Study

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.

Actions

This study is summarised as evidence for the following.

Action Category

Install exclusion and/or escape devices for mammals on fishing nets

Action Link
Marine and Freshwater Mammal Conservation

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

Action Link
Marine Fish Conservation
  1. 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)

  2. 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)

Output references
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