Report on assessing trawling impact in regional seas

Type Report
Date 2014
Language English
Ref. Benthis - Deliverable 7.6
Other localization
Author(s) Nielsen J. Rasmus4, Bastardie Francois4, Buhl-Mortensen Lene5, Eigaard Ole4, Gümüs Aysun7, Hintzen Niels T.1, Kavadas Stefanos6, Laffargue PascalORCID3, Mehault Sonia3, Notti Emilio9, Papadoupoulou Nadia6, Polet Hans2, Reid David8, Rijnsdorp Adriaan D., Rochet Marie-Joelle3, Robert Alexandre3, Sala Antonello2, Smith Chris6, Virgili Massimo2, Zengin Mustafa7
Affiliation(s) 1 : IMARES, Wageningen UR, PO Box 68, 1970 AB IJmuiden, The Netherlands
2 : Institute for Agricultural and Fisheries Research, Animal Sciences Unit—Fisheries and Aquatic Production, Ankerstraat 1, 8400 Oostende, Belgium
3 : IFREMER, Unité Ecologie et Modèles pour l'Halieutique,44000 Nantes, France
4 : National Institute for Aquatic Resources, Technical University of Denmark, Charlottenlund Castle, 2920 Charlottenlund, Denmark
5 : Institute of Marine Research, PO Box 1870, 5817 Bergen, Norway
6 : Hellenic Centre for Marine Research, PO Box 2214, 71003 Heraklion, Crete, Greece
7 : Central Fisheries Research Institute, Kasu¨stu¨, Trabzon 61100, Turkey
8 : Marine Institute, Galway, Ireland
Version Version 4
Note Grant Agreement number: 312088 Project acronym: BENTHIS Project title: Benthic Ecosystem Fisheries Impact Study Funding Scheme: Collaborative project Project coordination: IMARES, IJmuiden, the Netherlands Project website:
Abstract Baltic Sea
Benthic ecosystem impacts from demersal fishery in the western Baltic is assumed to come mainly from Nephrops trawling in the central and southern Kattegat, mussel dredging in the Belt Sea, and mixed cod trawling in the western Baltic Sea.These fisheries both impact the seabed, as well as produce substantial amounts of discards. The western Baltic Sea offer a unique opportunity to analyse the benthic effects of fishingthanks to the closure of Øresund to towed gears since the 1920s, and the introduction of the Kattegat MPA in2009 (including cod closure) and the western Baltic Sea Natura-2000 areas. Furthermore an extensive benthic nationalmonitoring and data collection effort has taken place in the area over a number of years, which can be usedto investigate spatial and temporal differences in the development of benthic communities in (chronically) towed and non-towed areas.
To evaluate benthic impacts of towed gears in the Baltic Sea a number of gear technological innovations to reduce effort, benthic contact of fishing gears, and discarding are evaluated. There are conducted experimental fisheries in relation to evaluation and comparison of direct ecosystem and habitat impacts, catch efficiency (target/by-catch/discard/invertebrates), selectivity (discard), energy efficiency, and economic efficiency (vessel specific cost-efficiency/cost-benefit analyses) of different gear modifications compared to standard gears. Furthermore, the case study evaluates potential fishing closures directed towards sensitive benthic habitats and communities.
A review of known regional benthic impacts of fishery with demersal towed gears in the area is evaluated with focus on the mussel dredging, demersal cod trawl fishery and Nephrops trawl fishery, where latter is also compared to Nephrops creel fishery. Subtidal dredging and benthic trawling in the area has been reported to affect the benthic fauna and flora as well as to change the structure of the sea bed. The impact of towed gears may also reduce substrate complexity owing to by-catch and movement of shells and stones. Among other this has been demonstrated to locally reduce survival of juvenile blue mussels as well as the population structure of sessile epibenthic organisms. Benthic towing also potentially impact on water transparency either directly owing to resuspension of sediment or as a result of a reduction in the filtering biomasser. Resuspension is induced at the bottom and in the water column during towing and at the surface when by-catch of sediment is released when heaving/washing the catch. Besides reducing transparency, resuspension of sediment has been found to increase levels of ammonia and silicate in the water column and to reduce the oxygen content.
