Individual Atlantic Salmon Research Articles

Temporal shifts in the marine feeding of individual Atlantic salmon inferred from scale isotope ratios.

Given the limited information on prey use during the marine residency period for Atlantic salmon, scales were collected from salmon at return to the River Namsen (Norway) for spawning after 1 year at sea, and scale material from the first and second summer marine feeding periods was analysed using stable isotope methods to understand dynamics of their trophic ecology. As the salmon increased in size from the first to second summer, they reduced their feeding niche and specialised more (narrowed the δ13C range) and increased their dependency on higher tropic level (δ15N) prey, likely fish. Changes in δ13C indicated a consistent pattern of movement towards the north and west between summer feeding periods. Hence, salmon during their first year at sea may have a migration route roughly resembling that of previous spawners, as inferred from earlier tagging studies. Feeding conditions and nutrient composition during the last summer at sea, i.e. in the months before returning to the river for spawning, impacted final body size and within-season timing of return. Fish undergoing the largest trophic niche shift (δ13C and δ15N combined) between summer feeding periods, returned earliest. The earliest returning fish had the fastest specific growth rates at sea. Hence, salmon encountering abundant high-quality fish food during the marine migration, particularly during the last months, may reach a size and energetic state whereby it is better to return early to a safer environment in freshwater than risk being eaten by a big predator at sea. Both trophic status (δ15N), resource use (δ13C) and growth rates were significantly correlated between feeding periods. Nutrient composition during the first summer at sea did not impact the fish body length after the following winter, but growth conditions during the first summer evidenced carry-over effects from the first to the second summer of feeding.

Statistical modelling of voluntary feed intake in individual Atlantic salmon (Salmo salar L.)

Precision feeding aims to provide the correct amount of feed to farmed animals for optimal growth and performance and to avoid feed waste. However, knowledge underlying the meal-to-meal variability in voluntary feed intake of farmed species is still limited. This study examined the relationship between meals, feed deprivation time and the feed (pellets) consumed by Atlantic salmon post smolts. The data was collected from individual fish handfed to satiety without social interaction in three independent short-term (6-12 days) experiments. The fixed variables of our model (feed deprivation time (i.e., time between meals), number of pellets provided, day, previous meal size, and fish growth) explained most of the feed intake (number of pellets ingested) (R2 0.68). Results show that fish ingested more pellets over the course of the trials as they grew, resulting in a positive correlation between feed intake and fish growth (final minus initial fish weight). The time between meals and prior meal size (the number of pellets ingested in the previous meal) significantly affected feed intake in the following meal. Our results suggest that it is possible to optimise meal size by considering the size of the previous meal and the time since it was given.

Iteroparity and its contribution to life-history variation in Atlantic salmon

Evolution of iteroparity is shaped by the trade-off between current and future reproduction. We studied variation in iteroparity among 205 050 individual Atlantic salmon caught in 179 rivers spanning 14° of latitude. The proportion of repeat spawners (iteroparous individuals) averaged 3.8% and ranged from 0% to 26% across rivers. Females were more often repeat spawners than males and had lower cost of reproduction in terms of lost body mass between spawning events. Proportion of repeat spawners for a given sea age at maturity, and the ratio of alternate to consecutive repeat spawners, increased with increasing population mean sea age at maturity. By combining smolt age, sea age at maturity, and age at additional spawning events, we identified 141 unique life-history types, and repeat spawners contributed 75% of that variation. Our results show that repeat spawners are important for life-history variation and suggest that the association between mean sea age and the frequency of repeat spawning is adaptive rather than a pleiotropic side effect arising from selection on sea age.

