Atlantic Salmon Smolts Research Articles

Hydropower-induced selection of behavioural traits in Atlantic salmon (Salmo salar)

Renewable energy projects such as hydropower facilities contribute towards meeting the world`s growing energy demands and urgent need for mitigating climate change. However, such infrastructure has the potential to substantially alter the environment which, in turn, can induce new challenges related to for instance fish migration conditions. As a consequence, local adaptations related to pre-development migration conditions may be affected for influenced populations. To explore selection regimes operating at a river hydropower plant, we monitored Atlantic salmon smolt individuals during their seaward migration. When passing the hydropower plant, the smolts chose between a surface fish passage or a submerged turbine intake. Smolts were scored for behavioural type (basal locomotor activity, net restrain (a measure of escape responses) and willingness to leave a familiar environment) prior to their migration choice, and we found that smolts with high basal activity had higher probability of using the fish passage than the turbine intake. In addition, migration route choice was a partly consistent trait in that fish that had previously passed a hydroelectric facility by using a fish passage rather than the turbine intake were significantly more likely to use it again when faced with the same choice. Higher mortality among turbine migrants could potentially reduce or eliminate particular behaviour types within populations- and the corresponding population genetic diversity that is essential to cope with future environmental challenges.

Dietary electrolyte balance of Atlantic salmon (Salmo salar) freshwater feeds: Impact on osmoregulation, mineral metabolism and performance in seawater

Dietary electrolyte balance is the equilibrium of monovalent cations and anions that influence the acid-base balance of the feed (dEB = Na + K − Cl, mEq kg−1). Dietary electrolytes/minerals can influence the physiological changes during smoltification in Atlantic salmon. In this context, we aimed to study if the dEB of the freshwater feeds can be used to pre-adapt the hypoosmotic functionality and the associated effects on mineral metabolism. The dEB of commercial freshwater Atlantic salmon feeds in Norway varied from −9 to 400 mEq kg−1 feed. Three experimental feeds were formulated to study incremental levels of dEB reflecting the low (L-dEB, −50 to 0), median (M-dEB, 200–250) and high (H-dEB, 350–400). Triplicate groups of Atlantic salmon parr (36 g) were fed one of the three feeds for 8 weeks in freshwater at 12 °C. The fish were transferred to full strength seawater in indoor tanks and fed a commercial diet for 6 weeks. Growth was not differentially affected by dEB levels, neither in the freshwater phase nor in the seawater. Plasma electrolytes (Na+ and Cl−) and gill mRNA expression of sodium potassium ATPase (NKA a1b, seawater isoform) were significantly lower in L-dEB fed fish. In the intestine, carbonate precipitates 24 h after seawater transfer was higher in fish fed both L-dEB and H-dEB feeds compared to the M-dEB fed fish. Whole body and plasma mineral levels were significantly affected by dEB levels in freshwater feeds. Interestingly, the carryover effect of dEB in freshwater feeds was significant after 6 weeks in seawater for plasma and whole-body Zn status, with the H-dEB fed fish showing significantly increased body Zn status compared to L-dEB and M-dEB fed fish. The study revealed that mineral metabolism and intestinal response to seawater transfer can be pre-adapted by modulating the electrolyte and/or mineral balance in freshwater feeds in Atlantic salmon. Further, dEB did not affect long term development of cataract or vertebral deformities.

Combining acoustic tracking and hydrodynamic modelling to study migratory behaviour of Atlantic salmon (Salmo salar) smolts on entry into high-energy coastal waters

