Ocean migration of Atlantic salmon

Paper II is not available in Munin. Paper II: Strøm, J.F., Thorstad, E.B., Chafe, G., Sørbye, S.H., Righton, D., Rikardsen, A.H., & Carr, J. (2017). Ocean migration of pop-up satellite archival tagged Atlantic salmon from the Miramichi River in Canada. Available in ICES Journal of Marine Science, 74(5), 1356-1370.Atlantic salmon is experiencing population declines throughout most parts of its distribution range, with changes in the marine environment considered important contributors to the overall decline. Most of the current knowledge about Atlantic salmon’s ocean distribution originates from mark-recapture and genetic studies associated with high-sea fisheries and sampling surveys, and therefore, little is known about individuals’ movements while at sea. For a greater understanding of the ecological conditions encountered in the marine environment, detailed information of the ocean migration and behaviour is needed. The aims of this thesis were to record the ocean migration and behavioural ecology for post-spawned Atlantic salmon from the Alta River, Norway, and the Miramichi River, Canada, and to quantify the marine mortality of post-spawned adults from large parts of the species’ distribution range, using archival telemetry. Results in this thesis documented that Atlantic salmon from the Alta River displayed an overall fidelity towards Arctic areas, with individual migration routes extending from the eastern Barents Sea to the Jan Mayen Island. These results support recent studies, suggesting that the utilization of these waters has previously been underestimated, particularly for the northernmost populations. In comparison, Atlantic salmon from the Miramichi River displayed an exclusive utilization of the Gulf of St. Lawrence and the Labrador Sea during their ocean migration. Individual migration routes diversified shortly after ocean entry, and after existing the Gulf of St. Lawrence, the overall distribution spanned large parts of the Labrador Sea. The overall variation in ocean migration documented in this thesis indicates that individuals from the same population may experience different ecological conditions during the marine phase, potentially contributing to within-population variation in growth and survival. During the ocean migration, Atlantic salmon displayed an overall pelagic behaviour. Individuals spent most of their time in the upper 10 m of the water column, diving frequently during daylight hours. For Atlantic salmon from the Alta River, there was a clear seasonal trend in diving intensity, with less diving during the winter, independent of individuals’ spatial distribution. In contrast, for Atlantic salmon from the Miramichi River, the overall trend in diving behaviour depended strongly on which habitat they occupied. As diving behaviour is likely a suitable proxy for foraging in Atlantic salmon and other pelagic fishes, these results indicate that individuals from both populations displayed flexible foraging strategies during their ocean migration, governed by seasonal variation and habitat-specific characteristics. Furthermore, this thesis documented that a variety of large marine animals, including large fish and toothed whales, preyed on Atlantic salmon during their time at sea. Predation by endothermic fish was most common, occurring primarily in the Gulf of St. Lawrence and in waters spanning from west of the Irish Shelf to the Bay of Biscay. Fewer predation events and mortalities were recorded for Atlantic salmon from northern Europe, which contrasted the higher predation rates and total mortality observed for populations from Canada, Spain, and Ireland. In conclusion, this thesis revealed several novel aspects of the ocean migration of individual Atlantic salmon and large geographical variation in the ocean mortality. This information is important towards a greater understanding of the ecological process governing Atlantic salmon’s growth and survival in the ocean, which should be further investigated in future research

Marine food consumption by adult Atlantic salmon and energetic impacts of increased ocean temperatures caused by climate change

Atlantic salmon Salmo salar are experiencing widespread population declines, and reductions in growth and survival in the marine environment are contributing factors. Our aims were to estimate marine food consumption of adult salmon and to determine how energetics would be directly afected by the increased ocean temperatures associated with climate change. We tagged previous spawners on outward migration (body size 76–119 cm) with archival tags and used a bioenergetic model to combine in situ temperature recordings with individual data on body growth. Average energy consumption was estimated to be 331–813 kJ per day, which is equivalent to 5–11 prey fsh with an average body mass of ca. 15 g. Energy content of prey was the most important factor determining food consumption required to maintain growth. Conversely, the increases in ocean temperatures expected with climate change were predicted to have limited physiological efects on energy budgets and limited impact on the food consumption needed to maintain growth. We conclude that climatic warming will impact Atlantic salmon primarily through changes in prey availability and ecosystem structure rather than the direct efects of temperature on physiological performance

