Fit large, supported escape openings (such as Fisheyes, Bigeyes and radial escape sections) to trawl nets
Overall effectiveness category Awaiting assessment
Number of studies: 8
Background information and definitions
Large, supported openings in trawl nets are catch reduction devices consisting of a large slit or hole, held open by floats and weights (e.g. Bigeye) or by supporting frames (e.g. Fisheye). They are typically used in prawn/shrimp trawls with the aim of allowing unwanted fish catch to escape while retaining the target crustacean species. Unlike other escape devices used in prawn/shrimp trawls such as size-sorting grids, these devices work by exploiting the behaviour of fish that are able to swim back and escape through the exit openings. Here, we also include radial escape sections that consist of a section of multiple large rigid openings running around the circumference of the net, because they too enable fish that can swim forward away from the codend to escape through the openings, unlike the prawns/shrimps. Use of large, supported openings in trawls potentially facilitate the escape of unwanted fish, including immature small fish, from the codend. They may also be used in addition to size-sorting escape grids to further increase the overall selectivity of the trawl net by providing another area for fish escape.
Evidence of the effect of this intervention when used in combination with another intervention to reduce unwanted catch in trawl nets is summarized under ‘Fishing gear modification - Fit a size-sorting escape grid (rigid or flexible) and large, supported escape openings to trawl nets’.
Supporting evidence from individual studies
A replicated, paired, controlled study in 1993 of three coastal seabed areas in the Gulf of Mexico, USA (Rogers et al. 1997) found that shrimp trawl codends fitted with large, supported openings (single or radial), reduced the unwanted fish catch compared to unmodified codends without escape openings. For seven of eight different designs of escape opening devices, fish catch was reduced by 22–55% in number (with: 74–248 fish/tow, without: 124–422 fish/tow), and by 27–62% in weight (with: 1,127–2,750 g/tow, without: 2,152–5,530 g/tow). One of eight designs had a 50% lower weight of unwanted fish than a net without escape openings (with:1,245 g/tow, without: 2,489 g/tow) but catch numbers were similar (with:177 fish/tow, without: 170 fish/tow). Target commercial shrimp Penaeidae catches were reduced by 5–39% in number (5–35% in weight) across all eight designs of escape opening devices, however three were not statistically different to catches without a device. Data were collected in spring and autumn 1993 during the shrimp fishing seasons from 36 twin trawl deployments (12/area/season). They were done using a net with one of four designs of supported escape openings towed simultaneously with an unmodified net. In the autumn, modified versions of each of the original four escape opening designs were tested in the same way. Escape opening devices consisted of either a single large framed opening or a section of large mesh openings running around the net circumference. Each design also had a mesh guiding funnel inside the net leading to the escape sections (see original paper for full specifications). Fish and shrimp in each codend were identified, counted and weighed in the laboratory.Study and other actions tested
A replicated, paired, controlled study in 1995 of a fished area of seabed in the Gulf of Carpentaria off Australia (Brewer et al. 1998) found that prawn trawl nets fitted with one of two designs of large, supported escape openings (single or radial) typically reduced the catch of unwanted fish compared to unmodified standard trawl nets. Compared to catches in the standard net, average catch weights of unwanted fish in nets fitted with a radial cylinder design of escape opening were lower in two of two trials, by 20–40%. Unwanted catch of fish in trawl nets fitted with a single escape opening (Fisheye) were similar by weight between trawls in three of three cases (69–90% of the unwanted fish catch in standard catches). Catch weights of commercial target prawns Penaeidae in trawls fitted with the radial escape section were similar in one of two cases with the radial cylinder escape design relative to the standard and reduced by 12% in the other. The Fisheye system caught similar weights of target prawns in three of three cases. Data were collected in scientific trials in February (two trials) and October 1995 from deployments of standard twin trawl prawn nets (45 mm codend mesh). Nets were fitted on one side with one of two designs of large supported escape openings and towed simultaneously with unmodified trawls or other combinations of net designs also being tested (see original paper for full gear specification). A total of 29 hauls (30 min or 2 h) were made with the radial escape design, 28 hauls with the Fisheye escape design and 71 with the standard trawl. Fish and prawns in codend catches were counted and weighed.Study and other actions tested
