Hatchery-reared Atlantic Salmon Smolts Research Articles

Determining early marine survival and predation by endothermic predators on acoustically tagged Atlantic salmon (Salmo salar) post-smolts

Many Atlantic salmon ( Salmo salar) populations have experienced significant declines for decades throughout North America and Europe. Mortality due to marine mammal predation during their early marine life could be an important factor contributing to these declines and limiting their population recoveries. However, quantifying predation events, and particularly the extent of marine mammal predation on Atlantic salmon, remains a challenge. In this study, we estimated the contribution of mesothermic and endothermic species predation to the mortality of Atlantic salmon post-smolts during their early marine life using acoustic telemetry. Predation events were inferred from changes in temperatures and depths experienced by acoustically tagged hatchery-reared Atlantic salmon smolts. No salmon were consumed by mesothermic predators, with most endothermic predation events being classified as marine mammals. Post-smolt mortality during the study period was low overall in both years (13.1%–16.7%), with endothermic predation accounting for 33.1%–42.9% of all marine mortality events (5.2%–5.6% mortality). Our results suggest that the current low return of adult Atlantic salmon observed in this area in recent years was not heavily influenced by endothermic predation on post-smolts in the first weeks at sea.

Partitioning survival during early marine migration of wild and hatchery-reared Atlantic salmon (Salmo salar L.) smolts using acoustic telemetry

The marine migration of Atlantic salmon (Salmo salar) has been prioritised for research internationally as populations of S. salar have declined significantly throughout the species’ range. The main objectives of this study were to use acoustic telemetry to partition survival during the early migration phase, investigate potential causes of mortality and establish diurnal and tidal influences on movements. In 2017 and 2018 wild (n = 49) and hatchery (n = 81) S. salar smolts were tagged with acoustic transmitters. Migration was monitored through a brackish tidal lake, which discharges through a short estuary into northeast Clew Bay in Ireland situated in the northeast Atlantic. Partitioned survival through each area was similar for both wild and hatchery smolts and both groups followed the same migration routes, travelling along the main current out of Clew Bay. Total survival was high within the brackish lake (wild = 98% ± 1.21 and hatchery = 100%) and estuary (wild = 98% ± 1.01 and hatchery = 89% ± 9.28) compared to the marine environment (wild = 67% ± 4.39 and hatchery = 66% ± 7.91). Leaving the brackish lake during an ebb tide and entering the marine environment during daylight increased the probability of survival through the early marine period. The majority of smolts transited through the study area during ebb tides. Migration of hatchery smolts occurred mainly during hours of daylight while wild smolt migration showed no diel patterns. High mortality rates during the initial stages of the marine migration have consequences for the persistence of salmon populations and should be addressed through regionally tailored management measures and conservation efforts aimed at increasing the resilience of salmon stocks.

Effects of dam-induced delays on system-wide survival of Atlantic salmon smolts during high-flow, high-survival years in the Penobscot River, Maine, USA

System-wide survival of hatchery-reared Atlantic salmon ( Salmo salar) smolts was evaluated (2017–2019) in the Penobscot River and compared to survival estimates from previous years that spanned major changes (2005–2016). This system was transformed through two dam removals in 2012 and construction of a nature-like passage structure at a third. The main stem had three dams (five prior to 2012), while the main tributary had four dams (one with the new nature-like passage). We estimated survival using acoustic telemetry mark–recapture ( n = 1482) from 2017 to 2019. Six release sites and two release dates were included to assess system-wide survival. Survival from 2017 to 2019 was higher than previous years, with total cumulative survival > 0.75, independently of year and release sites, compared to survival < 0.5 in previous years. These years coincided with exceptional high flows not seen previously. We found an effect of delays on survival, longer delays associated with lower survival. Overall, survival in these years increased in all reaches relative to previous years except for one dam, Weldon Dam, which was a site of sustained high mortality.

Catchment-wide survival of wild- and hatchery-reared Atlantic salmon smolts in a changing system

We developed a hierarchical multistate model to estimate survival of Atlantic salmon (Salmo salar) smolts in the Penobscot River, USA, over a decade during which two mainstem dams were removed from the catchment. We investigated effects of (i) environmental factors, (ii) rearing history, and (iii) management actions, including dam removal, turbine shutdown, and installation of new powerhouses. Mean ± SD smolt survival per kilometre was higher through free-flowing reaches of the catchment (0.995 ± 0.004·km−1) than through reaches containing dams that remain in the system (0.970 ± 0.019·km−1). We observed maximum survival between 12 and 17 °C and at intermediate discharges (1200 m3·s−1). Smolt survival increased concurrent with dam removal and decreased following increases in hydropower generation. The greatest increase in smolt survival followed seasonal turbine shutdowns at a dam located on the largest tributary to the Penobscot River, while other shutdowns had little influence. Our model provides a useful tool for assessing changes to survival of migratory species and will be useful for informing stocking plans to maximize numbers of smolts leaving coastal systems.

