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

Abstract

Abstract:Understanding fish escaping behavior through any given bycatch reduction device represents the cornerstone for its application as a technical conservation measure. To evaluate the square meshes and sorting grids as successful size-selective bycatch reduction devices in finfish trawl nets, we carried out a simulated trawling experiment to assess the effects of illumination, towing speed, and mesh or grid orientation on undersize fish escaping behavior through the square meshes and rigid grids. Juveniles masou salmon of an average body length of 130 mm were used as the experimental fish. A speed-controllable motor was used to tow the framed net in a circular water tank. Infrared CCD Camera fixed to the towed net, through a rotary connector, was used to observe fish behavior. Results demonstrated that, in contrast to the active escaping under light conditions, there was no active voluntary escaping under dark conditions. As a result, no fish could escape through the parallel and the sloping backward meshes and only 13 % passively escaped through the sloping forward meshes by increasing the towing speed. On the other hand, the probability of encounter and passive fish escaping through the longer openings between the sloping backward grids increased under dark conditions resulting in the highest escaping ratio that more increased from 67 % to 87 % by increasing the towing speed from 1 to 1.5 knot, and also shortened the swimming time before escaping from 5 to 1.5 minutes (the highest escaping ratio and the shortest swimming time before escaping under dark). Frame by frame analysis of some escaping events demonstrated an easy and passive fish escape through the grids without changing the normal swimming direction. Results support the use of the sloping backward grids (at the top panels as in the "Sort-V" grid system or at the bottom panels as in the form of "Evaflex" grid system 2 of a convenient grid material, area and slot size) as a size-selective bycatch reduction device well ahead of the codend of finfish trawl, especially demersal, working under dark at high towing speeds.

Fish Behavior near Bottom Trawls

“…4.15 d) -either slowly working their way through a mesh with a single tail -beat followed by motionless behavior or with rigorous acceleration through a mesh followed by continuous high -speed swimming away from the trawl net. In the absence of vision, fi sh may not be able to respond to a BRD and only passive fi ltration is possible, and then only if correctly orientated (Gabr et al 2007 ). This suggests that both the visual capability of fi sh and ambient light level are important parameters infl uencing fi sh escape.…”

Evaluating the behavioral impairment of escaping fish can help measure the effectiveness of bycatch reduction devices

“…The condition and survival of fish escaping trawls by passing through codend meshes or grids has been extensively studied, with much of the mortality shown to be related to physical contact with the net or grid (Chopin and Arimoto, 1995;Suuronen et al, 1996;Gabr et al, 2007). The behavior and condition of fish escaping from trawls due to deflecting grids, in which physical contact with the grid is either presumed or known to be much less, is an area of research that has received very little direct study (Soldal and Engås, 1997).…”

Survey of Fish Behavior Analysis by Computer Vision

“…Fish behavior is a crucial consideration in commercial trawling where catch reduction is important [152], and, notably, selectivity may lead to biased assessments of fish abundance in fisheries. A range of biogeographical and depth patterns have been identified in effort to remedy this [153][154][155].…”