Spatiotemporal distribution and catching status of elver (Anguilla japonica) in the Yangtze River Estuary

The Yangtze Estuary is the largest estuary in the Northwest Pacific Ocean. From January to May every year, a large number of Japanese eel (Anguilla japonica) elvers arrive at the area under the influence of the Kuroshio and Taiwan Warm Current. The Yangtze Estuary is an important upstream migration channel for Japanese eel elvers, and the most important producing area of eel elvers in China. The protection and utilization of elvers resources here will not only help to maintain the healthy and sustainable development of elvers related industries in China, but also help to maintain biodiversity and ecological security in the Yangtze River Basin. This study carried out investigation of Japanese eel elvers for four years in the Yangtze Estuary from 2017 to 2020. Seven stations were set up to basically cover the main migration distribution area of elvers , which could better reflect the distribution and migration in the Yangtze Estuary. By analyzing the fishing capacity and resource status of Japanese eel elvers in the Yangtze Estuary in the past four years, it was found that the the main bloom period of glass eel in the Yangtze Estuary was from February to April, and the main fishing areas were distributed in Dongwangsha, Sheshan and near Nanhuizui water area. In recent years, with the strengthening of the fishing management and the implementation of the ban on fishing in the Yangtze Estuary, the fishing capacity of elvers in the Yangtze Estuary has gradually decreased, and the effective net area has decreased by 36%, from 78.72×10⁴ m² in 2017 to 50.40×10⁴ m² in 2020. The main fishing waters of elvers have also shifted from the inside of estuarial mouth (Dongwangsha and Jiuduansha) to outside the mouth (Nanhuizui and near Luchaogang). The CPUE results of catching elvers in bloom season showed that the average catch in 2017 was (4 474±256) ind./100 m²; in 2018 and 2019, it gradually decreased to (1 917±335) and (1 365±257) ind./100 m² respectively; the maximum CPUE value in 2020 was (5 220±1 063) ind./100 m². The total yield of elvers is relatively high in 2017 (33.34 million ind.) and 2020 (20.74 million ind.), and low in 2018 (13.37 million ind.) and 2019 (9.86 million ind.), showing a cyclical fluctuation trend of about 2-3 years. It is suggested to strengthen the monitoring of Japanese eel elvers resources, scientifically evaluate and analyze the changes of elvers resources, and analyze the ecological impact of elvers fishing, so as to provide technical guarantee for the research and formulation of scientific management policies and measures, and provide scientific basis for the evaluation of the effect of the Yangtze River fishing ban.