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Providing evidence to improve practice

Action: Fit one or more mesh escape panels/windows to trawl nets Subtidal Benthic Invertebrate Conservation

Key messages

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  • Seven studies examined the effects of adding one or more mesh escape panels/windows to trawl nets on subtidal benthic invertebrate populations. Six were in the North Sea (Belgium, Netherlands, UK), two in the Thames estuary (UK), one in the English Channel (UK), and one in the Gulf of Carpentaria (Australia).

 

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (7 STUDIES)

  • Overall survival (1 study): One replicated, paired, controlled study in the English Channel and the North Sea found that fitting nets with either one of seven designs of square mesh escape panels (varying mesh size and twine type) led to higher survival rates of invertebrates that escaped the nets compared to unmodified nets.
  • Unwanted catch overall abundance (7 studies): Three of seven replicated, paired, controlled studies in the North Sea, the Thames estuary, the English Channel and the Gulf of Carpentaria  found that trawl nets fitted with one or more mesh escape panels/windows/zones reduced the unwanted catch of invertebrates compared to unmodified nets. Two found mixed effects of fitting escape panels on the unwanted catch of invertebrates and fish depending on the panel design. Two found that trawl nets fitted with escape panels  caught similar amounts of unwanted invertebrates and fish compared to unmodified nets.

OTHERS (7 STUDIES)

  • Commercially targeted catch abundance (7 studies): Three of seven replicated, paired, controlled studies in the North Sea, the Thames estuary, the English Channel and the Gulf of Carpentaria, found that trawl nets fitted with one or more mesh escape panels/windows/zones caught similar amounts of all or most commercial species to unmodified nets. Three found mixed effects of fitting escape panels on the catch of all or most commercial species depending on the species and/or panel design. One found that trawl nets fitted with escape panels reduced the catch of commercial species compared to unmodified nets.

Supporting evidence from individual studies

1 

A replicated, paired, controlled study in 1999 in two soft seabed areas in the southern North Sea, Belgium and Thames estuary, UK (Fonteyne & Polet 2002a) found that a modified trawl net with either diamond or square mesh escape zones (“bycatch reduction device”) did not reduce the amount of unwanted invertebrate catch overall compared to a standard unmodified net, and had mixed effects on the catch of individual species. The overall weight of unwanted invertebrates caught was not significantly different from standard nets for the nets with diamond mesh escape zones (diamond: 91; standard: 103 kg) or square mesh escape zones (square: 137; standard: 142 kg). Of the 11 unwanted invertebrate species caught, nets with diamond mesh escape zones reduced the catch of one, increased the catch of two, and caught similar amounts of the remaining eight, compared to standard nets. Nets with square mesh escape zones caught similar amounts of all species. Both designs of escape zones caught similar amounts for five of the six commercial species caught and reduced the catch of one species (plaice Pleuronectes platessa) by 15–18%. The escape zone (large diamond or large square mesh; 400 mm) were fitted to a beam trawl net (4 m) behind the ground rope. Fishing took place simultaneously with one modified and one standard unmodified net by attaching the two nets to an 8 m beam. Hauls (10 for diamond mesh; 6 for square mesh) were conducted in March 1999 in 20–50 m water depth. Total catch weights were recorded, and all invertebrate species were separated, weighed and identified to species level.

A replicated, controlled study in 1999–2000 in two soft seabed areas in the southern North Sea, Belgium and Thames estuary, UK (Fonteyne & Polet 2002b) found that overall when fitted to trawl nets, square mesh windows ( “bycatch reduction device”) of three different sizes reduced unwanted catch of invertebrates, compared to nets without a device, and had mixed effects on the catch of individual species. The windows decreased the overall weight of unwanted invertebrates caught by 64–83% compared to unmodified nets. The 120 mm window significantly decreased catches of six of 16 species (45–95% reduction). The 150 mm window significantly decreased catches of 11 of 17 species (34–90% reduction). The 200 mm window significantly decreased catches of four of 16 species (92–97% reduction). The 120 mm window increased the catch of one of eight commercially targeted species (by 111%) compared to nets without a device, with no differences for the remaining seven. The 150 mm window did not impact the catch of any commercially targeted species. The 200 mm window decreased catches of two of eight commercially targeted species (by 23–45%) compared to nets without a device, with no differences for the remaining six. Windows of either 120 mm, 150 mm or 200 mm mesh size were fitted to a beam trawl net (4 m) just in front of the codend. Nets were deployed between November and February 2000 at 20–50 m depth during paired hauls (one net with and one without a device; 5–16 hauls/window type; by attaching the two nets to one 8 m beam). All unwanted invertebrates were identified, counted, and weighed. Commercial catches were identified and weighed. No comparisons were made between windows of different mesh sizes.