AbstractMigration from fresh water to the marine environment is a crucial, transitional stage in the development of Atlantic salmon (Salmo salar). This study used a combination of acoustic tracking, instrument data, and hydrodynamic modelling to examine behaviour of juvenile salmon (smolts) during their transition from fresh water to the marine environment. The study focuses on a high-energy coastal environment in northern Scotland, which is currently being developed for renewable energy extraction and where there is potential for negative impacts on salmon with energy extraction devices and structures. Thirty-four smolts were captured in the River Wick in Caithness and tagged with acoustic tags transmitting at 69 kHz. The Telemac–Mascaret modelling suite was utilized to construct a three-dimensional model of the study area and surrounding waters to estimate smolt-current interactions during detection times. Timing of migration was linked to low-light conditions, with smolts mainly exiting the river at night and when the moon was below the horizon. The movement of most of the tags conformed with modelled tidal currents and the tracks matched modelled marine tidal patterns. Smolts were detected only on a single tide suggesting that they rapidly cleared the vicinity of the receiver array.

Trying to choose the less bad route: Individual migratory behaviour of Atlantic salmon smolts (Salmo salar L.) approaching a bifurcation between a hydropower station and a navigation canal

Contrary to small- and medium-sized rivers, little attention has been paid to the downstream migration of Atlantic salmon smolts in large-sized rivers and the size-related impact of hydropower stations. From 2014 to 2016, we investigated the downstream migration of n = 72 acoustic-tagged smolts in the Meuse river at a bifurcation zone between a hydropower station equipped with three Kaplan turbines and a navigation canal. A hydrodynamic model that solves the depth-integrated shallow water equations on a Cartesian grid using a finite volume technique was used to infer the influence of water discharge and flow velocity on the smolts' behaviour upstream of the hydroelectric complex. Of the migrating smolts, 41.5% performed back and forth movements before approaching the complex for the first time, sometimes over long distances and at a slow pace, leading to significant delays (3–298 h). Beyond about 250 m3 s−1, the water flow direction changes towards the hydropower station with a gradual acceleration. A median water discharge of 161 m3 s−1 and associated median flow velocity of 0.14 m s−1 tended to favour a more direct and downstream movement towards the hydropower station. On the other hand, the navigation canal was mainly approached at low water discharge (median 132 m3 s−1), due to a higher flow velocity (median 0.11 m s−1) at the entrance. Of the released smolts, only 38.6% passed through the complex, of which 36.4% migrated by the navigation canal and 63.6% by the hydropower station, with a median research time of 04:44. Among all the released individuals, the escapement rate at the end of the study site was 2.9% by the canal and 8.3% by the Meuse river. This site, which offers two non-optimal, unattractive and unsafe migration routes, turns out to be problematic for successful downstream smolt migration.

Biological and environmental influences on the migration phenology of Atlantic salmon Salmo salar smolts in a chalk stream in southern England

Abstract Migration enables animals to access important resources throughout their lifetime but exists in a trade‐off with elevated mortality risk. In spring, juvenile Atlantic salmon (smolts) migrate from their natal rivers for marine feeding grounds, with the timing of their marine entry a potentially important determinant of their long‐term survival. However, there is relatively little known on how the interaction of biological and environmental factors affect smolt migration phenology at the individual level, and how these vary throughout the duration of the smolt seaward migration (run). Using 15‐year tag, recapture, and detection datasets of individual smolts (marked with passive integrated transponder tags) from a chalk stream in southern England, the influences of a range of biological and environmental variables were tested on the run timing of individual smolts, measured as the timing of their arrival in a lower river reach. The probability of smolts arriving earlier in the lower river reach was elevated following winters that were relatively warm, and when there were larger positive daily changes in water temperature and discharge during the run. Early migrants tended to be larger individuals and from sites lower in the catchment, from where the smolts had to migrate relatively shorter distances. Later migrants were more likely to migrate in schools, but with schooling behaviour also more likely to occur during daylight than at night. The relative influence of some of these variables altered throughout the run. Relative changes in daily water temperature were not important during the middle period of the smolt run but were important at the start and end of the run. Relative changes in daily discharge were most influential towards the end of the run, when even relatively small changes in discharge had a strong influence on migration. These results reveal the importance of a wide range of biological and environmental variables on the phenology of smolt migrations, and how their influence can alter throughout the run. With predictions of annually increasing river temperatures, more frequent and intense discharge events, and associated shifts to earlier migration, these results emphasise that such changes in climate are likely to have substantial consequences on the future success of smolt migrations and thereby future numbers of returning adult spawners.

