Sea Entry Research Articles

Comparing the migration behavior and survival of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) smolts.

Many organisms rely on migrations between habitats to maximize lifetime fitness, but these migrations can be risky due to a suite of factors. In anadromous salmonids, the smolt migration from fresh water to sea is a critical life stage, during which smolts can experience high mortality from multiple sources. This study investigated the migratory behavior and survival of Atlantic salmon (Salmo salar) and anadromous brown trout (Salmo trutta) smolts during their seaward migration using acoustic telemetry between March and May 2021. Due to the extinction of wild salmon in the River Gudenaa after the construction of the Tange hydropower plant, this study used hatchery-reared salmon originating from a nearby Danish river. A total of 75 hatchery-reared salmon smolts, 75 hatchery-reared trout smolts, and 75 wild trout smolts were tagged with acoustic transmitters and released into River Gudenaa, Denmark. The downstream movements of tagged fish were monitored using acoustic receivers deployed in the river and fjord. Hatchery-reared trout initiated migration first, followed by hatchery-reared salmon, with wild trout being the last to migrate. There was no difference in riverine progression rates among the three smolt groups, but noticeable differences emerged once in the fjord: trout (wild and hatchery) slowed down, whereas hatchery-reared salmon maintained their speed. Riverine migration was predominantly nocturnal for all smolts; however, daytime migration increased at the fjord arrays. Day-of-year significantly influenced diurnal patterns in the river and fjord, where daytime migration increased later in the year. Hatchery-reared salmon and wild trout had reasonably good overall survival from river to sea entry (≥66%), whereas hatchery-reared trout had poor survival (c.26%). The fjord was the major bottleneck for survival of hatchery-reared trout. We found no strong evidence for differences in progression rate or diurnal patterns between wild and hatchery-reared trout to explain the lower survival. This study demonstrates that salmon and trout differ in their life-history strategy already in the post-smolt phase, and that stocking is a sub-optimal strategy to aid wild populations.

Diverse migration patterns of anadromous white-spotted charr Salvelinus leucomaenis revealed from otolith microchemistry

Anadromous migration of salmonid fish has been extensively studied, primarily focusing on the dichotomous framework of anadromy and residency. However, there remains a limited understanding of intermediate migratory behaviors within the anadromous framework. Our study aimed to classify the lifetime migration patterns of the anadromous white-spotted charr Salvelinus leucomaenis within and among populations using otolith annuli and Sr:Ca ratios. Initially, the migratory histories of anadromous charr were divided into two stages: “virgin sea-run stage” and “veteran sea-run stage”. The former was further categorized into three types: ocean entry at age ≥1+ years old, early descending, and brackish use. The latter was grouped into four types: annual migrants, frequent migrants, retired migrants, and ocean residents. We found that the proportion of migration patterns varied among rivers, with multiple patterns coexisting within the same river. Migration patterns typically involving 1–5 years spent in freshwater rivers followed by annual oceanic migrations were the most abundant, although diverse patterns were also observed. In the virgin sea-run stage, some individuals experienced the ocean at age 0+ years or brackish environments before their first sea entry. In the veteran sea-run stage, we found individuals who had resided in either the ocean or rivers for over a year. Retired migrants, characterized by stopping oceanic migration at a certain age and subsequently spending time in rivers, were exclusive to southern rivers. Conversely, ocean residents who spend one or more years in the ocean were more frequent in northern regions. Consequently, the lifetime migration patterns of anadromous white-spotted charr may exhibit stronger ocean dependency at higher latitudes. The implications of this study highlight the complexity and flexibility of migratory behaviors within and among white-spotted charr populations.

Interannual variation in survival of wild Atlantic salmon smolts through a dynamic estuarine habitat

AbstractMigration timing is critical for diadromous fish, especially for survival. Migration through fresh water and estuarine transitional waterbodies may be an important early life survival bottleneck through potential exposure to anthropogenic pressures and predators. Monitoring smolt movement and survival through riverine and estuarine habitats is important for identifying causes of smolt mortality and the potential for directing focused, mitigating management actions. Acoustic telemetry was used to track 186 Atlantic salmon smolts during migration from the river Boyne to the Irish Sea on the east coast of Ireland during 2019–2021. Migration success from release to sea entry that ranged from 47% to 81% was linked to biotic (predation) and abiotic effects (water flow and water temperature). Predation was associated with interannual variation in smolt survival, with 5.4% of smolts consumed by marine mammals and 26.3% lost to unknown fates. Our findings contribute to the understanding of predation on smolts and also how water temperature, flow and tidal phase can influence this critical life stage of Atlantic Salmon, a species in decline across its range.