Atlantic Salmon Upstream Migration Delay in a Large Hydropower Reservoir

AbstractSpawning success of Atlantic Salmon Salmo salar is challenged when migratory routes to natal streams are obstructed by hydropower generation stations and reservoirs that lack directional cues, potentially causing migratory delay. This study used 74 acoustic‐tagged adult Atlantic Salmon during their spawning migrations to quantify migratory success, rates, and delay through the Mactaquac Reservoir in the Saint John River, New Brunswick, during three migration seasons in 2014–2016. Tag loss or mortality was considerable, reducing the effective sample size to 34 successfully tracked adults. Of these, 41% experienced fallback over the dam, 12% were unsuccessful in exiting the reservoir, and 47% were successful in exiting the reservoir on the way to the spawning grounds. Migration rates were significantly slower in the reservoir (median ± SE = 9.3 ± 1.9 km/d) than upriver (39.0 ± 4.1 km/d), and the tagged Atlantic Salmon spent 31–53% of their time reversing direction and thus travelled longer distances (73 ± 58 km) than the minimum 37‐km midline route through the reservoir. Traveling the distance of the reservoir at the upriver migration rate could have shortened their journey by a median of 3.8 d. Sensor tags indicated that individual Atlantic Salmon experienced temperatures of 10–20°C (median ± SE = 16.0 ± 0.03°C) and migrated at depths of 5–35 m (23.4 ± 0.1 m) within the reservoir. Given that some of the energy needed for reproductive development and activities was likely appropriated by migratory delay and that a moderate proportion (47%) of adults emigrated from the reservoir, volitional passage may not be a successful management strategy in the studied reservoir. However, a substantially reduced sample size negates conclusive remarks about which management strategy would maximize survival and spawning success, and further study is needed.

Heart rate and swimming activity as indicators of post-surgical recovery time of Atlantic salmon (Salmo salar)

BackgroundFish telemetry using electronic transmitter or data storage tags has become a common method for studying free-swimming fish both in the wild and in aquaculture. However, fish used in telemetry studies must be handled, anaesthetised and often subjected to surgical procedures to be equipped with tags, processes that will shift the fish from their normal physiological and behavioural states. In many projects, information is needed on when the fish has recovered after handling and tagging so that only the data recorded after the fish has fully recovered are used in analyses. We aimed to establish recovery times of adult Atlantic salmon (Salmo salar) after an intraperitoneal tagging procedure featuring handling, anaesthesia and surgery.ResultsBased on ECG and accelerometer data collected with telemetry from nine individual Atlantic salmon during the first period after tagging, we found that heart rate was initially elevated in all fish and that it took an average of ≈ 4 days and a maximum of 6 days for heart rate to return to an assumed baseline level. One activity tag showed no consistent decline in activity, and two others did not show strong evidence of complete recovery by the end of the experiment: baseline levels of the remaining tags were on average reached after ≈ 3.3 days.ConclusionOur findings showed that the Atlantic salmon used in this study required an average of ≈ 4 days, with a maximum interval of 6 days, of recovery after tagging before tag data could be considered valid. Moreover, the differences between recovery times for heart rate and activity imply that recovery time recommendations should be developed based on a combination of indicators and not just on e.g. behavioural observations.

Individual Recognition of Atlantic Salmon Using Iris Biometry

Traditional tagging methods for fishes can have issues relating to both animal welfare and economic costs. Biometric data such as iris patterns can be captured via digital cameras, which allows for non-invasive tagging and inexpensive and rapid analysis. The purpose of this study was to investigate if the iris of Atlantic Salmon (Salmo salar) is a suitable biometric template for long-term identification of individuals. Atlantic Salmon were individually tagged in the body cavity using PIT tags at the juvenile pre-smolt stage, and the left eye was photographed six times over a 533-day period. Changes in iris stability were assessed both qualitatively and using iris-recognition software. Identification of individual Atlantic Salmon using the iris was not successful over the entire period, as the iris pattern changed significantly with time. Over a shorter time period (four months) with frequent samplings, iris software was able to correctly identify individual fish. The results show that iris identification has potential to replace other methods for Atlantic Salmon over short timeframes.

