Use a larger mesh size for fishing trap-nets

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • One study evaluated the effects on freshwater mammals of using a larger mesh size for fishing trap-nets. The study was in the River Indal (Sweden).





  • Human-wildlife conflict (1 study): One controlled study in the River Indal found that a fishing trap-net with a larger mesh size in the first two sections had fewer grey seals feeding around it and less damage caused by seals.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A controlled study in 2000–2001 at the mouth of the River Indal, northern Sweden (Lunneryd et al. 2003) found that a fishing trap-net with a larger mesh size in the first two sections had fewer grey seals Halichoerus grypus feeding around it and less damage by seals than a conventional trap-net. Fewer seals were observed surfacing (average 0.2 seals/h) and feeding on fish (0 seals) around the modified trap-net than a conventional trap-net (surfacing: average 1.6–4.1 seals/h, feeding: 0.1–0.3 seals/h). The modified trap-net had fewer holes caused by seals (total 6) than the conventional trap-net (total 269), although statistical significance was not assessed. Catches of target salmon Salmo salar and trout Salmo trutta were higher in the modified trap-net during one trial, and similar in modified and conventional trap-nets during two trials (see original paper for data). A modified and conventional trap-net were alternated between two fishing sites on opposite sides of a river mouth during three trials (each lasting 15–25 days). Both had a 100-m leader net with 3–4 funnel-shaped sections leading to a ‘seal-safe’ fish chamber. The first two sections had mesh sizes of 400 mm (modified trap-net) or 200 mm (conventional trap-net). Target fish catches and holes were recorded every other day during each of the three trials in June–August 2000. Seals were observed daily from the shore and with a video camera above each trap-net during two of the three trials in July–August 2001.

    Study and other actions tested
Please cite as:

Berthinussen, A., Smith, R.K. and Sutherland, W.J. (2021) Marine and Freshwater Mammal Conservation: Global Evidence for the Effects of Interventions. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

Marine and Freshwater Mammal Conservation

This Action forms part of the Action Synopsis:

Marine and Freshwater Mammal Conservation
Marine and Freshwater Mammal Conservation

Marine and Freshwater Mammal Conservation - Published 2021

Marine and Freshwater Mammal Synopsis

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 21

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 ProgrammeRed List Champion - Arc Kent Wildlife Trust The Rufford Foundation Save the Frogs - Ghana Mauritian Wildlife Supporting Conservation Leaders
Sustainability Dashboard National Biodiversity Network Frog Life The international journey of Conservation - Oryx Cool Farm Alliance UNEP AWFA Bat Conservation InternationalPeople trust for endangered species Vincet Wildlife Trust