Strontium signal lag in otoliths of juvenile sockeye salmon (<i> Oncorhynchus nerka</i>) during transition from the freshwater to marine environments

The shift in strontium (Sr) concentrations in fish otoliths is a commonly used proxy for identification of marine environment entry during diadromous migrations. However, there is still controversy about the appearance of the Sr-based sea entry mark relative to the true point of entry. In this study, the Sr signal lag was assessed in otoliths of juvenile sockeye salmon (Oncorhynchus nerka) under experimental conditions replicating the transition to seawater during juveniles’ seaward migration. A Sr signal delay was observed to average at 8.2 (SD = 5.1) days for fish with a body length of 140–170 mm (and a weight of 30–70 g). This lag may be species-specific and should be taken into consideration when estimating marine entry timing and marine residence duration of juvenile salmon.

Optimum timing and size at release of chum salmon: Improving survival by modifying hatchery practices

AbstractAn existing conceptual model that assessed the timing of and size at release of juvenile Japanese chum salmon in Japanese waters is modified using data obtained from young salmon collected in coastal waters off Kombumori, eastern Hokkaido, Japan. The early life histories of Tokachi River late‐migrating chum salmon are estimated based on microstructural analysis of thermally marked otoliths from recaptured fish. A revised model is presented that reveals the optimal timing of and size at release of chum salmon juveniles to be dependent upon their size at sea entry and sea surface temperature (SST) near Tokachi River mouth. Juvenile salmon are assumed to grow at a fixed rate after sea entry and to reach a target fork length upon arrival in Kombumori waters when SST conditions are within a specific optimal range. However, because of annual variation in SST, the size at and timing of juvenile chum salmon sea entry following their release from hatcheries represents a moving target, which flexible hatchery release practices are required to meet. Return rates of adult chum salmon to Tokachi River correlate positively with the number of days elapsed between the period when SST is suitable near the river mouth where they enter the sea, and SST conditions in Kombumori waters, which suggests that in years when SST increases rapidly the survival of salmon tends to decline. Windows of optimal times of and sizes at release are proposed for juvenile chum salmon to improve their survival and hatchery success.

Significance of sea entry pathway of chum salmon Oncorhynchus keta fry, inferred from the differential expressions of Na+,K+-ATPase α-subunit genes in the gills

Na+,K+-ATPase (NKA) α-subunit 1a (α1a) and 1b (α1b) gene expressions in the gills are changeable in response to ambient salinity in a few salmonids. In this study, the expressions were compared among ambient salinities and used to infer sea entry migration of chum salmon Oncorhynchus keta fry. The expression of α1a decreased from the 2 days after seawater (SW) transfer from freshwater (FW) and was significantly lower in SW-acclimated fry than that in FW-fry. On the other hand, the expression of α1b peaked on the first to second day after SW transfer and then settled to a level 2-fold higher than in FW-fry. In fry caught in the waterfronts of the beaches, the expression levels were quite similar to those on the first and second days after SW transfer, whereas, in fry caught off beach, the expressions were identical to those of SW-acclimated fry. These suggest that fry adapt to SW with moving along the shoal in the bay, and move to off beach after completing SW adaptation. One of the physiological significances in a usage of waterfront may be to transform the gills to SW type. Only fry on the 2 days after SW transfer failed to exhibit condition factor-dependency of burst swimming, probably due to physiological perturbation, which may be related to poor predation avoidance. The physiological approach used in this study inferred sea entry migration of fry; furthermore, it shows the possible significance of adaptation to SW in the shoal is to reduce predation risk.

Early-season brown trout (Salmo trutta) migrants grow and survive better at sea.

The timing of seaward migration is a key life‐history trait for many anadromous fish species, with growth and survival at sea depending on a match/mismatch scenario between the timing of the sea entry and optimal conditions. Based on a 25‐year study with 15,226 individually tagged brown trout (Salmo trutta) in a Norwegian river, we analysed how the within‐season timing of sea migration impacted growth and survival. In both first‐time and veteran migrants, marine growth was highest for early migrating individuals, large individuals, and those with a low condition factor when entering the sea. Survival was highest for individuals entering the sea early in the season. In first‐time migrants, survival increased with body length. Survival also increased with the number of other smolts migrating simultaneously. As the early smolts were the most successful, it may seem strange that many smolts migrate later in the season. We suggest that late‐migrating smolts may not be of a size and/or physiological state suitable for early marine conditions, and may make the best of a bad situation.

