The behaviour of anadromous Arctic charr during their first marine migration

Abstract

Climate change-induced alterations of aquatic ecosystems, increased water temperature and human activity are substantial stressors to anadromous Arctic charr, and these are all expected to increase in the future. The knowledge of the marine behaviour of anadromous Arctic charr, especially post-smolts, is limited and few studies has described their residency at sea in detail. Therefore, more studies are needed to get a complete understanding of anadromous Arctic charr’s feeding migration and how stressors as climate change and human activity interact with this life-history strategy. The use of electronic tags in behavioural studies have recently revealed the marine migratory behaviour of Arctic charr, but these studies include almost solely adult individuals. In this thesis, I have investigated the migratory behaviour of 50 Arctic charr first-time migrants (post-smolt) in the Balsfjord, northern Norway, during their first summer at sea. Fish were tagged with acoustic tags with depth sensors and monitored by 76 acoustic receivers in both fresh water (n = 72) and at sea (n = 4) in the period 28th June–08th October 2018. Arctic charr post-smolts were found to reside between 0–78 days at sea and experienced high survival with 82 % (n = 37) of the fish returning to fresh water. Fish were observed to re-enter their native river for assumed overwintering. Fish detections at four cross-fjord transects indicated the littoral habitat to be the most utilized habitat compared to the pelagic. Post smolts showed a fidelity to depths between 0–3 m while at sea and a diel shift in depth use was observed. Furthermore, fish depth use varied between fjord sections and throughout the summer. Fish resided within 45 km from their native river, within the fjord system and utilized mostly mid-fjord areas and the eastern side of Balsfjord. Despite within-fjord residency, fish moved distances beyond 300 km during the summer and these total travelled distances correlated positively with fish size. This study is one of the first to provide a detailed description of the spatiotemporal marine area use of Arctic charr post-smolts. The results presented here provide important information for coastal area management and conservation of anadromous Arctic charr populations and may also act as a reference during the ongoing climate change.