Study

Behaviour analysis of undersized fish escaping through square meshes and separating grids in simulated trawling experiment

  • Published source details Gabr M., Fujimori Y., Shimizu S. & Miura T. (2007) Behaviour analysis of undersized fish escaping through square meshes and separating grids in simulated trawling experiment. Fisheries Research, 85, 112-121

Actions

This study is summarised as evidence for the following.

Action Category

Use a different design or configuration of size-sorting escape grid/system in trawl fishing gear (bottom and mid-water)

Action Link
Marine Fish Conservation

Fit a size-sorting escape grid (rigid or flexible) to a fish trawl net

Action Link
Marine Fish Conservation

Fit mesh escape panels/windows to a trawl net

Action Link
Marine Fish Conservation
  1. Use a different design or configuration of size-sorting escape grid/system in trawl fishing gear (bottom and mid-water)

    A replicated study (year not stated) in a laboratory in Japan (Gabr et al. 2007) reported that changing the configuration of a size-sorting escape grid (orientation) allowed more small masu salmon Oncorhynchus masou to escape in the dark but not in the light, under simulated trawling conditions. Results were not statistically tested. In dark conditions, the proportion of salmon that escaped through the grids was 67–87% for a backward sloping grid, 60% for a forward sloping grid and 47–53% for a flat/parallel grid. In the light, 100% of salmon escaped through each of the backward and forward sloping grids, and 87–100% escaped with the flat grid. Six trials were conducted (sampling date/year unspecified) using three grid orientations at each of two towing speeds (1 and 1.5 knots); three in dark and three in light conditions. For each trial, five juvenile salmon (12–14 cm length) were released into a circular canal 75 cm wide and 50 cm deep and forced to swim inside a framed net driven around the canal by a motor, to simulate a trawl deployment. The rigid sorting grid (38 mm bar spacing) was fixed to the bottom net frame at three orientations: flat, forward facing or backward facing. Fish were forced to swim for a maximum of 30 min and escapes were monitored by video camera.

    (Summarised by: Leo Clarke)

  2. Fit a size-sorting escape grid (rigid or flexible) to a fish trawl net

    A replicated study in a laboratory in Japan (Gabr et al. 2007) found that small masu salmon Oncorhynchus masou were able to actively escape through rigid size-sorting escape grids fitted to a finfish trawl, regardless of grid orientation, under simulated trawling conditions, and escape ability was not typically affected by towing speed, but was affected by the light conditions. Across all grid orientations, salmon escape rate through grids was 47–100%. For flat-fitted and forward-sloping grids, more salmon escaped in light conditions compared to dark, regardless of towing speed (light: 87–100%, dark: 47–60%) and for a backward-sloping grid, increasing the speed to 1.5 from 1 knot increased escapes in the dark (to 87% from 67%) so that they were similar to escape rates in the light (100%). In addition, salmon escape through the grids was higher than square mesh panels, for flat and backward-sloping orientations at both light conditions and towing speeds (grids: 47–100%, square meshes: 0–67%) whereas escape through the forward-sloping grids was higher than escaping through the forward-sloping square meshes in the dark at the higher towing speed only. Six trials were conducted for each grid orientation at each towing speed (1 and 1.5 knots), three in dark and three in light conditions. For each trial, five juvenile salmon (12–14 cm length) were released into a circular canal 75 cm wide and 50 cm deep and forced to swim inside a framed net driven around the canal by a motor, to simulate a trawl deployment. The rigid sorting grid (38 mm bar spacing) was fixed to the bottom net frame at three orientations: flat, forward facing or backward facing. Fish were forced to swim for a maximum of 30 min and escapes monitored by video camera. The same trials were done with a square-mesh (60 mm mesh size) panel. The year the study took place is not reported.

    (Summarised by: Leo Clarke)

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

    A replicated study (yet not stated) in a laboratory in Japan (Gabr et al. 2007) found that small masu salmon Oncorhynchus masou were able to actively escape through square mesh escape panels fitted to a finfish trawl under simulated trawling conditions, regardless of panel orientation, and escape ability was not typically affected by towing speed, but was affected by the light conditions. Across all panel orientations, more salmon escaped in light conditions than dark, irrespective of towing speed (light: 20–100%, dark: 0–40%), with none able to escape at all in the dark through either a flat-fitted panel or a backward-sloping panel at the higher speed, and only 13–40% through forward-sloping panels. In light conditions, 100% of salmon escaped through a forward-sloping panel at both towing speeds. Increasing the towing speed to 1.5 from 1 knot increased the escape rate through a backward-sloping panel from 33 to 67% but reduced it from 40 to 20% in the flat-fitted panel. Six trials were done to test three panel orientations and two towing speeds (1 and 1.5 knots); three in dark and three in light conditions. For each trial, five small salmon (12–14 cm length) were released into a circular flow tank and forced to swim for a maximum of 30 min inside a framed net. The square mesh panel (60 mm mesh size) was fixed to the bottom net frame at three orientations: flat, forward- or backward-facing, and escapees monitored by video camera. The year the study took place is not reported.

    (Summarised by: Natasha Taylor)

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