Validating non-invasive growth measurements on individual Atlantic salmon in sea cages using diode frames

Most Atlantic salmon production takes place in large sea cages where there are limited opportunities to record growth on individual fish. This limits management capabilities for monitoring mean weight and thus total biomass, pellet sizes, feed volumes and the estimation of time until harvest. Furthermore, cause and effect of perturbations and arrested growth cannot be established or interceded on and genetic selection is limited to stocking and harvest measurements. The fusion of passive integrated transponder (PIT) identification and diode frames offers the possibility to noninvasively monitor the growth on thousands of individual fish. However, the accuracy of diode frame measurements for population- and individual level growth and biomass estimation has not been assessed. We stocked over 5000 individually PIT tagged Atlantic salmon post smolts in a net-cage in the sea and monitored growth using a diode frame and PIT tag reader. At the end of the growth period all fish were measured for body length and weight using the intensive gold standard methods of manual recording. At the population level diode frames were highly accurate with a mean difference of 0.002% for length and 4% for weight. Individual level length and weight records were repeatable 0.34 and 0.35, respectively. A single measurement at individual level from diode frames were moderately concordant with the gold standard measures (concordance correlation coefficient (CCC) of 0.52 for length and 0.57 for weight). By exploiting the repeatability and high throughput of diode frame measurements it was possible to increase the number of records per fish to a maximum of 5 resulting in CCC of up to 0.88 for length and 0.81 for weight. Diode frame measurements may hold promise for continuous growth measurements at sea needed in genetic evaluations.

Genetic variation in growth pattern within a population of farmed Atlantic salmon (Salmo salar) during a standard production cycle

Selection for faster growth to market size has a long history in selective breeding programs for aquaculture species. In Atlantic salmon, selection for faster growth has been carried out for more than four decades, i.e. about ten generations. Genetic improvement of growth rate is indicated as favourable correlated to improved feed utilization and, from an economic perspective, it can be argued that improved feed utilization is the main breeding objective: improved fish production per unit of nutrients (protein or energy) provided. However, also reduced production time in challenging grow-out environments, i.e. reduced exposure to sea lice and other disease agents, may be highly valuable from an animal welfare and environmental perspective. Reduced production time in challenging grow-out environments may also be economically favourable by reducing the need for mechanical or chemical treatments of the fish.Body weights at stocking and harvest are commonly recorded in breeding programs for Atlantic salmon. Selection for faster growth rate is usually based on recorded harvest weights, which represent the accumulated growth during the entire production cycle. Alternatively, selection could be based on other growth parameters, i.e. Thermal Growth Coefficient (TGC), which specify growth in specific production periods. Production of Atlantic salmon in open sea cages entails large variation of environmental parameters, i.e. water temperature, water quality, salinity, day light etc., that influences the growth.In this study, growth of individual Atlantic salmon in an open sea cage were investigated (as a part of the project BEHAVEGENES, NFR 2014–2017) to describe potential GxE for growth in different production periods. Test fish were transported to sea in autumn, 8 months after first feeding, and harvested after 16 months at a mean harvest weight of 5.5 kg. Individual body weights were recorded at five different time points allowing estimation of growth during a total of four subsequent production periods. Sexual growth dimorphism was discovered well before onset of sexual maturation; already 4 months after sea transfer, at average population body weight ~500 g, males were measured on average 2.8% heavier than females. Heritability estimates for body weight were all high (0.42–0.72), while heritability estimates expressed by growth calculated as TGC were mediate to high (0.35–0.56). Genetic correlations between growth (TGC) in different production periods varied between −0.20 and 0.82. The correlation between growth during an 8-month summer period (average body weights increase from 500 g to 3.9 kg) and a 4.5-month winter period (average body weight increased from 3.9 kg to 5.8 kg) was not significant (rg 0.16 ± 0.17), suggesting genetic variation for growth pattern and that different genes are involved in different growth periods.

