Study

Optimising codend configuration in a multispecies demersal trawl fishery

  • Published source details Broadhurst M.K., Millar R.B., Wooden M.E.L. & Macbeth W.G. (2006) Optimising codend configuration in a multispecies demersal trawl fishery. Fisheries Management and Ecology, 13, 81-92.

Actions

This study is summarised as evidence for the following.

Action Category

Use a square mesh instead of a diamond mesh codend in a trawl net

Action Link
Marine Fish Conservation

Use a larger mesh size

Action Link
Marine Fish Conservation
  1. Use a square mesh instead of a diamond mesh codend in a trawl net

    A replicated, paired, controlled study in 2004 in two seabed areas in the South Pacific Ocean off New South Wales, Australia (Broadhurst et al. 2006) found that square mesh codends in a mixed species bottom trawl fishery reduced the catches of discarded whiting Sillago spp. compared to conventional diamond mesh codends. Data were reported as statistical model results. Results varied between vessels, and in one of three cases the number and weight of total discarded whiting was lower in square mesh codends (35 and 41 mm) than diamond (41 mm/150), but there were no differences between codends for retained total whiting, or species individual categories. For a second vessel, there were no clear reductions in any whiting catches between a 35 mm square mesh and diamond mesh (41 mm/150), but a 41 mm square mesh had lower weight of total retained whiting and lower number and weights of total and retained stout whiting Sillago robusta. For the third vessel, there were no main differences in whiting catches between square (31 mm) and diamond mesh (41 mm/100) codends. In addition, square mesh codends improved selection for stout whiting compared to diamond, and the length at which 50% of fish are predicted to escaped increased with increasing size of the square mesh (35 mm: 14–15 cm, 41 mm: 17–18 mm. Catch data were collected on three commercial prawn trawlers, fishing with the two outer nets of a triple trawl gear configuration, in April-December 2004. Seventy-one paired trawl deployments were carried out in 27–51 m depth using one of five test codends on one side - two square mesh codends (nominal 35 and 45 mm mesh), and three diamond mesh codends (two 41 mm mesh of 100 and 150 mesh circumference and one 45 mm mesh) - and a small mesh (24 mm) control codend on the other. All trawl nets also included a square mesh escape panel. See original study for gear details. The weights, numbers and total lengths of total, retained and discarded stout and red spot whiting Sillago flindersi were recorded.

    (Summarised by: Leo Clarke)

  2. Use a larger mesh size

    A replicated, paired, controlled study in 2004 of two areas of seabed in the Tasman Sea off New South Wales, Australia (Broadhurst et al. 2006) found that a larger size of square mesh in trawl codends did not typically reduce the catches of discarded whiting Sillago spp. compared to a smaller mesh size, however both caught less than a standard diamond mesh. All data were reported as statistical model results. Catch number of all discarded whiting was lower in a 41 mm square mesh than a 35 mm square mesh, and both were lower than a standard 41 mm diamond mesh, for one of two vessels only. By weight, total discarded whiting was similar between the two square mesh sizes, but both were lower compared to the standard diamond mesh, for one of two vessels only. Target king prawn Penaeus plebejus catches were similar across codend designs, although weight and number of legally sized whiting were lower in larger square mesh. In April-December 2004, a total of eight deployments/trawl codend type were carried out on two commercial prawn trawlers rigged with three trawls. Two square mesh codends (41 mm and 35 mm) were compared with a conventional 41 mm diamond mesh codend. Each square mesh codend was deployed simultaneously with the standard diamond codend in paired tows using the outer two trawls. All codends also had a square mesh escape panel. See original study for gear details.

    (Summarised by: Leo Clarke)

Output references
What Works 2021 cover

What Works in Conservation

What Works in Conservation provides expert assessments of the effectiveness of actions, based on summarised evidence, in synopses. Subjects covered so far include amphibians, birds, mammals, forests, peatland and control of freshwater invasive species. More are in progress.

More about What Works in Conservation

Download free PDF or purchase
The Conservation Evidence Journal

The Conservation Evidence Journal

An online, free to publish in, open-access journal publishing results from research and projects that test the effectiveness of conservation actions.

Read the latest volume: Volume 18

Go to the CE Journal

Discover more on our blog

Our blog contains the latest news and updates from the Conservation Evidence team, the Conservation Evidence Journal, and our global partners in evidence-based conservation.


Who uses Conservation Evidence?

Meet some of the evidence champions

Endangered Landscape Programme Red List Champion - Arc Kent Wildlife Trust The Rufford Foundation Save the Frogs - Ghana Bern wood Supporting Conservation Leaders National Biodiversity Network Sustainability Dashboard Frog Life The international journey of Conservation - Oryx British trust for ornithology Cool Farm Alliance UNEP AWFA Butterfly Conservation People trust for endangered species Vincet Wildlife Trust