ICES mar. Sei. Symp., 196: 178-182. 1993

Size selectivity of rigid sorting grids in bottom trawls for Atlantic cod (Gadus morhua) and haddock{Melanogrammus aeglefinus)

Roger B. Larsen and Björnar Isaksen

Larsen, R. B., and Isaksen, B. 1993. Size selectivity of rigid sorting grids in bottom trawls for Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus). - ICES mar. Sei. Symp., 196: 178-182.

During 1990 and 1991, selectivity experiments with rigid sorting grids in the aft sections of a trawl were carried out on board different trawlers along the coast of northern Norway. Selectivity data on cod (Gadus morhua L.) and haddock (Melanogrammus aeglefinus L.) are presented and discussed in relation to other methods and the observed behaviour of escapees. The stainless steel sorting grids had a bar distance of 55 mm and covered an area of 3.2 m2, replacing a part of the upper panel in the extension piece ahead of the codend. To determine the size selectivity achieved with this system, the trawl codend was blinded by a 48 mm inner net, and fish escaping through the grids were collected into a 48 mm cover placed over the grids. Selection ranges on cod and haddock ranged from 4.6 cm to 8.5 cm, while 50% retention lengths of 47.8 cm to 50.2 cm were obtained. A camera on a towed vehicle observed fish behaviour inside the trawl and during escape. Additionally, a scanning sonar moni­tored gear geometry during the experiments.

Roger B. Larsen: The Norwegian College o f Fishery Science, University o f Tromsø, N- 9037 Tromsø, Norway, Bjørnar Isaksen: Gear Division, Institute o f Marine Research, PO Box 1870 Nordnes, N-5024 Bergen, Norway.

Introduction

There seems to be general agreement among gear scien­tists around the world that slight changes of mesh size in normal diamond-mesh codends may not serve the basic intentions of improved or altered selectivity (MacLen- nan, 1992). During studies of codend selectivity, many variables have been shown to influence the codend size selection of fish in trawls and other mobile gears (Pope et al., 1975). One of the largest problems to overcome is the reduction in effective mesh opening as the codend catch increases (Engås et al., 1989). The use of divers and underwater cameras in studies of fish behaviour inside gears is fundamental to understanding how to exploit this behaviour for selectivity purposes (Wardle, 1989).

The basic idea behind both square-mesh panels and rigid sorting grids is the need for stable openings in the codend through which effective escape can occur despite changing towing speeds, catch sizes, etc. Notwithstand­ing very promising results with square-mesh netting in codends (Cooper and Hickey, 1989; Robertson, 1989), this technique has been found unusable in the Barents

Sea, owing mainly to problems with redfish (Sebastes spp.) masking meshes and reducing selectivity at larger catch volumes (Isaksen and Valdemarsen, 1989; Larsen, 1989a).

Observations of diamond-mesh codend escape show that fish often panic inside the codend; some of the escaping fish are squeezed through the mesh and some hit the mesh opening and wriggle out (Main and Sang- ster, 1988). Ordinarily codend escape will only occur through a small area of the codend, i.e. a few rows of mesh in front of the accumulating catch (Engås et a l., 1989). As an attempt to overcome some of the selectivity problems already mentioned, the development of sort­ing grids for bottom fish trawls was begun in late 1989 (Larsen, 1989b).

Materials and methods

The experiments presented in this paper were carried out on fishing grounds along the coast of Finnmark, northern Norway, from 27 August to 9 September 1990 (on board the 52 m “Anny Kramer”) and 25 August to 8

178

15x8* PL Escape area< 7 6x8’ PL

Chain: 4 240 mm

Chain: 2 720 mm

;T K O O O O :rj 12

1500 mm< 1200 mm y j 1200 mm

1 1200 mm

<3=Direction of tow

1800 mmx 1200 mm

Figure 1. (Top) An indication of where the sorting grids are placed in the existing trawl, (middle) a sketch of the working principle of rigid sorting grids (Sort-X model), and (bottom) details of the construction. The grid area covers 3.2 m2 and the system made of stainless steel is buoyed with 8" (0200) plastic (PL) floats.

September 1991 (on board the 47 m “Skjervöyfisk”). In both periods normal bottom trawls were used, i.e. 424 meshes x 150 mm "Alfredo” designs equipped with 75 m bobbin ground gears. As a standard the “butterfly rig” , i.e. short bridles (10.3 m) and long sweeps (140 m), 7-8 m2 and 1800-1950 kg otter boards were used. Tow­ing speed varied from 4.0 to 4.5 knots.

