Search

Norway is the world’s largest producer of farmed Atlantic salmon and is home to ∼400 rivers containing wild salmon populations. Farmed escapees, a reoccurring challenge of all cage-based marine aquaculture, pose a threat to the genetic integrity, productivity, and evolutionary trajectories of wild populations. Escapees have been monitored in Norwegian rivers since 1989, and, a second-generation programme was established in 2014. The new programme includes data from summer angling, autumn angling, broodstock sampling, and snorkelling surveys in >200 rivers, and >25 000 scale samples are analysed annually. In 2014–2017, escapees were observed in two-thirds of rivers surveyed each year, and between 15 and 30 of the rivers had >10% recorded escapees annually. In the period 1989–2017, a reduction in the proportion of escapees in rivers was observed, despite a >6-fold increase in aquaculture production. This reflected improved escape prevention, and possibly changes in production methods that influence post-escape behaviour. On average, populations estimated to experience the greatest genetic introgression from farmed salmon up to 2014 also had the largest proportions of escapees in 2014–2017. Thus, populations already most affected are those at greatest risk of further impacts. These data feed into the annual risk-assessment of Norwegian aquaculture and form the basis for directing mitigation efforts.

Atlantic salmon in aquaculture act as reservoir hosts and vectors of parasites like salmon lice and this parasite is shown to harm wild salmonid populations. In this study, n = 29,817 tagged Atlantic salmon were studied in four release trials. Half of the released fish were given prophylactic treatment against lice, the other half represented sham control fish. We used a nested design comparing years with low and high lice density and seasonal dynamics in infestation pressure. The released Atlantic salmon thus experienced highly variable lice infestation pressures, which we linked to survival and growth in returning fish. The fish were released in a protected ‘National Salmon Fjord’ and n = 559 Atlantic salmon were recaptured after spending 1–4 years at sea. In most experimental groups 1%–2.5% of the fish were recaptured at return. However, survival of unprotected fish was extremely low for the trial released at the highest density of lice: only 0.03% of these Atlantic salmon returned to the river, compared to 1.86% in the protected group. 1. Atlantic salmon in aquaculture act as reservoir hosts and vectors of parasites like salmon lice and this parasite is shown to harm wild salmonid populations. 2. In this study, n = 29,817 tagged Atlantic salmon were studied in four release trials. Half of the released fish were given prophylactic treatment against lice, the other half represented sham control fish. We used a nested design comparing years with low and high lice density and seasonal dynamics in infestation pressure. The released Atlantic salmon thus experienced highly variable lice infestation pressures, which we linked to survival and growth in returning fish. The fish were released in a protected ‘National Salmon Fjord’ and n = 559 Atlantic salmon were recaptured after spending 1–4 years at sea. 3. In most experimental groups 1%–2.5% of the fish were recaptured at return. However, survival of unprotected fish was extremely low for the trial released at the highest density of lice: only 0.03% of these Atlantic salmon returned to the river, compared to 1.86% in the protected group. 4. Synthesis and applications. We document that high lice density can cause more than 50 times higher mortality risk in Atlantic salmon on their sea migration, even in a fjord with protected status. Fine‐tuned and hard‐to‐predict year‐to‐year differences in timing, both for the wild smolt migration and the population build‐up of lice released from aquaculture, means life or death to wild salmon. Management actions such as spatial segregation of farmed fish and lice (e.g. closed farm pens), and/or moving farms away from vulnerable habitats for wild salmonids (fjords and coastal areas), may be needed to ensure sustainable coexistence of wild and farmed Atlantic salmon. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2020 The Authors.

Background Mature male parr (MMP) represent an important alternative life-history strategy in Atlantic salmon populations. Previous studies indicate that the maturation size threshold for male parr varies among wild populations and is influenced by individual growth, environmental conditions, and genetics. More than ten generations of breeding have resulted in domesticated salmon displaying many genetic differences to wild salmon, including greatly increased growth rates. This may have resulted in domesticated fish with the potential to outgrow the size threshold for early maturation, or evolution of the size threshold of the trait itself. To investigate this, we performed a common-garden experiment under farming conditions using 4680 salmon from 39 families representing four wild, two wild-domesticated hybrid, and two domesticated strains. Results Domesticated salmon outgrew wild salmon 2–5-fold, and hybrids displayed intermediate growth. Overall, the numbers of MMP varied greatly among families and strains: averaging 4–12% in domesticated, 18–25% in hybrid, and 43–74% in the wild populations. However, when the influence of growth was accounted for, by dividing fish into lower and upper size modes, no difference in the incidence of MMP was detected among domesticated and wild strains in either size mode. In the lower size mode, hybrids displayed significantly lower incidences of mature males than their wild parental strains. No consistent differences in the body size of MMP, connected to domestication, was detected. Conclusions Our data demonstrate: 1- no evidence for the evolution of the size threshold for MMP in domesticated salmon, 2- the vastly lower incidence of MMP in domesticated strains under aquaculture conditions is primarily due to their genetically increased growth rate causing them to outgrow the size threshold for early maturation, 3- the incidence of MMP is likely to overlap among domesticated and wild salmon in the natural habitat where they typically display overlapping growth, although hybrid offspring may display lower incidences of mature male parr. These results have implications for wild salmon populations that are exposed to introgression from domesticated escapees. Electronic supplementary material The online version of this article (10.1186/s12862-018-1294-y) contains supplementary material, which is available to authorized users.

