Upstream migratory behaviour of wild and ranched Atlantic salmon Salmo salar at a natural obstacle in a coastal spate river

The upstream migratory behaviour of wild and ranched Atlantic salmon Salmo salar in a small Irish coastal spate river was investigated using acoustic telemetry. Prespawning migratory behaviour was investigated including movement patterns at a large natural waterfall in the lower reaches of the river. A strong diurnal pattern was observed for upstream migrants at the waterfall indicative of the need for daylight to ascend this complex natural obstacle to migration. Successful passage of the waterfall was also associated with distinct environmental conditions and no difference in migratory ability was detected between wild and ranched origin S. salar. Wild S. salar tended to exhibit a non‐erratic, stepwise upstream migration pattern after ascending the waterfall while ranched S. salar had an increased probability of displaying more erratic migratory behaviour. Wild S. salar penetrated further into the river catchment than ranched S. salar, although male ranched S. salar exhibited the greatest cumulative distance moved prior to the spawning period. The management implications of escaped or released ranched S. salar and movement at natural obstacles are discussed.     

The pre‐spawning migratory behaviour of Atlantic salmon Salmo salar in a large lacustrine catchment

The movements of adult Atlantic salmon Salmo salar were determined as they migrated to spawning habitats in a large lacustrine catchment, Lough Neagh, in Northern Ireland. The minimum average ground speed of S. salar through the lake was 2·1 km day^?1 and the mean residence time was 11 days. Tagged S. salar tended to actively migrate through the lake which represented a transitory habitat for adult S. salar. Migration time from the release site, through the lake, to a spawning tributary decreased during the migratory period. During the 4 year study period between 20·5 and 41·6% of tagged S. salar which entered the lake each year, explored at least one other channel before ascending the final spawning tributary. Exploratory behaviour was more likely in S. salar which spawned in the tributaries furthest from the sea. Exploratory behaviour was also more likely to occur during periods of reduced discharge in the natal stream. The fishery management implications of complex pre‐spawning behaviour in a mixed stock lacustrine system, are discussed.     

Heart rates of Atlantic salmon Salmo salar during a critical swim speed test and subsequent recovery

In this study, heart rate (HR) bio-loggers were implanted in the abdominal cavity of 12 post-smolt Atlantic salmon Salmo salar weighing 1024 ± 31 g and acclimated to 12°C sea water. One week after the surgical procedure, a critical swim speed (U_crit) test was performed on tagged and untagged conspecifics, whereafter tagged fish were maintained in their holding tanks for another week. The U_crit was statistically similar between tagged and untagged fish (2.67 ± 0.04 and 2.74 ± 0.05 body lengths s⁻¹, respectively) showing that the bio-logger did not compromise the swimming performance. In the pre-swim week, a diurnal cycle was apparent with HR peaking at 65 beats min⁻¹ during the day and approaching 40 beats min⁻¹ at night. In the U_crit test, HR increased approximately exponentially with swimming speed until a plateau was reached at the final speed before fatigue with a maximum of 85.2 ± 0.7 beats min⁻¹. During subsequent recovery tagged fish could be divided into a surviving group (N = 8) and a moribund group (N = 4). In surviving fish HR had fully recovered to pre-swim levels after 24 h, including reestablishment of a diurnal HR cycle. In moribund fish HR never recovered and remained elevated at c. 80 beats min⁻¹ for 4 days, whereafter they started dying. We did not identify a proximal cause of death in moribund fish, but possible explanations are discussed. Tail beat frequency (TBF) was also measured and showed a more consistent response to increased swimming speeds. As such, when exploring correlations between HR, TBF and metabolic rates at different swimming speeds, TBF provides better predictions. On the contrary, HR measurements in free swimming fish over extended periods of time are useful for other purposes such as assessing the accumulative burden of various stressors and recovery trajectories from exhaustive exercise.     

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.     