The sorting grids replaced a part of the upper panel in the extension piece, i.e. the tubular section between the belly and the codend (Fig. 1). The grids (Sort-X model) were made of stainless steel and consisted of three sections joined together. Two separate grids with fixed bar distance of 55 mm were connected to a poly- vinylchloride-canvas-covered steel frame, the main function of which was to guide escaping fish away from the trawl and to keep the system balanced during oper­ation. The system weighed about 90 kg, and neutral buoyancy was achieved with plastic floats. Once in the sea, the system opened and was kept in a steady position by using chains. The angle of attack is theoretically close to 25° in the first part of the system (Fig. 1).

To estimate the size selectivity obtained with grids,

escaping fish were collected in a 23 m long cover made of 48 mm polyethylene (PE). In total 7 plastic floats (200 mm) were placed along and on top of the cover (Fig. 2). To avoid biased selectivity data, the normal 135 mm polyamide (PA) codend of the trawl was simul­taneously blinded by a 48 mm PE inner net. Length was measured as total length to the nearest cm for samples of fish (cod and haddock) from both codend and cover. The length-frequency distributions, adjusted for the total numbers of fish, form the basis of further calculations of selection curves and parameters such as selection range and 50% retention length. Comparing the catch in the cover with that in the trawl codend, the sorting grid size selectivity (Sj) of fish in size group i was established as:

S| (% ) = NiCe/(N|Ce + NjCo),

where N, is the number of fish in size group i from the codend (ce) and the cover (co). Selection curves (ogives) were plotted as 3-point moving averages showing the retention percentage by the sorting grids for any fish size. Selection ranges (L75% -L 25%) and 50% retention lengths (L5o%) were calculated.

179

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150

80.5

-50_i

49.5

CHAFER, DBL. 150 mm PE 03

2x3190

199.5

2x31

Cover 23 m

■4 - Extension — Grids — --------- Extension --------- *■+— Codend — *8 m 4.2 m 15 m 9 m

Direction of tow

Figure 2. (Top) Construction of the 48 mm (full mesh size) polyethylene (PE) cover used to collect escaping fish during the experiments and (bottom) an illustration of the observed shape of it during operation. The lengths of the cover and different sections of the codend with extensions reflect the layout during the experimental periods.

Underwater observations

A towed and remotely operated vehicle (ROV) with a low-light underwater camera was used during the experimental periods to study both fish behaviour and the function of the sorting grid system during towing. The observations were made at depths of 50 to 90 m in natural daylight and lasted for about 30 min. Owing to a visibility of 10-15 m, it was possible to observe the behaviour of fish entering the area of the sorting grids and at closer distances, to examine how small fish escaped between the bars. In addition to these obser­vations, a Simrad FS 3300 scanning sonar placed on the ROV was used to make geometrical measurements of the sorting system and the cover on top of it.

Results

As fish approached the sorting grid area they became more active, turned and started to swim along the towing direction just in front of or underneath the sorting grids. After a few seconds, fish started to seek towards the sorting grids and fish small enough to pass between the

bars escaped rapidly by rising through the system with­out changing their normal swimming position (horizon­tal plane). Larger fish also attempted to escape.

Apart from a few hauls with broken covers owing to large quantities of escaping fish (exceeding an estimated 3-3.5 tonnes), the technique using the PE cover to collect escaped fish functioned as intended. ROV obser­vations had to be made in shallow waters owing to technical problems and limitations with the vehicle at greater depths.

The size and construction of the cover ensured a good spacing between the sorting grids and the upper part of the cover. By scanning sonar examinations, this distance was measured to be almost 4 m during to wings, thus leaving a large space of free water for the escaping fish. No panic movements of the escaping fish were seen as they passed between the bars of the grids and entered the cover. No major differences with respect to escape behaviour were seen between hauls with and without the cover over the grids.

The selectivity results when using the 55 mm sorting grids are presented as selectivity curves for cod and haddock (Figs. 3 and 4) obtained for single hauls during comparable conditions (depth, species, and size compo­sition) in the two periods. The 50% retention lengths and the selection ranges achieved with the 55 mm grids are given in Table 1. Total numbers of fish and numbers within the selection range are listed.

Discussion

Isaksen et al. (1990) found that 50% retention lengths decreased and selection ranges increased for both cod and haddock as catch sizes grew. Despite the masking effect discovered during covered codend hauls, similar results were achieved during trouser trawl codend ex­periments. The most likely explanation for these results

100

t T26 30

T o ta l leng th (cm )

Figure 3. Selectivity curves obtained for cod (Gadus morhua L.) during experiments in the periods 27 August to 9 September 1990 (solid curve) and 25 August to 8 September 1991 (broken curve) with the 55 mm Sort-X on fishing grounds along the coast of northern Norway.