Aquaculture escapees represent a threat to the genetic integrity of native populations, may spread infectious agents and display ecological interactions with wild fish. DNA-based identification methods are well established for tracing Atlantic salmon escapees back to their farms of origin. However, traditional genetic assignment approaches are not always able to single out the farm of origin in cases where several potential farm sources rear fish from the same genetic line, and display strongly overlapping allele frequencies. We investigated whether an alternative statistical approach, which involves ad hoc identification of sibling relationships, circumvents the challenge of overlapping allele frequencies. We analysed the following samples collected in 2013: (1) 221 farmed escapees captured in several rivers in the Ryfylke region of Norway, (2) 139 farmed escapees captured some 150 km away in an upstream fish migration trap in the River Etne, and (3) 779 farmed salmon sampled from 17 cages on 10 farms in Ryfylke. Siblingship tests increased the precision of identification of escapees back to their farm of origin over genetic assignment and population statistic approaches. Together with other non-genetic data, siblingship tests were also able to connect 2 seemingly independent escape events, demonstrating that some of the salmon escaping from 1 or 2 farms in Ryfylke took approximately 1 mo to migrate 150 km northwards before entering the River Etne. Finally, we demonstrated that the genetic background of the escapees captured in the River Etne during the course of an entire season was represented by the 3 major breeding programs in Norway.

Understanding Atlantic salmon Salmo salar post-smolt coastal migration behaviour is crucial for predicting their exposure to ecological challenges such as the parasite salmon louse Lepeophtheirus salmonis. We compared the migration of acoustically tagged, hatchery-reared Atlantic salmon post-smolts of wild and domesticated origins from the inner, middle and outer part of a 172 km long aquaculture-intensive fjord in western Norway. Additionally, we examined if the timing of the release or treatment with an anti-parasitic drug (prophylaxis) altered migratory behaviour. We found no significant differences in mean progression rates among the 3 release locations, among genetic groups or between treatments (range: 11.5−16.9 km d−1). However, individual variation in progression rates and migratory routes resulted in large differences in fjord residence times (range: 2−39 d). Ocean-current directions during and after release affected swimming speed, progression rate and route choice, and for most post-smolts, swimming speeds were much higher than their progression rates out of the fjord. The predicted lice loads based on lice intensity growth rates on smolts held in sentinel cages throughout the fjord indicated that individuals taking >10 d to exit the fjord in periods with high infestation pressure are likely to get lethally high sea-lice infestations. We conclude that, as migratory routes of S. salar post-smolts are hard to predict and migration times can stretch up to over a month, it is important to develop aquaculture management that keeps salmon lice levels down along all potential migration routes and during the full potential migratory period. Migratory behaviour · Salmo salar · Lepeophtheirus salmonis · Acoustic telemetry · Management · Fish farming · Parasite Migratory behaviour

Salmon lice Lepeophtheirus salmonis Krøyer may affect survival and growth of anadromous salmonids through physiological stress and/or behavioural changes. Using acoustic telemetry tracking, we investigated the behaviour of 30 infected sea trout Salmo trutta throughout the summer in a fjord with very high salmon lice infection pressure. Most of the tracked sea trout adopted a movement pattern expected to suppress salmon lice infestation, as they showed a strong preference for fresh or brackish water, spending most of the time close to a river outlet or even migrating into the river. Highly infested sea trout preferred shallower depths, associated with lower salinity. The fish lost to predation stayed further away from the river outlet than non-predated fish, and were likely subjected to a stronger infection pressure. Half of the tracked group were treated with a salmon lice prophylaxis, emamectin benzoate. The effect of treatment on infestation was monitored in a separate group held in a sea cage and found to be moderate; the mortality in this group was associated with infestation by motile lice stages. In contrast, treatment was not found to have an effect on tracked fish behaviour. It is likely that some physiological and behavioural responses to high salmon lice infection pressure may be present even after a prophylaxis treatment, in particular when the treatment is given after exposure to salmon lice infection. We conclude that increased salmon lice infection pressure associated with altered salmon farming practice may have the potential to influence the marine behaviour and growth of sea trout. The Norwegian Research Council (project no. 221404), the Institute of Marine Research, the Norwegian Institute for Nature Research (Strategic Institute Programme, project no. 160022/F40), and the Norwegian Food Safety Authority funded this project.