Revealing the full ocean migration of individual Atlantic salmon

BackgroundKnowledge of the complete horizontal migration and vertical movements of Atlantic salmon (Salmo salar L.) during the marine phase is important for understanding the link between individual processes and population recruitment. We studied the detailed migration behaviour of six post-spawned adult Atlantic salmon from a North Norwegian river, during the entire ocean migration, by using high-resolution light-based geolocation archival tags.ResultsThe fish performed long-distance migrations and utilized Arctic areas from the eastern Barents Sea, to areas of Jan Mayen Island, and north to Svalbard. Four of six individuals exhibited a strong fidelity towards Jan Mayen Island, particularly during winter, which may suggest that the ecological conditions in these waters may be important for overwintering. One fish displayed exclusive utilization of the Barents Sea, indicating that growth and survival of some individuals are entirely dependent on these waters. The fish displayed a pelagic behaviour and all individuals spent more than 83% of the time in the uppermost 10 m of the water column. There was a strong seasonal trend in diving intensity, with the number of dives increasing with day length, independent of individuals’ spatial distribution. Diving depth was positively correlated with the depth of the mixed layer.ConclusionsThe Atlantic salmon displayed large individual variation in horizontal migrations, utilizing different ocean areas north of the Arctic Circle. This means that, despite the overall fidelity towards northern latitudes, individuals from the same population may experience highly different ecological conditions, potentially contributing to within-population variation in growth and survival. The seasonal correlation between diving intensity and day length, and spatial correlation between diving depth and depth of the mixed layer, indicate that the overall depth use of the Atlantic salmon depends on both spatially independent seasonal factors, and dynamic oceanographic conditions that facilitate food aggregations. This is the first study of Atlantic salmon to present detailed information about individuals’ continuous horizontal migration and simultaneous vertical movements during the entire ocean migration, thus providing novel information on how habitat use change throughout the ocean residency.

Cloned and outbred Atlantic salmon display equal parasite dispersion when infected with the salmon louse

The salmon louse is an ectoparasitic copepod that infects salmonids in the marine environment. This parasite causes a major economic and biological challenge to the Atlantic salmon farming industry, and efforts to increase the resistance of salmon to this parasite are ongoing. We used a novel approach to investigate the relative importance of genetic and non-genetic factors on individual infection levels with this parasite. A collectively-reared group of 50 cloned (no genetic variation) and 50 outbred (maximum genetic variation) Atlantic salmon were challenged with lice in two replicate tanks. Lice abundance ranged from 19 to 58 and 13 to 50, and fish-weight corrected lice abundance from 16 to 56 and 11 to 50, for the cloned and outbred groups respectively. Thus, the dispersion in infection was identical among individuals within the two experimental groups. We therefore conclude that under the experimental conditions described, the large (>5-fold) variation in salmon lice abundance observed among individual salmon was primarily caused by random processes, and that host genetic background played no detectable role. While this result does not preclude the possibility of genetic variation for susceptibility, it demonstrates that random and/or non-genetic factors play a dominant role in infection dynamics in tank studies. Statement of relevanceThis is the first study to investigate the distribution of parasite burden on cloned (no genetic variation) and outbred (maximal genetic variation) fish. Our results clearly demonstrate that non-genetic factors represent the primary determinants of Lepeophtheirus salmonis infection-levels among individual Atlantic salmon infected in tanks. These novel results are highly relevant to the aquaculture industry where sea lice represent a major challenge to salmon farming, and breeding programs to decrease susceptibility to this parasite are initiated.

High host densities dilute sea lice Lepeophtheirus salmonis loads on individual Atlantic salmon, but do not reduce lice infection success

Host density likely plays a key role in hostparasite interactions, but empirical evi- dence in marine ecosystems remains limited. Classical models predict a positive relationship between host density and parasite infection parameters, but this depends on the parasite transmis- sion mode. Evidence from systems where mobile parasites actively seek hosts suggests that num- bers of parasites per host decrease with increasing host density ('dilution effect'). Copepodids, the infective stage of the salmon louse Lepeophtheirus salmonis, are mobile larvae that display a range of behaviours to detect their salmonid hosts. We hypothesized that high host density would decrease infection intensity, prevalence and degree of aggregation, but not infection success, which reflects parasite performance. We infected multiple groups of Atlantic salmon Salmo salar post-smolts at low (12 fish; 7.9 kg m �3 ) and high (96 fish; 68.5 kg m �3 ) densities, with the same number of L. salmonis copepodids in swimming chambers to enable more realistic swimming behaviours during infection. Infection intensity was 8.4 times higher in the low density treatment, but there were no differences in infection success and degree of aggregation. We observed 100% prevalence in the low density treatment, which was significantly higher than the high density treatment (68%). The dilution effect most likely explained the negative relationship between host density and infection intensity, as the individual risk of being 'attacked' by a parasite decreased as host density increased. Host density is crucial in salmonsea lice infection dynamics, and opportunities may exist within production environments to use the dilution effect of density to improve fish welfare outcomes.

