Options to Mitigate Bottom Habitat Impact of Dragged Gears

Home , Bottom trawling

ISSN 0429-9345 FAO PAPER FISHERIES 506 TECHNICAL Options to mitigate to mitigate Options habitat impact bottom gears of dragged

506 Options to mitigate bottom habitat impact of dragged gears FAO 9345 0429- 3 /11.07/1300 6 /1 7 ISSN 8 466E 5 0 3 1 TC/M/Y1 5 2 9 8 9 7 ISBN 978-92-5-105876- ISBN challenges to further develop bottom habitat friendly options. solutions may result in reduced capture efficiency for target species and efficiency solutions may result in reduced capture developed as practical solutions and implemented in commercial fisheries. developed as practical solutions and implemented activity might therefore be difficult. Thus, research communities face many activity might therefore be difficult. Bottom trawling is a diversified fishing method which uses numerous types Bottom trawling is a diversified fishing gears in use in commercial fisheries. These basic mitigation measures aim at gears in use in commercial fisheries. These maintain bottom contact, provide spread and herd the target species. These maintain bottom contact, provide spread bottom trawling the primary function of the forward parts of the trawl is to bottom trawling the primary function of that can be used to reduce the impact of trawling, some of which are already that can be used to reduce the impact of to a large extent depend on the bottom conditions in the area fished. During to a large extent depend on the bottom the impacted area while trawling. Implementation of reduced bottom-impact the impacted area while trawling. Implementation of gear designs, sizes, rigging and operational methods. Therefore, impact on of gear designs, sizes, rigging and operational reducing pressure on the bottom of various trawl components and minimizing reducing pressure on the bottom of various parts are the trawl doors, sweeps and bridles, and are essential for proper gear parts are the trawl doors, sweeps and bridles, Overall, however, there are presently few examples of low bottom-impact trawl there are presently Overall, however, acceptance of the technology by the fishing industry in their commercial fishing acceptance of the technology by the fishing performance and capture efficiency. This document describes the basic principles performance and capture efficiency. the bottom habitat will differ among the various bottom trawl fisheries and also among the various the bottom habitat will differ Cover photo: Picture showing “bottom-friendly” bottom trawl model in the flume tank of the Marine Institute of Memorial University of Newfoundland, Canada. Courtesy of Dana Morse of the Maine Sea Grant Program. FAO FISHERIES Options to mitigate TECHNICAL PAPER bottom habitat impact 506 of dragged gears

by John Willy Valdemarsen Terje Jørgensen Arill Engås Institute of Marine Research Bergen, Norway

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome, 2007 The designations employed and the presentation of material in this information product do not imply the expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations (FAO) concerning the legal or development status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. The mention of specific companies or products of manufacturers, whether or not these have been patented, does not imply that these have been endorsed or recommended by FAO in preference to others of a similar nature that are not mentioned.

ISBN 978-92-5-105876-3

All rights reserved. Reproduction and dissemination of material in this information product for educational or other non-commercial purposes are authorized without any prior written permission from the copyright holders provided the source is fully acknowledged. Reproduction of material in this information product for resale or other commercial purposes is prohibited without written permission of the copyright holders. Applications for such permission should be addressed to: Chief Electronic Publishing Policy and Support Branch Communication Division FAO Viale delle Terme di Caracalla, 00153 Rome, Italy or by e-mail to: [email protected]

By John Willy Valdemarsen, Terje Jørgensen and Arill Engås

Institute of Marine Research, Bergen, Norway

FOOD AND AGRICULTURE AND ORGANIZATION OF THE UNITED NATIONS Rome 2007

iii

Preparation of this document

This technical paper has been prepared within the FAO Fisheries and Aquaculture Department under the leadership of the Fishing Technology Service (FIIT) as a part of the Regular Programme on “Reviews of the impact of fishing gears on habitats and biodiversity”. It follows recent FAO documents focusing on issues related to impacts of bottom trawling, namely: “Impact of trawling and scallop dredging on benthic habitats and communities” (FAO Fisheries Technical Paper No. 472), “Mortality of fish escaping trawl gears” (FAO Fisheries Technical Paper No. 478), as well as related papers: “Discards in the world’s marine fisheries” (FAO Fisheries Technical Paper No. 470) and the “Global study on shrimp fisheries” (in preparation). This document was written by John Willy Valdemarsen, FIIT consultant with Terje Jørgensen and Arill Engås as co-authors – all highly recognized experts on the subject. There is considerable ongoing debate and research on the possible bottom impact resulting from different kinds of bottom trawling, but less attention is paid to mitigation measures. This paper specifically reviews the current research and introduced practices to mitigate bottom impact caused by dragged gears. iv

