Use a larger mesh size for fishing trap-nets

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
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How is the evidence assessed?

Effectiveness

50%
Certainty

30%
Harms

15%

Calculating overall effectiveness category

Background information and definitions

Trap-nets are stationary nets that have a series of net chambers, which fish can enter but not easily escape from. It has been suggested that using a larger mesh size for trap-nets may discourage marine and freshwater mammals, such as seals, from feeding on trapped fish, which may reduce the risk of entanglement as well as human-wildlife conflict. Fish chased by mammals could escape through a larger mesh and would be less likely to become entangled in the side panels of the trap-net. This may make trap-nets less attractive to feeding mammals.

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).

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (0 STUDIES)

BEHAVIOUR (0 STUDIES)

OTHER (1 STUDY)

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

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
Referenced paper
Lunneryd S.G., Fjälling A & Westerberg H. (2003) A large-mesh salmon trap: a way of mitigating seal impact on a coastal fishery. ICES Journal of Marine Science, 60, 1194-1199.

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

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This Action forms part of the Action Synopsis:

Marine and Freshwater Mammal Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Modify fishing pots and traps to exclude mammals Action Link	Likely to be beneficial	6	Synopsis Link
Reduce duration of time fishing gear is in the water Action Link	No evidence found (no assessment)	0	Synopsis Link
Enforce legislation to control illegal fishing using gear or methods that are harmful to mammals Action Link	No evidence found (no assessment)	0	Synopsis Link