Behavioural responses of wild anadromous Arctic char experimentally infested in situ with salmon lice

Salmon lice can impact the marine behaviour, growth, and survival of salmonids, but little is known about their effects on Arctic char. We present behavioural responses from the first dose-response experiment with wild anadromous Arctic char (n = 50) infested in situ with salmon lice (0.0–1.2 lice g−1 fish) in an area with low natural infestations. Infested fish spent less time at sea (mean ± SD = 22 ± 6 d) than non-infested fish (mean ± SD = 33 ± 5 d), and a significant dose response was evident, with even very low louse burdens (−1 fish) reducing the marine feeding time. Furthermore, a negative correlation was present between time spent close to their native watercourse and parasite burden, suggesting that salmon lice influence the marine habitat use of Arctic char. No impact of salmon lice was evident on the return probability, i.e. marine survival. However, the presence of louse-induced mortality cannot be excluded as the modest sample size was only sufficient to detect extreme effects. Reduced marine feeding time and altered marine habitat use will likely have substantial negative effects on growth and fitness, suggesting that impacts of salmon lice must be considered in the conservation of anadromous Arctic char

Behavioural responses of wild anadromous Arctic char experimentally infested in situ with salmon lice

Salmon lice can impact the marine behaviour, growth, and survival of salmonids, but little is known about their effects on Arctic char. We present behavioural responses from the first dose-response experiment with wild anadromous Arctic char (n = 50) infested in situ with salmon lice (0.0–1.2 lice g−1 fish) in an area with low natural infestations. Infested fish spent less time at sea (mean ± SD = 22 ± 6 d) than non-infested fish (mean ±SD = 33 ± 5 d), and a significant dose response was evident, with even very low louse burdens (<0.05 lice g−1 fish) reducing the marine feeding time. Furthermore, a negative correlation was present between time spent close to their native watercourse and parasite burden, suggesting that salmon lice influence the marine habitat use of Arctic char. No impact of salmon lice was evident on the return probability, i.e. marine survival. However, the presence of louse-induced mortality cannot be excluded as the modest sample size was only sufficient to detect extreme effects. Reduced marine feeding time and altered marine habitat use will likely have substantial negative effects on growth and fitness, suggesting that impacts of salmon lice must be considered in the conservation of anadromous Arctic char.publishedVersio

Sea trout Salmo trutta in the subarctic: home-bound but large variation in migratory behaviour between and within populations

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Behaviour and habitat use of first-time migrant Arctic charr: novel insights from a subarctic marine area

Anadromous Arctic charr Salvelinus alpinus is a cold-adapted salmonid that is vulnerable to climate warming and anthropogenic activities including salmon farming, hydropower regulation, and pollution, which poses a multiple-stressor scenario that influences or threatens populations. We studied the horizontal and vertical behaviour of Arctic charr tagged with acoustic transmitters (n = 45, mean fish length: 22 cm) in a pristine, subarctic marine area to provide insights into the behaviour of first-time migrants. Tagged fish spent up to 78 d at sea, with high marine survival (82% returned to their native watercourse). While at sea, they utilized mostly near-shore areas, up to 45 km away from their native river. Arctic charr showed large variation in migration distance (mean ± SD: 222 ± 174 km), and the migration distance increased with body size. Although the fish displayed a strong fidelity to surface waters (0-3 m), spatiotemporal variation in depth use was evident, with fish utilizing deeper depths during the day and in late July. These results represent baseline data on Arctic charr’s marine behaviour in a pristine fjord system and highlight the importance of near-shore surface water as feeding areas for first-time migrants. Furthermore, the observed dependency on coastal areas implies a vulnerability to increasing human-induced perturbations, on top of impacts by large-scale climate change in marine and freshwater habitats.publishedVersio

