ON THE NASE, CHONDROSTOMA-NASUS SPAWNING AREA SITUATION IN SWITZERLAND

The present study describes results of a survey performed in 1993 on t he distribution of the nase, Chondrostoma nasus (Linnaeus, 1758) and t he condition of its spawning areas in Switzerland. Characteristic feat ures of the spawning habitat and observations about spawning behaviour are described. Using the available data base, 48 registered spawning areas were compared according to their importance for reproduction of the nase in former and present times. It is estimated that the number of spawners which using these areas nowadays represent less than two p er cent of the spawning runs in former times. This drastic decline of the nase population is regarded as a result of progressive anthropogen ous water development. Comparing the former with the present situation a significant shift of the vertical range in which the main reproduct ion takes place is noticed. In former times, the main spawning runs we re observed below 300 m above sea-level. Because of anthropogenous dev elopment and water power use, however, these areas are now practically lost and of little significance. Now main reproduction takes place ab out 100 meters higher in altitude. The consequences of these changes f or the success of reproduction are discussed.