Validating timing of salmon smolt runs obtained by telemetry studies

AbstractTelemetry tags are increasingly used in management to monitor the migration timing of Atlantic salmon smolts (Salmo salar L.). It has been postulated that (1) effects from tagging and handling may alter migration behaviour, and (2) that the selection of fish during sampling is not representative of all migrating smolts, yielding bias in migration timing estimates. In the River Dale (Vestland, Norway), five groups of wild Atlantic salmon smolts (N = 385) were tagged in April–May and recaptured in a wolf trap. Migration timing was then compared to the untagged population. Migration timing differed between tagged (12 mm PIT) and untagged fish for 4/5 groups. Only fish tagged at the first time point did not have significantly different timing of migration from untagged counterparts. The relationship between length and the timing of migration was different for initial length and length at recapture; initial length suggested earlier migration of longer fish, but the extra time spent in the river prior to migrating for smolts that were initially smaller compensated for the size difference. The tagging protocol is crucial to obtaining representative migration timing results. Smolts should be tagged as early as possible and include the entire size distribution.

Surface water with more natural temperatures promotes physiological and endocrine changes in landlocked Atlantic salmon smolts

Hatchery salmonid smolts are often reared using groundwater with elevated temperatures to maximize growth. Previous work has shown that rearing hatchery smolts in surface water with a more natural thermal regime resulted in increased return rates of adult landlocked Atlantic salmon (Salmo salar). We evaluated whether landlocked Atlantic salmon reared in surface water with a natural temperature regime have altered physiological smolt characteristics compared with fish reared in groundwater with elevated winter temperatures. Hatchery fish were sampled three consecutive years from January to May. Additional fish were released as smolts, recaptured, and compared with fry-stocked smolts. Surface water smolts had earlier peaks of plasma T4, lower T3 levels, later peak cortisol, and lower gill Na+/K+-ATPase activity as compared with groundwater smolts. After release and recapture, surface water fish had elevated plasma T4 and gill Na+/K+-ATPase activity compared with groundwater fish, but less than stream-reared fish. Elevated plasma T4 in surface water fish in the hatchery and after release may have promoted imprinting and other aspects of smolt development, contributing to the higher adult return rates of a cohort reared in surface water.

Predicting the nationwide outmigration timing of Atlantic salmon (Salmo salar) smolts along 12 degrees of latitude in Norway

AbstractAimAccurate predictions about transition timing of salmon smolts between freshwater and marine environments are key to effective management. We aimed to use available data on Atlantic salmon smolt migration to predict the emigration timing in rivers throughout Norway.LocationIn this study, we used outmigration timing data of Atlantic salmon (Salmo salar) smolts from 41 rivers collected from 1984 to 2018 to make a predictive model for the timing of out‐migrating salmon smolts along 12 degrees latitude.MethodsSmolt migration data were collated from rivers where smolts are monitored with traps, video and tagging and matched to river‐specific metadata. Using a mixed‐effects generalized additive model, we tested for the effects of spatial location, air temperature, river flow and counting methods on the timing of 25% smolt emigration from rivers.ResultsAfter accounting for spatial effects and repeated measures (across years and among rivers), air temperature and counting method were significant drivers of the estimated timing of smolt emigration. In‐sample predictions yielded strong correlation with observed values, as did 10‐fold cross‐validation. Out‐of‐sample predictions suggested that the previous national estimates underestimated the migration timing in southern populations (linear model intercept = 39.73 days).ConclusionModel‐derived estimates of run timing can be used to more accurately predict the timing of outmigration in order to better calibrate environmental flows and regulate management of marine industries such as aquaculture that may affect migration success at this particularly sensitive life stage.

