Relative abundance trend of the blue shark Prionace glauca based on Japanese distant-water and offshore longliner activity in the North Pacific

摘要

The Kuroshio–Oyashio transition zone is one of the most important fishing grounds for pelagic fish, including blue shark and swordfish, which are primarily targeted by shallower Japanese longliners. We evaluated the fishing behavior of Japanese longliners based at the Kesennuma fishing port to estimate the standardized catch per unit of effort (CPUE) of blue shark. In our analysis, we first used data that had been filtered for blue shark sets and found that annual target shifts occurred seasonally and geographically; the greatest change in target species, from swordfish to blue shark, occurred in spring (April–June). Beginning in the early 1990s, the fishing grounds shifted from the southwest to the northeast of the North Pacific, and the number of sets targeting blue shark gradually increased. To incorporate this variable target behavior into the abundance index, the 10th percentile of the swordfish CPUE values was applied as the target indicator in the CPUE standardization model. The results indicate an upward population trend that has been caused by a decrease in fishing pressure from the early 1990s due to the prohibition of drift nets in 1992 and the subsequent decrease in longline vessels.