The innovations evaluated comprise: A) Lighter mussel dredges with less benthic impact; B) Smart mussel and cod fishery with previous acoustic or video monitoring or test fishery on the resources before fishery to optimize the catch and to reduce total effort as well as effort on more sensitive habitats resulting in reduced benthic impact; C) Semi-pelagic doors and alternative gear riggings to lift the doors off the seafloor in the western Baltic trawl mixed cod trawl fishery.For species like cod and plaice, which are herded by the sweeps/bridles, an off-bottom door rigging where these other gear components are on the bottom, may be a solution to maintain catchability and eliminate the seabed impact of the doors. The technical challenge with such rigging is to keep the trawl door distance above bottom nearly constant. D) Another focus is scenario evaluation of different effort (re-) allocation schemes with respect to benthic impacts and catch efficiency of W. Baltic mixed trawl fishery by reduction of overall effort or effort in more sensitive habitats through potential closures. The evaluation use the DISPLACE spatial explicit bio-ecomic model (Bastardie et al. 2013; 2014).E) Use short sweep lengths to reduce benthic impacts in the Nephrops trawl fishery (standard of 74 m compared to 5-10 m sweeps). Shorter sweeps are not expected to change the selectivity and catch of Nephrops, because Nephrops are not herded by the sweeps, while fish by-catch and discard is expected to be reduced because of less herding. Benthic impacts of demersal trawl fishing in an open fishing area, a short-term closed area, and a long term closed area in the Kattegat and the Sound is investigated. F) Evaluate benthic impacts of creels to be compared to benthic impacts of the Nephrops-fish mixed trawl fishery through overlapping fishery, creel fishery on soft bottom, and test attachment points of creels and inside creel shelters. The comparative analysis of all the above points of changed fishing methods with less benthic impact will involve comparative analyses of catchefficiency, benthic impacts, cost benefit (CBA), energy efficiency, and by-catch and discard.
Pilot investigations on use of a light mussel dredge indicate that: a) the weight of sediment retained and re-reduction in energy transfer to the sediment, c) catch efficiency tend to increase – reducing effort and area of impact and reducing fuel consumption - and accordingly increasing economic efficiency, and d) Sea floor tracks made by the two dredges could be distinguished by use of a side-scan sonar and the tracks were still detectable two months after fishing. Pilot investigations in relation to use of semi-pelagic trawl doors indicate similar catch rates between door settings, however, there are area and seasonal differences in the catch efficiency when using pelagic doors, possibly due to substrate or seasonal related behavior differences of cod in reaction to the gears. Pilot investigations with evaluation of mixed trawl fishery impact on W. Baltic and Kattegat sensitive habitats with respect to effort pressure is presented here and will be made finally reported under WP2 when the actual impact of the specific gear is known. Pilot investigations of bio-diversity and presence/absence of species from benthic sampling in open and short term closed trawl fishing areas in Kattegat is ongoing. Pilot investigations with creels in (Frandsen et al. 2013b) indicate that i) the creels sank very much down into the soft sediment (camera monitoring); ii) the creels were not directly lifted off the bottom but were dragged for several minutes making a footage (camera monitoring); iii) the bait attracted Hagfish (Myxine spp.) which scared the Nephrops in the creels => some escapement; iv) all by-catch thrown overboard immediately went to the bottom, and no predation from sea birds was observed, and there were observed no visible deviations in the cod's behaviour when they swam to the bottom; v) the catch rates were about 180 g/creel per day; vi) CBA indicate a daily profit about 3800 DKK per day which is comparable to trawl fishery for trawlers < 12 m with profit about 3050 DKK per day, however, larger trawlers have higher profit.
North Sea
The North Sea, a marginal sea of the Atlantic on the European continental shelf, is intensively used as an important European shipping lane, for fishery, for recreation and tourism and as a rich source of energy resources including fossil fuels, wind, and early efforts in wave power. A wide mix of fishing methods is being used in the North Sea ranging from active gear like beam trawls, otter trawls, twinrigs, dredges and rope seines (flyshooting) and passive gear like set nets, pots and lines. In terms of swept area, otter trawls are the most significant gears accounting for some 2/3 of the total. Beam trawls only account for 13 % of the swept area although they are economically the most important group. The average discard rate over all gears is some 40 %, based on weight and the small meshed beam trawls show the highest discard rates. Each of the fisheries in the North Sea has its typical geographical distribution, with in general otter trawls operating rather north and beam trawls rather south.
In the North Sea, a total of 38 habitat types occur although a few mainly soft sediment habitats dominate. The surface area is dominated by sublittoral sand, coarse sediment and mud. Bottom trawling mainly occurs in the soft bottom habitats that dominate the North Sea.There is a clear difference in preference for particular habitat types across métiers, although each métier is not restricted to one or two specific habitat types. The deep hard sediments are ‘relatively’ being fished most intensely, followed by sublitoral mud. In contrast, sublitoral sand that has by far the highest surface and swept area in absolute numbers has the lowest preference score.Fishing intensity has an uneven distribution within each habitat. Some parts of the habitat are trawled intensively while other parts are trawled lightly or are not trawled at all. The proportion of habitat fished more than once a year is less than 15%.
