Action

Modify the configuration of a mesh escape panel/window in a trawl net

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

  • Ten studies examined the effects of modifying the configuration (position/size and increased mesh size) of a mesh escape panel/window in a trawl net on marine fish populations. Four studies were in the Baltic Sea (Sweden/Poland). Two studies were in each of the North Sea (UK), the Irish Sea (UK) and the Kattegat and Skagerrak (Northern Europe). One study was in the Atlantic Ocean (Portugal). 

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (1 STUDY)

  • Survival (1 study): One replicated, controlled study in the Baltic Sea found that modifying the position of a mesh escape panel in a trawl net had no effect on the survival rate of cod.

BEHAVIOUR (0 STUDIES)

OTHER (9 STUDIES)

  • Reduction of unwanted catch (5 studies): Three of five replicated, paired studies (one controlled) in the Irish Sea, Atlantic Ocean and Kattegat-Skagerrak found that modifying the position or mesh size of a mesh escape panel/window in a trawl net reduced the unwanted catches of whiting in one of two cases, haddock and whiting, and boarfish, but caught similar amounts of horse mackerel and blue whiting. The other studies found that catches of unwanted cod or other fish were not reduced.
  • Improved size-selectivity of fishing gear (4 studies): Two of four replicated, controlled studies in the North Sea and Baltic Sea found that modifying the position and/or size of a mesh escape panel in a trawl net improved size-selectivity of haddock and whiting. One of these studies also found that increasing the mesh size of the panel had no effect on size-selectivity for haddock. The other two studies found that size-selectivity was similar for cod compared to standard trawls.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A replicated, paired study in 1994–1995 of two areas of seabed in the Irish Sea, UK and Ireland (Armstrong et al. 1998) found that trawl nets fitted with a square mesh escape panel located 7 m in front of the codend allowed more undersized whiting Merlangius merlangus to escape in one of two cases, compared to trawl nets with a panel 1 m in front of the codend. In twin trawl net deployments, fewer undersized (<27 cm) whiting were caught with the panel 7 m in front of the codend (72 fish/h) than with the panel immediately in front (131 fish/h). However, in parallel net deployments undersized catch was the same (88 fish/h) between trawl designs. Catch of legally sized whiting (>27 cm) was not statistically different between trawl designs in twin trawls (7 m panel: 40, 1 m panel: 50 fish/h) or parallel trawls (7 m panel: 85, 1 m panel: 83 fish/h). In summer of 1994 and 1995, a total of 43 twin and 40 parallel trawl net deployments were undertaken respectively, using two Nephrops norvegicus trawl nets fitted with a 3 m square mesh (75 mm) escape panel for 4–5 h each. In one trawl net, the panel was fitted 1 m ahead of the codend, consistent with UK regulations, and in the other trawl net it was fitted 7 m ahead of the codend, consistent with Irish regulations. Each trawl net was 70 mm diamond mesh. Twin trawl nets were towed by a single vessel and parallel net deployments were done by two vessels towing a single trawl each. Full details of the trawl designs are provided in the original paper. All fish caught were identified and the length recorded.

    Study and other actions tested
  2. A replicated, controlled study in 1998 of a fished area of seabed in the North Sea, off Scotland, UK (Graham & Kynoch 2001) found that one of two square mesh escape panels located in the codend of a trawl net improved haddock Melanogrammus aeglefinus size-selectivity compared to a panel located in the extension piece or using no panel, but increasing the mesh size of the codend panel had no effect. The length at which haddock had a 50% chance of escaping (selection length) was greater with a square mesh panel of 80 mm mesh located in the codend of the trawl (26 cm), compared to an 80 mm square mesh panel located in the extension piece ahead of the codend (23 cm), and no panel fitted (23 cm). Haddock selection length in a codend panel with a larger mesh size of 100 mm was not significantly higher than the other three cases (30 cm). The authors noted this was probably due to the low catch rates with the 100 mm mesh window. Trials were conducted from a commercial fishing vessel in June 1998, using a twin trawl net. On the starboard side, nets with a 100 mm diamond mesh codend fitted with either a 100 mm square mesh panel, an 80 mm square mesh panel, an 80 mm square mesh panel in the 100 mm diamond mesh extension, or no panel were deployed. A trawl net with a 40 mm codend was towed on the port side. Full details of the trawl designs are provided in the original paper. In total, 16 valid trawl deployments of 60–210 min were completed at 2.6–3.3 knots and 70–81 m depth. Fish were identified and lengths measured.