Abstract

Bottom trawling is a diversified fishing method which uses numerous types of gear designs, sizes, rigging and operational methods. Therefore, impact on the bottom habitat will differ among the various bottom trawl fisheries and also to a large extent depend on the bottom conditions in the area fished. During bottom trawling the primary function of the forward parts of the trawl is to maintain bottom contact, provide spread and herd the target species. These parts are the trawl doors, sweeps and bridles, and are essential for proper gear performance and capture efficiency. This document describes the basic principles that can be used to reduce the impact of trawling, some of which are already developed as practical solutions and implemented in commercial fisheries. Overall, however, there are presently few examples of low bottom impact trawl gears in use in commercial fisheries. These basic mitigation measures aim at reducing pressure on the bottom of various trawl components and minimizing the impacted area while trawling. Modifications to trawl doors that will reduce pressure on the bottom include the use of lighter doors and shortening of the warp-length-to-depth ratio (the extreme result of the latter option is off-bottom trawl doors). The impact by sweeps/bridles can be reduced with the use of discs or bobbins along the cable to raise them off the bottom and by off-bottom rigging of the lower bridle. Pressure of the ground gear can be reduced through the use of lighter gear components and gear components with lifting capabilities. Reduction of the bottom area that is directly affected by trawl doors can be achieved with the use of high aspect ratio doors and by reducing the shoe angle relative to the towing direction. The affected area by sweeps/bridles can be reduced by shorter bridles, reduced sweep/bridle angle and by discs and bobbins mounted along the length of the wires. Ground gear modifications include reduction of the length of the ground gear and arrangements that reduce the number of contact points between the gear and the bottom. Implementation of the above options may result in reduced capture efficiency for target species and acceptance of the technology by the fishing industry in their commercial fishing activity might therefore be difficult. Thus, research communities face many challenges to further develop bottom habitat friendly options. Alternatively, to reduce the impact of traditional bottom trawling, a viable option might be to catch some of the targets while they are off-bottom with midwater trawls. This is an option worth considering for fish resources that, as adults, spend part of their life cycles off bottom. Another alternative for exploiting near bottom organisms is to increase the use of stationary fishing gears which have much less impact on the bottom habitat. For some target species bottom set gillnets and longlines might be both practical and economical viable alternatives to bottom trawling, whereas other species like

shrimp and small non-shoaling fish species cannot be captured economically with such gears. For such species, bottom trawling is at present the only viable option. Finally, a general improvement of the efficiency of bottom trawls can contribute to the reduction of bottom habitat impact, as less effort is needed to catch the allotted quotas. The management regimes in place might, to a large extent, encourage rational exploitation of the resource, such as the individual transferable quotas ITQ management system.

Valdemarsen, J.W.; Jørgensen, T.; Engås, A. Options to mitigate bottom habitat impact of dragged gears. FAO Fisheries Technical Paper. No. 506. Rome, FAO. 2007. 29p.

vii

Contents

Preparation of this document iii Abstract iv List of figures viii

Introduction 1

Summary of what is known about the effect of dragged fishing gears on the seabed 3 Dragged gears 5 Bottom otter trawling 6 Gear design and bottom impact 6 Mitigation of bottom impact 8 Trawl doors 9 Weights 13 Sweeps/bridles 13 Ground gear 14 Bottom pair trawling 17 Bottom impact 17 Mitigation of impacts 18 Beam trawling 18 Bottom impact 18 Mitigation of effects 18 Off-bottom trawling 19 Alternative gears to catch demersal species 21 Bottom-set gillnets 21 Bottom-set longlines 21 Traps and pots 22 Seine fishing 22 Purse-seining 22 Increased catch rates to reduce seabed impact 23 Seabed maps 23 Multirig bottom-trawling 23 Gear monitoring instruments 24 Advantage of using lesser impacting gears 25 References 27 viii

List of figures

1. Bottom otter trawling – single trawl rigging 5 2. Double bottom trawl rigging 6 3. Bottom pair trawl rigging 6 4. Beam trawl 7 5. General layout of a bottom trawl. 7 6. Examples of common ground gear designs used in commercial fisheries 8 7. Examples of bottom trawl doors 8 8. Two examples of roller clump weights used while double trawling 9 9. Three examples of sweep/bridle arrangement 9 10. Two examples of trawl doors with equal surface area but with different height/length ratio 10 11. Impacted widths of two equal trawl doors when door angles are 45o and 25o 10 12. Pressure against the bottom floor when using short and long warps in relation to the fishing depth 11 13. Off-bottom rigging of the trawl doors 11 14. The roll angles of two Thyborøn trawl doors with various length of warp to depth ratios 12 15. Bridle rigging with the lower bridle off the bottom 14 16. Sweeps where the wire ropes are lifted off the bottom along their entire length using discs 14 17. A bottom trawl equipped with drop chains 16 18. A ground gear where a few bobbins have bottom contact, whereas rockhopper discs in the centre and rectangular plates in the wing section are lifted a few centimetres off the bottom (developed in the EU-funded DEGREE project) 17

Introduction

Harvesting of marine resources in the vicinity of the bottom requires fishing techniques that often interact with the bottom habitat. Dragged gears in particular may interact with the bottom substrate and organisms living on the seabed or in the bottom substrate. The degree of contact between the fishing gear and the bottom habitat depends on the bottom substrate, the area covered by the particular gear and the pressure exerted on the bottom by the various gear components. The ecosystem effect of the impact from dragged gear on the seabed has been intensively debated in recent years and several studies have been undertaken to resolve the issue (e.g. Dorsey and Pederson, 1998; Hall, 1999; Kaiser and de Groot, 2000; Linnane et al., 2000; Anon., 2001; NRC, 2002; Sinclair and Valdimarsson, 2003; Barnes and Thomas, 2005). In addition, several working groups and focused topic groups of the International Council for the Exploration of the Sea (ICES), especially the Working Group on Ecosystem Effects of Fishing Activities and the Working Group on Fishing Technology and Fish Behaviour have examined the issue (ICES, 1988; 1999; 2000a; b; 2004). A recent ICES special topic group, “Mitigation measures against seabed impact of fishing operations”, concentrated on measures to reduce seabed impact through gear designs and operations (ICES, 2004). Løkkeborg (2005) reviewed and discussed current knowledge about fishing impact in an FAO document. A brief summary of important findings in his study is presented in Chapter 2. The primary aim of this document, however, is to describe how physical impact of dragged gear, such as otter trawls and beam trawls can, be mitigated by modifications of the gears and the way they are operated. Some marine organisms targeted with demersal gear have diurnal and/or seasonal vertical migrations. For these targets an off-bottom harvesting fishing technique might be a bottom-friendly alternative. Technologies to capture off- bottom organisms are briefly described in this document. A significant quantity of demersal species are captured with gears such as seines, demersal long lines, set gillnets, traps and hook and line. Improving the efficiency and the operation of such gear might well result in an increased exploitation of many demersal resources with such fishing gears. This document therefore discusses which of these fishing gears could be alternatives to bottom-dragged gear for harvesting demersal resources and possible negative environmental impacts in using such alternative gears.