PIT tagged individual Atlantic salmon registrered at static depth positions in a sea cage: Vertical size stratification and implications for fish sampling

Individual dynamics within salmon sea cages are poorly understood. Large inter- and intra-individual variations in swimming depth and higher average body weight deeper in the cage have been observed. Sampling of fish for inspection purposes and estimates of body weight distributions based on automatic measures at a limited depth interval may thus be skewed. The present study investigates how 335 randomly PIT tagged Atlantic salmon of 3.4±0.96kg (bled weight, mean±SD) swam near square PIT antennas (0.6m) fixed at either 5 or 9m depth within a 14m deep cage holding a total of 3750 individuals. The individual variation in registration frequency was large, with 76 individuals never registered and 12 individuals registrered 30 times or more. Larger individuals were much overrepresented at 9m depth, resulting in 8.5% overestimation of average weight at this depth when repeated registrations were not accounted for. No overrepresentation of any size class was found at 5m depth. Half of the individuals registered at 9m depth were also registered at 5m depth, and 82% of the individuals registered at 5m depth were also registered at 9m depth. We interpret the result as fish of all sizes using a wide depth range, but larger individuals spending more time at deeper water than smaller individuals. Therefore, size estimates at a limited depth interval should be avoided.

Hypoxia tolerance and oxygen regulation in Atlantic salmon, Salmo salar from a Tasmanian population

For aquatic ectotherms, increasing water temperatures cause an exponential increase in metabolic rate and decreasing oxygen solubility. Fish species that regulate their metabolic rate to low dissolved oxygen concentrations are understood to be hypoxia tolerant whereas salmonid fish are considered to be classic metabolic conformers and their metabolic rate is dependent on the environmental oxygen concentration. This study examined Atlantic salmon, Salmo salar, undergoing a progressive hypoxia at optimal temperatures and at temperatures nearing the upper thermal tolerance limit for the species to determine if metabolic regulation occurred. Oxygen consumption was measured on individual Atlantic salmon (150.7 ± 40.8 g) in 66-L static respirometers; oxygen measurements were taken every 5 min until the fish lost equilibrium. Metabolic regulation was observed at all temperatures and occurred in 67, 50 and 50% of the fish at 14, 18 and 22 °C, respectively. The plateau metabolic rate (VO 2PL) was 293.4 ± 24.5 mg·kg·h − 1 at 22 °C which was significantly higher than in the 14 and 18 °C treatments (191.1 ± 24.5 and 203.9 ± 12.6 mg·kg·h − 1 , respectively). This difference was also reflected in the critical oxygen threshold (P crit) where the value for the 22 °C treatment (4.59 ± 0.32 mg·L − 1 ) was significantly higher than those of the 14 and 18 °C treatments (3.46 ± 0.14 and 3.39 ± 0.26 mg·L − 1 respectively). These results indicate that some fish from the Tasmanian population of Atlantic salmon have the ability to regulate metabolic rate to low oxygen concentrations and therefore show a relatively high degree of hypoxia tolerance.

Detection and mapping of mtDNA SNPs in Atlantic salmon using high throughput DNA sequencing