    Study and other actions tested
  3. A replicated, controlled study in 2000 of an offshore seabed area in the North Sea, Scotland, UK (Graham et al. 2003) found that changing the position of a square mesh escape panel in a trawl net improved haddock Melanogrammus aeglefinus and whiting Merlangius merlangus selectivity. The length at which haddock and whiting had a 50% chance of escaping was larger in nets with a square mesh panel located nearest (3–6 m) to the bag of the codend (haddock: 28.2 cm, whiting: 29.4 cm) than when the panel was located further away , (haddock, 6–9 m: 25.1 cm, 9–12 m: 25.5 cm; whiting, 6–9 m: 26.0 cm, 9–12 m: 27.1 cm) or when not using a panel (haddock:, 23.4 cm; whiting:, 25.0 cm). In August 2000, a twin trawl net was deployed with an experimental net on one side and a small-mesh codend net on the other, towed simultaneously. Nine experimental deployments were carried out with a square mesh panel 3–6 m from the cod-line, six with the panel 6–9 m from the cod-line, eight with the panel 9–12 m from the cod-line and nine without a panel. All codends were 100 mm diamond mesh and 100 mesh circumference. Panels were 3 m long with 90 mm square mesh. Fishing took place in the Buchan Deeps in depths of 68–106 m with tow durations of 2.0–3.5 hours. Fish were identified and lengths measured.

    Study and other actions tested
  4. A replicated, paired study in 1993–1994 of a seabed area in the Atlantic Ocean off Portugal (Campos & Fonseca 2004) found that increasing the mesh size of a square mesh escape window in a shrimp separator trawl net reduced the catch of unwanted boarfish Capros aper, but not horse mackerel Trachurus trachurus and blue whiting Micromesistius poutassou. In trials with a 120 mm separator panel, the escape of boarfish increased with increasing mesh size of the square mesh window, with 42% (1,430 kg) of the overall boarfish catch escaping through a 100 mm window compared to 10% (105 kg) with a 70 mm window mesh size. For horse mackerel, escape percentages were similar between window mesh sizes (100 mm: 33%, 70 mm: 34%). In trials with an 80 mm separator panel and 100 mm mesh escape window, boarfish escape was higher (44%) compared to the 120 mm panel with the smaller 70 mm mesh window, but similar to the 120 mm panel with the same size 100 mm mesh window. There were no differences in escape rates between each panel/window codend for horse mackerel or blue whiting (see paper for data). In addition, there were also no differences in escape rate of all three species between a codend with no panel and a 100 mm window alone, and the 70 mm window in the 80 mm panel codend. Percentage escape of the commercial target species rose shrimp Parapenaeus longirostris and Norway lobster Nephrops norvegicus was similar between mesh sizes with the 120 mm separator panel, however rose shrimp catches varied between the 120 mm and 80 mm mesh panels (see paper for data). Four fishing trials were done in July and September 1993 and May 1994 by research and fishing vessels. In each trial one of four trawl nets was tested: three with different separator panel/escape window mesh size combinations and one with the window alone (see paper for specifications). For each net type, six or seven experimental hauls were done, 26 in total, and catches were retained in the upper and lower codends. Fish escaping through the square mesh window were collected in a small mesh cover mounted over the escape window. Codend and cover catches were sorted by species, weighed and lengths recorded.

