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 (10-1 9 v. 2-3 days), and occupied a single ground each, whereas females moved be tween several places. After spawning, all grayling homed precisely into the pool-riffle sequences where they were tagged in late February, and remaine d here until late June. These observations indicate that resident grayling are far less mobile than autumn-spawning salmonids, and that the environmen tal factors triggering spawning migrations resemble more closely those of s pring-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. (C) 1999 The Fisheries Society o f the British Isles.