Study

The impact of turtle excluder devices and bycatch reduction devices on diverse tropical marine communities in Australia's northern prawn trawl fishery.

  • Published source details Brewer D., Heales D., Milton D., Dell Q., Fry G., Venables B. & Jones P. (2006) The impact of turtle excluder devices and bycatch reduction devices on diverse tropical marine communities in Australia's northern prawn trawl fishery.. Fisheries Research, 81, 176-188.

Actions

This study is summarised as evidence for the following.

Action Category

Fit one or more mesh escape panels/windows and one or more soft, rigid or semi-rigid grids or frames to trawl nets

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Subtidal Benthic Invertebrate Conservation

Fit one or more soft, semi-rigid, or rigid grids or frames to trawl nets

Action Link
Subtidal Benthic Invertebrate Conservation

Use a different design or configuration of size-sorting escape grid/system in trawl fishing gear (bottom and mid-water)

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Marine Fish Conservation

Fit a size-sorting escape grid (rigid or flexible) and large, supported escape openings to trawl nets

Action Link
Marine Fish Conservation

Fit large, supported escape openings (such as Fisheyes, Bigeyes and radial escape sections) to trawl nets

Action Link
Marine Fish Conservation

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

Action Link
Subtidal Benthic Invertebrate Conservation

Fit mesh escape panels/windows to a trawl net

Action Link
Marine Fish Conservation

Fit a size-sorting escape grid (rigid or flexible) to a prawn/shrimp trawl net

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Marine Fish Conservation
  1. Fit one or more mesh escape panels/windows and one or more soft, rigid or semi-rigid grids or frames to trawl nets

    A replicated, paired, controlled study in 2001 in areas of seabed in the Gulf of Carpentaria, northern Australia (Brewer et al. 2006) found that nets fitted with a mesh escape window (“bycatch reduction device”) and a grid (“turtle excluder device”), caught fewer large sponges and reduced the total weight of small unwanted catch (invertebrates and fish combined), compared to unmodified nets. Nets fitted with both escape window and grid caught 85% fewer large sponges and reduced the weight of small unwanted catch by 8%, compared to unmodified nets. The modified nets reduced the catch of commercially targeted prawns by 6%. The use of a “turtle excluder device” and a “bycatch reduction device” has been compulsory since 2000 in the Australian prawn fishery. Commercial vessels towed twin Florida Flyer prawn trawl nets from each side of the vessels in August–November 2001. Modified nets were fitted with both one of two designs of escape window (a “Bigeye” design or a square-mesh escape window) and one of 23 grid designs (rigid or semi-rigid frame with ≤120 mm bar spacing and an opening of ≥700 mm). A modified and an unmodified net were randomly assigned to either side of the vessel and towed simultaneously (324 modified nets examined for small unwanted catch, 150 for sponges; 703 unmodified nets for small unwanted catch, 339 for sponges). Total weights of small unwanted catch (<300 mm), commercially targeted prawns, and counts of sponges (>300 mm) were recorded. The combinations of various device designs were not compared. The “Bigeye” design was later removed from the Australian list of approved escape zone designs.

    (Summarised by: Anaëlle Lemasson & Laura Pettit)

  2. Fit one or more soft, semi-rigid, or rigid grids or frames to trawl nets

    A replicated, paired, controlled study in 2001 in areas of seabed in the Gulf of Carpentaria, northern Australia (Brewer et al. 2006) found that the effects of fitting a grid (“turtle excluder device”) to trawl nets, on large sponges and small unwanted catch (invertebrates and fish combined) varied with the device orientation. Nets fitted with a device oriented either ‘downward’ or ‘upward’ caught 82–96% fewer large sponges, compared to unmodified nets, but only the ‘downward’ devices reduced the weight of small unwanted catch (by 8%; data not provided for the ‘upward’ device). Compared to unmodified nets, nets fitted with a ‘downward’ device reduced the catch of commercially targeted prawns by 6%, while those with an ‘upward’ device caught similar amounts. The use of a “turtle excluder device” has been compulsory since 2000 in the Australian prawn fishery (as well as the use of a “bycatch reduction device”). Commercial vessels towed twin Florida Flyer prawn trawl nets from each side of the vessel in August–November 2001. Nets with one of 23 grid designs (rigid or semi-rigid frame with ≤120 mm bar spacing and an opening of ≥700 mm) grouped as either ‘upward’ (9 devices) or ‘downward’ (14 devices) oriented (267 nets examined for small unwanted catch, 392 for sponges) and an unmodified net (339 for sponges, 703 for small unwanted catch) 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.

    (Summarised by: Anaëlle Lemasson & Laura Pettit)

  3. Use a different design or configuration of size-sorting escape grid/system in trawl fishing gear (bottom and mid-water)

    A replicated, paired, controlled study in 2001 of bottom fishing grounds in the Arafura Sea, Australia (Brewer et al. 2006) found that using a different configuration and type of size-sorting escape grid (upward or downward opening) in prawn trawl nets increased the escape rate of unwanted sharks Selachii and to a lesser extent rays Batoidea, but not sawfish Pristidae, compared to trawl nets with no grid. For sharks, catches were reduced by 20% with upward-excluding grids compared to no grid and were more effective than downward-excluding grids that reduced catches by 9% compared to no grid. For rays the opposite effect was found with catches reduced by slightly more from a downward-excluding grid (35%) than an upward-excluding grid (27%) compared to no grid. No grid system reduced catches of sawfish. Data comparing grid types was not tested statistically. Data were collected from up to 1,612 paired trawl comparisons (3,224 nets sampled over 442 nights of trawling) from 23 different vessels in August-November 2001, in the Gulf of Carpentaria, testing a range of catch reduction devices were tested. Nets with and without escape grids (varied designs) were towed simultaneously from one randomly assigned side of each vessel. Escape grid designs included 14 downward-excluding grids and nine upward-excluding grids, made either of stainless steel or aluminium and with or without guiding panels/funnels (see paper for specifications). All codend catches were sorted and identified by species, weighed and counted.