Institute of Marine Research, PostBox 1870 Nordnes, NO-5817 Bergen, Norway*Corresponding author: tel: +47 55236898; fax: +47 55238531; e-mail: haakon.otteraa@imr.noOttera˚,H.,andSkilbrei,O.T.Possibleinfluenceofsalmonfarmingonlong-termresidentbehaviourofwildsaithe(PollachiusvirensL.).– ICES Journal of Marine Science, doi: 10.1093/icesjms/fsu096.Received11 November 2013; revised 30 April 2014; accepted 4 May2014.ThecultureofAtlanticsalmonisoneofthemostdevelopedaquacultureindustriesintheworld.Theproductionfromsmolttomarketsizeusuallytakesplaceinseacagesinopenwaters,andthesestructurestendtoattractwildfish,astheydoforotherfarmedspecies.Forsalmonfarming,saithe(Pollachiusvirens)isoneofthemost-frequentlyobservedspeciesaroundseacages.Animportantquestioniswhetherthelargeconcentrationofsalmonfarmsinsomeareasmightalterthenaturalbehaviourandmigrationpatternofwildsaithe.Wetagged62wildsaithewithacoustictagsandfollowed their movements for up to 2 years in an area in Southwestern Norway with many salmon farms. Furthermore, nearly 2000 saithe weretagged with external T-bar tags to study migration beyond the study area. The recaptures of the T-bar tagged saithe from offshore areassuggest that the offshore migration routes of saithe are similar to published results from before salmon farming became significant in the area.However, a large proportion of the saithe population appears to remain in the release area and was observed at the salmon farms for much ofthetime.Weconcludethattheaquacultureindustryisinfluencingthelocalsaithedistribution. Large-scalepopulationeffectsaremoredifficultto prove, but it is possible thatthe dynamic relationship between the coastal and oceanic phases has been altered.Keywords: aquaculture, interaction, migration, saithe.

Escapes of cultured Atlantic salmon ( Salmo salar L.) smolts from net pens in the sea and a growing abundance of salmon lice larvae in the vicinity of fish farms have potential environmental impacts. One strategy to reduce these risks could be to shorten the period the fish are kept in open net pens in the sea by producing fish up to 1 kg on land-based farms. However, if fish of this size escape after transfer to net pens in the spring, how will they behave? Salmon smolts migrate to the open sea at this time of year, but fish previously kept under a constant photoperiod indoor may behave differently. In a simulated escape event, we compared the post-release behaviour of 0.5–0.8 kg salmon, which had been tagged with acoustic transmitters and held outdoors under a natural photoperiod in net pens (n = 20, NP), with fish kept in indoor tanks under continuous light (n = 20, CP) during the previous winter. The NP fish migrated rapidly out of a 22 km-long fjord after release in late May [0.46 body length per second (bl s − 1 )], while the CP fish moved more slowly (0.17 bl s − 1 ) and also displayed a wider range of behaviour, with four individuals remained in the vicinity of fish farms in the fjord for at least three weeks. The production regime clearly influenced the dispersal rates of the escaped Atlantic salmon and will therefore have important effects on the type of interactions that take place between escaped farmed salmon and the environment.

The current paper compares the use of adipose fin and muscle biopsies as non-lethal sampling procedures, and the suitability of triacylglycerol (TAG) versus total lipid (TOT) fatty acid composition (fatty acid methyl esters, FAME) to estimate dietary history in farmed and wild matur- ing Atlantic salmon Salmo salar. TAG FAME gives best resemblance to dietary history. Fatty acid composition differs between tissues, and sample series should preferably be from 1 tissue only. TOT FAME supplies informative data on dietary history in fatty tissues, but differs from TAG FAME when total lipid levels are low. The reason is a larger contribution of phospholipid FAME. In wild maturing salmon, lipid content in adipose fin and muscle is low. TAG provides good data on dietary history, while TOT is less informative. Escapees are well identified analysing 18:2n-6 and 18:3n-3 fatty acids, which are high in commercial feed. For estimating a wider range of dietary history after escape (switch to wild prey), or feed preferences in wild fish, other fatty acids have to be taken into account. This requires the use of multivariate methods, like principal com- ponent analysis. Adipose fin and muscle biopsies are good alternatives for non-lethal sampling using the TAG method. The minimum amounts of samples to be used are proposed to be 0.5 to 1 g of adipose fin and 60 to 100 mg of muscle. The method of storage (liquid nitrogen/dry ice) does not affect fatty acid compositions. Other tissues can also be used for fatty acid profiling. Snout is a good alternative, being lipid-rich even in wild fish with low muscle lipid.

The objective of this study has been to investigate the effect of light intensity (10, 21, 43, 200 and 650 lx) during smolting and its potential influence on gill Na + ,K + -ATPase (NKA) activity, endocrinology, ion regulation, growth, morphology, and spinal development in Atlantic salmon. Gill Na + ,K + -ATPase activity, plasma glucose and blood PCO 2 were all affected by time (study duration 176 days), while no differences were seen as a result of the different light intensity treatments. Growth rate was positively correlated to light intensity. Lower body silvering index, reduced fin area (5–7% lower in the 10 and 21 lux groups) and increased percentage of skeletal abnormalities (6.9% in the 10 lux group vs. 1% in the 650 lux group) at low light intensity were seen. In a second experiment juvenile Atlantic salmon parr were reared in August at LD12:12, at either 1 lx or 1000 lx and circulating plasma thyroxin (FT) levels measured. Plasma FT4 levels remained low in the low light intensity group (1 lx), whereas FT4 levels increased significantly from 2 to 5 h in the high intensity group (1000 lx). Total T4 levels were nearly 2-fold higher 5 h after lights-on in the high intensity group compared with the low. Overall, the present data suggests that a minimum light intensity of 43 lx is needed to secure optimal smolt quality, development, welfare and growth in Atlantic salmon.