Summary of what is known about the effect of dragged fishing gears on the seabed

Several studies have been conducted to evaluate the physical and biological effects of dragged gear operations on benthic habitats and communities. Basically two methods have been employed to study such phenomena. One method is to compare physical and biological parameters on a site before and after disturbance by experimental fishing or alternatively compare an exposed site with an adjacent undisturbed control site (Freese et al., 1999; Hansson et al., 2000; Kenchington et al., 2001). The other method is to compare commercial fishing grounds that have been heavily fished with areas that are lightly fished or not fished at all (Engel and Kvitek, 1998; Thrush et al., 1998; McConnaughey, Mier and Dew, 2000). The most serious effect of trawling has been demonstrated for hard-bottom habitats dominated by large sessile fauna. Organisms growing up from the bottom such as sponges, anthozoans and corals have been shown to decrease considerably in abundance in the path of the ground gear (Freese et al., 1999; Moran and Stephenson, 2000; Fosså, Mortensen and Furevik, 2002). Such habitats are thus severely affected by fishing operations. A few studies have been conducted to determine the impact of experimental trawling on sandy bottoms of offshore fishing grounds (Prena et al., 1999). These studies showed declines in the abundance of some benthic species, but trawling disturbance did not produce large changes in the benthic community structure. These habitats may be resistant to trawling because they are subjected to high degree of natural disturbances such as strong currents and large temperature fluctuations. Several studies have been conducted on the impact of shrimp trawling on soft bottoms (Hansson et al., 2000; Drabsch, Tanner and Connell, 2001; Sparks- McConkey and Watling, 2001). However, clear and consistent effects of trawling disturbance have not been demonstrated in these studies. On the other hand, soft- bottom habitats show pronounced temporal changes due to natural disturbance, and potential changes attributed to trawling may be masked by this natural variability and therefore difficult to demonstrate. Clear evidence of short-term effects of beam trawling has been demonstrated in several studies (Bergman and van Santbrink, 2000; Kaiser and Spencer, 1996). Intensive disturbance has been shown to cause considerable reduction in abundance of several benthic species. No disturbance effect of beam trawling was Options to mitigate bottom habitat impact of dragged gears

documented in areas exposed to natural disturbances e.g. wave actions and salinity fluctuations confirming the general trend that exposed habitats seem to be less vulnerable to disturbance caused by towed gears.

Dragged gears

Dragged gear in the context of this document refers to bottom otter trawls, pair trawls and beam trawls. Bottom otter trawls encompass a large variety of designs, riggings and dimensions. Common components are a pair of otter boards (trawl doors), sweeps/bridles and one or more trawl nets (Figure 1). In a multirig configuration, weights are additional parts of the trawl system (Figure 2). The main difference between a bottom otter trawl and a pair trawl is the lack of otter boards (Figure 3). Instead, the foremost contact points of a bottom pair trawl are often weights attached to the joinings of the towing warp and the sweep. For a beam trawl the mouth of the net is held open by a solid metal beam. Solid metal plates (shoes) are welded to the ends of the beam so that it can slide over the seabed (Figure 4). Often two parallel beam trawls are towed from outriggers by a single vessel. Bottom trawling encompasses a wide range of gear designs and methods of operations. Towing speed ranges from 2 knots (1 m·s- 1) to 6 knots (3 m·s-1) and fishing might be conducted at depths from 10 to 2 500 metres. The vessels operating bottom trawls might have towing bollard pull ranging from 200 to 70 000 kg. Reduced impact of bottom otter trawling on the seabed can basically be achieved by minimizing the impacted area and by reduction of the pressure of gear components on the bottom.

Figure 1 Bottom otter trawling – single trawl rigging Options to mitigate bottom habitat impact of dragged gears

Figure 2 Double bottom trawl rigging

Figure 3 Bottom pair trawl rigging

Weights

Sweep

Bridles

Bottom otter trawling Gear design and bottom impact A bottom trawl design generally consists of netting divided into two wings, a belly and a codend (Figure 5). The front part of a trawl is framed with a headline equipped with floats along the top and a fishing line along the bottom equipped with various types of ground gears the purpose of which is to protect the netting Dragged gears

Figure 4 Beam trawl

headline tickler chain codend

warp

bridle beam footrope shoe (or head)

Source: He et al, 2006 of the trawl from damage and to allow continuous towing without hook-ups. Typical ground gear designs in common use are illustrated in Figure 6. In a bottom otter trawl rigging the trawl is held open horizontally with a pair of trawl doors which have three main functions: • to open the trawl horizontally; • to provide the front bottom contact points of the trawl gear; and • to stimulate fish to swim towards the trawl path. As for trawlnets, there is a wide range of designs and sizes/weights of trawl doors in common use. Traditional trawl doors were designed in such a way that they relied on the ground shear to spread the trawl (rectangular and V-doors). Such doors lost much of their spreading force when lifted off the bottom. Recent development has been towards more hydrodynamic efficient doors, maximizing

