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.     

Cytogenetic studies of Atlantic salmon, Salmo salar L., in Scotland

Chromosome numbers for Atlantic salmon, Salmo salar L., from ten populations in Scotland were ascertained. The majority of fish had 2n = 58, NF = 74 karyotypes, and no polymorphisms between populations were found. The findings suggest that Atlantic salmon in Scotland are cytogenetically homogeneous.     

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.     

Infestations of Atlantic salmon, Salmo salar, by Gyrodactylus salaris in Norwegian rivers

Over the 6 years 1980–85, 212 Norwegian rivers have been examined for occurrence of Gyrodactylus salaris: it was found in 26 rivers and six salmon hatcheries scattered throughout the country from Troms county in the north to Sogn og Fjordane in western Norway. The distribution of G. salaris is connected with the stocking of fish from infected salmon hatcheries. The populations of salmon parr have been drastically reduced in the infected rivers. In later years catches of ascending salmon in these rivers have also sharply declined: in 1984 salmon fishery losses were estimated at 250–500 t.
Gyrodactylus salaris is most probably a recent introduction to Norwegian rivers. A primary aim is to exterminate this parasite from all infected rivers and hatcheries: so far this has been accomplished in one river and one hatchery.     

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.     

Gonadotropic cells in the Atlantic salmon,Salmo salar

Summary The gonadotropin-producing cells (GTH-cells) in the Atlantic salmon were studied light and electron microscopically before, during and after spawning, and after injections of luteinizing hormone-releasing hormone (LH-RH). The double immunofluorescent technique was applied using rabbit anti-carp GTH as the first antibody. Numerous immunofluorescent cells were observed throughout the pars distalis, but very few in the pars intermedia. These cells are basophilic and PAS-positive, and ultrastructurally classified as globular gonadotropes. Only one gonadotropic cell type could be identified; its size, morphology and fine structure vary considerably. In the same specimen the GTH-cells can be predominantly globular or vesicular in appearance, depending on the reproductive phase of the fish. At spawning and after LH-RH injection, many GTH-cells reach a vacuolar stage; the content of the vacuoles is not immunofluorescent. Another cell type, which resembles GTH-cells in semithin sections, did not show gonadotropic properties; its nature and functional significance are unknown. In addition, the present study revealed an increase in the synthetic and exocytotic activity of prolactin cells after LH-RH injections. It is suggested that LH-RH mediates this effect via LH and eventually via estradiol.The authors are indebted to Dr. N. Johansson and superintendent N. Steffner for generously providing facilities at their institutes in Älvkarleby. They also wish to thank Mrs. Y. Lilliemarck and Mrs. V. Lundin for their skillful technical assistance. The grants made available by the Swedish Natural Science Research Council (No. 2124-037) are also gratefully acknowledged. Thanks are also due to Mrs. S.M. McNab for linguistic assistance     

Avoidance responses to low frequency sound in downstream migrating Atlantic salmon smolt, Salmo salar

The possibility of using intense sound as an acoustic barrier for downstream migrating smolt of the Atlantic salmon ( Salmo salar ) was studied by observing, the reactions of smolt to 10 and 150 Hz sounds in a small river. At the observation site the river branched into a main course and a minor channel, the latter rejoining the main stream after 30 m. The sound sources were positioned at the lower end of the channel. The number of smolt re-entering the mam stream at the lower end of the channel was recorded during alternating periods with and without sound. Intense 150 Hz sound had no observable effects on the smolt, even at intensities 114 dB above the hearing threshold at this frequency. At intensities above 1.0. 10⁻²ms⁻² the 10 Hz sound was an effective deterrent for the smolt, which turned and left the channel at the upstream branching point.     

Differential expression of olfactory genes in Atlantic salmon (Salmo salar) during the parr–smolt transformation

