The first months at sea: marine migration and habitat use of sea trout Salmo trutta post‐smolts

The early migration and habitat use of brown trout Salmo trutta post‐smolts tagged with acoustic transmitters (n = 50) were investigated in a fjord system in central Norway from 30 April to 26 November 2014. The main aims were to investigate return rate, marine residence time and spatial use of the fjord system. Median seaward migration and return to fresh water dates were 22 May and 4 July, respectively. Of the 40 seaward migrating smolts, 26 returned to fresh water, giving a minimum return rate to fresh water of 65%. Entrance to the fjord from the river occurred mainly at night (80% of the S. trutta), however, no such diurnal pattern was observed during the return migration. Mean marine residence time was 38 days, but with large individual variation (22–99 days). The innermost parts of the study area were more utilized than the outer part of the fjord system during the sea residency, and with more use of the near shore habitat than the open, pelagic areas. Many post‐smolts also utilized the outer part of the fjord system, however, and 94% of the post‐smolts were recorded at least 14 km from the home river mouth. Marine survival and distribution in the fjord were size dependent with the largest individuals utilizing outer fjord areas and having higher return rates to fresh water. As far as is known, this is the first published study on temporal and spatial behaviour in the marine environment of first‐time S. trutta migrants during the full course of their first trip to sea.     

Migratory behaviour and survival rates of wild northern Atlantic salmon Salmo salar post‐smolts: effects of environmental factors

To study smolt behaviour and survival of a northern Atlantic salmon Salmo salar population during river descent, sea entry and fjord migration, 120 wild S. salar were tagged with acoustic tags and registered at four automatic listening station arrays in the mouth of the north Norwegian River Alta and throughout the Alta Fjord. An estimated 75% of the post‐smolts survived from the river mouth, through the estuary and the first 17 km of the fjord. Survival rates in the fjord varied with fork length (L_F), and ranged from 97·0 to 99·5% km^?1. On average, the post‐smolts spent 1·5 days (36 h, range 11–365 h) travelling from the river mouth to the last fjord array, 31 km from the river mouth. The migratory speed was slower (1·8 L_F s^?1) in the first 4 km after sea entry compared with the next 27 km (3·0 L_F s^?1). Post‐smolts entered the fjord more often during the high or ebbing tide (70%). There was no clear diurnal migration pattern within the river and fjord, but most of the post‐smolts entered the fjord at night (66%, 2000–0800 hours), despite the 24 h daylight at this latitude. The tidal cycle, wind‐induced currents and the smolts' own movements seemed to influence migratory speeds and routes in different parts of the fjord. A large variation in migration patterns, both in the river and fjord, might indicate that individuals in stochastic estuarine and marine environments are exposed to highly variable selection regimes, resulting in different responses to environmental factors on both temporal and spatial scales. Post‐smolts in the northern Alta Fjord had similar early marine survival rates to those observed previously in southern fjords; however, fjord residency in the north was shorter.     

Estuarine survival and migratory behaviour of Atlantic salmon Salmo salar smolts

To estimate mortality rates, assess the spatio‐temporal dynamics of natural mortality and examine migratory behaviour during the fresh to saltwater transition, 185 wild Atlantic salmon Salmo salar smolts were implanted with coded acoustic transmitters. Seaward migration of tagged S. salar from four river systems in an area of Nova Scotia, Canada known as the Southern Upland was monitored using fixed receivers and active telemetry over 3 years. Cumulative survival through the river, inner estuary, outer estuary and bay habitats averaged 59·6% (range = 39·4–73·5%). When standardized to distance travelled, survival rates followed two patterns: (1) constant rates of survival independent of habitat or (2) low survival most frequently associated with inner estuary habitats. In rivers where survival was independent of habitat, residency periods were also independent of habitat, post‐smolts exhibited few upstream movements, took a more direct route to the ocean and reached the ocean rapidly. Alternatively, in rivers where survival was habitat specific, residency was also habitat specific with overall increased residency, more frequent upstream movements and delayed arrival to the open ocean. The sudden disappearance of most (75–100%) smolts and post‐smolts assumed dead during the course of this study warrants further examination into the role of avian predators as a mortality vector.     

