Potential application of an underwater towing camera system for a trolling survey of southern bluefin tuna recruitment

An underwater video camera system was developed, which was capable of being towed at speeds as high as 8 knots during trolling. Using this system, fish behavior during trolling for southern bluefin tuna (SBT), Thunnus maccoyii, was observed. Trolling was conducted as a pre-recruitment monitoring survey of SBT off the southern coast of Western Australia during January and February, 2010. Of the eight trolling lines used, one was equipped with a depressor and attached to the underwater video camera system. A total of 714,071 frames of video images (approximately 6.7 hours) were recorded using the underwater camera, and images of SBT were detected in 4,465 of them. Video images of SBT were classified based on 3 types of fish behavior directed towards trolling gear; "No response", "Following the gear", and "Approaching the lure". When SBT hooking was unsuccessful "No response" behavior was frequently observed, and when SBT hooking was successful "Approaching the lure" behavior was mainly observed. These results clearly demonstrated that success of trolling was affected by behavior of fish towards trolling gear. Underwater camera allowed for detection of SBT even when catches were unsuccessful, and this is a highly advantageous feature as the alternative tool of SBT school detection for pre-recruitment monitoring surveys.