Contribution to the knowledge of the distribution and spawning grounds of Chondrostoma nasus and Chondrostoma toxostoma (Pisces, Cyprinidae) in Switzerland

Summary

This study was carried out to gather information on the previous and present distribution of Chondrostoma nasus (nase) and C. toxostoma (toxostome) in Switzerland. Special efforts were made to locate the remaining spawning grounds and to compare them with formerly elaborated data. The research was conducted by means of literature reviews, information from fisheries authorities and fishermen, and field observations during the 1993 spawning period. Observed spawning grounds were described by abiotic parameters such as river bed substrate, water depth and water velocity. Spawning of nase was only observed in 14 river stretches, whereas in the past: reproduction took place along 57 stretches. Over the last 10 years, breeders have also been observed at another 15 sites, indicating that spawning may still take place. However, it is suspected that most of the remaining spawning grounds have today disappeared. In 1993 the spawning peak of nase occurred on 21st/22nd April and lasted only two or three days. Nevertheless, in two rivers this species reproduced repeatedly on two other occasions, on 10th/12nd May and on 26th/28th May. All spawning grounds were characterised by gravel bed, water depth of 10 to 30 cm and water velocity of 0.7 to 1.1 m-s^-1. The only population of toxostome was localised on a 4.3 km long stretch of the River Doubs and only one reproduction site was identified. In 1993 spawning took place at the end of May and did not last for more than a few days. The spawning ground was characterised by gravel bed with no fine sediments, water depth of 5 to 40 cm and water velocity of about 0.5 m-s^-1.