New approaches to the reduction of non-target mortality in beam trawling.

Modify design or arrangement of tickler chains/chain mats in a bottom trawl

A replicated, paired, controlled study in 1999 on bottom fishing grounds in the North Sea between the Netherlands and UK (van Marlen et al. 2005) found that two different modifications to the way tickler chains were attached to a beam trawl (hanging parallel from, or in loops along, the beam) did not reduce the discarded fish catch, compared to a conventional tickler chain arrangement. For unwanted fish (undersized commercial target species and all other non-target species), there were no differences in catches between the modified and conventional tickler chain arrangements, except for small whiting Merlangius merlangus, of which fewer were caught by the modified gear (data not reported). Data were collected from a series of beam trawl deployments along six parallel strips on the seabed (2,000 m × 30 m) using two modified and one conventional tickler chain arrangement. In March-April 1999, a total of 72 deployments were carried out with three different configurations of parallel chains (numbers and spacing, connected pairs) hung along the beam. In October 1999, a total of 35 deployments were undertaken with three configurations of chains hung in loops from the beam. In addition, a standard trawl with conventional tickler chain arrangement (attached to the shoe plates on either end of the beam) was towed simultaneously during each deployment. All catch was weighed.

(Summarised by: Rosslyn McIntyre)

Reduce the number or modify the arrangement of tickler chains/chain mats on trawl nets

A replicated, paired, controlled study in 1999 in one area of seabed in the North Sea, Netherlands (Van Marlen et al. 2005a) found that none of three modified tickler chain arrangements for trawl nets reduced the amount of non-commercial unwanted invertebrates and fish catch (discard), compared to unmodified trawl nets. Nets modified with two of the three tickler chain arrangements tested caught similar amount of discard to unmodified nets (153–175 vs 145–166 kg/h). The third arrangement (25 cm spacing) caught more discard than unmodified nets (123 vs 112 kg/h). All modified nets caught similar amounts of commercial species to unmodified nets (35–50 vs 33–36 kg/h). In conventional tickler chain rigging, both ends of chains are attached at either ends of the beam. Three parabolic tickler chain arrangements, where attachment points are distributed along the beam, were tested on trawl nets: 25 cm spacing; 40 cm spacing; 25 cm spacing with 35 cm for the centre chain. In October 1999, each arrangement was compared a conventional tickler chain during 5–17 paired simultaneous deployments along parallel strips (2,000 x 30 m). Catches were sorted into commercial and discard species, and each group weighed.

A replicated, paired, controlled study in 1999 in one area of seabed in the North Sea, Netherlands (Van Marlen et al. 2005b) found that all three modified tickler chain arrangements for trawl nets reduced the amount of non-commercial unwanted invertebrates and fish catch (discard), compared to unmodified trawl nets. Nets modified with either of three tickler chain arrangements tested caught less discard than unmodified nets (46–80 vs 80–117 kg/h). However, all modified nets also caught lower amounts of commercial species compared to unmodified nets (43–49 vs 52–58 kg/h). In conventional tickler chain rigging, both ends of chains are attached at either ends of the beam. Three parallel tickler chain arrangements, where chains are distributed along the beam but only attached at one end, were tested on trawl nets: 21 chains, 50 cm spacing; 29 chains, 35 cm spacing; 29 chains, 35 cm spacing with 10 connected pairs. In March–April 1999, each arrangement was compared to a conventional tickler chain during 11–42 paired simultaneous deployments along parallel strips (2,000 x 30 m). Catches were sorted into commercial and discard species, and each group weighed.

(Summarised by: Anaëlle Lemasson & Laura Pettit)

Fit one or more mesh escape panels/windows to trawl nets

A replicated, paired, controlled study in 1999 in one seabed area in the North Sea, Netherlands (Van Marlen et al. 2005a) found that trawl nets modified by adding one of two designs of diamond mesh drop out panels (“bycatch reduction device”) caught less non-commercial unwanted species of invertebrates and fish (combined as discards) compared to unmodified trawl nets. Nets fitted with a 720 mm mesh panel caught less discard (75 kg/h) than unmodified nets (87 kg/h), but nets fitted with a 120 mm mesh panel caught similar amounts (33 kg/h) to unmodified nets (34 kg/h). All modified nets caught similar amounts of commercial species (14–17 kg/h) to unmodified nets (14–15 kg/h). In January 1999, a trawl net fitted with a panel design (escape zone; each panel had 19 diamonds of either 720 mm or 120 mm) was compared to an unmodified net during 14–18 paired simultaneous deployments along parallel strips (2,000 x 30 m). Catches were sorted into commercial species (fishery target and other commercially valuable species) and discards, and each group weighed.

A replicated, paired, controlled study in 1999 in one seabed area in the North Sea, Netherlands (Van Marlen et al. 2005b) found that for three of four panel designs, trawl nets modified by adding a diamond mesh drop out panel (“bycatch reduction device”) reduced the amount of non-commercial unwanted species of invertebrates and fish (combined as discards) compared to unmodified trawl nets. Nets fitted with either one of three drop out panel designs caught less discards (94–110 kg/h) than unmodified nets (123–128 kg/h). Nets fitted with the fourth design (16 meshes of 100 mm) caught similar amounts (102 kg/h) to unmodified nets (136 kg/h). All modified nets caught similar amounts of commercial species (14–17 kg/h) to unmodified nets (14–15 kg/h). Four panel designs (escape zone) were tested on trawl nets: 19 diamonds of 500 mm; 19 diamonds of 100 mm; 16 diamonds of 100 mm; 12 diamonds of 100 mm. In March 1999, each design was compared to an unmodified net during 5–12 paired simultaneous deployments along parallel strips (2,000 x 30 m). Catches were sorted into commercial species (fishery target and other commercially valuable species) and discards, and each group weighed.

(Summarised by: Anaëlle Lemasson & Laura Pettit)

Fit mesh escape panels/windows to a trawl net

A replicated, controlled study in 1999 on a bottom fishing ground in the North Sea, the Netherlands (van Marlen et al. 2005) found that fitting large diamond mesh escape panels (drop-out panel) to beam trawl nets, typically reduced the amount of discarded catch (fish and invertebrates), compared to a standard diamond mesh trawl. Overall, discarded catch was reduced by 3–26% in nets with large mesh panels, irrespective of panel configuration. Nets with panels of 19 large meshes caught less discarded catch in three of four configurations (19 panel: 75–97 kg/h, standard: 86–128 kg/h) and similar amounts in one (19 panel: 33 kg/h, standard: 34 kg/h), and a 500 mm mesh size performed better than 720 mm. Discarded catch was reduced in a 12-mesh, 500 mm panel (12 panel: 110 kg/h, standard: 123 kg/h) and was similar in a 16-mesh, 500 mm panel: (16 panel: 102 kg/h, standard: 136 kg/h). Catches of retained fish of target species, although lower in most configurations, were not significantly reduced in nets with panels (panel: 15–44 kg/h: standard: 14–48 kg/h). Six parallel strips of seabed, 2,000 m x 30 m, were sampled by two research vessels in January and March 1999 on the Oyster Ground. Data were collected from a total of 68 deployments using either an 8- or 12-m beam trawl and one of six configurations of large diamond mesh panel (3 mesh numbers, 2 mesh sizes, with or without a sheet – see paper for specifications), towed simultaneously with a standard diamond mesh trawl net without a panel. Target fish catch and discarded catch of fish and invertebrates combined were weighed.

(Summarised by: Rosslyn McIntyre)