Long‐term effects of photoperiod,temperature and their interaction on growth,gill Na+, K+‐ATPase activity,seawater tolerance and plasma growth‐hormone levels in Atlantic salmon Salmo salar

This study was undertaken to examine the long‐term effects of photoperiod, temperature and their interaction on growth, gill Na⁺,K⁺‐ATPase (NKA) activity, seawater tolerance and plasma growth‐hormone levels in Atlantic salmon Salmo salar pre‐smolts and smolts. The fish (mean ± s.e . initial body mass = 15·9 ± 0·4 g) were reared on two photoperiods (continuous light, LL, and simulated natural photoperiod, LDN, 60° 25′ N) and two temperatures (8·3 and 12·7° C) from June to May of the following year. Mean body mass was affected by photoperiod, temperature and their interactions. Both temperature groups on LL developed peak levels in gill NKA activity from October to November, 4–5 months prior to the natural season for the parr–smolt transformation. Fish at 12° C showed peak levels in NKA activity 4–6 weeks before the fish at 8° C. Fish in all four experimental groups showed maximum NKA activity within a similar size range (113–162 g). The present findings further indicate that smoltification in S. salar is to some extent driven by size, and that S. salar will develop smolt characteristics, e.g. a marked increase in NKA activity, within a similar size range. Faster‐growing S. salar will, thus, reach this size threshold at a relatively younger age.     

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.     

Time and size at seaward migration influence the sea survival of Salmo salar

Whether time of seaward migration of young Atlantic salmon Salmo salar influences their subsequent survival and growth was investigated in the River Imsa, south‐western Norway. Salmo salar were tagged when moving downstream through a trap near the outlet between 1976 and 2010 and recaptured on their adult return. Most descended as smolts in April and May, but some descended during the other months of the year. Annual variation in timing of the smolt migration was significantly correlated with variation in water temperature during spring. Mean total body length of the descending S. salar varied with month of seaward migration. The sea survival of S. salar emigrating from the River Imsa between January and May was 2·8 times higher than for those descending between June and December. The sea survival of the various cohorts decreased with increasing river temperature in April to May, prior to the smolt migration, and decreasing day number when the smolts moved to sea. The size of smolts descending the river between April and May did not affect the survival at sea as much as it affected the survival of migrants descending in any other month of the year. The majority of the downstream migrating S. salar were 2 years old, but proportionally, more 1 year olds moved downstream in the autumn than in the rest of the year. Mean duration between downstream migration of the young and the return migration of the grilse was shortest (12·7 months) for those descending in July and August and longest for those descending in October (21 months). Mean monthly specific growth rate was highest for those migrating downstream between May and July and lowest for those emigrating in September. Based on the present results, it was hypothesized that S. salar emigrating between April and August migrated directly out into the ocean, while those that emigrated between October and March stayed in the estuary until the subsequent spring.     

Migration and survival of Atlantic salmon Salmo salar smolts in a large natural lake

An investigation with acoustic telemetry of the passage of Salmo salar smolts through a large natural lake found heavy mortality occurred at the river‐to‐lake confluences (mean 31.2% km^?1), but was lower in the main body of the lake (mean 2.4% km^?1). Predation was a significant pressure on emigrating smolts as tagged pike Esox lucius aggregated at river‐to‐lake confluences during the peak of the smolt run. Tagged smolts mainly emmigrated into the lake in the late evening after dusk, possibly as a predator‐avoidance behaviour.     