Salmon lice increase the age of returning Atlantic salmon

The global increase in the production of domestic farmed fish in open net pens has created concerns about the resilience of wild populations owing to shifts in host-parasite systems in coastal ecosystems. However, little is known about the effects of increased parasite abundance on life-history traits in wild fish populations. Here, we report the results of two separate studies in which 379 779 hatchery-reared Atlantic salmon smolts were treated (or not) against salmon lice, marked and released. Adults were later recaptured, and we specifically tested whether the age distribution of the returning spawners was affected by the treatment. The estimates of parasite-induced mortality were 31.9% and 0.6% in the River Vosso and River Dale stock experiments, respectively. Age of returning salmon was on average higher in untreated [corrected] versus untreated fish. The percentages of fish returning after one winter at sea were 37.5% and 29.9% for the treated and untreated groups, respectively. We conclude that salmon lice increase the age of returning salmon, either by affecting their age at maturity or by disproportionately increasing mortality in fish that mature early.

Enriched rearing improves survival of hatchery-reared Atlantic salmon smolts during migration in the River Tornionjoki

Low survival of stocked fish has been associated with fitness declines of the captive reared fishes because of genetic domestication and unnatural rearing environments. The effects of broodstock origin (wild or captive) or rearing method (standard or enriched) on survival and migration of hatchery-reared Atlantic salmon smolts (Salmo salar) were investigated in the Tornionjoki River using radiotelemetry. Smolts that were reared with enriched methods had a twofold increase in survival (∼38%) compared with smolts that had been reared in a standard hatchery environment (∼19%). Nature-caught smolts had highest survival (∼57%). Smolts from enriched rearing had a higher initial migration speed than fish from standard rearing. Initial migration speed during the first 3 km was positively correlated to survival probability after 290 km for hatchery fish. There was no clear effect of origin on survival or migration speed. The results of this study show that enriching the rearing environment with methods easily applicable to large-scale production promotes smolt survival and migration speed during river migration, which is imperative for stocking success.

Effects of hatchery rearing and sea ranching of parents on the life history traits of released salmon offspring

Life-history traits such as growth, survival, sea migration and age at sexual maturity in the sea were compared between Carlin-tagged hatchery-reared Atlantic salmon smolts (Salmo salar) originating from hatchery broodstocks (reared) or naturally ascending spawners captured (sea ranched). The ranched parents included both wild (born in nature) and reared individuals (released as parr or smolts). All smolts were reared in similar hatchery conditions, and they were tagged and released as two-year-olds in the Simojoki River in 1986–2007. The recapture rates did not differ between the progeny groups, although the tagged smolts of the reared parents were larger than those of ranched parents at time of release. The captive salmon with ranched parents were at average heavier during their second winter at sea, but not later. Salmon with ranched parents more frequently migrated to feed in the Main Basin of the Baltic Sea, further from the home river of the salmon stock. The proportions of multi-sea-winter returners were 30% and 69% in salmon with reared and ranched parents, respectively. The different patterns of migration and sea age at maturity of these parental progeny groups suggest that differences in the life history of brood fish may cause distinct, possibly even genetic differences in the progenies.

Exorbitant mortality of hatchery-reared Atlantic salmon smolts Salmo salar L., in the Meuse river system in the Netherlands

Stocking Atlantic salmon Salmo salar L. in the Meuse river system is unsuccessful, since hardly any adults return upstream. To investigate problems during seaward migration, individual salmon smolts were tracked in the Dutch part of the river Meuse, using the Nedap Trail® system. The study included a comparison for two seasons, one with high (2010) and one with low (2011) discharge conditions. Cultivated smolts (Loire-Allier strain, n = 100 per year) were implanted with telemetry tags and released in the tributary Roer in March, at the beginning of the natural smolt run. The study area was split into four river sections each characterized by different conditions: tributary Roer, main river Meuse extensively dammed, main river Meuse free flowing and the estuary. Mortalities differed considerably between sections. In the free flowing river Meuse the mortalities were relatively low (10 and 25 %). Mortalities were high in the tributary (44 and 45 %), the dammed river (46 and 49 %) and in the estuary (89 and 90 %). Only 2 and 3 % of the smolts escaped into the North Sea. Results are discussed in relation to environmental factors light and discharge and the presence of man-made obstacles: weirs, hydropower plants and a sea lock. A lack of current and delays at man-made obstacles result in disorientation of fish, a higher risk of predation and disturbance of the smolt run timing. Mortality causes must decrease for sustaining a salmon population in the Meuse.