Figure 5 General layout of a bottom trawl

XBSQ EPPS TXFFQ CSJEMFT XJOHFOE nPBU IFBEMJOF CPEZPGOFU

XJOH TRVBSF FYUFOTJPO DPEFOE

VQQFSCSJEMF MPXFSCSJEMF mTIJOHMJOF

HSPVOEHFBS

Source: He et al., 2006 Options to mitigate bottom habitat impact of dragged gears

Figure 6 Examples of common ground gear designs used in commercial fisheries

Bobbin Roller

Rockhopper Cookie

Chain Wire

Source: He et al., 2006

spread forces and minimizing resistance. Another recent trend is to increase the size and weight of trawl doors. Figure 7 shows some common trawl door designs. While towing, the shoe of a trawl doors is often angled 2 540 degrees relative to the trawl track. When a door is 4 metres long, the width of the track is thus 2 metres with a door angle of 30 degrees. The track can be made narrower basically by reducing the angle of the shoe and by using doors with a higher height/length ratio (H/L). Both options are to some extent used in commercial trawl fisheries. In a double trawl rigging a weight is used to achieve bottom contact of the front part of the inner sweeps/bridles located in the centre between the two trawl nets. This weight might be heavier than the weight of a trawl doors (normally 30 percent heavier in double trawl riggings). The weights differ in shape and rigging, and their effect on the bottom will vary. Two roller clump designs are shown in Figure 8. Figure 7 The doors and weights are connected Examples of bottom trawl doors to the trawl wings by sweeps or bridles (wire/chain/ropes). These connections vary in length from a few metres up to 300 metres. The lower bridle or sweep has normally bottom contact during towing. A selection of arrangements of bridle/sweeps is illustrated in Figure 9.

Mitigation of bottom impact Trawl doors, weights, bridles/sweeps and the ground gear represent different components of otter trawl rigging, each with a specific impact on the bottom Dragged gears

Figure 8 Two examples of roller clump weights used while double trawling

habitat. Types of impact and possible modifications to each gear component are therefore described separately below.

Trawl doors The trawl doors are often considered as the most significant source of bottom impact and the relatively deep and visible furrows they create in the bottom substrate are frequently easily seen on side scan sonar and video recordings of the seabed. Two trawl doors with four-metre long shoes towed at a 30 degree angle of attack will create two two-metre wide furrows on each side of the trawl path. Depending on whether the trawl doors are in a long sweep rigging with 150 metres door distance or in a short bridle rigging with only 30 metres door distance, the affected bottom areas of the trawl doors are then about 3 percent and 10 percent of the total area between the two doors, respectively.

Figure 9 Three examples of sweep/bridle arrangement 10 Options to mitigate bottom habitat impact of dragged gears

Figure 10 Two examples of trawl doors with equal surface area but with different height/ length ratio, where door B has 33% less contact area with the bottom than door A B

A H/L=1.5:1

H/L=1:1.5

There are basically four methods to reduce the bottom impact of trawl doors: 1. use of trawl doors with a higher aspect (height/length) ratio (Figure 10); 2. use of trawl doors with a lower shoe angle relative to the towing direction (Figure 11); 3. use of a shorter warp length relative to the fishing depth to achieve bottom contact (Figure 12); and 4. off-bottom rigging of the trawl doors (Figure 13). High-aspect doors are more hydrodynamically efficient than low-aspect ratio doors. Such doors are commonly used as pelagic trawl doors. The main reason for not using such trawl doors for bottom trawling is that they require more attention to keep them stable and vertical during towing. The recent development of gear instruments that monitor the vertical roll and pitch angels of trawl doors while towing may better facilitate the use of such trawl doors for bottom trawling. In conventional rectangular or V-door Figure 11 designs the shoe is parallel to the trawl Impacted widths of two equal trawl doors door surface. In modern bottom trawl when door angles are 45o and 25o door design, however, the flat surface has been replaced with one or several curved surfaces. In these new curved designs the shoe angle is reduced, resulting in Trawl doors narrower furrows in the bottom. Another practical solution to reduce the shoe angle relative to the towing Trawl door path direction is described by Sterling and

Trawl door Eayrs (2006). The shoe consists of a frame path towed at zero angle of attack relative to the towing direction to which a light- weight polyethylene wing was attached. Dragged gears 11

Figure 12 Pressure (Ts and Tl) against the bottom floor when using short and long warps in relation to the fishing depth

Long warp

Short warp

Ts Tl

Figure 13 Off-bottom rigging of the trawl doors

Weight

Door performance is quite complex to describe but in simplified terms the pressure of a trawl door against the bottom substrate (BP) can be expressed as a function of the doors’ weight (W), the warp tension (T) and the warp slope angle (α): BP= W-T·sin(α). By shortening the warp length, the slope angle increases, resulting in an increased lift force T·sin(α). For a certain warp tension there is an angle when the lift force exceeds the weight of the door, resulting in a situation wherein the trawl door will lose bottom contact. In normal bottom trawling situations proper bottom contact of the door is regarded as important for efficient capture. Therefore, longer warp than required to keep bottom contact is often used in commercial fishing situations. This is done to be “on the safe side” and because methods to monitor “optimal” bottom contact requires special instrumentation. An indicator of bottom contact of the trawl doors is the roll angle of the door, 12 Options to mitigate bottom habitat impact of dragged gears

Figure 14 The roll angles of two Thyborøn trawl doors with various length of warp to depth ratios. The vertical orientations of the doors are indicated below the graph (vertical represents 0 degrees)

P o rt S ta rb o a rd L in e a r (P o rt) L in e a r (S ta rb o a rd)