Domestication-induced reduction in eye size revealed in multiple common garden experiments: The case of Atlantic salmon (Salmo salar L.).

Domestication leads to changes in traits that are under directional selection in breeding programmes, though unintentional changes in nonproduction traits can also arise. In offspring of escaping fish and any hybrid progeny, such unintentionally altered traits may reduce fitness in the wild. Atlantic salmon breeding programmes were established in the early 1970s, resulting in genetic changes in multiple traits. However, the impact of domestication on eye size has not been studied. We measured body size corrected eye size in 4000 salmon from six common garden experiments conducted under artificial and natural conditions, in freshwater and saltwater environments, in two countries. Within these common gardens, offspring of domesticated and wild parents were crossed to produce 11 strains, with varying genetic backgrounds (wild, domesticated, F1 hybrids, F2 hybrids and backcrosses). Size‐adjusted eye size was influenced by both genetic and environmental factors. Domesticated fish reared under artificial conditions had smaller adjusted eye size when compared to wild fish reared under identical conditions, in both the freshwater and marine environments, and in both Irish and Norwegian experiments. However, in parr that had been introduced into a river environment shortly after hatching and sampled at the end of their first summer, differences in adjusted eye size observed among genetic groups were of a reduced magnitude and were nonsignificant in 2‐year‐old sea migrating smolts sampled in the river immediately prior to sea entry. Collectively, our findings could suggest that where natural selection is present, individuals with reduced eye size are maladapted and consequently have reduced fitness, building on our understanding of the mechanisms that underlie a well‐documented reduction in the fitness of the progeny of domesticated salmon, including hybrid progeny, in the wild.

Effects of fasting and water temperatures during transition from freshwater to seawater on juvenile chum salmon growth and metabolism

We examined the combined effects of fasting in freshwater and water temperature during the transition from freshwater to seawater on growth/metabolic parameters in juvenile chum salmon Oncorhynchus keta. Juveniles of 1 g in freshwater were first subjected to a 5-day fasting period at 10 or 5 °C, acclimated to either 10 or 5 °C seawater and fed ad libitum for 30 days. Control fish were transferred from 10 °C freshwater to 10 °C seawater and fed ad libitum throughout the experimental period. Serum insulin-like growth factor (IGF)-I was measured to evaluate growth status/potential and liver glycogen as an index of energy storage. Fasting in freshwater for 5 days negatively affected body size. Growth of juveniles kept at colder temperatures was retarded in seawater for at least 20 days, which may partly be explained by a lower feeding rate in cold seawater. Serum IGF-I levels were lower in fasted fish in freshwater at both temperatures and colder seawater had a negative effect on restoring serum IGF-I levels after refeeding for 20 days. Liver glycogen content was low in fish fasted in freshwater for 5 days. After refeeding in seawater for 10 days, liver glycogen content increased significantly in juveniles kept at colder temperatures. Colder water temperatures in both salinities positively affected glycogen content for 30 days after transfer to seawater, suggesting that juveniles allocated energy stores to the liver rather than growth under suboptimal feeding and temperature conditions. The findings of the present study suggested that relatively cold freshwater could negatively affect juvenile chum salmon growth soon after sea entry.

The effects of nutritional state, sex and body size on the marine migration behaviour of sea trout

The sea trout (anadromous brown trout Salmo trutta) displays extensive among-individual variation in marine migration behaviour. We studied the migration behaviour of 286 sea trout (27-89 cm) tagged with acoustic transmitters in the spring, in 7 populations located in 2 distinct marine fjord systems in Norway. We examined whether individual nutritional state, sex and body size influenced marine migration behaviour in terms of (1) the decision to migrate to the sea or remain resident in freshwater and/or estuarine habitats, (2) seasonal timing of sea entry, (3) duration of the marine residency and (4) migration distance at sea from the home river. Most sea trout were in a poor nutritional state in the spring prior to migration. Sea trout with low body condition factors and low plasma triglyceride levels were more likely to migrate to sea, and low triglyceride levels were also associated with earlier sea entry. Poor body condition also increased the probability of individuals remaining at sea longer and migrating further offshore compared to fish in better condition. Females were more likely to migrate to the sea than males. Larger fish were also more likely to migrate to the sea instead of remaining in freshwater and estuaries, and dispersed over greater distances from the river than smaller fish. In conclusion, this study documented general trends across multiple populations and showed that nutritional state, sex and body size influence important aspects of the marine migration behaviour of sea trout.