Validation of a Swimming Direction Model for the Downstream Migration of Atlantic Salmon Smolts

Fish swimming performance is strongly influenced by flow hydrodynamics, but little is known about the relation between fine-scale fish movements and hydrodynamics based on in-situ investigations. In the presented study, we validated the etho-hydraulic fish swimming direction model presented in the River Mandal from Southern Norway, using similar behavioral and hydraulic data on salmon smolts from the River Orkla in Central Norway. The re-parametrized model explained the variation of the swimming direction of fish in the Orkla system in same degree as the original model performed in the Mandal system (R2: 84% in both cases). The transferability of the model when using it from one river to predict swimming direction in the other river was lower (R2: 21% and 26%), but nevertheless relatively high given that the two localities differed in hydraulic conditions. The analyses thus provide support for the fact that the identified hydraulic parameters and their interaction affected smolt behavior in a similar way at the two sites, but that local parametrization of the base model is required. The developed etho-hydraulic models can provide important insights into fish behavior and fish migration trajectories and can be developed into prediction models important for the future development of behavioral downstream migration solutions.

Variations in migration behaviour and mortality of Atlantic salmon smolts in four different hydroelectric facilities

AbstractMigration behaviour, route selection and mortality of Atlantic salmon, Salmo salar L., smolts were studied at four different hydroelectric facilities in the River Mustionjoki, Finland, in May 2017. Radio‐tagged smolts were released upstream of the power stations and tracked by stationary antenna‐receiver systems and hand‐held receivers. Tracking revealed a general tendency of smolts to move downstream with the main flow, but also variable behaviour and mortality related to diverse conditions characteristic of each power station. Average migration delay at the power stations ranged between 13.8 and 101.1 h (median: 1.7–61.5 h). Estimated mortality ranges were 0%–50% in the forebays, 4%–64% in the power stations and 2–30%/km during river migration after passage of the dam. This study provided essential information on behaviour and mortality in relation to local conditions at each power station required for successful application of fish bypass systems in a salmon restoration project.

Movement, Survival, and Delays of Atlantic Salmon Smolts in the Piscataquis River, Maine, USA

AbstractMovement, delays, and survival of hatchery Atlantic Salmon Salmo salar smolts were evaluated through the Piscataquis River, a tributary of the Penobscot River in Maine, USA. We explored the effects of the river’s four dams (Guilford, Dover, Browns Mill, and Howland dams) from 2005 to 2019. During this period, the downstream‐most dam (Howland Dam) transitioned from full hydropower generation to seasonal turbine shutdowns and later was decommissioned with the construction of a nature‐like fish bypass in 2016. We estimated survival through open‐river reaches and at each dam using acoustic telemetry (n = 1,611). Dams decreased survival, with per‐river‐kilometer (rkm) apparent survival averages of 0.972, 0.951, and 0.990 for Guilford, Dover, and Browns Mill dams compared to a per‐rkm survival of 0.999 for open‐river reaches. Turbine shutdowns increased survival at Howland Dam (to around 0.95), which was further increased by the nature‐like fish bypass (0.99). We used radiotelemetry in 2019 (n = 75) and demonstrated that approximately one‐third of the fish used the bypass, while the remaining fish used alternative routes. Smolts successfully passing the three upstream dams had lower apparent survival through Howland Dam than smolts that were released upstream of Howland Dam. Although smolts passing Browns Mill Dam had high survival, the dam caused extended delays, with median delay times surpassing 48 h in most years. Most of the delays caused by Browns Mill Dam occurred after fish had passed the dam and may indicate a sublethal effect of passage. Overall, while survival through Howland Dam has improved, passage and delays at the three upstream dams in aggregate represent a critical impediment to the effective use of the high‐quality spawning habitat found upstream.

