Long-term changes in CPUE of sharks and size of blue sharks caught by tuna longlines in the western North Pacific Ocean

Motivated by Ward & Myers (2005) that claimed higher impact of longline fishery on large pelagic community including blue shark, long-term comparison of shark CPUE were made using recent and historical data collected by Japanese research and training vessels in the western North Pacific. Standardized species combined shark CPUE (predominated by blue shark catch and assuming that the shark combined CPUE represents blue shark CPUE in this paper) in the 1930's, 1960's and 1990's did not show any difference but were comparable between these periods. Average body lengths of blue sharks showed a minor decline in some area. The maximum decline of 13% was recorded at higher latitudes, which corresponded to 36 % reduction in body weight, but for the rest of the area there were no declines. Results of the analysis indicated that both CPUE and body size of blue shark varied temporally and spatially, but did not show statistically significant difference in most cases. Therefore, it is concluded that tuna longline fishery did not give a significant impact on blue shark stock which is predominant shark species caught by longline in the western North Pacific Ocean.