The bitterling Acheilognathus macropterus, introduced from the continent of China to the Tone River system, including Lake Kasumigaura, is designated as an invasive alien species by the Invasive Alien Species Act of Japan, due to their potentially negative impacts on other threatened bitterling species through interspecific competition. Although ecological studies of immature and adult stages of A. macropterus have already been reported in both China and Japan, little is known about larval and juvenile stages in the wild. To ascertain habitat characteristics in early life stages of the species, spatial distribution of larval and juvenile A. macropterus and environmental variables were investigated at 131 sites in two river systems (Ono R. and Shintone R.) flowing into Lake Kasumigaura in June 2018. A total of 1,118 larval and juvenile specimens (5.8–18.4 mm in body length, BL) were collected using hand nets during the study period, ca. 93% of the total number being larvae. A generalized liner mixed model based on the data for larval density with a variety of environmental variables (i.e., water temperature, dissolved oxygen, water depth, flow velocity, vegetation density, mud content ratio in the bottom sediment, distance from a freshwater pearl farm, and wave height) at each site revealed that greater vegetation density with higher dissolved oxygen and distance to pearl farms utilizing the unionid hybrid mussel Sinohyriopsis schlegeli × S. cumingii were the most significant determinants of larval density. The results indicated that appropriate management of river vegetation and pearl farm factors are necessary for the establishment of essential controls, so as to manage the reproduction and expansion of A. macropterus in the Ono and Shintone River systems.