BackgroundApproximately half of the mitochondrial genome inherent within 546 individual Atlantic salmon (Salmo salar) derived from across the species' North Atlantic range, was selectively amplified with a novel combination of standard PCR and pyro-sequencing in a single run using 454 Titanium FLX technology (Roche, 454 Life Sciences). A unique combination of barcoded primers and a partitioned sequencing plate was employed to designate each sequence read to its original sample. The sequence reads were aligned according to the S. salar mitochondrial reference sequence (NC_001960.1), with the objective of identifying single nucleotide polymorphisms (SNPs). They were validated if they met with the following three stringent criteria: (i) sequence reads were produced from both DNA strands; (ii) SNPs were confirmed in a minimum of 90% of replicate sequence reads; and (iii) SNPs occurred in more than one individual.ResultsPyrosequencing generated a total of 179,826,884 bp of data, and 10,765 of the total 10,920 S. salar sequences (98.6%) were assigned back to their original samples. The approach taken resulted in a total of 216 SNPs and 2 indels, which were validated and mapped onto the S. salar mitochondrial genome, including 107 SNPs and one indel not previously reported. An average of 27.3 sequence reads with a standard deviation of 11.7 supported each SNP per individual.ConclusionThe study generated a mitochondrial SNP panel from a large sample group across a broad geographical area, reducing the potential for ascertainment bias, which has hampered previous studies. The SNPs identified here validate those identified in previous studies, and also contribute additional potentially informative loci for the future study of phylogeography and evolution in the Atlantic salmon. The overall success experienced with this novel application of HT sequencing of targeted regions suggests that the same approach could be successfully applied for SNP mining in other species.

Outlier SNPs show more genetic structure between two Bay of Fundy metapopulations of Atlantic salmon than do neutral SNPs

Atlantic salmon of Eastern Canada were once of considerable importance to aboriginal, recreational, and commercial fisheries, yet many populations are now in decline, particularly those of the inner Bay of Fundy (iBoF), which were recently listed as endangered. We investigated whether nonneutral SNPs could be used to assign individual Atlantic salmon accurately to either the iBoF or the outer Bay of Fundy (oBoF) metapopulations because this has been difficult with existing neutral markers. We first searched for markers under diversifying selection by genotyping eight captively bred Bay of Fundy (BoF) populations for 320 SNP loci with the Sequenom MassARRAY™ system and then analysed the data set with four different F(ST) outlier detection programs. Three outlier loci were identified by both BayesFST and BayeScan whereas seven outlier loci, including the three previously mentioned, were identified by both Fdist and Arlequin. A subset of 14 nonneutral SNPs was more accurate (85% accuracy) than a subset of 67 neutral SNPs (75% accuracy) at assigning individual salmon back to their metapopulation. We then chose a subset of nine outlier SNP markers and used them to inexpensively genotype archived DNA samples from seven wild BoF populations using Invader™ chemistry. Hierarchical AMOVA of these independent wild samples corroborated our previous findings of significant genetic differentiation between iBoF and oBoF salmon metapopulations. Our research shows that identifying and using outlier loci is an important step towards achieving the goal of consistently and accurately distinguishing iBoF from oBoF Atlantic salmon, which will aid in their conservation.

Consistency of standard metabolic rate in relation to life history strategy of juvenile Atlantic salmon Salmo salar

Temporal consistency of relative standard metabolic rate (rSMR) of individual Atlantic salmon Salmo salar juveniles in three stocks was investigated. The stocks differed in their geographical origin and natal habitat (fresh water, brackish water, seawater). Oxygen consumption measurements of hatchery-reared fish hatched in February 2005 were conducted three times: in early spring 2006, in autumn 2006, and in late spring 2007. The results partly disagreed and partly agreed with earlier studies, in which temporal consistency of SMR in juvenile salmonids have been addressed. In the first period from early spring to autumn, no correlation between rSMR statuses of individuals was found whereas in the second period from autumn to late spring, fish were mainly observed to maintain their relative SMR level indicating a significant repeatability of individual rSMR status over the latter period. Furthermore, a relationship between rSMR status and life history strategy was found: post-smolts and smolts had higher SMR than non-smolts.

