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.     

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.     

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.     

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.     

The effect of catch‐and‐release angling at high water temperatures on behaviour and survival of Atlantic salmon Salmo salar during spawning migration

In this study, behaviour and survival following catch‐and‐release (C&R) angling was investigated in wild Atlantic salmon Salmo salar (n = 75) angled on sport fishing gear in the River Otra in southern Norway at water temperatures of 16·3–21·1° C. Salmo salar were tagged externally with radio transmitters and immediately released back into the river to simulate a realistic C&R situation. The majority of S. salar (91%) survived C&R. Most S. salar that were present in the River Otra during the spawning period 3–4 months later were located at known spawning grounds. Downstream movements (median furthest position: 0·5 km, range: 0·1–11·0 km) during the first 4 days after release were recorded for 72% of S. salar, presumably stress‐induced fallback associated with C&R. Individuals that fell back spent a median of 15 days before commencing their first upstream movement after release, and 34 days before they returned to or were located above their release site. Mortality appeared to be somewhat elevated at the higher end of the temperature range (14% at 18–21° C), although sample sizes were low. In conclusion, C&R at water temperatures up to 18° C had small behavioural consequences and was associated with low mortality (7%). Nevertheless, low levels of mortality occur due to C&R angling and these losses should be accounted for by management authorities in rivers where C&R is practised. Refinement of best practices for C&R may help to reduce mortality, particularly at warmer temperatures.     

Comparing upriver spawning migration of Atlantic salmon Salmo salar and sea trout Salmo trutta

Radio tagged wild Atlantic salmon Salmo salar(n = 30) and sea trout Salmo trutta(n = 19) were simultaneously released from a sea pen outside the mouth of the River Lærdalselva and their migration to spawning areas was recorded. The distance from the river mouth to a position held at spawning ranged from 2 to 24 km and did not differ between the species (mean ± s .d . 15·9 ± 4·3 and 14·9 ± 5·2 km for Atlantic salmon and sea trout, respectively). The duration of the migration phase, however, was significantly shorter for Atlantic salmon than for sea trout (8–12 days, respectively). All Atlantic salmon migrated straight to an area near the spawning ground, whereas 50% of the sea trout had a stepwise progression with one or more periods with erratic movements before reaching the spawning area. After the migration phase, a distinct search phase with repeated movements up‐ and downstream at or close to the position held at spawning was identified for the majority of the fishes (75%, both species). This search phase was significantly shorter for Atlantic salmon than for sea trout (mean 13–31 days, respectively). Mean ± s .d . length of the river stretch used during the search phase was larger for sea trout (3·3 ± 2·5 km) than for Atlantic salmon (1·2 ± 0·9 km). A distinct holding phase, with no movements until spawning, was also observed in the majority of the Atlantic salmon (80%, mean duration 22 days) and sea trout (65%, mean duration 12 days). For both species, a weak, non‐significant trend was observed in the relationship between time spent on the migration phase, and time spent on the search (r² = 0·43) and holding phase (r² = 0·24). There was a highly significant decrease, however, in the duration of the holding phase with an increase in the time spent on the search phase (r² = 0·67).     

Overwintering and migration behaviour of post-spawned Atlantic salmon Salmo salar in a large hydropower-regulated river and reservoir

Tracking 47 post-spawned adult Atlantic salmon Salmo salar L. in a hydropower-regulated river through autumn, winter and spring revealed that winter survival was 56% and 75% in two study years, respectively, with higher mortality of males than females (50% vs. 33% and 100% vs. 13%, respectively). Some kelts (n = 7) displayed nondirected movements that were interpreted as a reconditioning period for an average of 9–17 days prior to directed downstream movements indicating the initiation of migration. Survival after the initiation of migration in spring was 83% and 94% to the hydropower dam in the first and second study years, and decreased to 60 and 63%, respectively, after dam passage. There were no further losses in the downriver reach in the second year, with the first year having a cumulative survival estimate of 53% to the river mouth. Kelts approached the dam when the spillway gates were available as a passage option most of the time (64%–75%), but some kelts arrived at the dam or had not yet passed when spillways were closed (n = 6) and the only remaining passage option was restricted to the turbines. However, all but one kelt that must have passed via turbine were successful in reaching the river mouth. Migratory delay presumably due to searching behaviour caused by low water flow was estimated at approximately 6 days as migration rates were significantly slower in the reservoir (median ± s.e. 8.5 ± 2.5 km day⁻¹) than up- (29.7 ± 5.0 km day⁻¹) or downriver (22.1 ± 3.1 km day⁻¹). The proportion of time (median 30%) that kelts spent swimming upstream (searching behaviour) in the reservoir was a significant variable for migration success.     

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.     

Observations of the biology and parasites of postsmolt Atlantic salmon, Salmo salar, from the Norwegian sea

The main food items of postsmolt Atlantic salmon caught in the Norwegian sea during summer 1991 were Parathemisto spp. and herring ( Chipca harengus ). The most prevalent parasite species were Chloromyxion sp., Apatemon sp., H. adunction, A simplex and I. salmonis .     