Riverine and near coastal migration performance of hatchery brown trout Salmo trutta

To study migration performance and return rates of hatchery brown trout Salmo trutta smolts the first 5 months after release, 50 fish in each year (fork length, L_F, 158–288 mm) were in two subsequent years tagged with acoustic transmitters and recorded by automatic listening stations in the River Nidelva (central Norway), its estuary and in the marine environment. More than half of the smolts became anadromous migrants (52% in 2011 and 70% in 2012). The fish spent longer time in the estuary than in the marine environment and the results suggest that migratory behaviour of S. trutta smolts is not only restricted to be resident or anadrome–lacustrine, but that there is also an intermediary strategy of estuarine feeding. There were no differences in L_F or mass between groups of smolts with different migration patterns. Return rates from the sea within the first 5 months after release were in both years 16%. Median progression rate in the river was 0·090 L_F s^?1 but decreased significantly as the smolts entered the estuary (0·015 L_F s^?1). The long residential time in the estuary may increase the risk of negative effects of anthropogenic activities in estuaries, such as harbours and industrial development, and special attention should be given to evaluate effects of such activities.     

Riverine, estuarine and marine migratory behaviour and physiology of wild and hatchery-reared coho salmon Oncorhynchus kisutch (Walbaum) smolts descending the Campbell River, BC, Canada

Eighty coho salmon Oncorhynchus kisutch smolts (40 wild and 40 hatchery-reared) were surgically implanted with acoustic transmitters and released into the Quinsam River over 2 days. Differences in physiology, travel time and migratory behaviour were examined between wild and hatchery-reared fish. In addition, tagged and control fish of both wild and hatchery-reared stock were raised for 3 months following surgery to compare survival and tag retention. Detection ranges of the acoustic receivers were tested in the river, estuary and ocean in a variety of flow conditions and tide levels. Receivers were placed in the river, estuary and up to 50 km north and south from the river mouth in the marine environment. Wild smolts were significantly smaller by mass, fork length and condition factor than hatchery-reared smolts and exhibited significantly higher levels of sodium, potassium and chloride in their blood plasma than hatchery-reared smolts. The gill Na⁺K⁺-ATPase activity was also significantly higher in the wild coho smolts at the time of release. Ninety-eight per cent of wild and 80% of hatchery-reared fish survived to the estuary, 8 km downstream of the release site. No difference was found in migration speed, timing or survival between smolts released during daylight and those released after dark. Wild smolts, however, spent less time in the river and estuary, and as a result entered the ocean earlier than hatchery-reared smolts. Average marine swimming speeds for wild smolts were double those of their hatchery-reared counterparts. While hatchery smolts dispersed in both a northward and southward direction upon entering the marine environment, the majority of wild smolts travelled north from the Campbell River estuary. The wild coho salmon smolts were more physiologically fit and ready to enter sea water than the hatchery-reared smolts, and as a result had higher early survival rates and swimming speeds.     

Effects of seawater-acclimatization and release sites on survival of hatchery–reared brown trout Salmo trutta

To test whether seawater–acclimatization of hatchery–reared anadromous and freshwater resident brown trout before release increased the survival of adults, smolts were retained 0, 2, 4 and 8 weeks in sea water before release. Total recapture rate increased for smolts retained 4 and 8 weeks in sea water before release relative to the controls. This trend was more pronounced for Freshwater resident than for anadromous stocks, Offspring of anadromous fish stayed longer at sea than offspring of freshwater resident fish. Recapture rates in fresh water were higher for brown trout released in the river than in the fjord in the R, Drammen area, but not in the R. Imsa. In both cases, most fish were recaptured in the sea. Moving into the R. Imsa (relative to other rivers) appeared higher for fish released at the mouth of the river (93%) than in the fjord (47%). Judged from the recapture rates, sea survival appeared to be the same whether released in the fjord or at the mouth of the river.     

Brown trout Salmo trutta released to support recreational fishing in a Norwegian fjord

One- and two-year-old hatchery reared juveniles of seven freshwater resident and anadromous populations of Scandinavian brown trout were released in the outer and inner part of the Oslofjord and in the River Akerselva, flowing through the city of Oslo. Recapture rates were highest (mean 20.3%) for river released fish and lowest for those released in the outer (16.8%) and inner (12.1%) fjord. In general, recapture rate increased with fish size at release (r=0.76). When released in fresh water, most of the recaptures were from fresh water and when released in the fjord, most recaptures were from the fjord. In general, freshwater resident stocks showed a higher degree of freshwater residency than anadromous stocks. However, mean migratory distance was longer for freshwater resident than anadromous fish. Trout moved longer distances at sea when released in the outer than in the inner fjord. Specific growth rate and size at recapture varied among release sites and stocks; they were highest for fish released in the outer fjord and lowest for river-released trout. There was no consistent difference in sea growth between freshwater resident and anadromous stocks. Estimated total yield was highest for fish released in the outer fjord, whereas there was no significant difference in yields between trout released in the river and the inner Oslofjord.     