The impact of salmon lice on the survival of migrating Atlantic salmon smolts was studied by comparing the adult returns of sea-ranched smolts treated for sea lice using emamectin benzoate or substance EX with untreated control groups in the River Dale in western Norway. A total of 143 500 smolts were released in 35 release groups in freshwater from 1997 to 2009 and in the fjord system from 2007 to 2009. The adult recaptures declined gradually with release year and reached minimum levels in 2007. This development corresponded with poor marine growth and increased age at maturity of ranched salmon and in three monitored salmon populations and indicated unfavourable conditions in the Norwegian Sea. The recapture rate of treated smolts was significantly higher than the controls in three of the releases performed: the only release in 1997, one of three in 2002 and the only group released in sea water in 2007. The effect of treating the smolts against salmon lice was smaller than the variability in return rates between release groups, and much smaller that variability between release years, but its overall contribution was still significant (P < 0.05) and equivalent to an odds ratio of the probability of being recaptured of 1.17 in favour of the treated smolts. Control fish also tended to be smaller as grilse (P = 0.057), possibly due to a sublethal effect of salmon lice.

To be able to design effective management to alleviate wild fish from parasite infestation pressure from fish farms, it is pivotal to understand when post-smolts migrate past areas of potential exposure to salmon lice Lepeophtheirus salmonis. Here, data from release groups of coded-wire-tagged Atlantic salmon Salmo salar smolts and their subsequent recaptures in a trap net in the outer fjord 12 to 97 km from the various release sites were used to estimate the smolts’ progression rate and their arrival time in an outer fjord in Norway. The arrival time estimates to the outer fjord are compared with modelled infestation pressure from local fish farms. The overall progression rate varied from 0.8 to 31.2 km d-1 (0.05 to 2.20 body lengths s-1), with mean and median values of 8.8 and 7.8 km d-1, respectively (0.60 and 0.54 body lengths s-1). The progression rate varied with water discharge from the rivers into the fjords, fish length, condition factor and smolt origin. Simulated arrival time and capture of wild smolts suggest that smolts from the different rivers arrive in the outer fjord system with a difference of up to 4 wk. The arrival time for the rivers with the longest migration was estimated to be from mid-May throughout June. Infestation pressure from fish farms increased from the beginning of June in 2 of 3 study years, suggesting that an increase in lice exposure from fish farms will overlap with smolts from late-migrating populations in some but not all years.

Each year, hundreds of thousands of farmed Atlantic salmon escape from fish farms into the wild. Some of these escapees enter freshwater, and manage to interbreed with native populations. To hinder further genetic introgression in native populations, the use of sterile triploid salmon within commercial aquaculture is being examined. However, if triploid escapees migrate into freshwater, they may still have ecological impacts on local populations. In the present study, we used microsatellite DNA genotyping to determine the ploidy of 3794 farmed escapees captured in 17 Norwegian rivers in the period 2007–2014. Although a previous study has reported an average of 2 % triploids in Norwegian fish farms during this exact period, here, we only observed 7 (0.18 %) triploids among the escapees captured in freshwater. In addition, we identified three trisomic escapees. For the triploids where the within-river capture location was determined, they were only observed in the lower reaches and not on the spawning grounds. It is concluded that propensity for triploid Atlantic salmon to migrate into freshwater following escape from a fish farm is significantly lower than for normal diploid salmon escapees. Therefore, commercial production of triploids should not only be seen as an effective way of stopping genetic introgression, it will also significantly reduce the numbers of escapees entering rivers, which in turn limits ecological interactions and potential disease transmission. publishedVersion

Parasites can, in theory, have large impacts on the survival of fish populations. One method to evaluate such impacts on anadromous species is to apply manipulative field experiments in which parallel groups of antiparasitically treated and non-treated fish are simultaneously released and then subsequently recaptured as returning adults. A systematic review and meta-analysis on all such Norwegian studies on Salmo salar provided a data set for the time period 1996 to 2011 on 118 release groups comprising 657 624 fish released and 3989 recaptured. The overall risk ratio (RR) was estimated to be 1.18 (95% CI: 1.07–1.30). The effect varied strongly between groups, (Higgins I2 = 40.1%). Over 70% of this heterogeneity could be explained by the release location, time period and baseline survival. The most important predictor variable was baseline survival. In groups with low recapture in the control group (low baseline survival), the effect of treatment was high (RR = 1.7), while in groups with high recapture in the control group (high baseline survival), there was no effect of treatment (RR ~ 1.00). The most prevalent parasite in the region affected by the drugs administered was Lepeophtheirus salmonis. Hence, the meta-analysis supports the hypothesis that L. salmonis contributes to the mortality of S. salar during outward migration. However, the effect of treatment was not consistent, but was evidently strongly modulated by other risk factors. The results suggest that the population-level effects of parasites cannot be estimated independently of other factors affecting the marine survival of S. salar. Emamectin benzoate, fish farming, Lepeophtheirus salmonis, parasite, salmon louse, substance EX © 2015 John Wiley & Sons Ltd. This is the peer reviewed version of the article, which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1111/faf.12141/abstract. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