Comparative proteomic profiling of newly acquired, virulent and attenuated Neoparamoeba perurans proteins associated with amoebic gill disease

The causative agent of amoebic gill disease, Neoparamoeba perurans is reported to lose virulence during prolonged in vitro maintenance. In this study, the impact of prolonged culture on N. perurans virulence and its proteome was investigated. Two isolates, attenuated and virulent, had their virulence assessed in an experimental trial using Atlantic salmon smolts and their bacterial community composition was evaluated by 16S rRNA Illumina MiSeq sequencing. Soluble proteins were isolated from three isolates: a newly acquired, virulent and attenuated N. perurans culture. Proteins were analysed using two-dimensional electrophoresis coupled with liquid chromatography tandem mass spectrometry (LC–MS/MS). The challenge trial using naïve smolts confirmed a loss in virulence in the attenuated N. perurans culture. A greater diversity of bacterial communities was found in the microbiome of the virulent isolate in contrast to a reduction in microbial community richness in the attenuated microbiome. A collated proteome database of N. perurans, Amoebozoa and four bacterial genera resulted in 24 proteins differentially expressed between the three cultures. The present LC–MS/MS results indicate protein synthesis, oxidative stress and immunomodulation are upregulated in a newly acquired N. perurans culture and future studies may exploit these protein identifications for therapeutic purposes in infected farmed fish.

Synchrony and multimodality in the timing of Atlantic salmon smolt migration in two Norwegian fjords

The timing of the smolt migration of Atlantic salmon (Salmo salar) is a phenological trait increasingly important to the fitness of this species. Understanding when and how smolts migrate to the sea is crucial to understanding how salmon populations will be affected by both climate change and the elevated salmon lice concentrations produced by salmon farms. Here, acoustic telemetry was used to monitor the fjord migration of wild post-smolts from four rivers across two fjord systems in western Norway. Smolts began their migration throughout the month of May in all populations. Within-population, the timing of migration was multimodal with peaks in migration determined by the timing of spring floods. As a result, migrations were synchronized across populations with similar hydrology. There was little indication that the timing of migration had an impact on survival from the river mouth to the outer fjord. However, populations with longer fjord migrations experienced lower survival rates and had higher variance in fjord residency times. Explicit consideration of the multimodality inherent to the timing of smolt migration in these populations may help predict when smolts are in the fjord, as these modes seem predictable from available environmental data.

Frequency of vateritic otoliths and potential consequences for marine survival in hatchery-reared Atlantic salmon.

Otoliths are inner-ear structures of all teleost fish with functional importance for hearing and balance. The otoliths usually consist of aragonite, a polymorph of calcium carbonate, but may also take the form partly or entirely of vaterite, a different polymorph of calcium carbonate. Vateritic otoliths occur sporadically in wild fish, but with a higher frequency in hatchery-reared fish. Abnormal otoliths have direct consequences for the inner-ear functions of fish and may be a symptom of environmental stress. In this study, the authors assess the differences in the frequency of abnormal otoliths and degree of abnormality (% vaterite) for different groups of hatchery-reared Atlantic salmon (Salmo salar) smolt and adults. The groups differed in parental broodstock origin (number of generations in hatchery) and treatment temperature. Smolt from the same groups were also released to complete their ocean migration. The otoliths of the returning and recaptured adults were subsequently extracted to assess the difference in frequency and degree of abnormality between the adults and the smolt from corresponding groups. Return rate varied among groups (0.2%-2.6%). The frequency of vateritic otoliths was high (11.4%-64.4%) and differed among smolt groups. The lowest return rates corresponded with the highest frequency of abnormal otoliths for the groups, suggesting that abnormal otoliths may have negative consequences for marine survival. Furthermore, indications of an effect of fast growth on the formation of abnormal otoliths were found for only one of the experimental groups, and for none of the groups after correcting for Type 1 error. This contradicts previous reports, suggesting rapid growth as the main cause of abnormal otoliths. Adult return rates were generally low, but abnormal otoliths were common, with high coverage (% vaterite).