Migratory and reproductive behaviour of female adult Atlantic salmon, Salmo salar L., in a spawning stream

Migration and spawning behaviour of eight Atlantic salmon adult females were analysed by radio-tracking in relation to the degree of sexual maturity in a spawning tributary of the R. Sélune. Six of them were grilse and four of them were ripe. All the fish migrated upstream until reaching their spawning site at a distance of 4–12 km from the trap. The daily migration rate up to this site was inversely correlated with the length of the female. Spawning occurred in all fish at the same time when the water temperature increased dramatically. Spawning lasted 1–10 days according to the fish. After spawning, females quickly moved downstream only small distances and then stayed in approximately the same location until death. This study concluded that ripeness did not influence behaviour on the spawning migration and describes certain characteristics of the reproductive phase in a spawning tributary.     

Migratory behaviour and growth of hatchery-reared post-smolt Atlantic salmon Salmo salar

Individually lagged, 1+ and 2+ hatchery-reared smolts of Atlantic salmon were released in spring and early summer at the mouth of the R. Imsa, south-western Norway. The post-smolts moved mainly northwards in the sea with the coastal current. The estimated mean migratory speed (± s.d. ) of those captured in the sea along the Norwegian coast was 7.45 (± 6.26) km day ⁻¹; in the fjords it was 1.63 (± 2.33) km day⁻². Many of the post-smohs ascended rivers the same year as released; 37.3% of the total number recaptured were caught in R. Imsa, upstream from the site of release, and 5.8% were caught in other rivers throughout middle and southern parts of Norway. The fish recaptured in rivers was probably sexually mature and entered rivers to spawn. Mean specific growth rate for post-smolts caught in the sea was higher than for those caught in R. Imsa (P <0.001) but not for those caught in other rivers (P> 0.05). Post-smolts ascending R. Imsa were smaller at release than those ascending other rivers. However, there was no size difference at release between post-smolts captured in the sea and those recaptured in rivers other than the R. Imsa.     

Heart rate as an indicator of metabolic rate and activity in adult Atlantic salmon, Salmo salar

Telemetered heart rate (f_H) was examined as an indicator of activity and oxygen consumption rate (VO₂) in adult, cultivated, Atlantic salmon, Salmo salar L. Heart rate was measured during sustained swimming in a flume for six fish at 10° C [mean weight, 1114 g; mean fork length (f. l.), 50·6 cm] and seven fish at 15° C (mean weight, 1119 g; mean f. l., 50·7 cm) at speeds of up to 2·2 body lengths/s. Semi–logarithmic relationships between heart rate and swimming speed were obtained at both temperatures. Spontaneously swimming fish in still water exhibited characteristic heart rate increases associated with activity. Heart rate and Vo₂ were monitored simultaneously in a 575–1 circular respirometer for six fish (three male, three female) at 4° C (mean weight, 1804 g; mean F. L., 62· cm) and six fish (three male, three female) at 10° C (mean weight, 2045 g; mean f. l., 63·2 cm) during spontaneous but unquantified activity. Linear regressions were obtained by transforming data for both f_H and Vo₂ to log values. At each temperature, slopes of the regressions between f_H and Vo₂ for individual fishes were not significantly different, but in some cases elevations were. All differences in elevation were between male and female fish. There were no significant differences in regression slope or elevation for fish of the same sex at the two temperatures and so regressions were calculated for the sexes, pooling data from 4 and 10° C. There was no significant difference in the mean ± S. D. Vo₂ between the sexes at 4° C (male, 66·0 ± 59·6 mgO₂ kg^?1 h^?1; female, 88·0 ± 60·1 mgO₂ kg^?1 h^?1) or 10° C (male, 166·2 ± 115·4 mgO₂ kg^?1 h^?1; female, 169·2 ± 111–1 mgO₂ kg^?1h^?1). Resting Vo₂ (x?± s. d.) at 4°C was 36·7 ± 8.4 mgO₂ kg^?1 h^?1, and 10° C was 72·8 ± 11·9 mgO₂ kg^?1 h^?1. Maximum Vo₂ (x?± S. D.) at 4° C was 250·6 ± 40·2 mgO₂ kg^?1 h^?1, and at 10° C was 423·6 ± 25·2 mgO₂ kg^?1 h^?1. Heart rate appears to be a useful indicator of metabolic rate over the temperature range examined, for the cultivated fish studied, but it is possible that the relationship for wild fish may differ.     

Over-summering behaviour of Atlantic salmon Salmo salar returning to rivers in the Cromarty Firth, north–east Scotland

In 2005, a high percentage (50%) of Atlantic salmon Salmo salar fitted with acoustic transmitters, which entered rivers in the Cromarty Firth area of north–east Scotland dropped back to the estuary and ascended adjacent rivers after 33–80 days of freshwater residence. Fish residing in non-natal rivers generate mixed stock fisheries, and movements of fish between rivers could increase the risk of disease transmission between catchments.     

