title: Migration of Atlantic salmon (Salmo salar) smolts and post-smolts from a Scottish east coast river
Creators: name: Main, Robert Andrew Kenneth
Publisher: name: University of Glasgow
Year: 2021
Subjects: subject: Atlantic salmon, Salmo salar, early marine migration, smolt, post-smolt, Aberdeenshire Dee, acoustic transmitters (tags), Telemetery, post-smolt swimming speed.

description: The near-shore migratory behaviour of Atlantic salmon (Salmo salar, Linnaeus 1758) migrating to sea for the first time is poorly understood. This study aims to assess whether salmon smolt survival is consistent along the River Dee in Aberdeenshire, through Aberdeen harbour and early in their marine migration out to ten kilometres from shore. This study also looks at the patterns of directional movement in the sea during the first 10 km of post-smolt migration to distant feeding grounds and provides estimates of missed detections based on range testing and simulation results. One hundred and sixty wild salmon smolts were implanted with acoustic transmitters (tags) in three tributaries of the Aberdeenshire Dee in 2017 (n=60) and 2018 (n=100). Several tags each year were capable of transmitting temperature and depth (2017 n=15, 2018 n =40) readings. These temperature depth tags were used as a proxy for predation. Smolt progress down river and out to sea was monitored remotely by a large array of Acoustic Listening Stations (ALSs), moored in the river (individual receivers), harbour and marine environment (gates of ALSs). One marine gate (IN), consisting of 35 receivers was deployed in 2017, four kilometres from the mouth of Aberdeen Harbour. In 2018, a second marine gate (OUT) was added, consisting of 98 additional receivers 10 km from the mouth of Aberdeen Harbour. In addition, an Acoustic Doppler Current Profiler (ADCP) was deployed in 2018, to measure marine currents allowing for determination of the actual swimming vectors taken and speeds of post-smolts at sea. Using detections as a proxy for survival and lack of detection as a proxy for mortality (this must be treated with some caution as some tags may not be in the original study animal and might still be counted as surviving or fish passing a receiver without being detected, tag failure or tag ejection might account for some of the presumed mortality); mortality was different in Aberdeen Harbour and in early marine migration between years. The upper river in 2017 shows that of the 46 fish tagged at Dinnet Burn only 25 (54%) were later detected in the river. In 2018 five (83%) of the six fish tagged at Dinnet were later detected in the river. In 2017, all 33 tags that left the lower river passing the last river ALS (R12) and entered Aberdeen Harbour (H1) successfully migrated through the harbour (leaving the last gate of ALS (H2)). However, in 2018, of the 83 tags that left R12, three tags (3.6%) were not detected entering at the H1 gate and a further three tags failed to successfully migrate through Aberdeen Harbour. The highest mortality rate (5.3 % km-1) was recorded was between H2-IN gates in 2017, where seven smolts (21.2% of the remaining smolts) were not detected again. In 2018 the highest mortality rate (5.1 % km-1) was between R12-H1 gates where three smolts (3.6% of remaining smolts) were not detected again. Mortality and mortality rates were similar to that found in other studies. One tag (2.2% of the remaining temperature and depth tags) in this study showed a temperature spike indicating a predation event by a warm-blooded predator (either bird or mammal). Two further tags (4.4% of the remaining temperature and depth tags) showed unlikely depth profiles suggesting evidence of predation by a marine fish. The most parsimonious model predicting smolt river migration success (binomial) showed year of tagging as by far the largest effect. Variables that had a marginal effect (explaining some variation) include: Tag burden (by length), tag burden (by weight), group size and river flow at time of release. The bearings taken by individual fish between H2-IN were not significantly different between years (in 2017 106o from north and 2018 96o from north). However, in 2018, bearings were significantly different between the H2-IN gate (96o from north) and between IN-OUT gate (128o from north). The ADCP data were used to account for the effects of current. After correction, the mean actual headings taken by individual fish between H2-IN gates were not significantly different between years but the mean heading between H2-IN and between IN-OUT gates in 2018 remained significantly different. Between IN-OUT gates fish were swimming actively on a mean heading of 158o from north (circular sd ± 37o) and at a median speed of 0.57 ms-1 equivalent to 3.98 body-lengths s-1. This study highlights how smolt migration patterns vary greatly between years in a river with very few manmade structures, a busy harbour and during the construction of a wind farm. However the study also indicates that the majority of smolt losses occurred during the river migration in 2017 and 2018, however when considered as a percentage loss per kilometre the greatest loss occurs in the early marine environment in 2017 (5.30 (%) km-1).