Long-term changes in fish and crustacean catch by set-net in Lake Shinji

Abstract

Lake Shinji in San-in district, Japan, is home to many freshwater, brackish water, diadromous and marine fish species as well as crustaceans. Among these some marine and brackish water species and a crustacean species migrate upward from Lake Nakaumi during the warmer seasons. Catching these fishes and crustaceans by the set net and other methods has been actively practiced. However, there was a decline in the population of major fishes such as Hypomesus nipponensis, etc., decreased after the abnormal weather in the cool, rainy summer of 1993 and hot, dry summer of 1994. A previous study published in 2019 claimed that zooplankton and Meiobenthos such as Chironomus plumosus suddenly decreased in Lake Shinji after the introduction of neonicotinoid insecticides in 1993, these decreases may have caused the decline of H. nipponensis and Anguilla japonica. However, apart from the aforementioned two species and Salangichthys microdon, variation in the catch of other fish and crustacean species was not considered. Therefore, we investigated it using catch data for eight species by the set net practice from April 1979 to March 2009 kept in The Lake Shinji Fishery Cooperative Association. Post March 1995 the catch of H. nipponensis, A. japonica and Gobiidae decreased from September to March every year, and that of Decapoda also decreased from September to December, however, Mysidae increased from January to March, although yearly fluctuations were frequently observed. We considered the variation in these catches comparing with the annual variation in salinity and water temperature and the introduction of neonicotinoid insecticides from 1993.