Evidence for sea lice-induced marine mortality of Atlantic salmon (Salmo salar) in western Ireland from experimental releases of ranched smolts treated with emamectin benzoate

Sea trout ( Salmo trutta ) stock collapses in coastal areas of western Ireland subject to salmon aquaculture were contemporaneous with high abundances of larval sea lice ( Lepeophtheirus salmonis ) on juvenile sea trout. Whereas sea trout remain in near-shore waters throughout their marine migration, Atlantic salmon ( Salmo salar ) smolts typically move quickly offshore into oceanic waters. It might therefore be predicted that salmon smolts would be less vulnerable to coastal stressors and less likely to be negatively affected by infestations of sea lice early in their marine phase. Groups of microtagged, hatchery-reared Atlantic salmon smolts were fed either untreated pellets or pellets incorporating the in-feed sea louse treatment SLICE (emamectin benzoate) prior to eight experimental releases in three marine locations over a 3-year period. In total, 74 324 smolts were released and analysis of tag recaptures from returning adult salmon showed that emamectin-treated smolts experienced increased survivorship and were 1.8 times more likely to return compared with control fish. These results suggest that sea lice-induced mortality on adult Atlantic salmon returns in Ireland can be significant, and that sea lice larvae emanating from farmed salmon may influence individual survivorship and population conservation status of wild salmon in these river systems.

Experimental study of the influence of photoperiod and temperature on the swimming behaviour of hatchery-reared Atlantic salmon (Salmo salar L.) smolts

The impacts of photoperiod and temperature on the swimming behaviour of hatchery-reared Atlantic salmon smolts were investigated, with a view to optimising salmon restoration strategies in the Loire-Allier basin. A novel experimental design was developed to study individual tagged fish in 9m diameter hatchery tanks. Movements and swimming speeds were recorded over a period of eight months (January to August), repeated over four consecutive years (2005–2008). Fish switched from positive rheotactic behavior in January-February, to negative rheotactic behaviour at mid-March or April depending on the year, a period corresponding to the start of the downstream migration of wild smolts from the upper river spawning areas. Daily variation in movement and swimming speed occurred with high swimming speeds with the flow during the day and much lower speeds during the night. Swimming speeds could exceed water current velocity, showing hatchery-reared salmon can actively swim with the flow during the day. Experimental changes of photoperiod (constant light or constant darkness) allowed confirmation of the role of photoperiod in the daily variation of swimming activity. Analysis of the relationship between water temperature and swimming speeds over the four year experiments suggested a major role for water temperature in the modulation of swimming activity. Maximal swimming speed was recorded at 10.5°C and swimming speed was reduced by 80% for temperatures under 4 or above 17°C. Fish stopped moving above 20°C. The data highlight the importance of temperature for the modulation of downstream movement, as well as the potential deleterious impact of the anthropogenic increase in river temperature below the spawning and juvenile habitat in the Allier tributary on the success of downstream migration. Better understanding of temperature effects on actual population-specific in-river migration patterns is required to allow managers to identify mitigation measures needed to improve migration success.

Survival of reared and wild Atlantic salmon smolts: size matters more in bad years

Abstract We compared the marine survival of Carlin-tagged wild and hatchery-reared Atlantic salmon smolts of the Simojoki river, northern Baltic Sea. All the reared and released smolts were the offspring of native spawners returning to the river. Reared smolts were adipose-fin-clipped and released from the hatchery several weeks before tagging. The wild and reared smolts were simultaneously caught and tagged at a smolt trap located at the Simojoki river mouth. The study was conducted in two years, 1991 and 1993, when post-smolt survival in the Baltic Sea was different. Tags were returned by fishermen and return rates were used to estimate the survival of the smolt groups. We applied generalized linear models with survival as response variable and the year, origin, and smolt size as explanatory variables. On average, wild smolts had a 4.5 times higher survival rate than reared fish of the same smolt size. The difference in observed tag recovery rates as such was only about twofold or less, as the larger size of the reared smolts compared with the wild ones compensated for their lower survival rate. The better survival of wild than reared smolts was more pronounced in the low-survival year (1993 smolt year class) than in the high-survival year (1991 smolt year class).

