Hydroacoustic estimates of fish abundance and behavior characteristics in Yangzonghai Lake

Yangzonghai Lake is one of the nine major lakes in Yunnan plateau. In order to evaluate the distribution characteristics and resource status of fish, we had carried out four catch investigations in March, May, August and October in 2022 and conducted two hydroacoustic detections (Split beam echosounder, EY60) in June 20−21, 2022. The results show that a total of 24 fish species were collected, belonging to 23 genera, 14 families and 7 orders. Among them, Neosalanx taihuensis, Hypophthalmichthys molitrix, Aristichthys nobilis, Rhinogobius giurinus, Hemiculter leucisculus and Hyporhamphus intermedius were the dominant fish species in Yangzonghai Lake. Small fish such as N. taihuensis and R. giurinus accounted for more than 90% of the total number of fish species. Overall, the spatial distribution of fish had no significant differences between day and night. High-density areas were all distributed in two shallow water areas to the north and south of the lake. The vertical distribution and behavioral characteristics of fish showed significant differences between day and night. In the nighttime, a large amount of weak sound signals were concentrated at 5−10 m water layer, while they are scattered in the daytime. Besides, 75% of fish swam upstream in the nighttime, while vice verse in the daytime. The number of small and medium-sized fish tended to rise slightly in the nighttime, while larger fish were more active in vertical migration in the nighttime. The average densities of fish in Yangzonghai Lake in the daytime and in the nighttime were estimated as 0.033 and 0.038, respectively. The results show that a new community structure has been formed in Yangzonghai Lake, with exotic fish species as dominant and decreasing variety and quantity of indigenous fish species. It is recommended to increase the fishing of small fish such as N. taihuensis, and continue to carry out proliferation and release of silver carp, bighead and pelagic indigenous carnivorous fish to adjust the fish community structure, so as to achieve the goal of water quality regulation.