Effects of environmental factors on migratory behaviour of northern Atlantic salmon.

Abstract

Over the last decades, the abundances of many Atlantic salmon populations have declined drastically both due to anthropogenic and natural factors. The period when salmon enter the sea for the first time, starting as smolts in the river and subsequently as post-smolts in the fjord, is regarded as one of most critical periods in the salmon life history. In addition, salmon can also suffer from high mortality during their return migration through fjords and estuaries both due to predation and coastal fisheries. Despite an increasing number of recent Atlantic salmon migration studies, few have been conducted in northern areas which host some of the largest and most productive salmon populations in the world. <br>The main aim of this thesis was therefore by use of electronic acoustic tracking and video observation to study how the environment influence on the behaviour patterns of northern Atlantic salmon during their smolt migration in rivers (Tana and Alta), and as post-smolts and homing salmon during their estuary and fjord migration (Alta). Since northern areas have 24 h daylight during summer, special attention was given to the impact of light intensity on salmon migratory behaviour. This also includes a study of direct impact from light intensities on post-smolt swimming behaviour in a south-west Norwegian fjord (Hardanger Fjord) with distinctive day and night periods. Secondly, the observed behaviour of the northern Atlantic salmon was related to other important environmental factors like temperature, river flow, current, tides and wind. Finally, the survival rate of post-smolt during fjord migration, as well as the fjord residency and migratory speeds of both northern post-smolts and homing salmon, were studied for the first time for a northern salmon population. The results were compared with earlier reported findings from southern populations. <br>The results showed that the northern smolts, post-smolts and homing salmon migrated during both day and night in both the river and fjord. In contrast, the post-smolts in the south-western Hardanger Fjord showed distinctive changes in day and night behaviour by swimming deeper in the water column during the day than during the night. However, the northern smolt behaviour was affected by river flow and water temperature, and the post-smolts in the estuary and fjord seemed to be affected by the tidal cycle and wind-induced currents. The homing salmon were also periodically affected by wind-induced currents, and similar to the post-smolts, they migrated mainly close to the surface. Further, the homing salmon generally followed the coastline towards the river mouth, and as they approached the estuary, migratory speed was reduced by 75% and the average swimming depth reduced from 2.5 m to 0.5 m. There was no evidence that river entry of these fish was affected by tidal cycles or river flow. The post-smolts used on average only 0.8 days to migrate the first 17 km outward through the fjord (20.5 km day-1). In contrast, the homing adult salmon used 20% more time over the same distance (16.5 km day-1). Finally, the post-smolt survival rate was estimated to be 75% from the estuary and through the first 17 km of the fjord. <br>The findings of seemingly no difference between day and night migratory behaviour for the northern smolts and post-smolts in the present thesis may be due to the fact that nocturnal migration, as often observed for southern populations, does not provide the northern fish any benefit in regard to sight feeding predators due to the 24 h of daylight. The observed change in day and night swimming depth in the Hardanger Fjord indicated that light intensity may also affect the swimming depth of post-smolts. The relationships between migratory behaviour of northern smolts and post-smolt and water temperature, river flow and tidal cycles found in this thesis, could, as well as the adaptation to the light intensities, be antipredatory strategies. <br>In total, it seemed like the first-time migrants were more influenced by light, river flow, tidal cycle and fjord currents than the homing salmon. This may be due to their smaller size and higher vulnerability to predation, supported by the fact that 25% of the tagged post-smolts did probably not survive the first 17 km of the fjord migration. The high mortality rate was similar to earlier findings in southern populations during the first few days after sea entry. However, the finding that the homing salmon migrated close to the surface and shoreline, combined with their longer residency in the inner fjord, may greatly have increased their risk of being caught by net fishing targeting salmon along the shoreline in this area. <br>In conclusion, this thesis indicates that the migration behaviour of northern and southern salmon differ somewhat, and that this is related to local adjustments to the existing abiotic environmental factors typically for the different latitudes, in particular the light regimes. These local adaptations may be due to phenotypic plasticity and/or different genotypes. The study also highlight the post-smolts and homing salmon phases in fjords and estuaries as important bottlenecks of survival in the Atlantic salmon life cycle, and that variation in both natural and anthropogenic factors during these phases may have large impacts on their migration behaviour, performance and subsequently the total return rate of salmon to their home rivers. An evaluation of the impact from subsequent effects of interventions along the coastline in areas with migratory Atlantic salmon is highly recommended in order to avoid any negative effects on the seaward and homing migration.