Study

Can bycatch reduction devices be implemented successfully on prawn trawlers in the Western Indian Ocean?

  • Published source details Fennessy S. & Isaksen B. (2007) Can bycatch reduction devices be implemented successfully on prawn trawlers in the Western Indian Ocean? African Journal of Marine Science, 29, 453-463

Actions

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
Marine Fish 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

Action Link
Marine Fish Conservation
  1. Fit mesh escape panels/windows and a size-sorting grid (rigid or flexible) to a trawl net

    A replicated, paired, controlled study in 2005 of an area of seabed in the Indian Ocean, off Mozambique (Fennessy & Isaksen 2007) found that a prawn trawl net with a square mesh escape window and a size-sorting escape grid reduced the overall discarded catch (fish and invertebrates) compared to a conventional trawl net. Average catch rates of discards (90% fish, 10% invertebrates) were lower in the net with a square mesh panel and size-sorting grid (30 kg/h) compared to a conventional trawl net without a panel or grid (56 kg/h). In addition, average catch rates were also reduced by a square mesh panel (panel: 37 kg/h, conventional: 50 kg/h), or grid (grid: 36 kg/h, conventional: 52 kg/h) alone, compared to a conventional trawl without either. Catch rates of retained fish were similar between nets (panel/grid: 4 kg/h, conventional: 5 kg/h), and the catch of targeted prawn (mostly Fenneropenaeus indicus) was lower in a panel/grid net (panel/grid: 6 kg/h, conventional: 8 kg/h). Data was collected in February 2005, from a total of 23 trawl deployments (6–21 m) using a twin-rigged trawler towing a test net and a conventional diamond mesh trawl net side by side. Test nets were the conventional design fitted with either: a square mesh escape panel (143 mm mesh size) and a rigid grid (‘Nordmøre’, 100 mm bar spacing) (eight deployments); a square mesh panel alone (11 deployments); or a grid alone (four deployments). See original paper for gear specifications.

    (Summarised by: Natasha Taylor)

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

    A replicated, paired, controlled study in 2005 of an area of seabed in the Indian Ocean, off Mozambique (Fennessy & Isaksen 2007) found that fitting a prawn trawl net with a square mesh escape panel, with or without a rigid size-sorting escape grid, reduced the amount of overall discarded catch (fish and invertebrates), compared to a conventional trawl without square mesh panels. Average catch rates of discards (90% fish, 10% invertebrates) were lower in nets with square mesh panels, either alone or in combination with a grid (panel: 37.4 kg/h, panel+grid: 29.7 kg/h; no panel: 50.4–56.4 kg/h). In addition, panel nets did not reduce catches of retained fish (panel: 4.0 kg/h, panel+grid: 3.6 kg/h; no panel: 4.9 kg/h), but target prawn catch (mainly Fenneropenaeus indicus) was reduced in panel and grid nets only (panel: 12.8 kg/h, panel+grid: 5.61 kg/h; no panel: 7.52–12.4 kg/h). Trials took place on the Sofala Bank trawl grounds off the Zambezi River in February 2005 by a twin-rigged trawler towing test trawl nets with escape panels alongside a conventional trawl in depths of 6–21 m. A Data were collected from 11 tows with a 143 mm square mesh escape panel inserted on the top of the net near the junction of the extension piece and codend, and eight tows of the square mesh panel combined with an aluminium size-sorting grid (Nordmøre), 100 mm bar spacing (see paper for specifications). Codend catches were sorted into commercial/non-commercial portions, counted and weighed.

    (Summarised by: Chris Barrett)

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

    A replicated, paired, controlled study in 2005 of a seabed area in the Indian Ocean, off Mozambique (Fennessy & Isaksen 2007) found that fitting a prawn trawl net with rigid size-sorting excluder grids reduced the overall amount of discarded catch (fish and invertebrates) and caught fewer sharks and rays, compared to a conventional trawl without a grid. For two of two grid designs, average catch rates of discards (90% fish, 10% invertebrates) were lower in nets with grids than without (with: 37–48 kg/h, without: 49–83 kg/h). Large sharks and rays were caught in fewer hauls with grids than without (with: 0–2 hauls, without: 4–9 hauls) but there was no statistical difference for smaller sharks and rays (with: 5 hauls, without: 5–9 hauls). In addition, average catches of the target prawn species Penaeidae were similar between nets (with: 12–24 kg/h: without: 13–23 kg/h). Trials took place in February 2005 by twin-rigged trawler. A total of 16 paired trawl deployments were done with a net fitted with one of two grid designs (both Nordmøre) and a conventional trawl without a grid. Grids were aluminium, 100 mm bar spacing, fitted with a guiding funnel either with or without a cover flap in front of the grid escape opening (see paper for specifications). Codend catches were sorted into commercial/non-commercial portions, counted and weighed, and lengths of selected species measured.

    (Summarised by: Chris Barrett)

Output references

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, terrestrial 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