Changes in growth and migration patterns of sea trout before and after the introduction of Atlantic salmon farming

Abstract Marine growth has strong implications for reproductive potential and ultimate fitness of sea trout. Hence, the effects of anthropogenic factors on marine growth are important when evaluating population responses and implementing management measures. Temporal changes in growth patterns of sea trout from three Norwegian and two Irish watercourses were examined, covering time spans of 25–65 years. Elemental chemistry Ba:Ca profiles and visual reading of fish scales were used to estimate smolt length and lifetime growth after first sea entry. Reduced growth after the first sea entry coincided with periods of nearby (<14 km) salmon-farming activity in impacted watersheds in both countries. Increased Ba:Ca levels were also recorded during these periods, likely indicating reduced residency in marine habitats caused by premature return to freshwater and estuaries. An increase in estimated length at first sea entry coinciding with salmon-farming activity, for groups of fish sampled after sea migration, suggests a size-selective marine mortality, with the smallest individuals experiencing a larger mortality.

Sea-entry conditions of juvenile chum salmon Oncorhynchus keta that improve post-sea-entry survival: a case study of the 2012 brood-year stock released from the Kushiro River, eastern Hokkaido, Japan

We examined the sea-entry conditions (i.e., timing and body size) of juvenile chum salmon that improve their survival during their coastal residency. On 25 June 2013, we sampled 365 juvenile chum salmon [57.5–98.6 mm fork length (FL)] off Konbumori, eastern Hokkaido, ⁓ 20 km east of the Kushiro River mouth, which originated from a hatchery in the Kushiro River. Sea-entry conditions of these Konbumori juveniles back-calculated using otolith daily increment analysis were compared with data from 373 juveniles released from the same hatchery that were captured at the mouth of Kushiro River (i.e., just before sea entry) from April to July 2013. Most of the Konbumori fish were estimated to have entered the sea from 25 May to 5 June, when coastal surface temperatures constantly exceeded 5 °C, which is considered favorable for juveniles. The estimated FLs at sea entry of the Konbumori fish were larger than FLs of fish sampled at the river mouth during a comparable period, which suggests that size-selective mortality existed. Back-calculated post-sea-entry growth rates of fish with larger FL at sea entry, particularly those with FL > 65 mm, tended to be high enough for survival among the Konbumori fish. Assuming growth-dependent mortality, this case study suggests that the release of larger-sized juveniles under favorable coastal temperature conditions improves their survival.

Analysis of spatio-temporal distribution characteristics and fog forming conditions of Zhejiang coast in recent ten years

Based on the hydrometeorological observation data of Zhejiang coastal observation station from 2009 to 2018, the temporal and spatial distribution characteristics and fogging conditions of sea fog along the coast of Zhejiang Province were analyzed in this paper. The result shows that the average annual fog day along the coast of Zhejiang Province is 112 days according to the situation that the sea fog occurs as long as there is one station along the coast of Zhejiang Province. The fog season is from March to June and the sea fog occurs in the middle of the night and early in the morning. The spatial distribution of sea fog along the coast of Zhejiang Province is more in the middle of the two ends and the average annual fog days of each station are in the range of 19.2-82.0 days. Under the condition of fog, most of the relative humidity in Zhejiang coastal area is greater than or equal to 90%, the average temperature difference between gas and water is-0.5 - 2.1°C, and the direction of fog wind is mainly eastward. More than 93.6% of the sea fog occurs under the condition that the wind force is less than or equal to level 5. There are four main types of weather situation affecting the occurrence of sea fog in coastal areas of Zhejiang Province, including inverted trough type, low pressure sea exit type, weak cold air type and high-pressure sea entry type. When the continuous sea fog process occurs, the weather situation usually goes through the high-pressure sea entry type, the inverted trough type and the low pressure sea exit type. The wind direction will change with the variation of weather situation and the wind force will be less than or equal to 4 levels. When the wind is greater than level 5, the wind changes from west to southwest, and the sea fog tends to dissipate. Thick fog or strong fog occurs mainly in the middle of the night, early in the morning and around the evening.