Migration of Atlantic Salmon, Salmo salar, Smolt through the Estuary Area of River Ellidaar in Iceland

Synopsis We tagged both wild and hatchery Atlantic salmon, Salmo salar, smolts from River Ellidaar (64 ° 08′ N, 21 ° 50′ W) with ultrasonic tags. We caught the wild smolts in a smolt trap and selected the largest individuals from the run. We implanted the transmitters in the abdominal cavity of the fish and then released them in River Ellidaar close to the estuary. We used four ultrasonic receivers; one in the river’s estuary, one outside the estuary and two further away on both sides of an island in the estuary zone. The receivers recorded all transmitters within a 600 m radius. The hatchery smolts were larger than the wild smolts. Some of the smolts were lost on the way through the estuary. Only 4 out of 9 wild smolts and 14 of 17 hatchery smolts were recorded all the way through. The tags and the tagging likely affected the survival of the smolts especially the smaller fish. There were no differences in the smolt migration between the 2 years of study and no differences in the migration behavior between the wild and the hatchery smolts. After being released the smolts stayed on average for 10 h in the river then migrated into the estuary were they stayed for 54 h on average. Then they migrated straight through the area to the sea at approximately 0.2 fish lengths per second. We recorded large differences in the migration.     

Residency and movement patterns of Arctic charr Salvelinus alpinus relative to major estuaries

Estuarine residency and marine movements of 43 anadromous Arctic charr Salvelinus alpinus (mean ± s.d . fork length = 523 ± 97 mm) were examined using acoustic tracking in inner Frobisher Bay (IFB; 63° N; 68° W), Canada, from July to September 2008 and 2009. A mean ± s.d . migration duration of 63 ± 7 days occurred from late June to early September. Detected S. alpinus were either continuously (maximum 34 days) or intermittently present in estuarine zones, on average residing approximately one third of time tracked and returning once every 9 days. Significantly higher estuarine residency during the final 15 migration days suggested that a transition phase may occur prior to freshwater re‐entry. Low travel rates during flood tide suggested individuals staged before accessing intertidal and estuarine zones. Although the two main estuaries were c. 22 km apart, 19% of tagged individuals used both. Individuals remained relatively close to freshwater overwintering systems, although late‐migration inter‐estuarine movements may have indicated natal homing. Approximately half of the individuals exhibited extra‐estuarine travel, mostly during mid‐migration, but remained within 3 km of shore ranging < 30 km straight line distance (SLD) of either estuary. It was concluded that IFB S. alpinus (1) spent a significant portion of their migration within or adjacent to the estuaries and (2) had a restricted marine distribution within 30 km SLD of the river mouths.     

Residence time and habitat use of coho salmon in Humboldt Bay, California: an acoustic telemetry study

We monitored the movement of coho salmon (Oncorhynchus kisutch) smolts with acoustic transmitters from freshwater, through the freshwater/estuary ecotone, through the estuary, and to ocean entry to determine residence time and habitat use in Humboldt Bay, California. Tagged fish were monitored with a fixed receiver network and mobile tracking conducted from a boat. Coho salmon observed during the two-year study resided in Humboldt Bay beginning at least as early as late April and resided through the beginning of July. Coho salmon smolts spent more time in the freshwater/estuary ecotone compared to the lower estuary and spent an average of 10–12 days migrating to Humboldt Bay. Coho salmon smolts resided in Humboldt Bay, a marine embayment, for an average of 15–22 days prior to leaving the bay for the open ocean. Coho salmon smolts, as observed from mobile tracking, used deep channels and channel margins more often than floating eelgrass mats, pilings, and docks. In addition, tagged fish were more often detected in the central portions of Humboldt Bay characterized by deep channels with narrow intertidal margins. There were fewer detections in other portions of the bay characterized by shallow channels with large intertidal mudflats and eelgrass meadows. Relatively short transmitter life (70 days) precluded determining the latest date of coho salmon smolt residency in Humboldt Bay. In addition, tag size limited use to the largest emigrating smolts and may not represent the behaviors of the smaller-sized smolts which were more abundant.     