Pop‐up satellite archival tag effects on the diving behaviour,growth and survival of adult Atlantic salmon Salmo salar at sea

The effects of large, externally attached pop‐up satellite archival tags (PSATs) were compared with those of small implanted data storage tags (DSTs) on adult Atlantic salmon Salmo salar during their ocean migration in regards to depth utilization, diving depth, diving rate, diving speed and temperatures experienced. Additionally the return rate and growth of individuals tagged with PSATs was compared with those of small acoustic tags and DSTs. Overall, the depth distribution of individuals tagged with PSATs was similar to that of those tagged with DSTs, reflecting the pelagic nature of S. salar at sea. Individuals tagged with PSATs, however, dived less frequently and to shallower depths, and dived and surfaced at slower velocities. Sea surface temperatures experienced by individuals tagged with PSATs were similar to those experienced by those tagged with DSTs for the same time of year, suggesting that there were no large differences in the ocean migration. Return rates did not depend on whether individuals were tagged with PSATs or not, indicating that survival at sea was not impacted by PSATs in comparison to small internal tags. Individuals tagged with PSATs, however, had a smaller increase in body mass than those tagged with acoustic tags or DSTs. It was concluded that PSATs are suitable for use in researching large‐scale migratory behaviour of adult S. salar at sea, but that some effects on their behaviour from tagging must be expected. Effects of PSATs may be largest in the short term when S. salar are swimming in bursts at high speeds. Even though individuals tagged with PSATs performed deep and frequent dives, the results of this study suggest that untagged individuals would perform even deeper and more frequent dives than tagged individuals.     

Deposition of Atlantic salmon Salmo salar carcasses in a Scottish upland catchment

Radio telemetry was utilized to track 38 Atlantic salmon Salmo salar across space and time during and following their spawning run on the Conon system, Scotland. The data collected were used to assess the proportion of S. salar able to migrate successfully from an upland oligotrophic area of the catchment and the distribution of the carcasses of those fish that remained. Of these fish, 35% successfully migrated from the study area after the spawning period. The fish that remained were distributed approximately equally between riverine and lacustrine areas, but with a strong tendency to accumulate in regions of relatively slow water flows.     

Conventional tagging and acoustic telemetry of a small surgeonfish, <Emphasis Type="Italic">Zebrasoma flavescens</Emphasis>, in a structurally complex coral reef environment

Passive acoustic telemetry and conventional tag/re-sight techniques were used to study daily movement patterns of adult yellow tang, Zebrasoma flavescens, over a period of months. Range testing and visual observations revealed the limitations of using small acoustic transmitters to monitor movements of small coral reef fish in a topographically complex and noisy coral reef environment. Visual observations of conventionally tagged and albino fish suggest individuals return each day to forage over the same few hundred m² of shallow, turf algae dominated boulder and reef flat habitat for periods of at least weeks to months. Acoustic telemetry data suggest lower frequency of repeated use of daytime foraging, nighttime refuge and sunset spawning sites. However, integration of observation and acoustic telemetry data revealed that many fish were not detected while they were within the empirically tested range of the receivers. These observations indicate that data from passive acoustic telemetry can underestimate the frequency and duration of repeated use of specific areas. Yellow tang adults made daily crepuscular migrations of up to 600 m between foraging and spawning or sheltering sites at consistent times relative to sunset and sunrise. While there was high individual variability in migration distance, almost all individuals moved in the same direction (from south to north) at sunset. This study provided valuable information for evaluating ongoing fishery management efforts using marine protected areas in Hawaii.     

Programmed acoustic tags reveal novel information on late-phase marine life in Atlantic salmon,Salmo salar

This pilot study used programmed acoustic tags implanted into Salmo salar smolts, in conjunction with an extensive offshore marine receiver array, to investigate late-stage migratory behaviour and survival of returning adult salmon. A total of 100 smolts were tagged in 2020, and a number of individuals were successfully detected as returning adults in 2021. After detection efficiency was accounted for, 5–9 adults were estimated to have returned to the offshore array c. 45 km from the river mouth. A total of three fish were subsequently detected in the river. Losses of between 40% and 66% were evident during the final stages of ocean migration, and one tagged fish provided direct evidence of a predation event.     