A replicated, paired, controlled study in 2001 of four fished areas of seabed in the Gulf of Carpentaria off Australia (Brewer et al. 2006) found that prawn trawl nets modified with large, supported escape openings (Fisheyes or Bigeyes) or a square mesh escape panel reduced the unwanted catch of sharks Selachii, but not rays Batoidea or sawfish Pristidae, compared to conventional diamond mesh nets. Shark catches were 17% lower in nets modified with either a supported escape opening or a square mesh escape panel (data were combined) compared to conventional nets. There were no differences in catches of rays and sawfish between modified and conventional nets (data reported as statistical results). In addition, when used in combination with any type of size-sorting escape grid, shark and ray catches in modified nets were reduced by 18% and 36%, respectively, compared to conventional nets, but sawfish catches were unaffected. Commercial target prawn Penaeidae catch was reduced by 4% in nets modified with one type of large escape opening (Bigeye). Data were collected in August-November 2001 by observers onboard 23 different commercial prawn fishing vessels, from 1,612 deployments (3–4 h) using twin trawls. One trawl net was modified with one of three escape opening/panel designs, with or without size-sorting grids, and was towed with a conventional net on the other side of the trawl (see original paper for gear specifications and numbers of deployments/trawl net type). All catch was identified and counted.Study and other actions tested
A replicated, randomized, paired, controlled study in 2001 in a fished area of seabed in the Coral Sea off the coast of Queensland, Australia (Courtney et al. 2006) found that a prawn trawl net modified with large, supported escape openings (radial escape section) reduced the overall amount of unwanted catch (fish and invertebrates) compared to a standard net without escape openings. Total unwanted catch (up to 250 fish and invertebrate species combined) was 19% lower in the net with escape openings relative to the average catch rate of the standard net (11 kg/ha). In addition, when used in a net that also had a size-sorting escape grid, the combined system reduced unwanted catch by 24% relative to the standard net catch. Commercial target eastern king prawn Penaeus plebejus catches were similar between trawl types (data reported as statistical result). Data were collected from 90 experimental paired trawl deployments at 45 locations on a chartered fishing vessel over ten nights in October 2001. Four codend types were towed in blocks of pairs on either side of a twin trawl: one with a radial escape section only, one with a radial escape section and a size-sorting grid (turtle excluder device), one with a grid only (see paper for data) and one a standard codend. After each deployment unwanted catch was weighed and a subsample frozen and sorted by species in a laboratory.Study and other actions tested
A replicated, paired, controlled study in 2002 of a fished area of seabed in the Gulf of Carpentaria, off Australia (Heales et al. 2008) found that using a large, supported escape opening (new design of Fisheye) in a prawn trawl net reduced the amount of unwanted small catch (fish and invertebrates combined) compared to a standard prawn trawl net with no large escape opening. The average catch weight of small unwanted fish and invertebrates was lower with the Fisheye escape opening compared to without (with: 136–219 kg, without: 183–254 kg). In addition, there was no difference in the average catch weights of the commercial target species of tiger prawns, Penaeus esculentus and Penaeus semisulcatus, between trawl nets (with: 13–18 kg, without: 13–19 kg). Data were collected in November 2002 from 29 comparative trawl deployments by a commercial trawler on prawn fishing grounds in the south-western area of the Gulf. The vessel towed a pair of identical prawn trawl nets, both fitted with a compulsory downward-excluding size-sorting grid (Super-shooter type). One of the trawl nets also had a new design of large escape opening (Yarrow Fisheye): a rigid frame on the upper trawl section, creating a semi-round escape opening (see paper for specifications). The combined use of size-sorting excluder grids with other catch reduction devices was made compulsory in Australia’s Northern prawn fishery in 2000. Catches were separated into small unwanted catch (fish and invertebrates combined) and target prawn species and weighed.Study and other actions tested
