An assessment of bycatch reduction devices in a tropical Australian prawn trawl fishery

Published source details Brewer D., Rawlinson N., Eayrs S. & Burridge C. (1998) An assessment of bycatch reduction devices in a tropical Australian prawn trawl fishery. Fisheries Research, 36, 195-215.
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This study is summarised as evidence for the following.

	Action	Category
	Fit mesh escape panels/windows and a size-sorting grid (rigid or flexible) to a trawl net Action Link
	Install escape devices on fishing gear: Sea turtles Action Link
	Install escape devices on fishing gear: Snakes & lizards Action Link
	Fit a size-sorting escape grid (rigid or flexible) and large, supported escape openings to trawl nets Action Link
	Fit large, supported escape openings (such as Fisheyes, Bigeyes and radial escape sections) to trawl nets Action Link
	Install exclusion and escape devices on fishing gear Action Link
	Fit mesh escape panels/windows to a trawl net Action Link
	Fit a size-sorting escape grid (rigid or flexible) to a prawn/shrimp trawl net Action Link

Fit mesh escape panels/windows and a size-sorting grid (rigid or flexible) to a trawl net

A replicated, paired, controlled study in 1995–1996 of two fished areas of seabed in the Gulf of Carpentaria, Australia (Brewer et al. 1998) found that prawn trawl nets fitted with a square mesh escape panel in combination with a rigid size-sorting grid caught fewer unwanted fish, sharks and stingrays (Elasmobranchii) compared to an unmodified conventional trawl net. In the first of two trials, catch weights of unwanted fish in nets with a square mesh window and a rigid grid (Nordmøre) were reduced by 29–39% relative to a conventional net. In the second trial (commercial conditions), a square mesh window and grid (Super Shooter) caught fewer sharks (3) and stingrays (0) compared to a conventional trawl net (sharks: 12, stingrays: 2). Shark and stingray data were not tested statistically. In addition, target prawn Penaeidae catches were reduced by 17–34% in the square mesh window/Nordmøre system and average catch weight decreased from 41 kg in conventional nets to 38 kg in the square mesh window/Super Shooter system. Catch data were collected from deployments of two modifications of a standard prawn trawl net with a diamond mesh codend, each with a square mesh escape window grid fitted behind a rigid size-sorting grid (Nordmøre or Super Shooter) (see original paper for gear specifications). The modified nets were towed in paired deployments with other modified net designs and/or standard unmodified nets in Australia’s Northern Prawn Fishery area; during experimental trials in October 1995 (73 paired tows, 2h duration) and in commercial trials in October 1996 (24 paired tows, 2–3 h duration).

(Summarised by: Leo Clarke)
Install escape devices on fishing gear: Sea turtles

A randomized, paired, controlled study in 1995–1995 in seabed areas in the Gulf of Carpentaria, northern Australia (Brewer et al. 1998) found that trawl nets fitted with one of seven escape zone designs (“bycatch reduction devices”) caught similar numbers of sea turtles compared to unmodified nets. No statistical tests were carried out. Nets fitted with escape zones caught turtles at a similar rate (0.14 turtles/tow, 17 individuals) as unmodified nets (0.13 turtles/tow, 9 individuals). The unwanted catch included three species of turtles and three of snakes. The effect of escape zones on the commercially targeted prawn catch varied by design (see original paper for details). Escape zone designs tested included ‘fisheye’, ‘radial escape section’, ‘square mesh window’ and square mesh windows fitted with a number of modifications (see original paper for details). Vessels towed twin Florida Flyer prawn trawl nets from each side of the vessel in trials of one-month duration (sea turtles: February and October 1995). Nets fitted with one of the designs of escape zone and an unmodified net were randomly assigned to either side of the vessel.

(Summarised by: Katie Sainsbury)
Install escape devices on fishing gear: Snakes & lizards

A randomized, paired, controlled study in 1995 in seabed areas in the Gulf of Carpentaria, northern Australia (Brewer et al. 1998) found that trawl nets fitted with one of seven escape zone designs (“bycatch reduction devices”) caught similar numbers of sea snakes compared to unmodified nets. No statistical tests were carried out. Nets fitted with escape zones caught sea snakes at a similar rate as unmodified nets (escape zones: 0.5 snakes/tow, 7 individuals; unmodified: 0.4 snakes/tow, 15 individuals). The unwanted catch included three species of snakes. The effect of escape zones on the commercially targeted prawn catch varied by design (see original paper for details). Escape zone designs tested included ‘fisheye’, ‘radial escape section’, ‘square mesh window’ and square mesh windows fitted with a number of modifications (see original paper for details). Vessels towed twin Florida Flyer prawn trawl nets from each side of the vessel in scientific trials of one-month duration (sea snakes: October 1995). Nets fitted with one of the designs of escape zone and an unmodified net were randomly assigned to either side of the vessel.

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

A replicated, paired, controlled study in 1995 of an area of seabed in the Gulf of Carpentaria, Australia (Brewer et al. 1998) found that prawn trawl nets fitted with rigid size-sorting escape grids (two designs) in combination with either a large, supported escape opening (Fisheye) or a square mesh escape window, caught fewer unwanted fish compared to an unmodified conventional trawl. Data were reported as percentage reductions. The weight of unwanted fish caught in experimental trials of the modified nets was lower than conventional trawls: by 14–16% with a combined Super Shooter grid and Fisheye escape opening, by 28–31% with a Nordmøre grid and a Fisheye, and by 29–39% with a Nordmøre grid and a square mesh window. The Super Shooter/Fisheye system caught similar weights of commercial target prawns Penaeidae relative to conventional trawls (91–95%), while both Nordmøre systems caught less (Nordmøre/Fisheye: 14–18%, Nordmøre/square mesh: 17–34%). Standard prawn trawl nets were fitted with a combination of catch reduction devices: one of two size-sorting escape grids (‘Super Shooter’ or ‘Nordmøre’) and either a single large escape opening (‘Fisheye’) or a square mesh escape window in the codend. Paired trawl deployments, each with a different modified trawl, were conducted in experimental trials in October 1995 (146 paired tows, length 120 min). Full details of the gear designs are provided in the original study.