2 s

e 0 e

r 2.1 2.2 2.3 2.4 2.5 2.6 2.7 g

e -2 D -4

-6

-8

-10 Warp length/depth ratio

as most trawl doors heel inward when too long warp is used, while they heel outwards when the towing warp is too short. This effect on the door is illustrated on the graph in Figure 14 (Valdemarsen, unpublished). The optimal horizontal spread of a trawl door is normally achieved when it is vertical and therefore an optimal warp length often corresponds with optimal door efficiency. Certainly the most effective method to reduce trawl door impact on the bottom habitat is to lift the doors off the bottom as illustrated in Figure 13. This measure, however, has a technical as well as a catchability disadvantage and will therefore not work in all fishing situations. Off-bottom trawl doors are mainly an option for target species that are not herded by doors and sweeps/bridles along the bottom, such as shrimp and nephrops. For such target species the mouth area of the trawl itself is the key parameter for the catching efficiency. The technical challenge with such rigging is to keep the trawl door distance above bottom nearly constant. He et al. (2006) reported on the development and testing of such semi-pelagic rigging in the shrimp fishery in the Gulf of Maine (United States of America). In these experiments the door height was set above the seabed by adjusting the length of the warps when the distance of the doors to the bottom was monitored with acoustic instruments. Similar catch rates were obtained with this semi-pelagic trawl door rigging as with traditional trawl doors. Monitoring the height of the trawl doors above the bottom requires appropriate instruments which can be used to adjust the door height by altering trawl warp length or, alternatively, the towing speed. An active control of the trawl door depth can also be achieved technically by adjusting the towing point and back- strops of the doors while towing. Dragged gears 13

Weights The bottom impact of weights depends on the shape and design of the weight as well as the warp length/depth ratio. Reduced physical bottom impact of weights can thus be achieved by the use of: • a shorter towing warp to reduce the downward pressure of the weight; and • a narrower weight with rollers. The clump used in double-rigged trawling matches the trawl doors with regard to bottom contact. The sweeps/bridles behind these front bottom contact points should have equal tension and bottom contact. An alteration of the warp length/ depth ratio will therefore affect bottom contact of the clump as well as the pair of trawl doors. Active control of warp length requires proper monitoring of the bottom contact of the trawl door and the clump, which can be achieved by the use of sensors measuring the roll and tilt angle of the doors as already described. The shape of a clump varies between producers as indicated in Figure 8. To reduce the width of the impacted area, designing a narrower clump is an alternative. The use of wheels under the weight might also reduce the pressure exerted on the bottom.

Sweeps/bridles The sweeps/bridles in most bottom trawl situations have the largest contact area with the bottom. With 200-metre sweep lengths angled at 25 degrees relative to the towing direction, each sweep will be dragged across an 85-metre wide corridor totalling 170 metres in width for a single tow. The impact of sweeps/bridles dragged across the bottom is poorly understood but certainly affect protruding objects on the bottom. Possible mitigation measures are to: • reduce the sweep/bridle length or angles of attack; and • lift the sweeps/bridles off the bottom. Short bridles are used in trawls when the wingspread is the important catch efficiency parameter, as in shrimp and nephrops trawls. In some trawl riggings the wings are therefore attached directly to the doors, e.g. for outrigger prawn trawlers. However, in situations when there is a need for vertical height to the trawl, two or more bridles connect the wings to the trawl doors. The lengths of such bridles vary, but are commonly 30–50 metres in deepwater shrimp trawling. The impacted area of such short bridles can be made smaller by reducing the bridle angle. A possible solution to achieve this is to include spreading devices in the trawl itself in such a way that the wingspread is maintained while the door spread is reduced. This option has proven to work well with the self-spreading ground gear developed by Hansen and Valdemarsen (2006). When trawling for species that are efficiently herded inward towards the central trawl path by the sweeps, the sweep length is probably the most important factor affecting capture efficiency. Shortening of the sweep length is therefore not an option in fisheries where long sweeps are part of the common rigging, as it will result in lower catch rates. 14 Options to mitigate bottom habitat impact of dragged gears

Lifting bridles and sweeps off the bottom is probably a more feasible option to reduce their bottom impact. For relatively short bridles (20-40 m), a possible technical option is to avoid bottom contact between the trawl door and the wing as illustrated in Figure 15. This option might require some heavy weights on the wing tip to ascertain proper bottom contact at this point. With longer sweeps/bridles, inserting discs or bobbins at certain intervals along the sweep is a possible option to raise the sweep cable off the bottom as illustrated in Figure 16. The diameter of the rollers and their spacing determines the clearance below the sweeps as well as their weight in water. This solution has successfully been developed and tested in the Bering Sea (Rose, 2006).

Ground gear As illustrated in Figure 5, a wide range of ground gear designs are used on bottom trawls. The length of ground gears range from approximately 10 metres in small inshore trawls to nearly 100 metres in some large deepwater shrimp trawls. As the ground gear frames the bottom mouth of the trawl, it is of vital importance for both trawl protection and to get fish into and not under the trawl.

