Effects on the Escapement of Juvenile Bastard Halibut Paralichthys olivaceus of Actively Stimulating Devices Inside a Model Cod End

Abstract

The effects of actively stimulating devices (ASD) on juvenile flatfish escape were studied to increase escape rates from the cod end by encouraging fish to approach the net wall. Two kinds of ASD were designed: a fluttering net panel, i.e., a free-end flag-like net panel, and a double conical rope array. Escape responses of juvenile bastard halibut were observed in a circulating water channel using two model cod ends, one made with diamond-shaped 43-mm-mesh-size polyethylene (PE) as a high-contrast cod end and the other with polyamid (PA) mono-ply as a low-contrast cod end. Retention rate was significantly lower with the double conical rope-array ASD in the PE cod end than with conventional PE cod ends only or the fluttering net-panel ASD inside the PE cod end. Mean retention rate with the low-contrast PA cod end was also significantly lower than that with the high-contrast PE conventional cod end. Therefore, active fluttering devices using a double conical rope array together with less visible low-contrast netting in the cod end could help to reduce the bycatch of juvenile flatfish by weakening their optomotor response and actively driving fish to the side net panel.

Escape response of juvenile fish in a shaking codend determined from tank experiments

“…The escape rates of red seabream in this study were generally lower than those in previous experiments (Kim and Whang, 2010), most likely because of the lower temperature or the slower flow velocity in this study. Nevertheless, the positive effects of a shaking motion or an ASD on the escape of juvenile seabream were consistent with those in previous studies (Kim and Whang, 2010) There was however no difference in the escape rate of bastard halibut even when a rope ASD was used (Kim, 2011). This might be due to the different experimental conditions such as water temperature, light levels, flow velocity or fish vitality.…”

“…Accordingly this study confirmed the effects of an active stimulating device (ASD) on fish as they approach the netting panel or rope array, in bycatch reduction for juvenile fish. The escape rate of bastard halibut from a conventional codend, with either a net or rope ASD, in a steady state in this study was greater than in previous experiments using the same codends and flow velocity (Kim, 2011). However, the higher temperature used in these experiments may have influenced the bastard halibut's swimming ability and encouraged them to escape (Wardle, 1993).…”

“…ASDs consist of a conical rope array or a fluttering netting panel inside the codend that generates variable visual or contact stimuli for juvenile fish in addition to flow-related effects. The retention rate of juvenile fish such as red seabream (Kim and Whang, 2010) or bastard halibut (Kim, 2011) the sweeping motion of the fluttering netting panel inside the codend could be limited by turbulent flow and the catch. Therefore, a new method to encourage fish to approach the netting panel of the codend has been suggested where the codend is moved up and down by forming a cap-like canvas (Kim, 2013b).…”

“…As an alternative method, active stimulating devices (ASDs) have been tested in tank experiments (Kim and Whang, 2010;Kim, 2011Kim, , 2013a. ASDs consist of a conical rope array or a fluttering netting panel inside the codend that generates variable visual or contact stimuli for juvenile fish in addition to flow-related effects.…”

Effects on bycatch reduction in a shaking cod end generated by canvas in a shrimp beam trawl

“…However, reduction in juvenile catch as a primary objective has gained increasing attention for the response of fish, such as the optomotor response, maintaining relative position and speed to prevent escape through the mesh, and masking the effect of the blocking mesh by larger fish, especially those caught in towed fishing gear (Wardle, 1993;Kim et al, 2008). Therefore, active stimulating devices might provide a useful method for disturbing the optomotor behavior or to drive fish close to the mesh by fluttering the net or twine generated in the turbulent flow inside the cod end (Kim and Whang, 2010;Kim, 2011). More recently, trials have been carried out in water tanks with a shaking cod end moving up and down generated by an unbiased cap-like canvas attached at the end of the cod end to encourage the escape of juvenile fish positioned passively close to the net.…”