SCIENTIFIC COMMITTEE FOURTH REGULAR SESSION 11-22 August 2008 Port Moresby, Papua New Guinea THE USE OF UNDERWATER VIDEO TO CHARACTERIZE THE SPECIES, SIZE COMPOSITION AND VERTICAL DISTRIBUTION OF TUNAS AND NON-TUNA BYCATCH AROUND FLOATING OBJECTS WCPFC-SC4-FT-SWG/WP-3

Research surveys attempting to use acoustic devices (i.e. echo sounders and sonar) to avoid bycatch and STFO are significantly hindered by the lack of any way to accurately verify what is actually under their vessel and being recorded on their instruments. Purse seine fishermen have a distinct advantage as they can refine catch estimates with every successful set and no doubt become quite proficient at determining fish species by acoustic means. However, their ability to do so or to estimate fish size is difficult to test and has not been adequately documented. A self contained underwater video and recording system was tested on tuna schools aggregated to two anchored oceanographic buoys that act like FADs at 2°N and 5°N, 155°W. Skipjack and bigeye tuna were easily identified by body and fin morphology, swimming behaviour and the appearance of echo sounder images. Bigeye tuna were often observed to swim with a characteristic " waddling " movement with clearly visible tail beats. Yellowfin tuna were more difficult to positively identify but this was not fairly tested as few yellowfin were aggregated to the buoys visited during the cruise. Judging fish size from video images was found to be more difficult but could be greatly assisted by the development of identification guides based on external, visual characteristics of live tuna. The short duration of observations indicates that much more field work will be required in different areas and under a variety of conditions, including trials on commercial purse seine vessels. However, these preliminary tests were highly encouraging and will be continued in Hawaii and on the PTTP tagging vessel in the western Pacific during 2008/09.