Stress-reducing methods for releases of Atlantic salmon (Salmo salar) smolts in Norway

One-year-old, hatchery-reared Atlantic salmon ( Salmo salar ) smolts were Carlin-tagged to test the effects of light sedation and salt water addition during transporting as well as releasing methods on migratory behavior and marine survival in northern Norway. A total of 1600 salmon were tagged for the migratory behavior study in the Halselva River. The fish were transported in water containing either 1 g l −1 metomidate or 11 g l −1 balanced sea salt (Instant Ocean), or a combination of the two. The groups were then allowed to recover for 2 days in net pens in the river, after which they were permitted to migrate freely from the net pens located 1 km upstream from the fish trap. These groups were compared to the untreated groups, which were released directly into the river at the same releasing site at the same time. The results showed that brackish water with metomidate as transport medium, and/or recovery after transport led to improved recaptures of smolts compared to untreated controls. In addition, two groups of approximately 3000 Carlin-tagged Atlantic salmon were transported in 1998 by helicopter from the research station to the nearby Alta River (1-h transport time, including loading) and either released directly into the river or kept in a net pen in the river for 7 days before release. The same releases were performed in 1999. Recaptures of adult salmon in the Alta River were 0.43% and 1.43% (1998 releases) and 1.23% and 1.89% (1999 releases) for smolts released either directly or allowed 7 days to recover in the river before release, respectively. These results shows that the use of stress-reducing methods, especially recovery after transport, can improve both the migratory behavior and marine survival of smolt releases.

Recovery from loading and transport stress in Atlantic salmon ( Salmosalar L.) smolts

The main objective of this study was to analyse the recovery from loading and transport stress of hatchery-reared Atlantic salmon smolts. The experiment was carried out during the regular transport of fish from Eidfjord to Bondhus river (4.5 h), from Lundamo to Surna river (2.5 h) and from Eikesdal to Eira river (0.5 h). Blood samples were obtained from the smolts before transport (control), 30 min after loading and 1, 24 and 48 h after transport. Additionally, blood samples were obtained after a 24 h seawater challenge test before and after transport. Blood samples were analysed for haematocrit, plasma cortisol, lactate, glucose, chloride and sodium. Plasma cortisol concentrations increased up to 15 times from resting values, with a peak 1 h after transport. A severe discrepancy in both fresh- and seawater osmoregulatory ability was observed 24 and 48 h after transport. It is suspected that the low recapture rates and survival of the hatchery-reared salmon smolts were, in part, caused by the handling and transport of the smolts prior to the release.

Influence of Transmitter Attachment Procedures on Swimming Performance of Wild and Hatchery-Reared Atlantic Salmon Smolts

Effects of transmitter attachment procedures on smolts of Atlantic salmon Salmo salar have primarily been examined in the laboratory with hatchery-reared fish. Biotelemetry researchers must extrapolate findings of tagging studies to wild, migrating smolts. Because hatchery rearing can affect salmonid development and because field tagging procedures and environmental conditions can differ from those in the laboratory, we measured critical swimming speed of hatchery-reared Atlantic salmon smolts equipped with internal, external, or gastric dummy radio transmitters in the laboratory. We then repeated the experiment with wild, migrating fish in ambient environmental conditions. Swimming ability of hatchery smolts was not significantly affected by any attachment procedure, and no significant differences were found between fish tested 1 or 16 h after tagging. Similarly, wild smolts were not impaired by gastric transmitters; however, performance of wild fish tested 1 or 16 h after internal or external attachment was significantly lower than that of wild controls. Thus, reaction of hatchery-reared Atlantic salmon smolts to attachment procedures can differ from that of wild migrants. Biotelemetry researchers should exercise caution in interpreting data obtained from wild Atlantic salmon smolts equipped with internal or external radio transmitters during the first 16 h after tagging.