2 

A replicated, paired, controlled study in 2002–2004 in six seabed areas in the western English Channel and the North Sea, UK, Denmark, Germany, Netherlands (Revill & Jennings 2005) found that nets fitted with either one of seven designs of square mesh benthos-release panels (“bycatch reduction device”) caught less non-commercial unwanted catch of invertebrates (discard), compared to unmodified nets, and invertebrates escaping the nets had high survival rates. The two designs that reduced discards the most compared to unmodified nets consisted of 150 mm mesh with 5 mm diameter double twine (with panel: 1,988 individuals caught, without: 9,802 individuals) and 150 mm mesh with 6 mm diameter single twine (with panel: 5,286 individuals, without: 21,128 individuals). Overall survival rates (all designs combined) of escaped invertebrates were high (93–100% depending on species). In addition, five of the seven designs caught a similar amount of commercially targeted species (including the two that led to the greatest reductions in discards). These five designs reduced invertebrate discard by 48–80%. The other two designs led to 17–20% losses of target species (reductions in invertebrate discards not shown). The designs were tested on commercial beam trawls at 20–80 m depth. One trawl fitted with a panel and an unmodified trawl were towed simultaneously (4–24 tows/design). All commercial fish caught during the trials were counted and measured, and benthic invertebrates were counted and identified to species level. Invertebrates that had escaped through the panels were caught in a sled fitted to the underside of the trawl, and their survival in tanks assessed over three days.

3 

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.

4 

A replicated, paired, controlled study in 2001 in seabed areas in the Gulf of Carpentaria, northern Australia (Soetaert et al. 2016) found that nets fitted with either one of two escape zone designs (“bycatch reduction device”) did not reduce the numbers of large sponges caught or weight of small unwanted catch (invertebrates and fish combined), compared to unmodified nets. Data were not provided. Nets fitted with a ‘Bigeye’ escape zone reduced the catch of commercially targeted prawns by 4.2% compared to an unmodified net, while nets fitted with a square-mesh escape panel caught similar amounts. The use of a “bycatch reduction device” has been compulsory since 2000 in the Australian prawn fishery (as well as the use of a “turtle excluder device”). Commercial vessels towed twin Florida Flyer prawn trawl nets from each side of the vessel in August–November 2001. Nets fitted with one of the two designs of escape zone (112 nets examined for small bycatch, 97 for sponges) and an unmodified net (703 for small bycatch, 339 for sponges) were randomly assigned to either side of the vessel. Total weights of small unwanted catch (<300 mm), commercially targeted prawns, and counts of sponges (>300 mm) were recorded. The “Bigeye” design was later removed from the Australian list of approved designs.

5 

A replicated, paired, controlled study in 2014–2015 in five seabed areas in the North Sea, UK and Belgium (Soetaert et al. 2016) found that overall, when fitted to trawl nets, all four designs of square-meshed window (“bycatch reduction device”) tested reduced the non-commercial unwanted catch of invertebrates (discard), compared to unmodified nets without a device. Fitting nets with either design (window of either 150 mm, 200 mm, 240 mm mesh, or a 240 mm window with electrical pulse; see paper for details) decreased the catch of all invertebrate discard species recorded by 9–100% compared to unmodified nets. The 150 mm window significantly decreased catches of 11 of 15 species (55–91% reduction). The 200 mm window significantly decreased catches of 10 of 14 species (9–92% reduction). The 240 mm window significantly decreased catches of nine of 18 species (38–97% reduction). The electrified 240 mm window significantly decreased catches of 15 of 19 species (58–100% reduction). All devices reduced catches of commercially targeted species compared to nets without a device, by between 5 and 22%. Invertebrate discard was compared in nets with and without a device. Nets were deployed by two vessels during 58 paired hauls for 1.5 hour (one net with and one without a device; 10–22 hauls/device type). All invertebrate discards were identified, counted, and weighed from 5–8 kg subsamples. Commercially targeted catches were weighed. No comparisons were made between windows of different designs.

Referenced papers

Please cite as:

Lemasson, A.J., Pettit, L.R., Smith, R.K., and Sutherland, W.J. (2019) Subtidal Benthic Invertebrate Conservation: Global Evidence for the Effects of Interventions. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.