Wünstel, A., Greven, H.

Further data on the biology of the river lamprey Lampetra fluviatilis in an anthropogenously altered river in Northrhine Westphalia

Summary: In continuation of our investigations on the biology of Lampetra fluviatilis in the lower reaches of the anthropogenously altered Dhünn, we counted in 1998 485 adults that spawned in 184 nests of 40,6 cm in length and 74,6 cm in width on average. There are no relations between the phases of the moon and the spawning. Of importance, however, was the increase of water temperature in the Rhine that induced immigration of lampreys into the Dhünn. In 1996 and 1998 the sex ratio was biased to the males at the beginning of the spawning period but approached the ratio 1 : 1 later. In 1997 the number of females always was greater that those of males. Diagonal constructions in the river did not prevent upstream migration of the lampreys when a zone of reduced water current was present behind them. However, animals were not able to come over a slab of concrete 4 m in width and covered with algae. To evaluate the barrier effect of  diagonal constructions not only their height and water velocity, but also the conditions behind them should be considered. Number of animals within the nests is not constant during the day.