To study relationships between fish length, photoperiod and the onset of precocious maturity in 0+ parr during intensive rearing of Atlantic salmon (Salmo salar L.), 0+ juveniles of 55–89 mm length were individually tagged with Passive Induced Transponders (PITs) in May and exposed to reduced daylength (LD12:12) in May, June or July, or kept as control fish under continuous light (LD24:0). Relationships between the length of the fish and maturity were studied by the use of probabilistic maturation reaction norms. The incidence of mature males and the proportion of fish of lower modal group size in autumn were highest in the groups exposed to short days in May, and lowest under continuous light. In contrast with the expectation that high growth rates promote maturation, the future mature male parr were smaller than the immature males at the start of the experiment, and they also grew more slowly during the subsequent maturation process. Variability in condition factor was low until autumn, when the condition factor of maturing males rose well above that of immature males during autumn as the fish grew from ~ 80 to ~ 95 mm length. Reduced daylength increased the probability of the onset of precocious maturity in the fish smaller than, but not above a threshold length of ~ 90–100 mm (9 – 11 g). Intensively reared parr of 60–90 mm, and possibly also smaller fish, may be particularly sensitive to photoperiod manipulations that may influence the probabilities of fish adopting one of the three life-history alternatives, to enter lower or upper mode or to mature precociously.

Escapes of farmed fish from coastal farms around the world are believed to have genetic and ecological consequences for wild fish populations. Each year there are numerous escapes of adult farmed Atlantic salmon Salmo salar. However, survival until maturity appears to be low. One possible explanation is that many of these salmon have difficulties switching to live prey. To address this possibility, growth rate and dietary preference were recorded in recaptured salmon released in 3 groups from August to October 2008 within a small fjord in south-western Norway (Masfjord). The fish were primed with a commercial diet containing 11% of the 'vegetable oil marker' 18:2n-6 before release. In the 14.5 to 35.1% of fish recaptured, growth rates were generally good (0.21-1.23% d -1 of fish recaptured in main fjord >2 mo after release), with body weight doubled or more for fish recap- tured after 19 wk and thereafter. However, fish recaptured in a small arm of Masfjord (Hopsvag) had a growth rate close to zero and were in poor condition. While stomachs of recaptured fish were gen- erally empty in the first weeks after release, pellets from fish farms were found in more than 80% of the stomachs after 12 wk and thereafter. Additionally, the level of 18:2n-6 in depot lipids (11% of fatty acids) of the muscle indicated that none of the fish recaptured the following autumn and winter in the vicinity of the release site had switched to wild prey diets. Salmon that escape during autumn are unlikely to compete for prey with wild fish in the vicinity of the release site.

The timing of the smolt run in the Dale River in western Norway was monitored from 2002 to 2007 after annual stocking in late autumn 2000 to 2005 with 5000–10800, 11–16 g, Atlantic salmon, Salmo salar L., parr. The releases yielded an annual smolt production of 1000–2000 individuals, mainly 1+ smolts. Almost 5700 stocked smolts were trapped during smolt migration and 60% of these were genotyped for family identification. The date for 50% descent varied by 14 days from year to year. For the most part, however, the 2+ stocked smolts and the majority of the wild smolts left the river in May, while the 1+ stocked smolts migrated 23–26 days later in June. It was concluded that the strategy of stocking large parr in late autumn may conflict with the natural timing of smolt migration the following spring.

Skilbrei, O. T., Holst, J. C., Asplin, L., and Holm, M. 2009. Vertical movements of “escaped” farmed Atlantic salmon (Salmo salar L.)—a simulation study in a western Norwegian fjord. – ICES Journal of Marine Science, 66: 278–288. To study the vertical distribution of fish that had been allowed to escape, farmed Atlantic salmon were tagged with acoustic tags equipped with depth sensors, and then released on five different dates in the course of a year from two fish farms in the Hardanger Fjord in western Norway. Release stimulated the fish to dive to deeper than 15 m during the first hours or days post-release, often down to 50–80 m. However, during the following 4 weeks, most of the escapees spent most of their time above the pycnocline at depths of 0–4 m. The fish were more widely distributed in the water column after release during winter, but still spent most of the time in the cold surface layers. There was a wide range in the vertical distribution of individual fish, and the proportion of detections below 14-m depth ranged from 0 to 90%. There was a significant diurnal cycle in all seasons except midsummer, when the fish were less abundant in the upper layer during daylight, especially on brighter days. The results suggest that salmon diving activity following escape may complicate the recapture of escaped fish at the farm site but that the subsequent tendency of most fish to stay near the surface, virtually irrespective of the time of year, may facilitate recapture.

