Diel Catch Variation of the Primary Fish Species Captured by Trammel Nets in a Shallow Eutrophic Lake in Jiangsu Province, China

Fish diel activity can affect the catch of fishing gear, such as gill nets, thereby influencing fishery resource assessment and management. This study investigated diel catch variations of primary fish species in Gehu Lake using monofilament trammel nets from April to November of 2016. Fish sampling occurred monthly, with nets set and fish caught at four-hour intervals in each month. The results showed that significant diel effects and diel × month interaction were found on Chinese silver carp (SC) and diel × month interaction on common carp (CC). Topmouth, humpback, and Wuchang bream (WB) displayed significantly higher catch per unit effort (CPUE) during twilight or daytime than at night, and no diel × month interactions were detected. For Chinese bighead carp (BC), Mongolian redfin (MR), Japanese grenadier anchovy (JGA), and crucian carp, no diel effect and diel × month interaction were observed. The study suggested that most activities occurring in daytime and at twilight were caused by visual orientation to prey for topmouth and humpback, and by the herbivorous feeding habitat of WB. Food competition between BC and JGA may drive a pronounced temporal partitioning of their diel activity. Overnight gillnet fishing could underestimate the population sizes of herbivores, such as WB, and visually oriented predators, for example, humpback, and might not influence the estimation for BC, JGA, and crucian carp. However, its effects on the stock estimation of SC and CC would vary with months. Notably, future winter investigations into diel catch in this lake could potentially augment the conclusions of the present study.