Movement patterns and spawning activity of individual nase Chondrostoma nasus (L.) in flow-regulated and weir-fragmented rivers

Eighteen adult nase Chondrostoma nasus (L.) (37.6-48.2 cm FL) from three highly fragmented Belgian rivers were tagged with surgically implanted radio transmitters and manually tracked from the banks of the rivers for 3-34 weeks. Their movements were mainly recorded during the circum reproduction period in 2001, 2002 and 2006 in river stretches delimited by physical barriers and/or minimum flow conditions. To reach the spawning grounds, nase displayed different patterns of movements that were mainly influenced by the configuration of the study site, the date of observation and the associated environmental conditions. Nase never cleared any physical obstacle but were able to spawn in minimum flow conditions or just downstream from physical obstacles. Nase were present on spawning grounds in late March in the warmest spring (2001) and from mid-April to early May in the coldest spring (2006). After spawning, a major proportion of C. nasus migrated a substantial distance downstream, whereas others remained near their capture site and showed frequent movements, sometimes corresponding to group displacements from one river to another.