Physiological characteristics of wild Atlantic salmon post-smolts during estuarine and coastal migration

Changes were measured in some of the major physiological variables associated with seawater adaptability, growth and energetics in wild Atlantic salmon Salmo salar smolts and post-smolts migrating from the river and through the estuary, fjord and coastal areas in the River Orkla and the Trondheimsfjord, Norway during late May to early June. Gill Na⁺,K⁺-ATPase activity increased to levels of 12–16 µmol ADP mg protein⁻¹ h⁻¹ in post-smolts caught in higher salinity zones, probably representing long-term levels of Atlantic salmon post-smolts in oceanic conditions. Muscle moisture was regulated within narrow limits (77·7–78·7%) in fish from all zones during both years, suggesting that post-smolts adapt to marine conditions without any long-term disturbance of hydro-mineral balance. Lipid and glycogen content showed a general trend towards depletion from the river, through the fjord and into the ocean. There was, however, no significant change in protein content. The present results confirm that smolts are naturally 'energy deficient' during downstream migration, and suggest that post-smolts also mobilize energy reserves during their early marine phase, while protein is allocated for somatic growth. Plasma growth hormone (GH) levels increased transiently during passage through the estuary and fjord, with lower levels observed in post-smolts caught off-shore, i.e. in fish which were feeding on marine prey and had adapted to the marine environment. These physiological changes may confer substantial selective advantages during the critical early marine phase of anadromous salmonids, and hence are adaptive for long-term survival in sea water.     

Geographical differences in marine feeding of Atlantic salmon post‐smolts in Norwegian fjords

Stomach content analyses were conducted on Atlantic salmon Salmo salar post‐smolt (average size, 119–154 mm fork length, L _F) caught in eight large Norwegian fjord systems along a north–south geographical axis during 1998–2001. In general, post‐smolts from southern Norway showed low feeding intensity in the fjords, whereas extensive feeding was observed in fjords in the northern and middle parts of Norway. The marine diet mainly included different crustaceans and in particular marine pelagic fish larvae (sand-eels Ammodytes spp., herring Clupea harengus and gadoids), but with a substantial spatial and annual variation in prey diversity and feeding intensity. Insects were most frequently taken in the estuary, although fishes often made a large contribution in mass. In contrast, fishes, and to some extent various crustaceans (particularly Hyperiidae, Gammaridae, Euphausiacea and Copepoda) dominated the diet in the middle and outer parts of the fjords, where post‐smolts also fed more extensively than in the inner part. The results indicate that extensive feeding immediately after sea entrance may be more common for post‐smolts in the northern and middle parts of Norway, than in the southern fjords. The observed differences in post‐smolt feeding may be due to spatial and temporal differences in prey availability within and between the different types of fjord systems, and this might influence post‐smolt growth and survival.     

Physiological preparedness and performance of Atlantic salmon Salmo salar smolts in relation to behavioural salinity preferences and thresholds

This study investigated the relationships between behavioural responses of Atlantic salmon Salmo salar smolts to saltwater (SW) exposure and physiological characteristics of smolts in laboratory experiments. It concurrently described the behaviour of acoustically tagged smolts with respect to SW and tidal cycles during estuary migration. Salmo salar smolts increased their use of SW relative to fresh water (FW) from April to June in laboratory experiments. Mean preference for SW never exceeded 50% of time in any group. Preference for SW increased throughout the course of smolt development. Maximum continuous time spent in SW was positively related to gill Na⁺, K⁺‐ATPase (NKA) activity and osmoregulatory performance in full‐strength SW (measured as change in gill NKA activity and plasma osmolality). Smolts decreased depth upon reaching areas of the Penobscot Estuary where SW was present, and all fish became more surface oriented during passage from head of tide to the ocean. Acoustically tagged, migrating smolts with low gill NKA activity moved faster in FW reaches of the estuary than those with higher gill NKA activity. There was no difference in movement rate through SW reaches of the estuary based on gill NKA activity. Migrating fish moved with tidal flow during the passage of the lower estuary based on the observed patterns in both vertical and horizontal movements. The results indicate that smolts select low‐salinity water during estuary migration and use tidal currents to minimize energetic investment in seaward migration. Seasonal changes in osmoregulatory ability highlight the importance of the timing of stocking and estuary arrival.     

