Performance of bycatch reduction devices varies for chondrichthyan, reptile, and cetacean mitigation in demersal fish trawls: assimilating subsurface interactions and unaccounted mortality

Published source details Wakefield C.B., Santana-Garcon J., Dorman S.R., Blight S., Denham A., Wakeford J., Molony B.W. & Newman S.J. (2017) Performance of bycatch reduction devices varies for chondrichthyan, reptile, and cetacean mitigation in demersal fish trawls: assimilating subsurface interactions and unaccounted mortality. ICES Journal of Marine Science, 74, 343-358.
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This study is summarised as evidence for the following.

	Action	Category
	Install exclusion devices on fishing gear: Snakes & lizards Action Link
	Install exclusion devices on fishing gear: Sea turtles Action Link
	Install exclusion and/or escape devices for mammals on fishing nets Action Link
	Use a different design or configuration of size-sorting escape grid/system in trawl fishing gear (bottom and mid-water) Action Link

Install exclusion devices on fishing gear: Snakes & lizards

A replicated study in 2012 in off the coast of Western Australia (Wakefield et al. 2017) found that exclusion grids with escape hatches (‘bycatch reduction device’) did not prevent sea snakes from entering the codend of trawl nets in a tropical teleost fishery. In total 331 of 351 sea snakes passed through the exclusion grid, however only 16 sea snakes were recorded as trawl catch. The authors note that sea snakes were observed escaping through the trawl net and may have done so after passing through the grids. Loss of commercially-targeted teleost species from all trawls was 1% of catch. In June–December 2012, catch (target and unwanted) from three commercial trawl vessels was monitored using in-net and onboard cameras during daylight. Vessels were fitted with either: upward-facing grid and escape hatch with diamond-mesh net (372 trawl hours on 2 vessels), downward-facing grid and escape hatch with diamond-mesh net (559 trawl hours on 2 vessels), or downward-facing grid and escape hatch with square mesh net (389 trawl hours on 1 vessel; see original paper for all specifications). Use of bycatch reduction grids with escape hatches was mandatory in this fishery from 2006.

(Summarised by: Katie Sainsbury)
Install exclusion devices on fishing gear: Sea turtles

A replicated study in 2012 in demersal waters off the coast of Western Australia (Wakefield et al. 2017) found that exclusion grids with escape hatches (‘bycatch reduction device’) prevented sea turtles from entering the codend of trawl nets in a tropical teleost fishery. All 11 sea turtles that entered trawl nets modified with an exclusion grid and escape hatch were expelled (downward-facing grid with square mesh net: 6 turtles; upward-facing grid in diamond mesh: 5 turtles) and 9 of 11 turtles exited in <2.5 minutes. Loss of commercially-targeted teleost species from all trawls was 1.2–1.4% of catch. In June–December 2012, the catch (target and unwanted) from three commercial trawl vessels was monitored using in-net and onboard cameras during daylight. Vessels were fitted with either: upward-facing grid and escape hatch with diamond-mesh net (372 trawl hours on 2 vessels), downward-facing grid and escape hatch with diamond-mesh net (559 trawl hours on 2 vessels), or downward-facing grid and escape hatch with square mesh net (389 trawl hours on 1 vessel; see original paper for all specifications). Use of bycatch reduction grids with escape hatches had been mandatory in this fishery since 2006.

(Summarised by: Katie Sainsbury)
Install exclusion and/or escape devices for mammals on fishing nets

A study in 2012 of a pelagic area in the Indian Ocean, Western Australia (Wakefield et al. 2017) found that less than 30% of common bottlenose dolphins Tursiops truncatus that entered exclusion and escape devices on trawl nets escaped alive through hatches. Two of seven dolphins that entered exclusion and escape devices on trawl nets escaped alive through an escape hatch in the roof of the net within 18 seconds and five minutes. The five other dolphins were retained at the grid of the exclusion device, one of which died and was expelled through an escape hatch. The seven dolphins were recorded interacting with exclusion and escape devices during five of 774 day-trawls carried out by a commercial fishery targeting groundfish. Exclusion and escape devices were installed between the body and ‘cod-end’ extension panel of each trawl net. The devices consisted of a steel grid angled either up or down towards an escape hatch and/or slit in the roof or floor of the net. Underwater video cameras recorded dolphins within the nets during each of the five trawls in June–September 2012.

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

A replicated, controlled study in 2012 of an area of seabed in the Indian Ocean off north east Australia (Wakefield et al. 2017) found that using a different type of size-sorting escape grid system (upward-angled) reduced the capture of two of four groups of unwanted sharks and rays (Chondrichthyes) compared to two (one modified and one standard) downward-angled escape grids. The percentages of individuals that escaped was greater from an upward-angled grid compared to the two downward grids (one with square mesh) for two groups of fish: ‘benthopelagic’ sharks that feed on bottom and free swimming prey (up: 50%, down: 25%, square mesh: 28%) and shark-like rays (up: 53%, down: 28%, square mesh: 25%). There were no differences in escape rates between grids for rays and skates (up: 72%, down: 67%, square mesh: 70%) or bottom-dwelling sharks (up: 82%, down: 78%, square mesh: 80%). From June–December 2012, three vessels completed a total of 774 deployments of trawl nets fitted with one of three catch escape devices: upward opening/inclined rigid escape grid (218 hauls), a standard semi-rigid downward grid used in the bottom trawl fishery (301 hauls), and a rigid grid (same as upward) modified in a downward inclined orientation and stitched into a section of 50 mm square mesh (255 hauls). See original paper for gear specifications. Escapes of sharks/rays were monitored using video footage recorded from within the nets and onboard the vessels.

(Summarised by: Chris Barrett)