Passing a seawater challenge test is not indicative of hatchery‐reared Atlantic salmon Salmo salar smolts performing as well at sea as their naturally produced conspecifics

Despite satisfactory reactions to seawater challenge tests indicative of appropriate physiological state, hatchery‐reared Atlantic salmon Salmo salar smolts stocked in the Eira River in Norway between 2001 and 2011 performed less well at sea in terms of growth, age at maturity and survival than smolts of natural origin. The mean rates of return to the river for hatchery‐reared and naturally produced S. salar were 0·98 and 2·35%. In the Eira River, c. 50 000 hatchery‐reared S. salar smolts of local origin were stocked annually to compensate for reduced natural smolt production following regulation for hydroelectric purposes, while a mean of 17 262 smolts were produced naturally in the river. This study demonstrates that, although captive S. salar perform well in seawater challenge tests, hatchery‐reared smolts are not necessarily as adaptable to marine life as their naturally produced counterparts. These findings suggest that production of hatchery‐reared smolts more similar to naturally produced individuals in morphology, physiology and behaviour will be necessary to improve success of hatchery releases. Where possible, supplementary or alternative measures, including habitat restoration, could be implemented to ensure the long‐term viability of wild stocks.     

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.     

Evidence of changing migratory patterns of wild Atlantic salmon Salmo salar smolts in the River Bush,Northern Ireland,and possible associations with climate change

The migration patterns, timing and biological characteristics of wild Atlantic salmon Salmo salar smolts in the River Bush, Northern Ireland, were examined over the period 1978–2008. A distinct change in the timing of the smolt run was detected with progressively earlier emigration periods evident across the time series. The shift in run timing ranged from 3·6 to 4·8 days 10 years^?1 for a range of standard migratory audit points. The timing of smolt emigration has been linked to ambient river temperature patterns. Distinct seasonal patterns were evident for biological characteristics of River Bush smolts with mean age and fork length decreasing throughout the emigration period. Marine survival patterns in 1 sea winter River Bush S. salar were strongly influenced by the run timing of the preceding smolt year such that later emigrating cohorts demonstrated increased survival. Possible mechanisms for this relationship based on local climatic variation have been explored, including the effect of potential thermal mismatch between freshwater and marine environments.     

Seaward migrating Atlantic salmon smolts with low levels of gill Na+, K+ -ATPase activity; is sea entry delayed?

Two groups of migrating wild Atlantic salmon (Salmo salar) smolts caught within a 1 week interval in the River Alta, northern Norway, were tagged with acoustic transmitters and measured for gill Na⁺, K⁺ -ATPase activity in order to compare their smolt status with timing of sea entry. The first group of smolts had low levels of gill Na⁺, K⁺ -ATPase activity and resided in the lower part of the river twice as long as the second group that had high levels of gill Na⁺, K⁺ -ATPase activity. This indicates that early migrating smolts may not be completely physiologically adapted for salt water and delay their sea entry, thereby also synchronizing their seaward migration with the later migrating smolts.     

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.     

Development of seawater tolerance and subsequent downstream migration in wild and stocked young‐of‐the‐year derived Atlantic salmon Salmo salar smolts

This study investigated the development of hypo‐osmoregulatory capacity and timing of downstream migration in wild Atlantic salmon Salmo salar smolts from the River Stjørdalselva and stocked young‐of‐the‐year (YOY), derived S. salar smolts from the tributary River Dalåa. Both wild and stocked S. salar smolts developed seawater (SW) tolerance in early May, persisting through June, measured as their ability to regulate plasma osmolality and chloride following 24 h SW (salinity = 35) exposure. Although the majority of downstream migration among the stocked S. salar smolts occurred later than observed in their wild counterparts, the development of SW tolerance occurred concurrently. The wild S. salar from Stjørdalselva and stocked YOY smolts from the River Dalåa started to migrate on the same cumulative day‐degrees (D°). The study revealed no downstream migration before development of SW tolerance. This emphasizes the importance of incorporating physiological status when studying environmental triggers for downstream migration of S. salar smolts. Overall, these findings suggest that the onset of smolt migration in stocked S. salar smolts was within the smolt window from an osmoregulatory point of view.     