    Study and other actions tested
  5. A replicated, controlled study in 1998 of seabed in a coastal bay in the Baltic Sea, Sweden (Suuronen et al. 2005) found that the survival of cod Gadus morhua escaping from trawl nets fitted with two different configurations of square mesh escape panels was similar compared to a standard trawl net with no escape window. Survival of escaped cod was 96-100% from a trawl with a single square mesh panel in the top of the codend, 92–100% from a trawl with two square mesh panels mounted on either side of the codend below the side seams, and 93-100% from a standard trawl with no escape panel. In addition, the number of fish with skin injuries was similar between trawl types (data reported as statistical results). Results where seawater temperatures were relatively high (>9oC) were excluded. In August-September 1998, a total of 19 trawl deployments were done: four with a 105 mm knotless square mesh top panel (Bacoma window) fitted in a standard 120 mm diamond mesh trawl codend, eight with a codend with 105 mm square mesh escape panels on both sides, and seven with a standard trawl net of 105 mm diamond mesh codend. Full gear specifications are given in the original paper. Deployments were carried out on a twin-rig fishing vessel for 3 h each at 30-55 m depth. Cod escaping through the square mesh panels were collected in cages attached to the end of the trawl nets, towed to the seabed and kept in cages for 12–14 days, after which survival and visible skin damage were recorded.

    Study and other actions tested
  6. A replicated, paired study in 2003 of seabed areas in the Kattegat and Skagerrak, Northern Europe (Krag et al. 2008) found that changing the position of a square mesh escape panel in a Norway lobster Nephrops norvegicus trawl net did not reduce the catches of unwanted cod and other fish. The total number of small unwanted cod and the weight of other unwanted fish caught were similar in a trawl with a square mesh panel in the extension piece (ahead of the codend) compared to a square mesh panel in the middle of the codend (cod: 420–423 fish, all other fish: 1,567–1,590 kg). The total number of Norway lobster caught was lower in the trawl with a square mesh panel in the middle of the codend compared to the square mesh panel at the front of the codend (middle: 16,458, front: 19,735 lobsters). In August-September 2003, a total of 24 experimental trawl deployments were conducted in the North Sea by a vessel towing twin 80 mm diamond mesh trawl net codends. One of the two nets had a 90 mm square mesh panel in the top panel of the extension section of the codend (6–9 m from end), and the other had an identical 90 mm square mesh panel in the centre (3–6 m) of the top panel of the codend. Tow duration was 7 hours at a speed of 2.5 knots. All fish were identified and weighed, and the most abundant species were counted.

    Study and other actions tested
  7. A replicated, paired study in 2003 on fishing grounds in the Kattegat and Skagerrak, northern Europe (Krag et al. 2008) found that increasing the mesh size of a square mesh escape panel in the codend of a Norway lobster Nephrops norvegicus trawl net did not typically reduce the amount of unwanted fish. For six of eight target fish species, numbers of unwanted small fish were similar between mesh sizes of the escape panel (90 mm: 391–1,161: 120 mm: 275–666) and lower with the larger mesh size for two species (90 mm: 188, 120 mm: 52). Weight of other unwanted fish catch was similar between escape panel mesh sizes (90 mm: 1,642 kg, 120 mm: 1,833 kg). In addition, nets with a larger escape panel mesh size had a lower overall number (9,739) and a lower number of small Norway lobster (5,168) compared to the smaller mesh size (overall: 10, 738, small: 6,175). In August-September 2003, two diamond mesh codends were tested, one fitted with a 90 mm mesh square mesh escape panel and one with a 120 mm square mesh escape panel. A total of 20 paired trawl deployments were done on a twin-trawl fishing vessel. Catches were sorted into commercial (lengths and weights) and non-commercial (weights) categories.