The downstream migration success of Atlantic salmon (Salmo salar) smolts through natural and impounded standing waters

Atlantic salmon (Salmo salar) smolt migration through standing waters is generally known to be associated with slow migration speed and low success. Most previous studies have however been conducted on impounded waters. The bathymetry and specifically the flow dynamics around the outlets differ markedly between impounded lakes and naturally draining lakes. To date no study has attempted to disentangle the effects of the impoundment from that of standing water itself. This question was tested in a single naturally draining standing water and two impounded standing waters within a single catchment using fish tagged with acoustic and PIT tags (in three and two lakes, respectively). Overall migration success (river to estuary) was very low (10%); migration success through the lakes was also very low (total loss rates range: 31 to 55%; 16 to 53%.km−1) compared with migration in the river downstream of the lakes (range: 3.9 to 10.8%.km−1). Migration success was no lower in impounded lakes compared with the naturally draining lake. 49% of directional movements were in a direction opposite to the migration pathway, indicating that a lack of navigational cues may be in-part responsible for low migration success. We tentatively conclude that successful migration through standing waters without strong directional cues may be more stochastic than previously thought. We suggest that at least for some impoundments (as shown here), the impoundment per se may not reduce migration success, its greater effect being the creation of the standing water through which smolts are required to migrate. However, any impoundment effect is likely to be dependent upon a combination of lake basin shape, the exit route from the lake and the navigational cues available to smolts.

Quantifying the effects of post-surgery recovery time on the migration dynamics and survival rates in the wild of acoustically tagged Atlantic Salmon Salmo salar smolts

BackgroundThe experimental effects of surgically implanting fish with acoustic transmitters are likely to have negative effects on survival and behaviour. Measuring the extent of these negative effects is important if we wish to extrapolate inferences from tagged animals to un-manipulated animals. In this study, we examine the effect of surgery and post-tagging recovery time on the survival and migration rate of acoustically tagged wild Atlantic Salmon (Salmo salar) smolts through freshwater, estuarine and ocean phases of migration. Four treatment groups were used: pre-smolt captured in the fall that overwintered in a hatchery and were tagged either 75 days prior to release (winter hatchery) or within 24 h prior to release (spring hatchery) and smolt captured during the spring smolt run, tagged 24 h prior to release and released during the day (day-released) or night (night-released).ResultsThe spring hatchery treatment group served as a reference treatment group such that recovery time (comparison to winter hatchery treatment) and hatchery effects (comparison to day-released and night-released treatments) could both be discerned. The hatchery effect increased migration rate, whereas short recovery times and captivity in a hatchery negatively affected survival. These effects were most pronounced within the first 5 days and/or 48 km downstream post-release, however, the residual recovery time effects appeared to persist during the transition from the estuary into salt water.ConclusionsEven with smolts originating from the wild and spending relatively little time within the hatchery environment, post-release survival was still negatively affected. Migration speed was faster for hatchery smolts, but is likely only due to their larger size. Recovery time effects were most prominent during the initial migration period in freshwater and again in the transition from the estuary to saltwater which may be due to added stress during these transitional zones. As surgery-related bias will likely never be completely removed from telemetry studies, it is important to quantify and account for these effects in situ when making inferences on the un-manipulated component of the population.

Bubble barriers to guide downstream migrating Atlantic salmon (Salmo salar): An evaluation using acoustic telemetry

Structures for guiding fish around migration barriers are frequently used for maintaining connectivity in regulated riverine systems. However, for non-physical barriers, experimental studies providing direct and detailed observations of fish–barrier interactions in rivers are largely lacking. In this study, we quantify the efficiency of bubble barriers (alone or in combination with light stimuli, and in both daylight and darkness) for diverting downstream migrating Atlantic salmon (Salmo salar). Both a laboratory-based migration experiment and a large-scale field experiment in a regulated river were used to evaluate efficiency of bubble barriers. In the latter, we used acoustic telemetry to provide in situ measurements of how downstream migrating Atlantic salmon smolts interact with bubble barriers. We show that bubbles divert smolts with high efficiency in both a laboratory flume (95%) and in natural settings (90%). This latter efficiency is higher compared to an already present physical barrier (46%) covering the upper two meters of the water column in the large river. The bubble barrier did not affect flume migration in darkness, suggesting that visual cues are crucial for the observed repelling effect of bubbles. We conclude that bubble barriers can be effective, largely maintenance free and low-cost alternatives to physical structures currently used to divert salmon away from high-mortality passages.