Mitigation of acidified salmon rivers – effects of liming on young brown trout Salmo trutta

In southern Norway, 22 acidified rivers supporting anadromous salmonids were mitigated with lime to improve water quality and restore fish populations. In 13 of these rivers, effects on Salmo trutta and Salmo salar densities were monitored over 10–12 years, grouped into age 0 and age ≥ 1 year fish. These rivers had a mean annual discharge of between 4·9 and 85·5 m³ s^?1, and six of them were regulated for hydro‐power production. Salmo salar were lost in six of these rivers prior to liming, and highly reduced in the remaining seven rivers. Post‐liming, S. salar became re‐established in all six rivers with lost populations, and recovered in the seven other rivers. Salmo trutta occurred in all 13 study rivers prior to liming. Despite the improved water quality, both age 0 and age ≥ 1 year S. trutta densities decreased as S. salar density increased, with an average reduction of >50% after 10 years of liming. For age 0 year S. trutta this effect was less strong in rivers where S. salar were present prior to liming. In contrast, densities of S. trutta increased in unlimed streams above the anadromous stretches in two of the rivers following improved water quality due to natural recovery. Density increases of both age 0 and age ≥ 1 year S. salar showed a positive effect of river discharge. The results suggest that the decline in S. trutta density after liming is related to interspecific resource competition due to the recovery of S. salar. Thus, improved water quality through liming may not only sustain susceptible species, but can have a negative effect on species that are more tolerant prior to the treatment, such as S. trutta.     

Net ground speed of downstream migrating radio-tagged Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) smolts in relation to environmental factors

The downstream migration of Atlantic salmon (Salmo salarL.) and sea trout smolt (S. trutta L.) was investigated using radio telemetry in the spring of 1999 and 2000. Forty wild sea trout smolts, 20 F1 sea trout smolts, 20 hatchery salmon smolts and 20 salmon smolts from river stockings were radio tagged and released in the Danish River Lilleaa. The downstream migration of the different groups of fish was monitored by manual tracking and by three automatic listening stations. The downstream migration of radio tagged smolts of both species occurred concurrently with their untagged counterparts. The diel migration pattern of the radio tagged smolts was predominantly nocturnal in both species. Wild sea trout smolt migrated significantly faster than both the F1 trout and the introduced salmon. There was no correlation between net ground speed, gill Na⁺,K⁺-ATPase activity or fish length in any of the different groups. The migration speed of wild sea trout smolts was positively correlated with water discharge in both years. In F1 sea trout smolts, migration speed was positively correlated with temperature in 1999. The migration speed of salmon smolts did not correlate to any of the investigated parameters.     

Evaluating the effects of controlled flows on historical spawning site access,reproduction and recruitment of lake sturgeon Acipenser fulvescens

Lake sturgeon Acipenser fulvescens spawn at the base of Kakabeka Falls, a 39 m waterfall on the Kaministiquia River, a tributary to Lake Superior. Access to this historical spawning site can be restricted or delayed due to hydroelectric flow fluctuations that coincide with the A. fulvescens spawning season. The objectives of this study were to determine (a) the necessary flow conditions that facilitate spawning site access; (b) quantity and duration of flow required for successful spawning and dispersal of larvae; and (c) evaluate recruitment of juvenile A. fulvescens in relation to flow. A. fulvescens spawning migrations were tracked using a stationary telemetry receiver that logged the movements of 166 A. fulvescens fitted with radio-transmitters. Unrestricted access to the spawning site was facilitated when spawning flow was controlled at 23 m³ s⁻¹ in 2004 and 17 m³ s⁻¹ in 2006. Fluctuating (0.5–8.5 m³ s⁻¹) and delayed spawning flows resulted in restricted and delayed access to the spawning site. Flow duration for successful egg incubation, hatch and larval dispersal was determined by sampling larvae using drift nets and quantified using cumulative temperature units (CTU). Over 10 years, 10,083 larvae were captured between 31 May and 20 July with 97% of the drift occurring prior to 30 June. From the date of first spawning to the end of larval dispersal took an average of 38.6 days, and the mean CTU value was 398.6. In general, a minimum flow of approximately 14.5 m³ s⁻¹ from the date of initial spawning to the accumulation of c. 400 CTU ensured successful hatch and larval dispersal. During the timeframe of this study, recruitment was variable. This study described the complex and variable reproductive life history of A. fulvescens and defined spawning flow requirements ecologically, which can be used to develop operational provisions at hydropower facilities to ensure successful reproduction.     

