Study

Selectivity experiments in the NE Mediterranean: the effect of trawl codend mesh size on species diversity and discards

  • Published source details Stergiou K.I., Politou C.-., Christou E.D. & Petrakis G. (1997) Selectivity experiments in the NE Mediterranean: the effect of trawl codend mesh size on species diversity and discards. ICES Journal of Marine Science, 54, 774-786.

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, randomized, controlled study in 1993–1994 in two seabed areas in the Aegean Sea, Greece (Stergiou et al. 1997, same experimental set-up as Petrakis & Stergiou 1997 and Stergiou 1999) found that a bottom trawl net with a square mesh codend did not allow more unwanted individuals and higher number of species (fish and invertebrates) to escape compared to a diamond mesh codend of the same mesh size. In two of two years, the average number of individuals (fish and invertebrates) and species escaping from the codend was similar between square and diamond mesh (individuals, square: 1,653–6,100/h, diamond: 1,486–8,167/h; species, square: 12–15/h, diamond: 12–16/h ). Experimental trawl deployments (using the same experimental set-up as Petrakis & Stergiou, 1997) were conducted in the Trikeri Channel in October 1993 (5 stations) and the North Euboikos Gulf in March 1994 (seven stations). A trawl net was randomly assigned either a 20 mm square mesh codend or 20 mm diamond mesh codend (12 hauls each codend), and towed for 45–60 min at depths between 73–210 m. Small mesh (10 mm) covers over the codend sampled the escaping fish catch. All individuals caught in the covers were identified and counted.

    (Summarised by: Rosslyn McIntyre)

  2. Use a larger mesh size

    A replicated, randomized, controlled study in 1993–1994 in two areas of coastal water in the Aegean Sea, Greece (Stergiou et al. 1997, same experimental set-up as Petrakis & Stergiou 1997) found that trawl codends of larger mesh size allowed the escape of more unwanted individuals and species (fish and invertebrates) compared to a conventional diamond mesh codend of smaller mesh size. For both sampling periods and for all species (fish and invertebrates combined), the average number of individuals (20 mm: 1,486–8,167 ind/h, 14 mm: 204–855 ind/h) and species (20 mm: 16 species, 14 mm: 9 species) that escaped was higher with the larger mesh compared to the standard. The ratios of commercial/non-commercial retained catch were higher in 20 mm diamond mesh codends (0.60–1.31) than in 14 mm diamond mesh codends (0.27–0.29). In October 1993 and March 1994, experimental trawl deployments were conducted in two areas (Trikeri Channel and North Euboikos Gulf, twelve stations in total) using a trawl fitted with either a 20 mm diamond mesh codend or a conventional 14 mm diamond mesh codend used by the fishery (12 hauls of each at each station). Codend type was randomly allocated and small mesh (10 mm) covers retained fish escaping through the meshes. For each deployment, the total number and weight caught by species in the codends and covers were recorded.

    (Summarised by: Rosslyn McIntyre)

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