Behaviour of adult sea trout Salmo trutta that survive or die at sea

We tagged 125 sea trout kelts (460–925 mm) in seven Danish rivers with positively buoyant DSTs. Fifty-three (42%) tags were recovered, enabling a comparison of behaviour in kelts that survived the marine period and kelts that did not. Data revealed an estimated mean survival time at sea of 14.3 days (range 1–65 days) for fish that died at sea. Fish that did not survive had lower weight/length ratios when tagged than survivors (P = 0.005) but exhibited a similar diel diving pattern while at sea. Both surviving and non-surviving fish gradually increased diving activity and the daily visited maximum depths after sea entry, but some performed fewer dives and resided in shallower depths than others. This difference was pronounced when comparing surviving fish (most active divers) with fish caught by anglers (least active divers). The results show that the first weeks at sea are critical for kelt survival and that physical status of kelts may affect behaviour and probability of survival. The preference for shallower waters and less diving activity in some individuals indicate that coastally based fisheries and recreational angling may select against specific behavioural phenotypes which should be investigated more intensively.

The effect of physiological and environmental conditions on smolt migration in Atlantic salmon Salmo salar

AbstractHydropower development has negatively influenced Atlantic salmon (Salmo salar) populations. Compensatory hatchery‐rearing programmes exist, but released fish suffer from high mortality that may be related to the lack of experience from natural environments in hatchery‐reared smolts and their large body size and high energetic state. Here, we used acoustic telemetry to test how body size, energetic state, and the environmental conditions of the river affect migration in hatchery‐reared smolts. The study was conducted in three consecutive years between 2011 and 2013 in the lower part of the River Umeälven, Sweden. For individual fish, there was no effect of body size but the energetic state of the fish had a negative effect on sea entry. The most important factor affecting sea entry rate was the water discharge in the old river bed that differed among years. Smolts were more likely to enter the sea in years when the discharge was high or when the discharge increased substantially shortly after release. Hatchery‐reared fish had higher migration speed at a slower flowing section compared with a faster flowing section, which was likely a result of large hesitation to enter the rapid section. The increase in water discharge led to an increase in fish migration speed disproportional to the increase in water velocity. Our results highlight the importance of water discharge for the smolts during smolt migration, and we argue that concern should be given to migrating fish when managing regulated rivers.

First report of growth rate of juvenile chum salmon Oncorhynchus keta captured in the Sea of Okhotsk offshore

The growth rate of 16 juvenile chum salmon Oncorhynchus keta (180–286 mm fork length) captured in the Sea of Okhotsk offshore during autumn 2002 was estimated using daily-incremental analysis of otoliths. The specimens originated from Hokkaido, Japan, and Sakhalin and Kamchatka, Russia. The average (range) of estimated mean daily growth in fork length was 1.19 (1.02–1.54) mm/day through the entire early ocean life and 1.05 (0.84–1.21) mm/day during the first 30 days after sea entry. These growth rates were higher than those reported previously for coastal resident or migrant juvenile chum salmon along the coast of Japan.

Evidence of cormorant‐induced mortality, disparate migration strategies and repeatable circadian rhythm in the endangered North Sea houting (Coregonus oxyrinchus): A telemetry study mapping the postspawning migration

AbstractLife history theory predicts a trade‐off between migration and residency where migration is favoured when it infers elevated fitness. Although migration to more favourable environments may offer higher growth rates, migrants often experience increased mortality due to predation. Here, we investigated mortality and migration behaviour of the North Sea houting (Coregonus oxyrinchus), an anadromous salmonid endemic to the Wadden Sea. We used acoustic telemetry to map the migration of the only remaining indigenous population by applying stationary hydrophones combined with manual tracking. Data suggested a total mortality of 26%, with 30% of the total mortality attributed to predation by great cormorants (Phalacrocorax carbo sinensis), highlighting that North Sea houting conservation could be jeopardised by increased cormorant predation. Risk of cormorant predation was size‐dependent, with smaller fish suffering higher risk of predation. The study found North Sea houting to exhibit disparate migration strategies and identified a lentic area in the estuary as an important habitat. Two newly established artificial lakes within the river system significantly reduced the migration speeds, possibly indicating constrained navigation through the lakes. The migration into the Wadden Sea correlated with temperature perhaps indicating osmoregulatory constraints of sea entry. Unlike many salmonid species, migration occurred both day and night. Moreover, fish exhibited repeatable individual differences in diel activity patterns, suggesting that individuals differ consistently in their migratory activity throughout the 24‐hr period. Our study provides novel information on salmonid migration, which is crucial for the development of science‐based conservation strategies.