    Study and other actions tested
  8. A replicated, paired, controlled study in 2008–2009 of seabed areas in the Irish Sea, off Northern Ireland, UK (Briggs 2010) found that prawn trawl nets with a modified design and position of square mesh escape panel caught fewer unwanted haddock Melanogrammus aeglefinus and whiting Merlangius merlangus compared with the standard trawl design and configuration used in the Norway lobster Nephrops norvegicus fishery. The percentage total catch of unwanted haddock and whiting was lower in the trawl with a modified square mesh panel (haddock: 32%, whiting: 26%) than the trawl with a standard panel (haddock: 68%, whiting: 74%). In addition, capture of the target Norway lobster were the same (modified: 50%, standard: 50%). Trials were conducted in October 2008–March 2009 using a twin-rig vessel towing two trawl nets simultaneously. One was an experimental net with two 120 mm square mesh panels located 8.9 m from the codend and separated by a strip of 80 mm diamond mesh 12 meshes wide. The other was a standard 80 mm mesh net with a standard 80 mm square mesh panel 2.1 m from the codend. A total of 16 trawl deployments of 3–4 hours were completed, and nets were regularly swapped between port and starboard sides.

    Study and other actions tested
  9. A replicated, controlled study in 2000 of areas of seabed in the Bornholm Deep, Baltic Sea, between Poland and Sweden (Madsen et al. 2010) found that trawl nets fitted with square mesh escape windows either in the bottom or top of the codend did not affect cod Gadus morhua selectivity, compared to a standard codend with no square mesh windows. Overall, the length at which cod had a 50% chance of escape was similar between trawl types (bottom windows: 44.6–50.3 cm, top window: 49.3–50.3 cm, no windows: 48.0–49.3 cm). However, this decreased with increasing catch weights in bottom windows in one of two cases, and with no window in one of two cases. In addition, more cod escaped with bottom windows in one of two cases (top: 6,906 fish, bottom: 18,765 fish, none: 13,202 fish) and with a top window in the other case (top: 27,616 fish, bottom: 17,026 fish, none: 16,089 specimens). Trials were conducted on two commercial fishing vessels in April–June 2000. A total of 18 trawl deployments were completed using a trawl net with a 122 mm square mesh top window fitted into a 122 mm mesh codend, 16 tows with two 122 mm square mesh windows fitted into the bottom of a 122 mm mesh codend, and 17 tows using a trawl net with a standard 136 mm mesh codend. Full gear specifications are given in the original paper. Tow duration was 8–11 hours at a speed of 2.5–2.6 knots and depths of 91–95 m. A cover was applied to each codend to retain and sample escaped cod.

    Study and other actions tested
  10. A replicated, controlled study in 2012 of a seabed area in the western Baltic Sea, northern Europe (Herrmann et al. 2015) found that changing the size and position of a square mesh escape panel in the codend of a trawl net did not increase the likelihood of escape of small Atlantic cod Gadus morhua, compared to a standard trawl design with a square mesh panel codend (Bacoma). The length at which cod had a 50% chance of escape was similar in codends with small square mesh panels located either towards the back (30.2–40.3 cm) of the codend or at the front of the codend (30.6–41.7 cm), compared to the standard large square mesh panel codend (35.2–41.8 cm). For three of the four codends with the small square mesh panel at the front, the length at which cod had a 50% chance of escaping was smaller (21.3–41.7 cm) (two codends not tested for significance). Trials were conducted in March/April and September 2012 in the Arkona Basin. A total of 41 alternate haul deployments were done with six separate codends, comprising different combinations of size/position of square mesh panel, one of them the established Bacoma design, the other five with a 50% smaller square mesh panel, four of which had the panel at the front of the codend. A cover was applied to each codend to retain and sample escaped cod. Full gear specifications are given in the original paper. Catches from both the codends and the covers were sorted, and all cod counted and lengths measured.

    Study and other actions tested
Please cite as:

Taylor, N., Clarke, L.J., Alliji, K., Barrett, C., McIntyre, R., Smith, R.K., and Sutherland, W.J. (2021) Marine Fish Conservation: Global Evidence for the Effects of Selected Interventions. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

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Marine Fish Conservation

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Marine Fish Conservation
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