For the Benthis North Sea case study, the beam trawl has been selected as a focus and is deployed as different types of beam trawls. These types are defined by their target species (mainly shrimp and sole) and by the type of stimulation in the netopening, which is determining for the benthic impact. As such, a bobbin rope can be the only stimulus and this is usually used to target shrimps ; tickler chains or a chain mat can be rigged into the netopening and flatfish are the target ; or an electric pulse field can be rigged for targeting sole or shrimps. A preliminary analysis suggests that compared to traditional beam trawls, the pulse trawlers differ in the choice of fishing grounds.
Western waters
Fishing activity spreads out over large areas of the European western waters and some trawling fisheries operate for more than one century on those continental shelves. They represent a large variety of metiers utilizing various types of fishing gears (dredges, beam trawl, otter trawl, Danish and Scottish seines). From the coast to the shelf break, they operate in various benthic habitats, generating impacts on benthic community as well as on habitats themselves. The severity of impacts is a function of fishing spreading out and level, covered habitats and operated fishing gears.
The Nephrops fishery in the Bay of Biscay (BoB) mainly operates twin-trawl gears adopted from 1985 onward. Nephrops reaches the third and second rank in terms of biomass and value respectively for the exclusive trawlers of the French Atlantic coast. Fishing activity for Nephrops concentrates on soft sediment habitats (muddy-sands) of the specific "Grande Vasière" area. Despite a fishing effort reduction's trend and adoption of various selectivity devices, BoB trawlers still generate a high rate of discards (50% of their catches), impose a significant level of physical disturbance to the bottom (trawl-induced fine particle remobilization = 10 to 30 percent of the storm-induced erosion) and generate additional mortality to associated benthic species.
The scallop dredge fishery started along the Irish coast in the 1970s as a small scale inshore fishery and expanded into offshore waters during the 1990s. Others towed gears, otter trawling and beam trawling, occur in the same area. When total effort of dredgers is increasing, bottom trawling is stable and beam trawl activity is declining. Dredgers utilize series of toothed spring loaded dredges suspended on a beam whose generic effects on benthic habitats are well known and unequivocal. The fishery operates on sedimentary sand, gravel habitats and cobble/kelp reef areas. Despite a number of Special Areas of Conservation (SACs), the scallop fishery encroaches onto these areas and the impact of the fishery may be inconsisten with the conservation objectives. Dredge design modifications and/or encouraging scallop fishery to concentrate fishing effort on the most productive/less sensitive areas will help to mitigate the effects of the fishery on benthic habitats.
From the Bay of Biscay to the Norway shelf, the list of vulnerable marine ecosystems (VME) includes as diverse habitats as cold water corals (CWC), sponges' grounds or pennatulaceans communities. As compared to others exploited habitats, fisheries interacting with VME's can generate immediate and long lasting impacts, even at relatively low effort level. In such complex and fragile habitats (e.g. CWC), generation of impacts comes not only from trawling activity but from metiers utilizing passive gears too (e.g. demersal longline, gill nets, pots). Only strong spatial regulations could help to protect those habitats from severe unwanted and uncontrolled fishing damages.
The western waters case studies proposed a mix of strategies that involve the development of less impacting gears (e.g. modified trawls or alternative metiers) and new management options including spatio-temporal rules. That combination will help to reach fisheries sustainability and good environmental status as considered in Habitats Directive and the Marine Strategy Framework Directive (MSFD).
In the Mediterranean highly impacting bottom fishing (trawling, dredging, etc.) mainly affects shelf areas, where seabed surfaces are mainly muddy and sandy. Although these fishing grounds are very suitable to trawling fisheries, they represent an important part of the ecosystem since they are inhabited by a wide variety of benthic organisms.Scientific studies carried out in the Adriatic Sea showed that hydraulic dredging causes the destabilization and partial modification of sediment conditions, decreasing the habitat complexity and leading to fluctuations within benthic communities (Brambati and Fontolan 1990). In particular the macrobenthic community appears to be altered by intense hydraulic dredging activity (Morello et al.,2005).Rapido trawling has one of the most severe impacts onbenthos, both because it captures epifaunal and infaunal componentsand also because of the high direct and delayed mortalityassociated with contact with this gear (Giovanardi et al.,1998). Asingle Rapido tow may induce changes in the structure ofbenthic infauna that last up to 9 months before completerecovery takes place (Pranovi et al., 2000).Owing to their different catching principles, otter trawls are likely to have a different physicalimpact on the seabed from those of the Rapido trawls anddredges (Lucchetti and Sala, 2012).The most evident physicaleffects of trawlingare produced by the otterboards, while other parts have not a significant impact (Lucchetti and Sala, 2012). Eastern Mediterranean studies on otter trawling impact (Smith et al., 2000) showed that trawlingcaused functional change on the macrofaunal and megafaunal community structure,where sessile or discretely mobile filter-feeding organismswere replaced by mobile scavengers and opportunists, changes to physico-chemical properties and chemical fluxes (Smith et al., 2003, 2005, 2007). Bottom trawl is also believed to have contributed to the disappearance of sea-grassbeds in the northern Adriatic Sea, where meadows that werepresent at the beginning of the 20th century have almostdisappeared (Zavodnik and Jaklin 1990). Therefore, for a substantial reduction of bottom impact, some additional measures related to type of gear are needed, as well as avoidance of certain sensitive habitats (e.g. Posidonia, maerl). A reduction of fishing effort by reducing dredge fleet or the number of Rapidos on board, or introducing pelagic otterboards may lead to a reduction of physical impact on the seabed and a better protection of benthic habitats.