The anadromous salmon life cycle includes two migratory events, downstream smolt migration and adult homing migration, during which they must navigate with high precision. During homing migration, olfactory cues are used for navigation in coastal and freshwater areas, and studies have suggested that the parr – smolt transformation has a sensitive period for imprinting. Accordingly, we hypothesized that there would be significant changes in gene expression in the olfactory epithelium specifically related to smoltification and sampled olfactory rosettes from hatchery‐reared upper growth modal juvenile Atlantic salmon at 3‐week intervals from January to June, using lower growth modal nonsmolting siblings as controls. A suite of olfactory receptors and receptor‐specific proteins involved in functional aspects of olfaction and peripheral odor memorization was analyzed by qPCR. Gene expression in juveniles was compared with mature adult salmon of the same genetic strain caught in the river Gudenaa. All mRNAs displayed significant variation over time in both modal groups. Furthermore, five receptor genes (olfc13.1, olfc15.1, sorb, ora2, and asor1) and four olfactory‐specific genes (soig, ependymin, gst, and omp2) were differentially regulated between modal groups, suggesting altered olfactory function during smoltification. Several genes were differentially regulated in mature salmon compared with juveniles, suggesting that homing and odor recollection involve a different set of genes than during imprinting. Thyroid hormone receptors thrα and thrβ mRNAs were elevated during smolting, suggesting increased sensitivity to thyroid hormones. Treatment of presmolts with triiodothyronine in vivo and ex vivo had, however, only subtle effects on the investigated olfactory targets, questioning the hypothesis that thyroid hormones directly regulate gene expression in the olfactory epithelium.     

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.     

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.     

Lunar periodicity and the timing of river entry in Atlantic salmon Salmo salar

Historical catch records of Atlantic salmon Salmo salar from three rivers discharging to the Baltic Sea in an area free from tides and from strong effects of the moon on illumination were analysed to investigate whether timing of S. salar river entry was associated with lunar cycles directly. Although a significant effect of lunar phase on river entry was detected, with more fish entering rivers around the full moon than other phases, the effect of the lunar cycle was very small compared with other sources of variation. Hence, the biological role of lunar cycle as a determinant of the timing of S. salar runs in the investigated populations was negligible, suggesting that lunar cycle per se does not play a role in the timing of S. salar river entry.     

Growth differences between the Atlantic salmon parr,Salmo salar,of nursery brooks and natal rivers in the River Teno watercourse in northern Finland

Synopsis A comparison was done between growth in Atlantic salmon,Salmo salar, parr from natal rivers and small tributaries in the subarctic River Teno watercourse in northern Finland (70° N, 28° E). Salmon do not spawn in these tributaries but juveniles enter the brooks from their spawning rivers. Parr from these brooks (age groups 1 + and 2 +) were larger, so were their recorded annual growth increments, than those caught simultaneously in the River Teno. First year growth was also better in brook parr, although both groups had spent their first year in the main river. The brook water temperature was lower than that in the main rivers through-out the growing season. On the other hand, drifting food resources were more abundant in brooks. It has been suggested that better growth and a longer residence in fresh water result in the larger smolts being produced in the brooks relative to the main river. Differences in habitat selection and subsequent differences in the smolt age and size of fish from the same place of origin reflect the distinct life histories of the salmon in the River Teno.     

Counterintuitive migration patterns by Atlantic salmon Salmo salar smolts in a large lake

What little is known about the seaward migration of Salmo salar smolt migration through standing waters indicates that it is both slow and results in high mortality rates, compared with riverine migration. This may be partly because smolts in lakes need to swim more actively and require more complex directional cues than they do in rivers. In this telemetry study of smolt migration through Loch Lomond, S. salar smolts made repeated movements in directions away from the outflowing river, which considerably increased migration time.     

Infestations by Gyrodactylus sp. of Atlantic salmon, Salmo salar L., in Norwegian rivers

Young Atlantic salmon, Salmo salar , brown trout, Salmo trutta , and Arctic char, Salvelinus alpinus , from eight rivers in North and Mid Norway were examined for Gyrodactylus. The fish were collected from 1975 to 1980. A Gyrodactylus salaris type was observed infecting salmon from six of these rivers. No trout or char were infected. A high frequency and intensity of infection of salmon were observed in all but one of the rivers surveyed. In the Saltdalselva, only one specimen of Gyrodactylus infecting one fish was observed. No subsequent mortality of salmon was observed in this river while there were signs of a high mortality of salmon in the other rivers. The salmon parr were more frequently attacked than the fry, the mortality of salmon seemed to have happened in most of the rivers one year after the first observations of Gyrodactylus were made. The mortality of salmon and the high infection rate of Gyrodactylus in these rivers appears unique, and as far as known to the authors there are no other described cases of mortality due to Gyodactylus in Atlantic salmon in natural waters. The reasons for the outbreak of Gyrodactylus in these rivers are not known. Two theories are discussed: one that the fish were weakened by environmental factors and the other that Gyrodactylus was introduced from some of the infected salmon hatcheries in Scandinavia.     

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.     