Mid-winter activity and movement of Atlantic salmon parr during ice formation events in a Norwegian regulated river

A telemetry study in a Norwegian regulated river was conducted through a 12-day period in mid-winter 2003. The objective was to study activity (defined as number of movement per hour) and movement (defined as distance moved per hour) during different ice formation events. Twenty-four Atlantic salmon (Salmo salar L.) parr were radio tagged and continuously monitored by both manually tracking (N = 24) and by fixed recording stations (N = 15). Detailed data on climate, flow and ice formation and its spatial distribution were collected and used in the analyses. Fish activity was not found to be affected by their size (L _F). There was a significant difference in activity between diel periods with highest activity during dusk (5–6 p.m.). Between high and low flow (mean ± SD, 21.1 m³ s⁻¹ ± 1.7 SD and 11.1 m³ s⁻¹ ± 1.7 SD, respectively) no significant difference in activity was found. During the experiment extensive anchor ice growth occurred mainly in the riffle part with thickness up to 50 cm. Juveniles tend to avoid riffle section during anchor ice formation and exploited ice covered areas, indicating critical and preferable habitats respectively. Further, a significant difference in movement was found between five selected ice events with highest mean movement during an anchor ice event and lowest mean movement during an ice break up with no anchor ice formation. No significant difference in activity or movement between parr exposed to frazil ice and parr not exposed were found.     

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.     

Pre-spawning migration of adult Pacific lamprey, <Emphasis Type="BoldItalic">Entosphenus tridentatus</Emphasis>, in the Willamette River,Oregon, U.S.A.

We describe the migration distances and timing of the adult Pacific lamprey, Entosphenus tridentatus, in the Willamette River Basin (Oregon, U.S.A.). We conducted aerial surveys to track radio-tagged fish upstream of a major waterfall and hydropower complex en route to spawning areas. We detected 24 out of the 43 fish that passed the waterfall-hydropower complex. Of the detected fish, 17 were detected multiple times. Their maximum migration distance upstream in the mainstem Willamette approximated a normal distribution. The maximum distance migrated upstream did not significantly correlate with total body length (r = −0.186, P = 0.385) or date that the fish passed Willamette Falls (r = −0.118, P = 0.582). Fish migrated primarily during the spring to early summer period before stopping during the summer, when peak river temperatures (≥20°C). However, at least three fish continued to migrate upstream after September. Behavior ranged from relatively slow migration, followed by holding; to rapid migration, followed by slow migration further up in the basin. This study provides a basis for informing more detailed research on Pacific lamprey in the future.     

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.     

Migratory movements of pacu, Piaractus mesopotamicus, in the highly impounded Paraná River

A mark‐recapture study was conducted in 1997–2005 to investigate movements of stocked pacu, Piaractus mesopotamicus, in the Paraná River Basin of Brazil, Paraguay, and Argentina. Fish raised in cages within the Itaipu Reservoir and in ponds were tagged externally (n = 2976) and released in the Itaipu Reservoir (53.2%) and bays of its major tributaries (46.8%). In total, 367 fish (12.3%) were recaptured. In all, 91% of the pacu moved away from the release site; upstream movements were more extensive than downstream movements. Pacu traveled upstream a maximum of 422 km (average of 41.3 km) at a maximum rate of 26.4 km day⁻¹ (av. 0.8). Downstream movements were limited in terms of number of individuals and distance moved. Fish released during the wet season moved farther than those released during the dry season, and feeding rather than spawning might have been the compelling reason for movement. Although fish passed downstream through dams, none of the marked fish were detected to have moved upstream through the passage facilities. Pacu showed movement patterns not radically different from those of other neotropical migratory species, but their migratory movements may not be as extensive as those of other large migratory species in the basin.     

