A preliminary investigation of spawning migrations of grayling in a small stream as determined by radio-tracking

Adult grayling Thymallus thymallus migrated from 230 to 4980 m up the Aisne stream, Belgium, to spawn between 18 and 29 March, under decreasing floods, increasing temperature and low turbidity.Males (n=4) arrived on spawning grounds several days earlier than females (n=2), stayed there longer (10-19 v. 2-3 days), and occupied a single ground each, whereas females moved between several places. After spawning, all grayling homed precisely into the pool-riffle sequences where they were tagged in late February, and remained here until late June. These observations indicate that resident grayling are far less mobile than autumn-spawning salmonids, and that the environmental factors triggering spawning migrations resemble more closely those of spring-spawning cyprinids than of other salmonids. The implications of these restricted mobility patterns are discussed within the scope of population structure, and impact of river management.