Ocean migration and recaptures of tagged, triploid, mixed-sex and all-female Atlantic salmon (Salmo salar L.) released from rivers in Ireland

Fiftyfour thousand diploid and triploid Atlantic salmon were tagged with coded-wire micro-tags and released to the sea as smolts from two freshwater sites in Ireland in 1996 and 1997. Over 36,000 were mixed-sex groups in which the triploids (MS3N) were treated batches and the diploids (MS2N) were untreated batches of a single group of fertilized eggs at each site and year class. Over 17,000 were all-females, the triploids (AF3N) being treated batches and the diploids (AF2N) untreated batches of a single group of fertilized eggs at each site and year class. Adult tagged fish were subsequently recovered in the coastal fishery and in freshwater traps and angling fisheries in the rivers of release during 1997, 1998 and 1999. Recoveries from migration ranged from 0.08 to 9.79%. Diploid recoveries were within the normal ranges for salmon ranched from these sites, being between 0.64 and 1.82% at one site and between 3.85 and 9.79% at the other. Triploid recoveries, which occurred in the coastal fishery and in freshwater, ranged from 0.08 to 0.40% at the first site and from 0.98 to 2.05% at the other. Overall, triploid recoveries were between 12 and 24% of their diploid siblings within each release group. There were two peaks of recapture in the coastal fishery, the first in mid-June and the second in mid to late July. All-female diploids appeared to enter the fishery in advance of mixed-sex diploids. Triploids were the latest to arrive. There was no evidence of increased straying in triploids relative to diploids and the mean lengths, weights and condition factors were not significantly different between the ploidy groups. Triploidisation therefore reduced the home water harvest of treated salmon to less than 25% but did not completely eliminate triploid recaptures. The way in which triploidisation may influence the return of salmon from the oceanic feeding grounds is discussed.     

Tidal and diel timing of river entry by adult Atlantic salmon returning to the Aberdeenshire Dee, Scotland

The timing of salmon migration from the estuary of the Aberdeenshire Dee into the river in relation to tidal phase and time of day was studied by combined acoustic and radio-tracking of individual fish and by analysing records of untagged fish from a resistivity fish counter 0.8 km upstream from the tidal limit. Up-estuary movements that led to river entry were predominantly nocturnal and tended to occur during the ebb tide. Penetration into the non-tidal reaches of the river also tended to occur at night, but the timing of salmon movements was no longer significantly associated with tidal phase. The tracking data suggested that the reduction in the strength of the association between salmon migration and tidal phase resulted from variability in rates of progress from the estuary to the river. This variability may have been random, or related to changes in migratory behaviour during entry to fresh water. A simulation of upstream progress by groups of salmon illustrated the decay of the relationship between salmon movements and tidal phase and the advance of the average tidal phase of observed salmon movements with increasing distance of the observation point from where movements were initiated. The magnitude of these effects depended on the average rate of upstream progress and variability in the rate of progress, being greatest when upstream progress was slow and variable. These results highlight a limitation of point observations of migration with regard to identifying environmental stimuli for migration and quantifying their effects.     

Evidence of a genetic component in the seasonal return pattern of Atlantic salmon, Salmo salar L.

We tested the null hypothesis that differences in the seasonal return patterns between stocks of Atlantic salmon, Salmo salar L., are a result of a direct response to the environment, and not under genetic control. Two stocks were used in the experiments, originating from the R. Figga and R. Imsa, respectively. In their native habitat fish from the former are known to return to the home stream as adult salmon early in the summer, while from the latter return during late summer and autumn. By rearing these stocks in the same hatchery and releasing smolts of both stocks together at three sites in southern Norway, it was demonstrated that salmon from the R. Figga stock returned earlier to coastal Norway than salmon from the R. Imsa stock, as maturing adults. Thus, we reject the hypothesis that these stocks are genetically identical in this trait. Within both stocks, multi-sea-winter fish returned earlier than one-sea-winter fish. Within stocks, there was no significant difference in time of return between salmon released as 1- and 2-year-old smolts, or between fish reared from parents ascending the R. Imsa early or late in the season.     

Spawning behaviour of a farmed escaped female Atlantic salmon (Salmo salar)

Excavation of stranded redds revealed differences in spawning behaviour between farmed and wild Atlantic salmon. The redd of a farmed fish contained more egg pockets (nine v . average of two) and fewer eggs per pocket (averages: 459 v . 707). No other pocket measures differed.