Use a larger mesh size for fishing trap-nets
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
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.
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