Survival and behaviour of post‐smolt Atlantic salmon in coastal habitat with extreme tides

The early marine migration of 55 Atlantic salmon post‐smolts tagged with acoustic transmitters was automatically monitored using 13 to 25 km long arrays of receivers deployed inside the Bay of Fundy, a coastal system on the east coast of Canada. The survival of post‐smolts from groups with short‐ and long‐term transmitters in coastal habitat up to 10 km from the river was 92 to 100%, indicating a successful transition to salt water and departure. Migration for 68 to 77% of post‐smolts followed a direct route and it was rapid (transit time usually <12 h). Post‐smolts initially migrated in a south to south‐west direction ( i.e . orientation towards the mouth of the bay) and they were aggregated near the coast. Post‐smolts with long‐term transmitters were monitored 20 km from the river where they continued to be aggregated, moving near the coast through a 'common corridor', and their survival to that point was at least 84%. Post‐smolts from both groups travelled heading out of the coastal system during ebb tides. Flood tides interrupted migration, and they caused changes in travel direction and delays in departure for post‐smolts not leaving by a direct route. Monitoring of coastal habitat inside the Bay of Fundy intercepted 62% of migrating post‐smolts with long‐term transmitters returning after an initial absence of 2 to 22 days. Returning post‐smolts displayed a resident behaviour, using the habitat monitored inside the Bay of Fundy during July and August.     

Predation on salmon smolts, Salmo salar L., in the estuary of the River Surna, Norway

The post-smolt stage of salmon, Salmo salar L., is subject to heavy mortality in Norway. Each year 35 000 hatchery-reared smolts are stocked in the River Surna. Smolt stocking at sea and in the fjord resulted in recovery rates of adult salmon which were up to three times better than those resulting from river stockings. This was supposed to be due to by-passing some predators. In 1984 and 1985 predators and their predation on smolts were studied in the R. Surna estuary. Cod, Gadus morhua , were found to prey heavily on smolts, and a total mortality up to 24.8% was found in a small restricted area. Cod are thought to assemble in the Surna estuary in the spring, foraging on the smolt run.     

Interaction between migration behaviour and estuarine mortality in cultivated Atlantic salmon Salmo salar smolts

Migration behaviour and estuarine mortality of cultivated Atlantic salmon Salmo salar smolts in a 16 km long estuary were studied using two methods: (1) acoustic telemetry and (2) group tagging in combination with trap nets. Progression rates of surviving individuals through the estuary were relatively slow using both methods [0·38 L_T (total length) s^?1 v. 0·25 L_T s^?1]. In 2012, the progression rate was slow from the river to the estuary (0·55 L_T s^?1) and the first part of the estuary (0·31 L_T s^?1), but increased thereafter (1·45–2·21 L_T s^?1). In 2013, the progression rate was fast from the river to the estuary (4·31 L_T s^?1) but was slower thereafter (0·18–0·91 L_T s^?1). Survival to the fjord was higher in 2012 (47%) compared to 2013 (6%). Fast moving individuals were more likely to migrate successfully through the estuary compared to slower moving individuals. Adult recapture of coded‐wire‐tagged S. salar was generally low (0·00–0·04%). Mortality hot spots were related to topographically distinct areas such as the river outlet (in 2012) or the sill separating the estuary and the fjord (in 2013). At the sill, an aggregation of cod Gadus morhua predating on cultivated smolts was identified. The results indicate that slow progression rates through the estuary decreases the likelihood of smolts being detected outside the estuary. The highly stochastic and site‐specific mortality patterns observed in this study highlight the complexity in extrapolating mortality patterns of single release groups to the entire smolt run of wild S. salar.     

Survival and behavior of juvenile steelhead trout (Oncorhynchus mykiss) in two estuaries in Oregon, USA

Anadromous salmonids are viewed as a prized commodity and cultural symbol throughout the Pacific coast of North America. Unfortunately, several native salmonid populations are threatened or at risk of extinction. Despite this, little is known about the behavior and survival of these fish as the juveniles transition from freshwater to the ocean. Our primary objectives were to estimate survival of juvenile steelhead migrating between trapping sites and the ocean and evaluate whether survival in the estuary varies temporally (within a year) or spatially (within and between estuaries) within the same distinct population segment. We also evaluated whether flow or fork length were correlated with survival and collected information on variables that have been demonstrated to affect smolt survival in other studies to lend insight regarding differences in survival estimates between basins. We compared run timing, migration rate, survival, condition factor, age composition and time of residence in the estuary for steelhead outmigrants from each basin and measured parasite loads in outmigrating steelhead to evaluate potential differences in parasite density and parasite community between basins. In 2009, we implanted acoustic transmitters in 139 wild steelhead smolts in two small rivers on the Oregon Coast. In general, only 40–50 % of the wild steelhead smolts tagged at upstream smolt traps were detected entering the ocean. The majority of mortality occurred in the lower estuary near the ocean. Wild steelhead smolts typically spent less than 1 day in the estuary in both basins. Using similar data from previous studies in the Nehalem and Alsea basins, we showed that survival appears to be negatively correlated with flow in most releases, and in 2009 fork length was not correlated with survival. Our observations provide baseline information on factors that could influence smolt survival through the estuary as well as smolt to adult survival in these basins, and emphasize the importance of monitoring smolt survival in the estuary.     