    (Summarised by: Leo Clarke)

  4. Fit a size-sorting escape grid (rigid or flexible) and large, supported escape openings to trawl nets

    A replicated, paired, controlled study in 2001 of bottom fishing grounds in the Gulf of Carpentaria, Australia (Brewer et al. 2006) found that prawn trawl nets fitted with a large, supported opening (a ‘Bigeye’) and a size-sorting escape grid reduced the catch of unwanted sharks and rays Batoidea, but not sawfish Pristidae, compared to a conventional diamond mesh trawl net. Shark catches were reduced by 18% and ray catches by 36% in trawls with a ‘Bigeye’ opening in combination with an excluder grid, compared to a conventional trawl. There was no statistical difference in overall catch of sawfish between gear types (grid/Bigeye: 17 fish, conventional: 32 fish). Trawl nets with a Bigeye/grid reduced commercial target prawn Penaidae catches by 6% compared to the conventional trawl. In August-November 2001, data were collected from 23 prawn fishing vessels towing twin trawls, one modified and one conventional, with a total of 1,612 tows for 3–4 hours each, on either side of the vessel. Modified trawls were fitted with an upward-angled grid with an escape outlet plus a ‘Bigeye’ escape opening nearer to the codend and were paired with a conventional trawl used by the prawn fishery. See original study for full escape panel specifications. All catch was identified, weighed, and counted.

    (Summarised by: Leo Clarke)

  5. Fit large, supported escape openings (such as Fisheyes, Bigeyes and radial escape sections) to trawl nets

    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.

    (Summarised by: Leo Clarke)

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

    A replicated, paired, controlled study in 2001 in seabed areas in the Gulf of Carpentaria, northern Australia (Brewer et al. 2006) 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.

    (Summarised by: Anaëlle Lemasson & Laura Pettit)

  7. Fit mesh escape panels/windows to a trawl net

    A replicated, paired, controlled study in 2001 of bottom fishing grounds in the Gulf of Carpentaria, Australia (Brewer et al. 2006) found that prawn trawl nets fitted with either a top square mesh escape panel or a large, supported opening (‘Bigeye’), reduced unwanted catch of sharks Selachii, but not rays Batoidea and sawfish Pristidae, compared to conventional diamond mesh trawl nets. Shark catches were reduced by 17% in trawl nets fitted with one of two types of fish escape opening (results combined for nets with either a square mesh panel or a Bigeye opening) and catches of rays and sawfish were similar. In addition, total target prawn catch was similar in nets with a square mesh panel and reduced by 4% with the Bigeye escape opening. Data were collected from up to 1,612 paired trawl comparisons (3,224 nets sampled over 442 nights of trawling) from 23 different vessels in August-November 2001, in which a wide range of catch reduction devices were tested. Standard prawn trawl nets fitted with either a square mesh panel or a Bigeye large escape opening and standard nets without an escape panel/opening were towed simultaneously from one randomly assigned side of each vessel (see paper for specifications). All codend catches were sorted and identified by species, weighed and counted.

    (Summarised by: Leo Clarke)

  8. Fit a size-sorting escape grid (rigid or flexible) to a prawn/shrimp trawl net

    A replicated, paired, controlled study in 2001 of bottom fishing grounds in the Gulf of Carpentaria, Australia (Brewer et al. 2006) found that prawn trawl nets fitted with rigid or semi-rigid size-sorting escape grids reduced the amount of unwanted shark Selachii and ray Batoidea catch, but not sawfish Pristidae, compared to conventional trawl nets. Across all trawl nets fitted with escape grids, shark and ray catches were reduced by 13% and 31% respectively. Trawl nets fitted with upward-angled grids and top escape openings reduced unwanted shark catch by 20% and rays by 27%, while downward-angled grids and bottom escape openings reduced shark and ray catches by 9% and 35% respectively. No grid system reduced catches of sawfish. Total prawn Penaeidae sp. catch was reduced by 6% with grid-modified trawls, except trawls with an upward-angled grid, which caught similar prawn numbers to conventional trawls. Data were collected from up to 1,612 paired trawl comparisons (3,224 nets sampled over 442 nights of trawling) from 23 different vessels in August-November 2001, in which a wide range of catch reduction devices were tested. Nets with escape grids and nets without a grid system installed were towed simultaneously from one randomly assigned side of each vessel. Escape grid designs varied, with no two vessels having the same design. These included 14 downward-excluding grids and nine upward-excluding grids, made either of stainless steel or aluminium and with or without guiding panels/funnels (see paper for specifications). All codend catches were sorted and identified by species, weighed and counted.

    (Summarised by: Leo Clarke)

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