A temporal perspective on population structure and gene flow in Atlantic salmon (Salmo salar) in Newfoundland, Canada

Studying population structure and gene flow patterns on temporal scales facilitates an evaluation of the consequences of demographic, physical, and environmental changes on the stability and persistence of populations. Here, we examine temporal genetic variation within and among Atlantic salmon ( Salmo salar ) rivers in Newfoundland and Labrador, Canada, using samples collected over a period of six decades (1951–2004). Our objective was to evaluate temporal changes in population connectivity associated with the closure of a commercial marine fishery. Despite demographic instability, we find that population structure remained temporally stable over more than 50 years. However, age structure can affect results when not taken into consideration, particularly in populations of large effective size where genetic drift is not strong. Where weak signals of genetic differentiation did not complicate analyses, contemporary migration was often asymmetric, yet low, suggesting patterns of intermittent gene flow. Nevertheless, we find some links between changes in population dynamics and contemporary gene flow. These findings may therefore imply that management decisions impacting the contemporary population dynamics of individual Atlantic salmon rivers can also affect the genetic stability of this species as a whole.

Size‐dependent growth of individual Atlantic salmon Salmo salar alevins from hatch to first feeding

Variation in egg size, hatch timing and size at hatch, and their influence on individual growth rates of Atlantic salmon Salmo salar alevins up to first feeding were examined in pure strain and hybrid crosses of fish from Scotland and Canada. At the intra-female, intra-cross type and inter-cross type levels, specific growth rates prior to first feed were strongly size dependent, with smaller and later hatching alevins growing significantly faster. The magnitude of this size-dependent growth was greatest in the hybrid crosses. This resulted in a 40% reduction in the coefficient of variation (c.v.) in alevin size from post-hatch to first feeding at the intra-female level, and a reduction of both intra- and inter-cross differences in alevin sizes in the same period.

Swimming depth and thermal history of individual Atlantic salmon ( Salmo salar L.) in production cages under different ambient temperature conditions

Salmon production cages at sites with a pronounced thermal stratification give individual fish an opportunity to choose their thermal environment. The behavioural responses of individual salmon to such stratification, however, are poorly documented. Information about spatial distributions and temperature experience of individual Atlantic salmon (initial weight 1.5 kg) was gathered over a period of 4 months (mid-August to early-December) using data storage tags. Fish were stocked at normal or high densities in triplicate 2000 m 3 production cages at 5.6–14.5 (ND) or 15.7–32.1 (HD) kg m − 3 , and valid data were collected for 12 ND and 11 HD salmon. There were large inter- and intra-individual variations in swimming depth, with indications that the salmon performed behavioural thermoregulation in an attempt to maintain body temperature within the range of 8–20 °C. Stocking density influenced the average swimming depth and body temperature, indicating competition for preferred thermal space in periods of unfavourably high temperature (towards 20 °C) in large parts of the cage volume. Analysis of temporal behavioural patterns demonstrated a higher variability during day than night and that 60 to 70% of the individuals displayed cyclic diel patterns in either swimming depth or body temperature in at least one out of three sub-periods. The results are discussed in relation to bio-energetic and thermal stress theory and possible consequences for growth variation in salmon cages. Generally, this study suggests that individual swimming depth and body temperature is in part a response to available temperature interacting with stocking density and time of day, while some individual variation cannot be ascribed to the measured variables.

Development of a sensitive and controlled real-time RT-PCR assay for viral haemorrhagic septicaemia virus (VHSV) in marine salmonid aquaculture

A survey was undertaken to determine the potential distribution of viral haemorrhagic septicaemia virus (VHSV) in marine cage-based salmonid farms in Scotland. A rapid, accurate and sensitive quantitative real-time RT-PCR (qRT-PCR) assay was developed, targeting a conserved region of the nucleoprotein (N) gene of the virus. The qRT-PCR assay was shown to be more sensitive than the conventional VHSV RT-PCR. A validation protocol included several different virus isolates as the target and confirmed that the assay could detect all European VHSV genotypes (I, II and III). Both endogenous and exogenous controls were designed to control for integrity of template and distinguish between true VHSV positives and contamination with the positive control material. In total, the universal European VHSV qRT-PCR assay with exogenous positive control was applied to screen 2040 individual Atlantic salmon Salmo salar and 150 individual rainbow trout Oncorhynchus mykiss. No evidence of the presence of VHSV in association with either salmonid species in Scottish marine farms was detected. However, both marine Atlantic salmon and rainbow trout are still considered possible carriers of VHSV, which remains a potential threat to freshwater farming. Therefore, a continued surveillance of these species in marine environment is recommended.