Changes in swimming depths of Atlantic salmon Salmo salar post‐smolts relative to light intensity

Eight hatchery‐reared Atlantic salmon Salmo salar post‐smolts, implanted with acoustic depth sensing transmitters and manually tracked for 5–12 h in the Hardangerfjord (Norway), spent most of their time (49–99%) at 1–3 m depth during the day, whereas four of seven fish tracked were found close (<0·5 m) to the surface at night, with a strong negative cross‐correlation between general swimming depth and surface light intensity. Hence, the actual swimming depth of post‐smolts during their early marine migration may depend on the light conditions, although the individual variation in vertical movement pattern was large. No cross‐correlations were found between light intensity and swimming depth during daytime periods with rapid changes in light intensity, indicating that other factors than light intensity were important in initiating the irregular dives that were recorded down to 6·5 m depth.     

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.     

Residency time, migration route and survival of Atlantic salmon Salmo salar smolts in a Canadian fjord

Atlantic salmon Salmo salar smolts (n = 181) from two rivers were surgically implanted with acoustic transmitters and released to determine migration route, residency time and survival in a 50 km long estuarine fjord located on the south coast of Newfoundland, Canada. Data obtained from automated receivers placed throughout the Bay d'Espoir fjord indicated that migrating smolts used different routes to reach the outer areas of the fjord. The duration of time that smolts spent in the immediate estuary zone also differed between the two localities (7 and 17 days) although the total time smolts were resident in the fjord was similar and extensive (40 days). Many smolts were resident for periods of 4-8 weeks moving back and forth in the outer part of the fjord where maximum water depths range from 300 to 700 m. Survival in the estuary zone was greater for smolts with prolonged residency in estuarine habitat. Overall smolt survival to the fjord exit was moderately high (54-85%), indicating that the initial phase of migration did not coincide with a period of unusually high mortality.     

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.     

Thiamin dynamics during the adult life cycle of Atlantic salmon (Salmo salar)

Thiamin is an essential water-soluble B vitamin known for its wide range of metabolic functions and antioxidant properties. Over the past decades, reproductive failures induced by thiamin deficiency have been observed in several salmonid species worldwide, but it is unclear why this micronutrient deficiency arises. Few studies have compared thiamin concentrations in systems of salmonid populations with or without documented thiamin deficiency. Moreover, it is not well known whether and how thiamin concentration changes during the marine feeding phase and the spawning migration. Therefore, samples of Atlantic salmon (Salmo salar) were collected when actively feeding in the open Baltic Sea, after the sea migration to natal rivers, after river migration, and during the spawning period. To compare populations of Baltic salmon with systems without documented thiamin deficiency, a population of landlocked salmon located in Lake Vänern (Sweden) was sampled as well as salmon from Norwegian rivers draining into the North Atlantic Ocean. Results showed the highest mean thiamin concentrations in Lake Vänern salmon, followed by North Atlantic, and the lowest in Baltic populations. Therefore, salmon in the Baltic Sea seem to be consistently more constrained by thiamin than those in other systems. Condition factor and body length had little to no effect on thiamin concentrations in all systems, suggesting that there is no relation between the body condition of salmon and thiamin deficiency. In our large spatiotemporal comparison of salmon populations, thiamin concentrations declined toward spawning in all studied systems, suggesting that the reduction in thiamin concentration arises as a natural consequence of starvation rather than to be related to thiamin deficiency in the system. These results suggest that factors affecting accumulation during the marine feeding phase are key for understanding the thiamin deficiency in salmonids.     

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.     

Water chemistry and its effects on the physiology and survival of Atlantic salmon Salmo salar smolts

The physiological effects of episodic pH fluctuations on Atlantic salmon Salmo salar smolts in eastern Maine, U.S.A., were investigated. During this study, S. salar smolts were exposed to ambient stream-water chemistry conditions at nine sites in four catchments for 3 and 6 day intervals during the spring S. salar smolt migration period. Plasma chloride, plasma glucose, gill aluminium and gill Na(+)- and K(+)-ATPase levels in S. salar smolts were assessed in relation to ambient stream-water chemistry during this migration period. Changes in both plasma chloride and plasma glucose levels of S. salar smolts were strongly correlated with stream pH, and S. salar smolt mortality occurred in one study site with ambient stream pH between 5·6 and 5·8 during the study period. The findings from this study suggest that physiological effects on S. salar smolts are strongly correlated with stream pH and that in rivers and streams with low dissolved organic carbon (DOC) concentrations the threshold for physiological effects and mortality probably occurs at a higher pH and shorter exposure period than in rivers with higher DOC. Additionally, whenever an acidification event in which pH drops below 5·9 coincides with S. salar smolt migration in eastern Maine rivers, there is potential for a significant reduction in plasma ions of S. salar smolts.