Lagrangian fronts and saury catch locations in the Northwestern Pacific in 2004-2019

A database with Pacific saury catch locations in 2004-2019 fishing seasons in the region of the Kuril and Hokkaido islands was created to study a connection of favorable fishing grounds with mesoscale fronts in the area. The locations of the fronts have been identified with the help of daily altimetry-based Lagrangian maps computed for a large number of artificial particles. Based on the Lagrangian analysis of the multi-year simulation results, we distinguished persistent Lagrangian fronts with accumulation of catches, which were associated with the Soya Warm Current, the 1st, 2nd and 3rd Oyashio branches and with quasi-stationary Hokkaido and Bussol anticyclones. The simulated Lagrangian fronts, which are accessible practically in the real time, have been shown to be useful proxies for locations of hydrological fronts with potential fishing grounds. The proximity of catch sites to locations of strong Lagrangian fronts has been shown statistically using the maximal gradients of the Lyapunov field as an indicator of fronts. The origin maps have been used for analysis of interannual variation in the spatial distribution of the nutrient-rich Oyashio and Okhotsk Sea water masses. Calculating surface Oyashio transport in the altimetry era, we connected the catastrophic decrease in saury catches in traditional fishing places in 2015-2019 with a change in the pathways of the Oyashio water. The significant increase of the Oyashio transport in the recent years was supposed to be favorable for formation of potential fishing grounds far east off Kurils and Hokkaido coasts. This factor together with a smaller stock-size saury immigration was supposed to explain a decrease in the actual catches after 2014 and a displacement of the fishing grounds to the open ocean.