Migration of Atlantic salmon smolts and post-smolts in the Rivière Saint-Jean, QC north shore from riverine to marine ecosystems

The characteristics of the Atlantic salmon (Salmo salar) smolt run of the Rivière Saint-Jean, Quebec, Canada, in 2009 and 2010 were determined using acoustic telemetry. Tagged smolts were tracked from freshwater release sites for 17 km, then through the estuary and for their entry into the Gulf of St. Lawrence. The smolt migration began in both years at water temperatures of 10 °C, but lasted twice as long in the cooler year. The smolts in 2009 crossed the river to ocean boundary faster than the smolts of 2010, despite being similar in size. Smolt speed over ground increased from the river to the estuary to the marine environment. Smolts migrated both during the day and the night, but most movements began near or just after sunset, with increasingly nocturnal movements in the ocean. Smolts crossed the estuary during ebbing tides and moved faster during the night than the day. The tide cycle also influenced estuarine smolt travelling rates, but only secondarily to the diel cycle.     

Trophically transmitted parasites in wild Atlantic salmon post‐smolts from Norwegian fjords

The community structure of trophically transmitted intestinal helminths of Atlantic salmon Salmo salar post‐smolts was highly variable among four fjords in Norway. There were no severely pathogenic parasite species. Post‐smolts from the southernmost Trondheimsfjord had a higher diversity of freshwater parasite species compared to the three northern fjords (Tanafjord, Altafjord and Malangen). In contrast, the highest diversity and proportion of marine species was found in the three northern fjords. Post‐smolts were generally more infected with marine parasites in the outer rather than inner parts of all of the fjords. The prevalence of the acanthocephalan Echinorynchus gadi (range: 13–42%) and marine trematodes (range: 14–47%) was higher in post‐smolts in outer zones of the northern fjords than in fish from Trondheimsfjord (0 and 6%, respectively). The within‐fjord variability and north‐south geographical gradient in parasite infection patterns reflected differences in marine feeding of the post‐smolts on potential intermediate hosts such as amphipods ( E. gadi ) and fish larvae (trematodes), which were higher in the northern fjords (range: 27–28 and 67–85%, respectively) than in Trondheimsfjord (5 and 19%, respectively). High intensities of marine parasites suggest that some post‐smolts from northern fjords may have a prolonged fjord‐feeding compared to those from Trondheimsfjord. Parasites of both freshwater and marine origin appear to be suitable as bio‐indicators of feeding and migratory pattern of Atlantic salmon post‐smolts and preadults during their seaward migration.     

Post-smolt migration of Atlantic salmon, Salmo salar L., from the Simojoki river to the Baltic Sea

The migration pattern of Atlantic salmon ( Salmo salar L.) post-smolts in the Baltic Sea was investigated based on tag recoveries of Carlin-tagged wild and hatchery-reared smolts released in the Simojoki river in 1972–2005. Exact date of sea entry was known only for the wild smolts. Tag recoveries of wild salmon in the estuary within 10 km from the river mouth were received on average 3.5 days (±2.0 SD) after release. Time required for emigration from the estuary was dependent on the sea surface temperature (SST) off the river ( R ² = 0.625, P = 0.004), being shorter in years with warmer than colder sea temperatures. Outside the estuary, the wild and hatchery-reared post-smolts migrated southwards along the eastern coast of the northern Gulf of Bothnia, the tag recoveries coinciding with the warm thermal zone in the SST occurring along the coastal area. After arriving in the southern Gulf of Bothnia in late summer the post-smolts mostly migrated near the western coast, reaching the Baltic main basin in late autumn. The relationships between the marine conditions and migration patterns are discussed.