Research on mitigation of the interaction of sea turtle with pelagic longline fishery in the western North Pacific

Abstract

We conducted shallow-set longline fishing operations on a research vessel in the western North Pacific, May-July, 2002-2004. We investigated sea turtle interaction with a pelagic longline fishery and preliminarily examined the potential gear modifications (bait and hook types) for reducing sea turtle bycatch or mortality. A total of 54 loggerhead sea turtle Caretta caretta were caught in 76 operations (about 74, 000 hooks), which were all alive. The mean straight carapace length was 67 cm (range: 52 - 82 cm); this suggested most of loggerhead sea turtles caught in this area were sub-adult. Loggerhead sea turtle catches ranged from 19.1 to 24.5 °C in sea surface temperature, which were concentrated in warm water masses in the Kuroshio extension. The Loggerhead sea turtle catch was more frequently observed on hooks hauled after sunrise. For bait type, the catches by mackerel bait were fewer than those by squid bait. In the circle hook trial in 2003, the deep-hooking rates (proportion of deep-hooking (at esophagus or pharynx) to total in numbers) on squid bait were 0.41 and 0.23 in 3.8 sun conventional tuna hook (n = 22) and 3.8 sun Tankichi type circle hook (which had similar size to 3.8 sun conventional tuna hook) (n = 13), respectively, but the difference was not significant (P > 0.05, extended Fisher’s exact test). In 2004 the large 18/0 circle hooks (which had a larger size, compared to 3.8 sun tuna hook) were tested, but its effect was not clarified due to very small catch numbers. Following these researches, we have been conducting sea turtle mitigation studies, with evaluation of their impacts on target and other non-target species catches.