Potential importance of lagoons as breeding grounds for bluegill Lepomis macrochirus in Lake Biwa, Japan

Exotic bluegill Lepomis macrochirus have been propagating in the littoral region of Lake Biwa and its tributary lagoons. To investigate the reproductive activity of bluegill in the prespawning season, fish samples were collected from three sites at the littoral region of the lake and their nearby lagoons. The number of females relative to the total number of specimens tended to be higher in lagoons than in the adjacent littoral sites in the lake. The females collected from lagoons had significantly larger ovaries than those from the lake. The means of gonadal somatic index of females in the lake ranged from 1.77 to 3.44, whereas those in lagoons ranged from 4.35 to 12.2. The higher temperature conditions of lagoons appeared to be responsible for the higher maturation of ovaries of the fish in lagoons. The environmental condition of lagoons is potentially suitable for the active growth of larvae, subsequently increasing their overwintering success. Therefore, earlier spawning of bluegill in lagoons might have contributed to the propagation of bluegill in Lake Biwa to some extent.