecklenburg-Western Pomerania, a state-funded pilot project is already implemented with the aim to evaluate options for regular retrieval operations with fishing vessels. This community case study provides the foundation for the state projects outlined below and the insights necessary for its evalulation.

Despite the success in detecting and retrieving substantial amounts of trawl netting and gillnets, several challenges remain in the presented method.

The datasets presented in this article are not readily available because the sonar data employed in this article are proprietary to WWF Germany and not publicly available. Requests to access the datasets should be directed to andrea.stolte@wwf.de.

AS: lead project manager, coordination of sonar excursions, retrieval campaigns, waste management solutions, data analysis. GD: co-project manager, lead WWF Ghostdiver app development, coordination of diving activities, sonar and retrieval campaigns. JL: project initiator and project leader 2014-2020, political implementation and initiation of state pilot projects. ML: coordinator WWF Ghostdiver app and data management. MG: development of analysis script for sonar data positions and first data base. CF: sonar trainer and long-term sonar search for ALDFG expert with Northwest Straits Foundation. WF: sonar driver and chief sonar analyst, verification diver and support of retrieval campaigns. CH: sonar driver and scientific diver including sonar data analysis and gear retrieval campaigns. HV: head of WWF international marine centre, funding and overarching project support. SW: lead scientist German Federal Environment Agency on marine litter and ALDFG, expert harm of plastic litter in the marine environment, scientific verification diver.

WWF Germany is sincerely grateful for unabated support by PreZero Lizenz GmbH (formerly Karl Tönsmeier Entsorgungswirtschaft GmbH & Co. KG) throughout the project lifetime since 2015. Without this long-term stability of base funding, this pilot project would not have been possible. The sonar searches were supported by the German Federal Environment Agency, who also supported the pilot excursion into the North Sea, through a Ministry for the Environment non-governmental organisations grant (FKZ 373820145), as well as several German Postcode Lotterie project grants (FV-1619, FV-4067), both of which also supported some of the retrieval activities. GD, ML and AS acknowledge support by NUE/BINGO Lottery for the development of the Ghostdiver App prototype and by the Federal Ministry for the Environment through the European Environment Initiative EURENI for WWF Ghostdiver’s final development and internationalisation (EURENI_21_D_020). The state pilot project in Mecklenburg-Western Pomerania is supported by the Ministry for Agriculture, Fisheries and the Environment of Mecklenburg-Western Pomerania through a State Promotion Institute grant (LFI-LM-FA-01-21). The authors are grateful to the German Edeka Corporation and all private donors for their ongoing support throughout this project. This study received funding from PreZeroLizenz GmbH, formerly Karl TönsmeierEntsorgungswirtschaft GmbH & Co., and the Edeka Corporation. The funders were not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.

This project would not be possible without the support of a substantial number of diving and sonar teams and local fisherfolk.This work would not have been possible without the intensive contributions of Wolfgang Frank, our main sonar driver and interpreter-cum-diver, Christian Howe, Philipp Schubert, Florian Hubert, Uli Kunz and all connected divers of the scientific diving team submaris, professional divers Jochen Firzlaff and Olaf Döhler, Stefanie Werner of the Federal Environment Agency for her support in the verification diving teams and all matters around marine litter, and the TSCW Warnemünde, Uli Baumhör, Ingo Oppelt, and all recreational divers carrying out verification dives. We are sincerely grateful to all fisherfolk involved in the project, including Karl-Heinz Neumann and Günther Baltsch, the fisheries associations Freest and Greifswald-Wieck, Sebastian Erler and his crew on vessel “Crampas”, Lars and Jörg Engels on trawler “Bergen”, Martin Saager and Thomas Peters for the very first fishing vessel sonar trials, and all fishers that will be engaged in the state-funded projects in the near future for their motivation, participation and patience. Our sincere thanks to Hannah Siegesmund for her intensive data work that enabled the build-up of the first sonar data base. For support during recycling trials and finding solutions for waste management, we are grateful to Dr. Michael Krüger’s and Falk Schneider’s ongoing enthusiasm and contributions. We also sincerely wish to thank the referees for their intense effort and detailed comments provided, which substantially helped improve the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.