The treatment of Atlantic salmon smolts with antiparasitic drugs may be a method to increase their survival after release to the sea. In the present study, age 1+ years tank-reared Atlantic salmon smolts that were the offspring of wild parents were treated against sea lice infection using (SLICE™) emamectin benzoate. This treatment was administered orally for seven days to each of three independent experimental groups during May and early June. The concentration of emamectin benzoate in muscle of the experimental groups was measured 14, 21, 35, 49 and 63 days post-initiation of treatment. To test the protection against sea lice, two separate challenges with Lepeophtheirus salmonis were performed such that the number of days from start of treatment to challenge ranged from 49 to 110 days for the three groups. Observed concentrations of emamectin benzoate in muscle were comparable to previous studies, and were close to zero 63 days post-initiation of treatment. However, a very high degree of individual variation in muscle concentration was observed. In contrast to other studies, no significant protection against sea lice was observed after 49 days post-initiation of treatment. Due to high variation between fish, and the observed short period of protection, it is concluded that the treatment of hatchery-reared smolts for sea ranching with emamectin benzoate prior to release will give a lower degree, and shorter period of protection against sea lice infection than indicated in previous studies.

Glover, K. A., Skilbrei, O. T., and Skaala, Ø. 2008. Genetic assignment identifies farm of origin for Atlantic salmon Salmo salar escapees in a Norwegian fjord. – ICES Journal of Marine Science, 65: 912–920. This study represents the first example of genetic assignment being used to identify the farm of origin for escaped Atlantic salmon. Following reports by local fishers of escaped farmed salmon in Romsdalfjord, western Norway, baseline samples were collected from 16 cages located on seven operational farms. The baseline samples, in addition to 29 escapees, were screened for 15 microsatellite loci. Pairwise FST values between baseline samples varied from

Experimental releases of Atlantic salmon smolts treated with emamectin benzoate (EB) against salmon lice have previously been used to estimate the significance of salmon lice on the survival of migrating smolts. In recent years, the salmon louse has developed reduced sensi- tivity to EB, which may influence the results of such release experiments. We therefore tested the use of 2 anti-lice drugs: EB was administered to salmon smolts in high doses by intra-peritoneal injection and the prophylactic substance EX (SubEX) was administered by bathing. A third, untreated control group was also established. Salmon were challenged with copepodids of 2 strains of salmon lice (1 EB-sensitive strain and 1 with reduced EB-sensitivity) in mixed-group experimental tanks. At 31 d post-challenge, the numbers of pre-adult lice on treated fish were around 20% compared with the control fish, with minor or no differences between the 2 treat- ments and lice strains. Both treatments therefore appeared to give the smolts a high degree of pro- tection against infestation of copepodids of salmon lice. However, significantly lower growth of the EB-treatment group indicates that bathing the fish in SubEX is less stressful for smolts than intra- peritoneal injection of EB.

To improve assessments of the environmental risks of aquaculture, a series of simulated escapes of farmed Atlantic salmon (Salmo salar L.) from seawater netpens were performed. Individually tagged post-smolts and adult Atlantic salmon were released from various locations at different times of the year. Post-smolts that escaped during their first summer were capable of rapid migration towards the open sea. A small fraction returned to spawn and were recaptured after 1–3 years at sea (0.4%, range 0.0–1.1%). A total of 13% of the post-smolts that escaped during autumn were reported in nearby fisheries during subsequent months, partly because they had grown large enough to be caught in the gillnets used, but more importantly because migratory behaviour diminished towards the end of the year. The mean recapture rate of adult salmon was high after releases in fjords (7–33%), lower after coastal releases (4–7%), and zero on the outer coast. Most of these recaptures were immature fish recaptured in sea relatively close to the release site during their first months post-release. Recaptures of adult escapees after 1–2 years in the wild were very rare (0.09%), probably because of their low survival. A Monte-Carlo method was developed to estimate the annual numbers of escapees from Norwegian fish farms based on reported catches of escaped farmed salmon in the sea and in rivers and the recapture probabilities reported here. The model provides a tool to estimate numbers of escapees independently from the reported numbers. Importantly, our analysis suggests that the total numbers of post-smolt and adult escapees have been two- to fourfold as high as the numbers reported to the authorities by fish farmers, depending on whether the incomplete sea fishery statistics are compensated for.

Skilbrei, O. T., and Wennevik, V. 2006. Survival and growth of sea-ranched Atlantic salmon, Salmo salar L., treated against sea lice before release. e ICES Journal of Marine Science, 63: 1317e1325. Cultured Atlantic salmon smolts were treated with Slice, orally administered emamectin benzoate, before release in the Dale River, western Norway, to study the potential effects of sea lice during the early stages of their marine phase. In all, 10 470 treated and untreated (control) fish from ten family groups were adipose fin-clipped, coded-wire tagged, and released on three different dates in 2002 (11 May, 25 May, and 7 June), which coincided with the natural smolt run. The percentage of released smolts recaptured as one-sea-winter salmon in 2003 did not differ between the treated and untreated groups released on the two dates in May 2002, but the recapture rate of fish from the treated group released on 7 June 2002 was almost twice that of the controls. The weights of the recaptured one-sea-winter salmon tended to decline from the first to the third release date, and one-sea-winter salmon from the treated groups were approximately 15% heavier than the controls. The difference in recapture rate between the treated and untreated groups increased after inclusion of the two-sea-winter and three-sea-winter salmon recaptured in 2004 and 2005, respectively. We conclude that the infestation level of salmon lice changed from non-lethal to lethal levels during the period of the smolt migration in 2002 and that non-lethal infestation levels may adversely affect Atlantic salmon populations by reducing the growth rate of fish and, consequently, their size at spawning.