Figure 15 Bridle rigging with the lower bridle off the bottom

Upper bridle

Lower bridle

Figure 16 Sweeps where the wire ropes are lifted off the bottom along their entire length using discs

Conventional sweeps

Modiﬁed sweeps

Source: after Rose, 2006 Dragged gears 15

Fish herded inward by the sweeps/bridles often continue to swim in the trawl mouth in front of the ground gear until they are fatigued and are overtaken by the gear. The capture success then depends on how many fish are overtaken by the trawl and end up in the codend, and how many are able to escape over or under the trawl. The under-trawl escapement for many species is significant (e.g. Ingolfsson and Jørgensen, 2006) and therefore the challenge for efficient capture is often related to the ground gear design. A ground gear design that balances proper trawl protection, minimum bottom contact and minimal under-trawl escapement of targets is obviously the ideal ground gear. As these somewhat contradictory objectives doubtfully can be incorporated fully in any ground gear designs, compromises must be found in practical situations. Three basic methods to reduce the bottom impact of a ground gear are to: 1. reduce the length and thus the width of the ground gear; 2. reduce the physical pressure from gear components on the bottom; and 3. reduce the number of bottom contact points along the ground gear. In situations where the target species are herded by sweeps/bridles into the trawl mouth, the length of the fishing line equipped with a ground rope can be relatively short and thus have a relatively narrow ground gear spread. The behaviour of the various target species might be different so that the optimal length of the ground gear will be species specific. Optimal gear lengths in various trawl fisheries have to be evaluated but many trawl gears will probably maintain capture efficiency with shorter ground gears than what is commonly used in that particular fishery. Reduction of the ground gear weight in water or the creation of an upward lift on the ground gear might technically solve the second option. An optimal gear weight is one that balances the lifting forces in such a way that the ground rope is touching, but not digging into the bottom sediment. In many trawls, however, extra weight is added to the ground gear to insure bottom contact. Use of sensors that monitor bottom contact while towing may be a solution to reduce the risk of losing bottom contact with light-weight ground gear. A possible solution to create an upward lift is the use of the self-spreading plate gear developed by Hansen and Valdemarsen (2006). As these plates can be rigged to have either a lifting or depressor function, rigging with lifting function along the wings is considered a feasible option. Fish seem to be herded inward towards the trawl mouth centre by the off-bottom ground gear similar to a ground gear with bottom contact. However, the escape behaviour of fish aggregating in front of the centre ground gear is often sideways where they seek openings under the fishing line. Therefore, the ground gear in the aft wing section close to the quarter section should have proper bottom contact to avoid escapement under the trawl. The third option is one which has already been introduced with some success in various trawl fisheries. It is basically to use fewer rollers or rock hopper discs with bottom contact than normal ground gear. In a large offshore shrimp trawl gear, He and Foster (2000) replaced 31 bobbins with a diameter of 24” and 21” 16 Options to mitigate bottom habitat impact of dragged gears

with 9 bobbins, resulting in a reduction in gear weight in water from 2 984 kg to 1 306 kg and a 70 percent reduction in affected bottom area. Although this ground gear did not decrease the shrimp catch, the reduced number of bobbins was more likely to incur gear damage, especially when fishing on grounds with rough seas and bottom conditions. In the Gulf of Maine in the United States of America, a trawl with a raised footrope was developed for the small mesh whiting fisheries. This trawl is locally called a sweepless trawl because in the northeastern United States a ground gear is called a sweep. Drop chains scattered along the fishing line are used to keep bottom contact, as illustrated in Figure 17 and on cover page. The bottom contact with this arrangement is limited to a number of points (bottom of the drop chain), instead of a continuous ground line. Because of its advantages, fishers in the Gulf of Maine area have adopted this ground gear concept voluntarily, although concerns have been raised about loss of target species. A solution using a combination of roller wheels, rock hopper discs and plates is under development within the European Commission-funded project “Development of Fishing Gears with Reduced Effects on the Environment” (DEGREE) (http://ec.europa.eu/research/fp6/ssp/degree_en.htm). The new ground gear design is illustrated in Figure 18. The basic requirements include a gear that has fewer contact points with the bottom, similar protection of the trawl as the rock hopper ground gear and no increase in loss of fish under the trawl. The contact points are roller wheels scattered along the ground gear, which is divided into a centre section of modified rock hoppers and two wing sections of plates. A chain runs through the centre axis of the rollers and through the lower part of the rock hopper discs and plates. This will lift the discs and plates off the ground because the rollers have a diameter large enough to reach below the discs and plates. Initial testing of this gear set up during research cruise in 2006 clearly demonstrated that it performed satisfactorily with regard to reduced bottom contact for parts of the ground gear it had no extra escapement of fish under the

Figure 17 A bottom trawl equipped with drop chains

Source: Pol, 2003 Dragged gears 17

trawl and the protection of Figure 18 the trawl was comparable A ground gear where a few bobbins have bottom contact, whereas rockhopper discs in the centre and rectangular to rock hopper ground plates in the wing section are lifted a few (5–7) centimetres gear (Valdemarsen, off the bottom (developed in the EU-funded unpublished). The off- DEGREE project) bottom discs between rollers did hit obstacles on the bottom, but it was obvious that on sandy flat bottom the rollers were the main bottom contact points, as seen by the sand clouds behind the rollers. Top view The plates along the wings create some turbulence which might disturb small particles and Rectangular plates thus create a sand or mud Bobbins cloud. The direct physical Rockhopper discs pressure of the plates are controlled by tilting the plates inwards to get an upward lift and by using rollers in between the plates and in between the centre discs. Also important for minimizing bottom contact of plates along the wings is to use a danleno bobbin in front of the gear that has a diameter large enough for the plates behind it to be lifted off the bottom. These three examples of lighter ground gear are certainly options to be considered when a more bottom-friendly trawl technique is developed. All three have the potential to reduce the physical pressure on the bottom and thus reduce the impacted area. Organisms targeted with a bottom trawl, however, often stay very close to the bottom. Therefore, for such species an opening under the fishing line of the trawl might result in reduced capture efficiency.

