Finless porpoise bycatch in Ariake Sound and Tachibana Bay, Japan

Abstract

Bycatch of narrow-ridged finless porpoises Neophocaena asiaeorientalis in gillnets was estimated for the Ariake Sound/Tachibana Bay population in Japan using interview-based surveys. In 2007 and 2008, the minimum number (actual number reported by all interviewees) of bycatch was 31 to 35 individuals (ind.) yr �1 . The number of interviewees was 131 to 136 in each year. The bycatch estimate was 238 (95% CI 123 to 353) to 270 (95% CI 148 to 391) ind. yr �1 . The estimate exceeded the potential biological removal level of this population, 27 ind., which was cal- culated on the basis of a published abundance estimate. The bycatch estimate accounted for 8% of the abundance. The bycatch rate was 0.237 ind. gillnetter �1 yr �1 in 2007 and 0.257 ind. gillnet- ter �1 yr �1 in 2008. Additional analysis of information on bycatch in the bottom-set gillnet fisheries collected from 1987 to 1992 suggested an increase in bycatch during fall and winter seasons, the possible existence of a calving ground, and age-sex related habitat use patterns. Neonates were found in a restricted area, whereas nursing calves and possible calves were seen in a wide range of waters, implying outward movement of mothercalf pairs from a calving ground. Neither neonates nor nursing calves were captured in waters deeper than 30 m.