Preliminary performance assessment of an underwater line setting device for pelagic longline fishing

Abstract

Baited branchlines were set from a tuna longlining vessel using an underwater setting device and their sink patterns compared with those of baited branchlines that were hand‐thrown. Using a paired /‐test at an hypothesised mean difference of 2 m, at a point 100 m astern of the vessel, baited branchlines set using the device were significantly deeper than those that were hand‐thrown. Baited branchlines set using both methods showed a high variation in then‐sink patterns; on some sets they sank faster than others. The underwater setting device has potential to reduce seabird bycatch substantially with minimal intrusion on the normal operation of a longline fishing vessel. It delivers baits underwater (removing the visual cue of a hand‐thrown baited hook to seabirds) and immediately places baited hooks outside the diving range of some vulnerable albatross species (Diomedea spp., Phoebastria spp., Thalassarche spp., and Phoebetria spp.).

Keywords:

• seabirds
• incidental mortality
• longline fisheries
• mitigation devices
• underwater setting
• sink rates
• time depth recorders