The North Atlantic subpolar gyre and the marine migration of Atlantic salmon Salmo salar: the ‘Merry‐Go‐Round’ hypothesis

One model for marine migration of Atlantic salmon Salmo salar proposes that North American and southern European stocks (<62° N) move directly to feeding grounds off west Greenland, then overwinter in the Labrador Sea, whereas northern European stocks (>62° N) utilize the Norwegian Sea. An alternate model proposes that both North American and European stocks migrate in the North Atlantic Subpolar Gyre (NASpG) where S. salar enter the NASpG on their respective sides of the Atlantic, and travel counterclockwise within the NASpG until returning to natal rivers. A review of data accumulated during the last 50 years suggests a gyre model is most probable. Freshwater parr metamorphose into smolts which have morphological, physiological and behavioural adaptations of epipelagic, marine fishes. Former high‐seas fisheries were seasonally sequential and moved in the direction of NASpG currents, and catches were highest along the main axis of the NASpG. Marking and discrimination studies indicate mixed continental origin feeding aggregations on both sides of the Atlantic. Marked North American smolts were captured off Norway, the Faroe Islands, east and west Greenland, and adults tagged at the Faroes were recovered in Canadian rivers. Marked European smolts were recovered off Newfoundland and Labrador, west and east Greenland, and adults tagged in the Labrador Sea were captured in European rivers. High Caesium‐137 (¹³⁷Cs) levels in S. salar returning to a Quebec river suggested 62·3% had fed at or east of Iceland, whereas levels in 1 sea‐winter (SW) Atlantic Canada returnees indicated 24·7% had fed east of the Faroes. Lower levels of ¹³⁷ Cs in returning 1SW Irish fish suggest much of their growth occurred in the western Atlantic. These data suggest marine migration of S. salar follows a gyre model and is similar to other open‐ocean migrations of epipelagic fishes.     

Influence of sea temperature and initial marine feeding on survival of Atlantic salmon Salmo salar post-smolts from the Rivers Orkla and Hals, Norway

The abundance of returning adult Atlantic salmon Salmo salar, in the River Orkla in mid‐norway (1 sea‐winter, SW, fish) and River Hals in north Norway (1–3 SW fish), was tested against the early marine feeding and the seawater temperature experienced by their corresponding year classes of post‐smolts immediately after entry into the Trondheimsfjord (Orkla smolts, 22 years of data) and Altafjord (Hals smolts, 17 years of data). In both river–fjord systems, there was a significant positive correlation between the abundance of returning S. salar and the mean seawater temperature at the time of smolts descending to the sea. The number of 1SW fish reported caught in River Orkla was positively correlated to the proportion of fish larvae in the post‐smolt stomachs in Trondheimsfjord. The abundance of returning S.salar was, however, neither correlated to forage ratio (R_F) nor other prey groups in post‐smolt stomachs in the two fjord systems. In the Altafjord, the post‐smolts fed mainly on pelagic fish larva (70–98%) and had a stable R_F (0·009–0·023) over the 6 years analysed. In the Trondheimsfjord, however, there was a higher variation in R_F (0·003–0·036), and pelagic fish larvae were dominant prey in only two (50 and 91%) of the 8 years analysed. These 2 years also showed the highest return rates of S. salar in River Orkla. These results demonstrate that the thermal conditions experienced by post‐smolts during their early sea migration may be crucial for the subsequent return rate of adults after 1–3 years at sea. Pelagic marine fish larvae seem to be the preferred initial prey for S. salar post‐smolts. As the annual variation in abundance of fish larvae is related to seawater temperature, it is proposed that seawater temperature at sea entry and the subsequent abundance of returning adult S. salar may be indirectly linked through variation in annual availability of pelagic fish larvae or other suitable food items in the early post‐smolt phase.     

River lamprey present an unusual predation threat to Atlantic salmon smolts in Lough Neagh,Northern Ireland

A new monitoring programme on the Lough Neagh catchment has documented a high incidence of river lamprey, Lampetra fluviatilis L., predation on Atlantic salmon smolts, Salmo salar L. In total 470 smolts were examined during the 2020 emigration period with 168 fish (36%) exhibiting lamprey scars of which 57 were lightly scarred and 111 were classed as heavily scarred. Lamprey predation was not size selective on Lough Neagh S. salar smolts.     