Marine migration and habitat use of anadromous brown trout (Salmo trutta)

The biology and ecology of anadromous brown trout (Salmo trutta) at sea is poorly understood. This study provided information on spatial and temporal distribution of sea trout in the ocean. The behaviour of 115 individuals (veteran migrants, 270–700 mm) was tracked by using acoustic telemetry in a fjord system during April–September in 2012–2013. Overall, fish spent 68% of their marine residence time close to river mouths (<4 km). Most fish registrations (75%) were in nearshore habitats, but pelagic areas were also used. The maximum migration distance of tagged fish was categorized as short (<4 km from river mouth, 40% of fish), medium (4 – ∼13 km, 18% of fish), or long (>∼13 km, 42% of fish). Long-distance migrants had poorer body condition in spring prior to migration, used pelagic areas more often, and returned earlier to fresh water than short- and medium-distance migrants. Marine residence time was 7–183 days and was positively correlated to body length and smolt age, but negatively correlated to the date of sea entry.

Genetic Identification of Chinook Salmon: Stock‐Specific Distributions of Juveniles along the Washington and Oregon Coasts

AbstractWe used microsatellite DNA data and genetic stock identification methods to delineate the temporal and spatial distributions of juvenile Chinook Salmon Oncorhynchus tshawytscha occupying coastal habitats extending from central Oregon to northern Washington. Juveniles were collected in trawl surveys conducted during spring, summer, and autumn over 15 years. Distributions (mean latitude and distance from shore) differed between yearling and subyearling life history types and between stocks; many of these differences were consistent across years. Yearlings were nearly all (98%) from Columbia River sources, and only 6% were naturally produced. In late May, yearlings from the lower Columbia and Willamette rivers were farther north than other yearlings, likely due to the early spring timing of their releases from hatcheries and subsequent out‐migration from the Columbia River. However, yearling distributions in late June reflected known migration behaviors. Yearlings from interior Columbia and Snake River sources were farthest north by June, whereas yearlings from other stocks were more spread out in latitude. Subyearlings sampled in early summer were also largely from the Columbia River (98%), but greater percentages of subyearlings from coastal rivers were present during the fall (24%). In contrast to yearlings, natural production accounted for nearly one‐third of subyearlings. Subyearlings of most stocks tended to remain relatively near their point of sea entry throughout the summer. Subyearlings from the Snake River fall‐run stock and upper Columbia River summer–fall‐run stock exhibited diverse distributions that included both southward and northward dispersal. Overall, distributions of Chinook Salmon stocks and life history types reflected differences in migration behavior but also reflected the influence of environmental factors and hatchery practices.Received November 13, 2014; accepted April 15, 2015

A critical life stage of the Atlantic salmon Salmo salar: behaviour and survival during the smolt and initial post‐smolt migration

The anadromous life cycle of Atlantic salmon Salmo salar involves long migrations to novel environments and challenging physiological transformations when moving between salt-free and salt-rich waters. In this article, (1) environmental factors affecting the migration behaviour and survival of smolts and post-smolts during the river, estuarine and early marine phases, (2) how behavioural patterns are linked to survival and (3) how anthropogenic factors affect migration and survival are synthesized and reviewed based on published literature. The timing of the smolt migration is important in determining marine survival. The timing varies among rivers, most likely as a consequence of local adaptations, to ensure sea entry during optimal periods. Smolts and post-smolts swim actively and fast during migration, but in areas with strong currents, their own movements may be overridden by current-induced transport. Progression rates during the early marine migration vary between 0.4 and 3.0 body lengths s(-1) relative to the ground. Reported mortality is 0.3-7.0% (median 2.3) km(-1) during downriver migration, 0.6-36% (median 6.0) km(-1) in estuaries and 0.3-3.4% (median 1.4) km(-1) in coastal areas. Estuaries and river mouths are the sites of the highest mortalities, with predation being a common cause. The mortality rates varied more among studies in estuaries than in rivers and marine areas, which probably reflects the huge variation among estuaries in their characteristics. Behaviour and survival during migration may also be affected by pollution, fish farming, sea lice Lepeophtheirus salmonis, hydropower development and other anthropogenic activities that may be directly lethal, delay migration or have indirect effects by inhibiting migration. Total mortality reported during early marine migration (up to 5-230 km from the river mouths) in the studies available to date varies between 8 and 71%. Hence, the early marine migration is a life stage with high mortalities, due to both natural and human influences. Factors affecting mortality during the smolt and post-smolt stages contribute to determine the abundance of spawner returns. With many S. salar populations in decline, increased mortality at these stages may considerably contribute to limit S. salar production, and the consequences of human-induced mortality at this stage may be severe. Development of management actions to increase survival and fitness at the smolt and post-smolt stages is crucial to re-establish or conserve wild populations.