Movement diversity and partial sympatry of coastal and Northeast Arctic cod ecotypes at high latitudes

Movement diversity within species represent an important but often neglected, component of biodiversity that affects ecological and genetic interactions, as well as the productivity of exploited systems. By combining individual tracking data from acoustic telemetry with novel genetic analyses, we describe the movement diversity of two Atlantic cod Gadus morhua ecotypes in two high-latitude fjord systems: the highly migratory Northeast Arctic cod (NEA cod) that supports the largest cod fishery in the world, and the more sedentary Norwegian coastal cod, which is currently in a depleted state. As predicted, coastal cod displayed a higher level of fjord residency than NEA cod. Of the cod tagged during the spawning season, NEA cod left the fjords permanently to a greater extent and earlier compared to coastal cod, which to a greater extent remained resident and left the fjords temporarily. Despite this overall pattern, horizontal movements atypical for the ecotypes were common with some NEA cod remaining within the fjords year-round and some coastal cod displaying a low fjord fidelity. Fjord residency and exit timing also differed with spawning status and body size, with spawning cod and large individuals tagged during the feeding season more prone to leave the fjords and earlier than non-spawning and smaller individuals. While our results confirm a lower fjord dependency for NEA cod, they highlight a movement diversity within each ecotype and sympatric residency between ecotypes, previously undetected by population-level monitoring. This new knowledge is relevant for the management, which should base their fisheries advice for these interacting ecotypes on their habitat use and seasonal movements.publishedVersio

Micro- and macro-habitat selection of Atlantic salmon, (Salmo salar), post-smolts in relation to marine environmental cues

Atlantic salmon is an economically and culturally important species. The species encounters several natural and man-made threats during its migration between fresh water and the ocean, which in combination may explain its ongoing decline. With the aim to better understand whether post-smolt behaviour is influenced by physical oceanographic conditions, the migratory behaviour of 173 post-smolts in a high-latitude Norwegian fjord was investigated, combining acoustic telemetry with site- and time-specific environmental variables from an oceanographic model. Most post-smolts (94%) performed a unidirectional migration out the fjord. Progression rates were relatively high (0.42–2.41 km h−1; 0.84–3.78 BL s−1) and increased with distance from the river. While post-smolts had an affinity for lower salinities in the inner fjord, statistical models failed to detect any significant relationship between the small-scale (within arrays) migratory behaviour and salinity, temperature, or coastal surface currents within the fjord. In the outer part, the post-smolts predominantly exited the fjord system through the strait with the highest surface salinities and lowest temperatures, independently of the current direction. Our findings indicate that the macro-habitat selection of the Atlantic salmon post-smolts was influenced by environmental factors: the post-smolts directed their migration towards “ocean cues.” However, this was not confirmed on the micro-habitat level.publishedVersio

Thermal habitat of adult Atlantic salmon Salmo salar in a warming ocean

The year‐round thermal habitat at sea for adult Atlantic salmon Salmo salar (n = 49) from northern Norway was investigated using archival tags over a 10 year study period. During their ocean feeding migration, the fish spent 90% of the time in waters with temperatures from 1.6–8.4°C. Daily mean temperatures ranged from −0.5 to 12.9°C, with daily temperature variation up to 9.6°C. Fish experienced the coldest water during winter (November–March) and the greatest thermal range during the first summer at sea (July–August). Trends in sea‐surface temperatures influenced the thermal habitat of salmon during late summer and autumn (August–October), with fish experiencing warmer temperatures in warmer years. This pattern was absent during winter (November–March), when daily mean temperatures ranged from 3.4–5.0°C, in both colder and warmer years. The observations of a constant thermal habitat during winter in both warmer and colder years, may suggest that the ocean distribution of salmon is flexible and that individual migration routes could shift as a response to spatiotemporal alterations of favourable prey fields and ocean temperatures