Lateral migrations of fishes between floodplain lakes and their drainage channels at the Lower Rhine: diel and seasonal aspects

摘要

Although the total migratory activity of fishes from floodplain lakes in the Lower Rhine (Germany) into the connected drainage channels, estimated using electrofishing, tyke nets and visual observations, was low during summer 1997, conspicuous diel migrations of adult bream Abramis brama in June and July were recorded in one of the channels studied. A considerable proportion of the population of the floodplain lake immigrated into the channel at dusk and left it at dawn. During two winters (1994-1995, 1997-1998) the abundance of fishes increased considerably in the channels. The winter migrations, studied in detail in one of the channels in 1997-1998, started in late October, when the water temperature dropped below 10degrees C. In contrast to the mean of 500 fishes per 100 m channel length between May and October, >5000 fishes were found from November to March, with maximum values of >50 000 fishes per 100 m channel length in February 1998. More than 90% of these shoals were 0+ year fishes, dominated by roach Rutilus rutilus, bream, perch Perca fluviatilis and rudd Scardinius erythrophthahmus. These shoals entered the channel at sunrise and left it around sunset. Generally, the 0+ year fishes in the shoals swam separately from the older fishes, and different reactions towards the attack of predatory fishes were observed. The migrating shoals of juvenile fishes during the winter were interpreted as a strategy to minimize predation risk.