Effects of age composition of Pacific bluefin tuna on their spatiotemporal distribution of active breeding in the south-western North Pacific and on its recruitment strength

Abstract

Context The south-western North Pacific is one of the main spawning grounds of the Pacific bluefin tuna (Thunnus orientalis) and is targeted by Japanese longline fisheries. Aims This study evaluated the influence of age composition of spawning groups in this area on the strength of recruitment through the spatiotemporal distribution of active breeding. Methods The variation in the recent year-class strength in the area was estimated on the basis of the otolith age estimation and catch data from 2007 to 2016. Key results In the age–frequency distribution, 9 years was the most frequent age class. The instantaneous total mortality after 9 years of age was 0.32. Conclusions On the basis of catch locations and age estimation results, the bias toward young individuals influenced the spatiotemporal pattern of active breeding to shifts to higher latitudes and earlier breeding. In this area, the abundance of the main spawning group (9- and 10-year-old fish) may affect the recruitment of 0-year-old fish, and the older fish are considered to serve as a ‘seed bank’. Implications These results are expected to contribute to the development of appropriate management strategies for spawning stocks in the area and demonstrate the magnitude of the influence of age composition on population fluctuations.