180

ce«S

o 1 ' - = * ' - + - •

24 28 40 44 4g

Total length (cm)

Figure 4. Selectivity curves obtained for haddock (Melano­grammus aeglefinus L.) during experiments in the periods 27 August to 9 September 1990 (solid curve) and 25 August to 8 September 1991 (broken curve) with the 55 mm Sort-X on fishing grounds along the coast of northern Norway.

is reduced effective mesh openings with increased catch and hence drag of the codend. The 50% retention lengths of cod and haddock obtained with 55 mm sorting grids are comparable to those achieved with small catches in 135 mm codends. During trouser trawl experi­ments (135 mm codend), selection ranges of 14.2 cm and 16.1 cm for cod and haddock, respectively, were ob­tained (Isaksen et al., 1990) for catch sizes comparable with those in the grid experiments.

The effects on grid selectivity of using the cover and codend blinder were examined during May 1990 (Lar­sen, 1990). The size composition of fish in the blinded trawl codend did not differ between hauls with and without the cover. Furthermore, only a few cod within the size range of 55 to 75 cm, i.e. the upper part of the selection curve, escaped the codend when the blinder was removed. The size distributions of smaller cod (35 to 55 cm) and haddock (40 to 70 cm) did not differ between the blinded (48 mm) codend and the normal 135 mm codend.

Fish behaviour and escape may be the result of a very complicated set of stimuli. On the deeper fishing grounds and during darkness keywords to explain grid escape may be water flow, grid angles and the size of the

escape area, whereas in shallower waters visual stimuli may be more important and vision may be the dominant sense. Owing to the technical limitations of underwater cameras, however, all observations on fish behaviour and escape have so far been made in shallow waters (working to a maximum depth of 150 m) and in con­

ditions of relatively high levels of light. During the grid escape observations (at depths of 50-90 m during day­time), fish did react to the grid section, i.e. they turned just in front of the grids and held station by swimming for a few seconds before escaping rapidly between the bars. In a comparable manner fish have been observed to escape, or make attempts to escape, as soon as they enter an area of square mesh netting in trawls (Engås et al., 1989). One likely explanation for this reaction may be a change of light penetrating the trawl where the net changes from normal diamond mesh to square mesh. Square mesh with a larger distance between opposite bars permits more light to penetrate the codend.

The given angle of attack of the first sorting grid reduces the distance between the grids and the lower panel of the net from about 80 cm to 20-30 cm. The aft section of the sorting system, i.e. the PVC-canvas covered frame, acts like a reversed kite during towing and hence ensures a narrow entrance into the codend behind it. The likelihood that fish will encounter the sorting system is therefore high owing to its geometrical shape, and the poor swimming ability and speed of smaller fish (Wardle, 1977) when related to normal trawl towing speeds.

Successful escape through a (codend) mesh is achieved when fish are able to hit the mesh opening precisely. However, to do so, fish must change their swimming position from a horizontal to an upright and vertical swimming position, which will be more difficult at higher water velocities. Finally, escape success will depend upon the strength of fish to wriggle their body through the flexible mesh. No principal differences between diamond-mesh and square-mesh codends are observed with respect to the escape pattern of fish (Engås et al., 1989). The major difference between a codend mesh and the opening between the bars in the grids is the length of the escape area. Every slot in the

Table 1. Single haul results on 50% retention lengths and selection ranges obtained with the 55 mm Sort-X in bottom trawls during 27 August to 9 September 1990 (“Anny Krämer”) and 25 August to 8 September 1991 (“Skjervöyfisk”).

Species Year

50% retention

length (cm)

Selection range

(s.r.) (cm)Number of

fishNumber

within s.r.

Cod ( Gadus 1990 49.6 8.5 1000 369morhua L.) 1991 49.5 7.4 440 135

Haddock 1990 47.8 6.3 4920 1401(Melanogrammus aeglefinus L.)

1991 50.2 4.6 3026 213

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grids has a fixed opening of 55 x 750 mm. Fish are thus given more time to escape by a factor of 6-7 compared with a diamond 135 mm mesh. The size and shape of the escape area of the grids may explain why fish appear to escape more easily, and smoothly, than through codend meshes.

Acknowledgements

We are grateful to colleagues and vessel crews for their assistance during the experiments. Thanks are due to the Directorate of Fisheries for financial support and com­ments during the experiments with sorting grids.

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