Improving bypass performance and passage success of Atlantic salmon smolts at an old fish-hostile hydroelectric power station: a challenging task

Old hydroelectric power stations often provide unsafe migration routes to fish, apart from passing through mobile gates during high flow. The installation of retrofitted bypass is considered to be a potential solution to improve fish passage at such old structures, but their performances are often insufficient. In the Amblève River (Belgium), a mobile 3.3 m high dam feeds two principal Francis turbines (12 + 14 m3 s−1) and is equipped with three mobile gates, a modern vertical slot upstream fish pass and a retrofitted downstream bypass functioning at a very low flow (1% of maximum turbined flow). A bypassed river section (length 8.4 km) downstream of the dam was set at 3 m3 s−1 and supplies a Francis microturbine. From 2015 to 2016, we used 1150 hatchery Atlantic salmon smolts (Salmo salar), and we placed antennae with automatic radio-frequency identification (RFID) stations to analyse the migration routes used (n = 5). We tested the attraction efficiency and the performance of the bypass with and without the placement of a guidance system at the entrance and examined the proportion of smolt passage at the different migration routes under three functioning configurations. The placement of the guidance system markedly improved the attraction efficiency and the overall passage efficiency. The median time spent at the entrance of the bypass was from 7 to 12 min, and the time spent near the entrance of the bypass was less than 1 h for 67.1% (release 1), 88.2% (release 2) and 63.7% (release 3) of the smolts. During the three release events, the smolts arrived near the entrance of the bypass mostly during the dusk and dark periods (87.5%, 96.0% and 95.5%, for releases 1, 2 and 3, respectively). In a configuration without opening a mobile gate, the bypass was the most used migration route, followed by the microturbine and the main turbine. Stopping the microturbine and opening a mobile gate has consequences of making it the first choice of passage followed by the main turbine and the bypass. The re-establishment of safe and quick downstream migration has to be considered with a holistic view of the functioning of the hydroelectric powerstation.

The biological basis of smoltification in Atlantic salmon

Chile is the second-largest producer of Atlantic salmon in the world, and the Chilean salmon production accounts for 27% of the world’s production. One important step of the productive cycle in freshwater is the smoltification process that prepares the fish for the marine life stage. This review describes the biological basis of smoltification in Atlantic salmon, with particular attention on branchial osmoregulatory adaptations. We also discuss some of the infectious diseases and problems in smoltification (two of the main causes of losses in Chilean aquaculture) that could be related from a physiological point of view.

Dynamics of BK channel expression in gills during smoltification of Atlantic Salmon under farm conditions

Smoltification is a crucial step in the aquaculture of Atlantic salmon. Currently, the Na+/K+ ATPase (NKA) test is used to determine whether fish have adapted to seawater when moved from freshwater. The mortality rate of unadapted fish in Chile is ~10% after smoltification. During this period, BK channel expression decreases in gills after two weeks in saline water under laboratory conditions. However, these findings have not yet been tested on an industrial scale. We characterized the changes in mRNA levels of BK channels in gills from Salmo salar during smoltification using salmon farming protocols and compare these results against gill NKA activity and found a significant decrease in expression of BK channel transcripts during the smolt stage. We also characterized the dynamic of the BK channel expression (mRNA) versus NKA activity by using transcriptomic and RT-PCR analyses under an industrial setting. BK channels are involved in a variety of physiological processes, and our results indicate that BK potassium channel expression in gills could form the basis of a new complementary test to determine the level of smoltification more precisely.