A replicated, paired, controlled study in 1997–1999 of two coastal seabed areas in the Atlantic Ocean off Florida, USA (Crawford et al. 2011) found that large, supported escape openings (four designs) fitted to a shrimp roller-frame trawl net reduced the overall unwanted catch of finfish in three of four cases compared to a standard net without escape openings. Across all trials, average catch weights (and numbers, see paper for data) of all non-target finfish were lower in nets with an extended-mesh funnel design of escape opening, both with and without a stimulator cone (funnel: 20–206 g/effort, standard: 53–309 g/effort) and in nets with a Florida Fisheye design and stimulator cone (Fisheye/cone: 107 g/effort, standard: 275 g/effort), compared to standard nets. Total finfish catch rates in nets with just a Florida Fisheye (no stimulator cone) were similar to standard nets (Fisheye/ no cone: 112–230 g/effort, standard: 111–248 g/effort), however, the effect varied between individual fish species (see paper for data). Data were collected from 158 paired trawl deployments targeting pink shrimp Farfantepenaeus duorarum in two areas: at Tarpon Springs in October 1997 and March and October 1998 (research vessel), and at Biscayne Bay in November/December 1999 (commercial vessel). One trawl net equipped with one of two escape opening designs (large-mesh extended-mesh funnel or Florida Fisheye), with and without a stimulator cone (nylon webbing designed to prevent finfish from reaching the codend), was towed on one side of each vessel. On the other side a standard net was towed at the same time. Finfish in the catches were sorted by species and weighed.Study and other actions tested
A replicated study in 2009–2010 of a fished area of seabed in the Pacific Ocean off Oregon, USA (Lomeli & Wakefield 2012) found that fish trawl nets fitted with large escape opening systems (two designs) allowed the escape of unwanted Chinook salmon Onchorynchus tshawytscha in two of two designs and widow rockfish Sebastes entomelas in one of two designs. Results were not tested statistically. In trials of two pairs of escape openings, 80–100% of Chinook salmon observed to enter the net escaped (escaped: 1–8 fish, entered: 1–11 fish) and 19–33% of rockfish (escaped: 4–8 fish, entered: 21–24 fish). In trials of a single pair of escape openings, 50% of salmon escaped (escaped: 8 fish, entered: 16 fish), but of the 53 rockfish that entered the net, none were observed to escape through the openings. Data were collected in September 2009 and May, August and September 2010 from video footage recorded during 32 trawl deployments (113–259 m depth) on a commercial vessel targeting Pacific hake Merluccius productus. Two designs of net were used, with either one (14 h video) or two (17 h video) pairs of large escape openings cut out of the netting on the upper portions of both side panels. Two square mesh ramps in front of the openings acted as a guide to actively swimming fish towards the escape openings (see original paper for gear specifications). A total of 23 tows were done with an open codend, the other nine with a closed codend. Fish entering and escaping through the large openings were identified and counted from the video recordings.Study and other actions tested
A replicated, paired, controlled study in 2007–2011 in an area of seabed in the Gulf of Mexico, USA (Parsons & Foster 2015) found that fitting a large escape opening (nested cylinder design) to a shrimp trawl net reduced the unwanted catch of immature red snapper Lutjanus campechanus, compared to a standard net. Across both trials, catch numbers of red snapper were lower in nets with escape openings compared to without (with: 638–877 fish, without: 1,197–1,265). Data were collected from paired trawl deployments carried out on commercial shrimp vessels, in September 2007 off Mississippi (32 deployments) and November 2011 off Texas (32 deployments). One side of a pair of shrimp trawl nets was fitted with a large escape opening design (nested cylinder bycatch reduction device) and the trawl net on other side had no escape opening device (see original paper for gear specifications). Tows were 2–6 hours. Numbers of all immature red snapper caught in each trawl codend were recorded.Study and other actions tested