Black Sea Case studies of effects of beam trawl fisheries on the coastal benthic ecosystem by CFRI (Central Fisheries Research Institute) and OMU (Ondokuz Mayıs University) conducted a series of sampling study in order to reveal the impact of bottom and beam trawl gears on benthic and demersal macro fauna along in SSA (Samsun Shelf Area) in 2013 and 2014. The major targets of these field study for the Black Sea Turkish coasts can be outlined as; (1) to define and draw out the structural and technical properties of beam trawls (algarna) conventionally used for rapa whelk fisheries, (2) to estimate the catch per unit effort in beam trawl fishery in SSA, (3) to reveal the species composition of benthic and benthopelagic macro fauna (invertebrates and fishes) besides of the target species; rapa whelk. (4) The monthly variation of bycatch in beam trawl fishery. In experimental design, the samplings were made by a pair of algarna in the same vessel which is equipped with two kinds of gears (commercial gear with the mesh size of 72 mm and the blind gear with the mesh size of 12 mm) to find out the species composition of benthic and macrobenthic fauna and its monthly variation. (5) The seasonally discard in bottom trawl samplings were carried out within the depth range of 30 and 120 m by using meshes varying between 400 and 900 and 40 mm diamond mesh size in codend in traditional bottom trawl by commercial vessels. The monthly samplings were realized by two kind of vessels in size > 18 m (12-17 m) and <18 m (18-32 m) which are common for Black Sea trawl fishery fleet. In each sampling period, the catches were recorded on board from at least two commercial vessels representing the study area. Fieldwork included estimating the total catch and the relative fractions per haul and recording the faunal composition as standardized for per haul duration or per day. The sediment samplings were taken from totally 40 stations and particle size analysis were realized. The stations for sediment sampling were assigned as on a vertical line to land at certain distances from each other and at four different depths. The data derived from PSA (Particle Size Analysis) and also the information of coordinates applied to the ArcMap Sediment Classification Tool (ArcGIS ver 9.2) to derive the habitat map of the substrate in SSA. The abundance of macro benthic fauna is greater in summer months than in winter, spring and fall. The fishing mortality is increasing in this period. 70.3% of total catch is composed of Rapa whelk and 29.7 % is the bycatch species. The reasons for the heavy pressure on red mullet and whiting populations were the low selectivity of meshes and the long operation durations. The high exploitation rate generally causes the catch of relatively small and immature individuals. Though the rate of discarded catch in weight is lower than the marketed catch, as it is considered in number of individual the discarded portion is larger than the market. The age composition of red mullet was composed of 0 and 1 age groups and of whiting are 0, 1 and 2 age groups. In the whiting fishery of the 2013 and 2014, the mean values of fishing effort are found to have no significant difference between seasons for the landing. The highest CPUE for the landing and discard is estimated in fall and the lowest in spring but the difference is not statistically significant. In red mullet fishery for the same years, the trend seems similar within the whiting fishery. The mean values of fishing effort in red mullet are also not significantly different between seasons for the landing. The highest CPUE for the landing and discard is determined in winter and the lowest in spring with no statistical difference. Though the rate of discard by weight seems less than of landings, the rate of discard by individual number is significantly high and cause great bio-economic losses. SSA can be accepted as a soft bottom habitat that is mostly composed of muddy sand and sandy mud. Stations having hard substratum is very limited. This soft bottom is highly available for all kind of drag-net fisheries and this makes this habitat highly sensitive because of heavy fishing pressure.
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Nielsen J. Rasmus, Bastardie Francois, Buhl-Mortensen Lene, Eigaard Ole, Gümüs Aysun, Hintzen Niels T., Kavadas Stefanos, Laffargue Pascal, Mehault Sonia, Notti Emilio, Papadoupoulou Nadia, Polet Hans, Reid David, Rijnsdorp Adriaan D., Rochet Marie-Joelle, Robert Alexandre, Sala Antonello, Smith Chris, Virgili Massimo, Zengin Mustafa (2014). Report on assessing trawling impact in regional seas. Benthis - Deliverable 7.6.