Development of passive acoustic tracking systems to investigate tooth whale interactions with fishing gear

Abstract

Depredation (=stealing) by toothed whale species of Coral Sea tuna longline catches threatens the viability of the fishery through direct removal of bait and hooked fish and behavioural modification of the target fish species. The false killer whales and short-finned pilot whales responsible for depredation on longline catches generate frequency modulated communication whistles, time constant broadband burst-pulses with a possible emotional context, and time variable broadband echolocation trains used in hunting. All vocalisations offer potential for passive acoustic tracking. A range of methods is being developed to mitigate depredation including acoustic, mechanical and chemical approaches. To assess the effectiveness of these methods, an integrated passive acoustic tracking system to determine movement trajectories of toothed whales relative to longline gear is under development for use in oceanic situations. A real-time tracking system for broadband clicks using a small aperture Mills Cross array to obtain an initial azimuth to source, is being integrated with a post-processing, wide aperture sonobuoy system for all vocalisations to obtain localisation in three dimensions. Data obtained from the small aperture array showed that comparable azimuth estimates were obtained for inshore toothed whales using both tracking systems. Trials over wide areas in oceanic conditions with larger arrays (1.5-1,000 m) to determine 3 dimensional trajectories, are yet to commence.