Bottom pair trawling Bottom impact A major difference between bottom pair trawling and bottom otter trawling is that trawl doors are not used. Instead, weights are normally used in the connection between the towing warp and the sweeps. In some pair trawl riggings the weights are removed, as in pair-seining which was developed in the North Sea to use the benefit of rope herding and at the same time cover a larger fishing area than a single-seine net operation. 18 Options to mitigate bottom habitat impact of dragged gears

Mitigation of impacts Mitigation of bottom impact by pair trawls is similar to that for bottom otter trawling with regard to the sweeps/bridles and the ground gear. One possible difference might be when very long sweeps are used for herding purposes. Such techniques are normally associated with smooth and flat bottom and lifting of the sweeps a few centimetres off bottom might be feasible by the use of rubber discs and rollers scattered along the sweeps. The impact of weights can partly be controlled by the length of warp in relation to the fishing depth as well as by using weight designs that minimize the impacted bottom area.

Beam trawling Bottom impact Beam trawling, or the concept of opening a trawl with a boom or spar, has existed since the 1400s. It became more important as a fishing method in the 1960s as a replacement for otter trawls where chains had been added between the two otter boards to enhance flatfish catches. Since then the beam trawls have increased in weight, number of chains used and size of the beam. Since 1988, beam width has been restricted to 12 metres in European Union waters. The penetration depth of a beam trawl depends on the weight of the gear and the towing speed, as well as on the type of substrate, and ranges from 1 to 8 cm (Paschen, Richter and Köpnick, 2000). Beam trawls cause direct mortality to non-target organisms in two ways. Firstly, the shoes, tickler chains or chain mat impact on animals on the seabed (Bergman and van Santbrink, 2000) and secondly, animals are caught in the net and die from sustained injuries during hauling or when the bycatch is processed and discarded (Lindeboom and de Groot, 1998).

Mitigation of effects Although several mitigation measures have been examinated over the last 30 years to reduce impact of beam trawling on the benthic environment, only electric fishing and benthos release panels are considered to have a positive potential. Research into the potential for electrical beam trawling began in shrimp fisheries when the typical jumping behaviour of the animals to electrical stimulation was noted. Similar potential was later identified for catching flatfish, especially for sole, and significant effort was dedicated to develop this technique in Belgium, Germany, the Netherlands, and the United Kingdom of Great Britain and Northern Ireland in the 1970s and 1990s. In 1992 a private company in the Netherlands developed an electrical stimulation system which has been tested in commercial fishing with encouraging results with regard to reduced bycatch of benthos (Marlen et al., 2006) Benthic release panels (also known as drop-out panels) have the potential to reduce the environmental impact of beam trawling too. Some successful designs have reduced benthic invertebrate bycatch by 75–80 percent without loss of commercial target species. 19

Off-bottom trawling

Trawl gear without any bottom contact during fishing is certainly not harmful to the bottom habitat. Trawling off-bottom is called pelagic trawling and is conducted in the water column anywhere from the surface to the vicinity of the bottom. Pelagic trawling is primarily used to exploit pelagic fish resources either in schools or in layers. However, some species are known to have seasonal and diurnal vertical migrations and are therefore available for both pelagic and bottom trawls. Catching such species with pelagic trawls when they are off-bottom is thus an option that will reduce the bottom impact significantly. There are several examples of pelagic trawling for species with both demersal and pelagic vertical distribution. In the North Atlantic a major pelagic trawl fishery has developed since 1975 for blue whiting. Prior to 1975 this resource was captured with bottom trawls in the North Sea. Until 1977 cod, haddock and saithe in the Barents Sea were captured with pelagic trawls as well as with bottom trawls. Poor size-selectivity and large catches of small-sized fish was the main reason for the introduction of a ban on pelagic trawling in the Barents Sea in 1977. Alaskan pollock (Theragra chalcogramma) was only captured with bottom trawls prior to 1990. Concerns about the bycatch of crabs and other ground fish species, such as Pacific halibut, initiated a switch to pelagic trawling for the pollock. As pelagic trawling soon proved to be as efficient as bottom trawling the industry quickly adopted this new trawling technique, resulting in a bottom trawl ban by the North Pacific Fisheries Management Council (NPFMC) since 1999 (NRC, 2002). In recent years, the size-selective properties of trawls have been improved with the introduction of new mesh configurations (square meshes, T-90 meshes and exit windows, etc.) and sorting grids. Pelagic trawl techniques have also become more efficient during the last two decades with the introduction of megasized large mesh trawls and advanced instrumentation to monitor trawl performance (Valdemarsen, 2001).

Alternative gears to catch demersal species

Most demersal organisms can be captured with other fishing gears than trawls. Bottom-set longlines and gillnets, as well as pots and traps are, obvious alternatives. Other alternative gear types are the seine net and, to a lesser extent, the purse seine. For some species, however, the only feasible technique is with trawls, as is the case with the deepwater shrimp and some small size fish species, e.g. sand eel (Ammodytes). Although the bottom impact of these alternative gears might be less than with a bottom trawl, they may have other disadvantages that outweigh the environmental bottom impact of a bottom trawl.

Bottom-set gillnets Gillnetting is one of the predominant and most efficient fishing gears used by all sizes of fishing vessels all over the world, from tropical regions to under-ice fishing in the Arctic region, and in marine as well as in freshwater. It is certainly one of the most energy effective methods in terms of fuel used per kilogram of fish caught, Gillnets are size-selective fishing gears and, by using appropriate mesh sizes, the size of the target can be chosen with great accuracy. The most obvious weaknesses of gillnetting are occasional poor quality of captured fish and ghost fishing of lost gillnets. A future expansion of bottom-set gillnetting as a possible replacement for bottom trawling, to a large extent depends on solving these two negative issues. These negative issues are mostly related to the operationa more responsible fishing operation may well solve these problems.

