Variable migration and delay in two stock components of an Atlantic salmon population

Abstract

Migration obstacles, such as waterfalls, can delay the upriver migration of salmon, but little is known about the magnitude of such delays and the impact on different stock components. Atlantic salmon (n = 60) were captured by angling and radio tagged below natural waterfalls near the mouth of the Laxa River, northeast Iceland. The movement of the tagged fish was followed throughout the fishing and spawning season. The distributions of the tagged salmon and salmon caught by sport fishermen in the Laxa River were consistent. Water temperature had a significant effect on the behaviour of tagged salmon ascending the waterfalls. Salmon that spawned in the lower parts of the main river were delayed longer below the waterfalls than salmon spawning in the upper river. Salmon that spawned in tributaries tended to delay longer below the waterfalls than salmon spawning in the main river, and the average delay tended to be positively correlated with fish length. For multi-sea-winter salmon, the period from tagging until becoming stationary at a holding site near their spawning ground was 35 days on average, which was significantly longer than one-sea-winter salmon, in which this period was 17 days on average.