Detection of Aeromonas salmonicida, causal agent of furunculosis in salmonid fish, from the tank effluent of hatchery-reared Atlantic salmon smolts

The fish pathogen, Aeromonas salmonicida, could be detected only by bacteriological culture from the kidney of dead or moribund fish in one tank in a hatchery rearing Atlantic salmon (Salmo salar L.) smolts. However, by using a DNA probe specific for this species, allied to a PCR assay, the pathogen could be detected in water, feces and effluent samples taken from this fish tank. Also, the presence of the pathogen was found in effluent samples from two fish tanks containing apparently healthy fish. Subsequently, the presence of pathogen in these tanks was confirmed by an increase in the daily mortality rate and by a plate culture from moribund fish.

Distribution of adult recaptures from hatchery-reared Atlantic salmon (Salmo salar) smolts released in and off-shore of the River Surna, western Norway

The hypothesis that motivation of adult salmon to enter fresh water is dependent on the location of release site was tested. Three groups of smolts were released into a river; into a fjord outside the river and into the open sea. Numbers and percentages of adult salmon caught in the commercial marine fisheries were highest for smolts released from sites outside the river. Adults from smolts released in the river did not migrate farther upstream than adults released from marine sites. Adults homed to the area of smolt release, whether or not the fish were released in sea or fresh water. The results also showed that both recapture rates and straying increased with increasing release distance from fresh water.Consequently, both rates of recapture and straying of ocean ranched Atlantic salmon are strongly dependent on release location.

Comparative aspects of the biology of naturally produced and hatchery-reared Atlantic salmon smolts ( Salmo salar L.)

The age composition of naturally produced (wild) smolts in the Burrishoole river system has given a consistent proportion of > 90% as 2+, with the remainder divided equally between 1+ and 3+. Hatchery-reared smolts produce 40% as 1+ and the remainder as 2+; the proportion of 1+ smolts can be increased by the use of warmed water in early stages of the life history. The size and condition of wild smolts has also proved to be consistent each year but they are considerably smaller than hatchery-reared smolts of comparable age. Grilse derived from both wild and hatchery-reared 1+ smolts are smaller than those from 2+ smolts. The survival rate from ova to wild smolts has averaged 0.61% over the past 10 years, compared with almost 50% for reared smolts. Returns to fresh water at the grilse stage are almost four times better for wild than reared smolts. Up to 81% exploitation of grilse from reared smolts by coastal nets has been calculated from coded-wire tagging results. No significant straying has occurred from either wild or reared smolts but the latter make repeated attempts to rediscover their homing point when upstream of it. Smolt rearing has given rise to abnormalities such as “giant” smolts and vertebral compaction, not reported in wild stocks. From 20 to 50% of the reared parr population are precocious males in their second freshwater winter.

Thinking, feeling and acting sad: Contributions to a theory of affective development

Life-history traits such as growth, survival, sea migration and age at sexual maturity in the sea were compared between Carlin-tagged hatchery-reared Atlantic salmon smolts (Salmo salar) originating from hatchery broodstocks (reared) or naturally ascending spawners captured (sea ranched). The ranched parents included both wild (born in nature) and reared individuals (released as parr or smolts). All smolts were reared in similar hatchery conditions, and they were tagged and released as two-year-olds in the Simojoki River in 1986–2007. The recapture rates did not differ between the progeny groups, although the tagged smolts of the reared parents were larger than those of ranched parents at time of release. The captive salmon with ranched parents were at average heavier during their second winter at sea, but not later. Salmon with ranched parents more frequently migrated to feed in the Main Basin of the Baltic Sea, further from the home river of the salmon stock. The proportions of multi-sea-winter returners were 30% and 69% in salmon with reared and ranched parents, respectively. The different patterns of migration and sea age at maturity of these parental progeny groups suggest that differences in the life history of brood fish may cause distinct, possibly even genetic differences in the progenies.

Survival of Hatchery-reared Atlantic Salmon Smolts Vaccinated with a Vibrio anguillarum Bacterin

Abstract Atlantic salmon (Salmo salar) smolts were vaccinated with a single intraperitoneal injection of a trivalent Vibrio bacterin to evaluate the feasibility of increasing survival. Mortalities of the treated and untreated groups for the 4-5 week post-injection period at the hatchery were negligible. There was no significant difference in the rate of adult return of the vaccinated smolts compared to the controls, indicating that immunization did not enhance marine survival.

Seaward migration of hatchery-reared Atlantic salmon (Salmo salar) smolts in the Penobscot River estuary, Maine: open-water movements

Seaward migration of hatchery-reared Atlantic salmon (Salmo salar) smolts in the Penobscot River estuary, Maine: open-water movements