Bottom-set longlines Bottom-set longlining is another low-fuel demanding fishing technique for demersal fish. Its impact on bottom is also considered to be marginal (Bjordal and Løkkeborg, 1996), so that longlining is seen to have many environmentally positive characteristics. The incidental catch of seabirds when setting longlines has partly been mitigated by the use of various devices resulting in the general belief that longlining is no longer considered a major environmental problem (Valdemarsen and Suuronen, 2003). The main weakness of longlining is therefore the cost of bait which sometimes makes fishing uneconomical. Longlining is an obvious alternative to exploit certain bottom fishes such as cod and haddock, whereas saithe is not captured efficiently with such gear. 22 Options to mitigate bottom habitat impact of dragged gears

Traps and pots Baited pots and unbaited traps are common alternative gears to exploit several bottom species. These gears can be operated by a wide range of vessel sizes and used in areas where bottom conditions are too rough for bottom trawling. Pots and traps are commonly used in small-scale fisheries in tropical regions. In temperate waters, where trawling is the most common gear to exploit bottom fish, there are only a few success stories of economical capture with traps and pots and they are therefore not in widespread use. However, research to improve the capture efficiency for species such as cod is ongoing. The bottom impact of pots and traps is considered to be low but ghost fishing of lost pots and traps is a major problem in some areas. Loss of fish traps is quite common in tropical regions occasionally affected by strong winds and rough sea, as in the Caribbean.

Seine fishing Seining is a fishing method which, in many aspects, has similarities with bottom trawling. The fishing method consists of a bag-shaped net that is dragged over the bottom. Instead of trawl doors that open the net horizontally, long ropes are set in a triangle surrounding the target fish which are then herded into the path of the net by the long ropes. Both the net and the ropes are in contact with the bottom and, except for lesser gear weight and no trawl doors, seines may have impact on protruding bottom organisms similar to a bottom trawl. Seine-netting can exploit the same resources as a bottom trawl, except for non-herded organisms such as shrimp and nephrops. Another drawback of a seine net might be the fishing depth, which is presently limited to approximately 500 metres.

Purse-seining Purse-seining is a traditional fishing method to catch schooling pelagic fish. Thus it is not an obvious alternative to dragged bottom gear for exploiting bottom resources. However, some fish species migrate vertically and therefore traditional bottom fish might occasionally be available for purse-seining. For capture of species such as cod and haddock, purse-seining has been used successfully when these are aggregating for e.g. spawning or feeding. As a purse seine is off-bottom during the fishing operation, the bottom impact of this gear is zero. 23

Increased catch rates to reduce seabed impact

Some management regimes (e.g. vessel quota regulated-fisheries) encourage the use of minimum effort to catch an allocated quota of fish. As seabed impact is very dependent on the duration of contact between a fishing gear and the seabed, improvement of the efficiency of the gear might be a significant contribution to reduce any adverse seabed impact. Several options that can improve catch rates for dragged gear are known and these are briefly introduced in this chapter.

Seabed maps For some target species, density is often related to bottom topography and sediment characteristics. As catch rate is related to density, better knowledge of the bottom topography and sediment types will help reduce fishing effort, particularly in quota-regulated fisheries. In the Canadian scallop fishery, seabed characteristics typical for high scallop density was mapped with multibeam technology. Fishers used detailed 3-D topography with information on bathymetry, sediments and benthic habitat to identify scallop beds. Vessel captains optimized dredge efficiency by towing directly on those areas. Average fishing time per metric tonne of scallop meat was reduced from 6.37 hours to 2.41 hours, fuel consumption was reduced by 36 percent and 74 percent less seabed was dredged (BIO, 2002). Although the above example refers to a stationary organismthe scallopdetailed information about the bottom topography and its sediments can be a useful planning tool to reduce trawling on a bottom with low density of fish species and shellfish.

Multirig bottom-trawling Multirigged bottom trawls sometimes produce higher catch rates per trawl than single-rigged trawls (Engås, unpublished ). Even with similar catch rates per trawl, the catch corrected impact from multirigged bottom-trawling is expected to be less than that for single rig trawling. A single-rigged trawl uses two trawl doors while a multirigged trawl uses two doors and one less weight than the number of trawls. Provided that the impact of a weight is less than that of a trawl door, the impact of the operation will be in favour of a multirigged trawl. 24 Options to mitigate bottom habitat impact of dragged gears

Gear monitoring instruments Gear monitoring systems are useful tools to optimize the efficiency of trawl gears. Instruments that monitor spread, bottom contact and symmetry of the trawl are examples of such equipment. 25

Advantage of using lesser impacting gears

Fishing is a commercial activity and therefore profit is its key driving force. Maximizing revenues and minimizing operation costs are what fishers consider as their main challenge. In recent years concern about the environmental impact of fishing operations has become an important issue and it is also of increasing concern for the fishing industry. In a context where consumers demand certificates on the origin of fish products, the method of fish capture will play an even greater role in the future. Certainly a certification process might encourage fishers to use low-impact fishing gears, as the alternative might be reduced market access and possibly a lower price for the captured fish as well. Another extreme threat is that bottom-trawling might be banned because of the potential negative impact on the bottom habitat. Several initiatives in that direction have been launched in recent years. Such external pressure is certainly a driving force towards implementing more environmentally-friendly fishing methods. This pressure may force fishers out of the industry, resulting in reduced harvest of food from the sea, or may lead to a change to using only low-impact gear.

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ISSN 0429-9345 FAO PAPER FISHERIES 506 TECHNICAL Options to mitigate to mitigate Options habitat impact bottom gears of dragged