The effects of sustained aerobic swimming on osmoregulatory pathways in Atlantic salmon Salmo salar smolts

Atlantic salmon Salmo salar smolts were exposed to one of the four different aerobic exercise regimens for 10 weeks followed by a 1 week final smoltification period in fresh water and a subsequent eight‐day seawater transfer period. Samples of gill and intestinal tissue were taken at each time point and gene expression was used to assess the effects of exercise training on both branchial and intestinal osmoregulatory pathways. Real‐time polymerase chain reaction (PCR) analysis revealed that exercise training up‐regulated the expression of seawater relevant genes in the gills of S. salar smolts, including Na⁺, K⁺ ATPase (nka) subunit α1b, the Na⁺, K⁺, 2 Cl^? co‐transporter (nkcc1) and cftr channel. These findings suggest that aerobic exercise stimulates expression of seawater ion transport pathways that may act to shift the seawater transfer window for S. salar smolts. Aerobic exercise also appeared to stimulate freshwater ion uptake mechanisms probably associated with an osmorespiratory compromise related to increased exercise. No differences were observed in plasma Na⁺ and Cl^? concentrations as a consequence of exercise treatment, but plasma Na⁺ was lower during the final smoltification period in all treatments. No effects of exercise were observed for intestinal nkcc2, nor the Mg²⁺ transporters slc41a2 and transient receptor protein M7 (trpm7); however, expression of both Mg²⁺ transporters was affected by salinity transfer suggesting a dynamic role in Mg²⁺ homeostasis in fishes.     

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.     

Does size matter? A test of size‐specific mortality in Atlantic salmon Salmo salar smolts tagged with acoustic transmitters

Mortality rates of wild Atlantic salmon Salmo salar smolts implanted with acoustic transmitters were assessed to determine if mortality was size dependent. The routinely accepted, but widely debated, ‘2% transmitter mass: body mass’ rule in biotelemetry was tested by extending the transmitter burden up to 12·7% of body mass in small [mean fork length (L_F) 138·3 mm, range 115–168 mm] downstream migrating S. salar smolts. Over the short timescale of emigration (range 11·9–44·5 days) through the lower river and estuary, mortality was not related to S. salar size, nor was a relationship found between mortality probability and transmitter mass: body mass or transmitter length: L_F ratios. This study provides further evidence that smolt migration studies can deviate from the ‘2% rule’ of thumb, to more appropriate study‐specific measures, which enables the use of fishes representative of the body size in natural populations without undue effects.     

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.     

Body and scale growth of wild Atlantic salmon smolts during seaward emigration

The relationship between scale and body growth for emigrating Atlantic salmon, Salmo salar, smolts was previously not understood and therefore was examined in this study using mark-recapture techniques. The size of smolts at time of recapture was significantly greater than when marked (P = 0.0002). The growth in length of smolts emigrating 5 km over an average of 20 days was 7.7 ± 6.1 mm per day. Instantaneous somatic growth (G _body) ranged from 7.0 × 10⁻⁴ to 5.1 × 10⁻³ (mean = 2.7 × 10⁻³ ± 1.3 × 10⁻³). The mean number of plus growth circuli present per scale was significantly greater for smolts when recaptured compared to when marked (P = 0.0014). The instantaneous growth rate of scales (G _scale) ranged from 1.4 × 10⁻³ to 11.5 × 10⁻³ (mean = 6.6 × 10⁻³ ± 4.3 × 10⁻³). The relationship between body size and scale radius showed positive allometry rather than isometry. The relationship of G _scale with G _body showed positive allometry indicating that scales grew at a slightly faster rate than the body during the emigratory period.