THE MECHANISM OF DAYLENGTH PERCEPTION IN THE RED ALGA ACROSYMPHYTON PURPURIFERUM1

The red alga Acrosymphyton purpuriferum (J. Ag.) Sjöst. (Dumontiaceae) is a short day plant in the formation of its tetrasporangia. Tetrasporogenesis was not inhibited by 1 h night-breaks when given at any time during the long (16 h) dark period (tested at 2 h intervals). However, tetrasporogenesis was inhibited when short (8 h) main photoperiods were extended beyond the critical daylength with supplementary light periods (8 h) at an irradiance below photosynthetic compensation. The threshold irradiance for inhibition of tetrasporogenesis was far lower when supplementary light periods preceded the main photoperiod than when they followed it (<0.05 μmol·m⁻²·s⁻¹ vs. 3 μmol·m⁻²·s⁻¹). The threshold level also depended on the irradiance given during the main photoperiod and was higher after a main photoperiod in bright light than after one in dim light (threshold at 3 μmol·m⁻²·s⁻¹ after a main photoperiod at ca. 65 μmol·m⁻²·s⁻¹ vs. threshold at <0.5 μmol·m⁻²·s⁻¹ after a main photoperiod at ca. 35 μmol·m⁻²·s⁻¹). The spectral dependence of the response was investigated in day-extensions (supplementary light period (8 h) after main photoperiod (8 h) at 48 μmol·m⁻²·s⁻¹) with narrow band coloured light. Blue light (λ= 420 nm) was most effective, with 50% inhibition at a quantum-dose of 2.3 mmol·m⁻². However, yellow (λ= 563 nm) and red light (λ= 600 nm; λ= 670 nm) also caused some inhibition, with ca. 30% of the effectiveness of blue light. Only far-red light (λ= 710 nm; λ= 730 nm) was relatively ineffective with no significant inhibition of tetrasporogenesis at quantum-doses of up to 20 mmol·m⁻².     

Heart rate as an indicator of stress during the critical swimming speed test of farmed Atlantic salmon (Salmo salar L.)

A swim tunnel is to fish as a treadmill is to humans, and is a device used for indirect measuring of the metabolic rate. This study aims to explore the fish stress (if any) during the critical swimming test routines (fish handling, confinement, and swimming) using heart rate (f_H, heartbeat per minute) bio-loggers in farmed Atlantic salmon (Salmo salar L.). In addition, the recovery dynamics of exercised fish using f_H were explored for 48 h post swim tests. Continuous f_H data were acquired following the surgical implantation and throughout the trials, such as during fish handling, swim tests (critical swimming speed, U_crit), and 48 h post swim tests. After 3 weeks of surgical recovery, f_H stabilized at 46.20 ± 1.26 beats min⁻¹, equalizing a ~38% reduction in f_H recorded post-surgical tachycardia (74.13 ± 1.44 beats min⁻¹). Interestingly, f_H was elevated by ~200% compared to baseline levels not only due to the U_crit (92.04 ± 0.23 beats min⁻¹) but also due to fish handling and confinement in the swim tunnel, which was 66% above the baseline levels (77.48 ± 0.34 beats min⁻¹), suggesting fish stress. Moreover, significantly higher plasma cortisol levels (199.56 ± 77.17 ng mL⁻¹) corresponding to a ~300% increase compared to baseline levels (47.92 ± 27.70 ng mL⁻¹) were identified after U_crit, predicting post-swim test stress (physiological exhaustion). These findings reinforce the importance of fish acclimation in the swim tunnel prior to the swimming tests. However, f_H dropped over the course of the 48-h post-swim test, but remained comparatively higher than the basal levels, suggesting fish should be given at least 48 h to recover from handling stress for better fish welfare. This study further explored the influence of fish tagging on U_crit, which resulted in reduced swimming capabilities of tagged fish (1.95 ± 0.37 body lengths s⁻¹) compared to untagged fish (2.54 ± 0.42 body length s⁻¹), although this was not significant (p = 0.06), and therefore future tagging studies are warranted.     

Tag retention and mortality of adult Atlantic salmon Salmo salar gastrically tagged with different sized telemetry transmitters

A batch of 1 sea winter pre‐spawning adult Salmo salar from the Bush river in Northern Ireland, U.K., were gastrically tagged with large (13 mm diameter) and small (9 mm diameter) dummy acoustic telemetry tags alongside untagged control fish. Survival differed between control and tagged fish and the estimated probability of survival by the end of the study for control fish with no tag was 0·94, small tags was 0·90 and large tags was 0·72. Tag loss through regurgitation was slightly higher for fish tagged with larger tags than for fish tagged with smaller tags and the estimated probability of tag loss for fish with a small tag was 0·10 and for large tags was 0·14.     

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.