Skilbrei, O. T., and Wennevik, V. 2006. The use of catch statistics to monitor the abundance of escaped farmed Atlantic salmon and rainbow trout in the sea. e ICES Journal of Marine Science, 63: 1190e1200. Catch statistics and scale samples were collected from a gillnet fishery targeting escaped farmed salmonids between 1 October and 28 February each year from 2001 to 2004 in Hordaland County, western Norway. Fish were classified into different groups, or escape incidents, using catch per unit effort (cpue) and size distribution of the catch from different geographical subregions. Reported escape incidents of both rainbow trout and salmon appeared to be followed by peaks in the cpue lasting four to six weeks, but a large proportion of the catch of escaped salmon appeared to stem from unreported, small-scale escape events. The wide size-range of fish caught suggests that the escapees originated from different escape incidents, and the variability between regions suggests that most catches were of local origin. Genetic comparisons among three groups of escapees indicated that DNA profiling may facilitate identification in monitoring programmes of escapees originating in different genetic groups. A low incidence of wild fish was found in the catches. Provided the conservation status of local wild salmonid stocks is taken into account, a fishery targeting escaped farmed salmonids may reduce the numbers of escapees, thus lowering the risk of introgression with wild salmon populations and removing potential sources of sea lice. Information on the relative abundance of escapees in the sea would also be provided by a fishery targeting escapees.

The aim was to study relationships between photoperiod in freshwater and growth rates prior to and following seawater exposures for fish from different stocks. Individually tagged 1+juveniles originating from four different stocks of Atlantic salmon were reared under the constant photoperiods LD10:14, LD12:12, LD14:10 and LD16:8 and under simulated natural increasing photoperiods from early spring starting from LD11:13 (SNP-11; fish from LD10:14 and LD12:12) or from LD15:9 (SNP-15; fish from LD14:10 and LD16:8). The groups were exposed to running seawater during May and June under LD22:2 and transferred to natural light regime in late July to monitor the subsequent growth rates in seawater for the next 6 months. The genetic origin of the fish was the factor of most influence for variation in the proportion and growth rates of the upper modal group fish in freshwater. The Dale stock showed the best growth performance, followed by the Vosso, Hatchery and the Lone stock. During the first 3–6 weeks in seawater, there was an evident and common drop in growth rates for all the stocks. The largest part of the variation in growth could be attributed to pre-smolt photoperiods, as the drop in growth rates was less pronounced for the fish previously held under increasing photoperiod in freshwater. During the next 5 months in seawater, the significance of stock was partly re-established. The Dale, Vosso and Hatchery stocks reached comparable sizes, significantly larger than the Lone stock. In addition, the initially slow growing groups under constant photoperiod in freshwater grew at the highest rate resulting in no differences in final sizes between treatment groups, possibly implying that their transfer to long daylength in seawater stimulated growth and/or completed the smoltification process. Relationships between photoperiod in freshwater, smoltification and growth rate before and after seawater exposure are discussed.

The main question which must be raised when planning to enhance natural populations through sea ranching is whether reared organisms are fit for a life in the wild, or more specifically, whether there are differences between reared and naturally resident individuals. The causes and effects of these differences are reviewed, and results from the Norwegian enhancement programme, which compared reared and wild individuals of Atlantic salmon, Salmo salar L., Atlantic cod, Gadus morhua L., and European lobster, Homarus gammarus L., are discussed with emphasis on morphological and behavioural differences. It was concluded that exposure to an artificial rearing environment during ontogeny can affect both the phenotype and the behaviour of the reared individuals, and thereby, their potential for survival after release into the wild as well. Suggestions are made as to how to diminish observed differences, and thereby, improve the survival potential.

Parasites may have large effects on host population dynamics, marine fisheries and conservation, but a clear elucidation of their impact is limited by a lack of ecosystem-scale experimental data. We conducted a meta-analysis of replicated manipulative field experiments concerning the influence of parasitism by crustaceans on the marine survival of Atlantic salmon ( Salmo salar L.). The data include 24 trials in which tagged smolts (totalling 283 347 fish; 1996–2008) were released as paired control and parasiticide-treated groups into 10 areas of Ireland and Norway. All experimental fish were infection-free when released into freshwater, and a proportion of each group was recovered as adult recruits returning to coastal waters 1 or more years later. Treatment had a significant positive effect on survival to recruitment, with an overall effect size (odds ratio) of 1.29 that corresponds to an estimated loss of 39 per cent (95% CI: 18–55%) of adult salmon recruitment. The parasitic crustaceans were probably acquired during early marine migration in areas that host large aquaculture populations of domesticated salmon, which elevate local abundances of ectoparasitic copepods—particularly Lepeophtheirus salmonis . These results provide experimental evidence from a large marine ecosystem that parasites can have large impacts on fish recruitment, fisheries and conservation.