(Summarised by: Leo Clarke)
Fit large, supported escape openings (such as Fisheyes, Bigeyes and radial escape sections) to trawl nets

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.

(Summarised by: Leo Clarke)
Install exclusion and escape devices on fishing gear

A randomized, paired, controlled study in 1995 in seabed areas in the Gulf of Carpentaria, northern Australia (Brewer et al. 1998) found that trawl nets fitted with one of three different grids accompanied by an escape hole (“turtle excluder device”) and a secondary escape zone (“bycatch reduction device”) reduced the catch of sea turtles and sea snakes, compared to unmodified nets. No statistical tests were carried out. Nets fitted with both a turtle excluder device and a bycatch reduction device caught fewer turtles (0.005 turtles/tow, 1 individual) compared to unmodified nets (0.10 turtles/tow, 11 individuals). Overall, nets fitted with both a turtle excluder device and a bycatch reduction device caught fewer sea snakes (0.36 snakes/tow, 45 individuals), than unmodified nets (0.42 snakes/tow, 15 individuals). However, some combinations of a turtle excluder device and a bycatch reduction device caught fewer sea snakes (flexible upward grid with fisheye in front of grid: 0.2 snakes/tow, 3 individuals; square upward grid with square mesh: 0.2 snakes/tow, 8 individuals) compared to unmodified nets or other combinations tested (see original paper for details). The unwanted catch included three species of turtles and three of snakes. The effect of grids and escape zones on the commercially targeted prawn catch varied by design (see original paper for details). Devices tested included a flexible, circular, upward tilted grid with top escape hole (‘AusTED’) and a secondary escape zone in front of the grid; a square, upward tilted grid with a top escape hole (‘Nordmøre’) and secondary escape zone after the grid; and a circular downward tilted grid (‘Super Shooter’) with a secondary escape zone after the grid. Secondary escape zones included different configurations of a ‘fish eye’ or ‘square mesh window’ (see original paper for full details). Vessels towed twin Florida Flyer prawn trawl nets from each side of the vessel in scientific trials of one-month duration (sea turtles: February 1995, October 1995, October 1996; sea snakes: October 1995). Nets fitted with one of the designs of grid and an unmodified net were randomly assigned to either side of the vessel.

(Summarised by: Katie Sainsbury)
Fit mesh escape panels/windows to a trawl net

A replicated, paired, controlled study in 1995–1996 in an area of seabed in the Gulf of Carpentaria, Australia (Brewer et al. 1998) found that prawn trawl nets fitted with a square mesh escape window reduced the catches of unwanted fish, and three variations of the window resulted in variable effects, compared to unmodified standard nets. Relative to standard nets, trawl nets fitted with a square mesh window alone caught 25–36% less unwanted fish catch. A square-mesh escape window and black canvas cylinder reduced unwanted fish catch by 33% in one trial, but catches were similar in another (16% less than standard). A square mesh window and ‘hummer’ grid reduced unwanted fish catch by 26% and a square mesh window made of glow-in-the-dark mesh caught similar amounts (17% less than standard). Catch weights of target prawns Penaeidae were only reduced in one of two tests of each of a square mesh window alone (35%) and in combination with a black cylinder (25%). Trials were done on a two-leg research vessel survey in February 1995. Data were collected from paired, 30 min deployments using a twin-trawl to tow different combinations of modified and standard nets in a semi-systematic block design. Catches from a standard prawn trawl net (45 mm codend mesh) fitted with a 150 mm square mesh panel (30 tows) or one of three panel variations (14–18 tows of each) were compared with unmodified standard net (35 tows) catches (see paper for specifications).

(Summarised by: Leo Clarke)
Fit a size-sorting escape grid (rigid or flexible) to a prawn/shrimp trawl net

A replicated, paired, controlled study in 1995–1996 of prawn fishing grounds in the Gulf of Carpentaria, Australia (Brewer et al. 1998) found that commercial prawn trawls fitted with size-sorting escape grids (rigid and flexible) typically caught fewer unwanted fish, sharks and stingrays compared to unmodified conventional trawls and the effect varied with grid type. Shark and stingray data were not tested statistically. In two of two experimental trials, two grid types (Nordmøre and AusTED) caught 27–35% less unwanted fish catch than unmodified trawls, but a third grid type (Super Shooter), while reducing unwanted fish catch by 17–21%, was not statistically different. In commercial trials, trawls with grids (Super Shooter and NAFTED) caught fewer sharks (with grids: 3–6, unmodified: 4–16), and stingrays (with grids: 0, unmodified: 0–15). Catches of the target prawn species were similar for two of the grids (Super Shooter and NAFTED) compared to unmodified trawls but were lower for the Nordmøre (50% less) and AusTED (22% less) grids. Fishing trials were carried out in the Gulf of Carpentaria, Australia, in February and October 1995 (experimental) and October 1996 (commercial). Standard prawn trawl nets (45 mm codend mesh) were fitted with one of four grid systems and replicate paired deployments with unmodified trawl nets were conducted by a dual-rigged trawler. Grids had either a top or bottom escape opening, and all had a guiding panel (full details of the grid system designs are provided in the original study).

(Summarised by: Leo Clarke)