Salmon lice, Lepeophtheirus salmonis, are naturally occurring parasites of salmon in sea water. Intensive salmon farming provides better conditions for parasite growth and transmission compared with natural conditions, creating problems for both the salmon farming industry and, under certain conditions, wild salmonids. Salmon lice originating from farms negatively impact wild stocks of salmonids, although the extent of the impact is a matter of debate. Estimates from Ireland and Norway indicate an odds ratio of 1.1:1-1.2:1 for sea lice treated Atlantic salmon smolt to survive sea migration compared to untreated smolts. This is considered to have a moderate population regulatory effect. The development of resistance against drugs most commonly used to treat salmon lice is a serious concern for both wild and farmed fish. Several large initiatives have been taken to encourage the development of new strategies, such as vaccines and novel drugs, for the treatment or removal of salmon lice from farmed fish. The newly sequenced salmon louse genome will be an important tool in this work. The use of cleaner fish has emerged as a robust method for controlling salmon lice, and aquaculture production of wrasse is important towards this aim. Salmon lice have large economic consequences for the salmon industry, both as direct costs for the prevention and treatment, but also indirectly through negative public opinion.

Source at https://doi.org/10.3354/aei00021. The search for effective strategies to prevent and mitigate accidental releases of aquaculture fishes is on-going. To test a new recapture strategy and evaluate the individual dispersal behaviour of escaped farmed Atlantic salmon Salmo salar L. at the northern limit of its range, 39 adult salmon (mean ± SD fork length and weight: 85.5 ± 5.0 cm and 7.4 ± 1.4 kg, respectively) were implanted with depth-sensing acoustic tags and released in a north Norwegian fjord during the spring of 2007. The fish were released from 2 aquaculture sites in the Altafjord system and tracked using both mobile and fixed receivers. The coastal marine bag-net fishery, in combination with in-river angling, was tested as a potential recapture strategy. Immediately following the simulated escape event, the fish dove to near-bottom depths, subsequently returning to surface levels within the following days. The fish dispersed rapidly (9.5 ± 19.2 km d–1), traveling outward to coastal waters along the edges of the fjord. The bag-net fishers and anglers recaptured 79% of the escaped fish within 1 mo post-release, 90% of which were from bag nets. While most of the fish left the fjord, 7 tagged fish (18%) entered the Alta River estuary (3 of which later migrated up the Alta River), and 1 returned to the Altafjord the following year, presumably to spawn. The results showed that recapture efforts need to be immediate and widespread to mitigate farm-escape events. Coastal bag nets were effective at recapturing escaped farmed salmon, compared to previously tested methods, and would be especially useful in areas where gill-netting is not permitted.

The influence of time of escape on survival to adulthood was tested in farmed Atlantic salmon Salmo salar smolt and post-smolts. Escape events were simulated in a small fjord in western Norway during and after the natural period of smolt migration by releasing 6 groups of individually tagged smolts and post-smolts (~2000 fish in each group) every second week from 27 May to 5 August 2005. With the exception of a lower return rate of the first released group (0.31%), the recapture rates were very similar (0.85 to 1.05%) and did not vary significantly with time of release. Of the 100 recap- tures, 57% were 1-sea-winter (1SW) salmon, 25% were 2SW and 18% returned as 3SW salmon. Release date did not influence the sea age but affected the weights of the recaptured adults moder- ately. Fifty-four percent of the fish were recaptured in the vicinity of the release site, most of them in the freshwater effluent from a hydropower plant. The rest were spread along the coast of Norway and in rivers (26% of the distant recaptures) over distances of 100s of kilometres. The present study shows that farmed salmon escaped during their first summer in sea cages — after the natural time for smolt migration in spring — are still capable of adopting the marine migratory pattern of their wild con- specifics. This suggests that escapes at this time of the year are a hazard to the conservation of wild salmon populations.

A large-scale escape experiment using 1031 adult Atlantic salmon Salmo salar L. was performed in the Hardangerfjord in western Norway to study the dispersal of escaped salmon, eval- uate the effect of a gill-net fishery targeting escaped salmonids and test whether surface trawling is an effective way of recapturing escaped salmon in a large fjord system. The salmon of mean weights 1.56 and 5.5 kg were released from 2 commercial fish farms in late September 2006. All fish were tagged with external tags, and 48 were also equipped with acoustic transmitters. A surface pair-trawl (50 m wide and 8 m deep) was constructed to optimize catchability and maneuverability in the fjord environment. Trawling was unsuccessful, and caught only 6 simulated escapees. Telemetry data con- firmed that the fish were available along the towing tracks, and we assume that towing speed and/or trawl size may have been suboptimal with regard to avoidance by fish in the fjord environment. Gill- netting proved to be an efficient method of recapture. The total reported recapture rate (of 114 fish- ers) was 40%, but a significantly higher recapture rate (67%) of the more highly rewarded acoustic transmitters, and the distribution of the fish in time and space, suggest that the actual catch may have been substantially higher. Approximately 90% of the catches were taken within 40 km of the release sites over the course of 4 wk. We conclude that a significant proportion of escaped adult salmon can be recaptured if the catch effort within the fjord basin is widespread and lasts for at least 4 wk.

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 treated 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.

One- and two-year smolts from 11 family groups of Atlantic salmon were individually tagged and reared in net pens. Growth rate and maturation until the second autumn in seawater were studied for both year classes. Effects of smolt length and family background were estimated using multiple regression analysis. There was a positive correlation between smolt size within a family and size after one year in the sea. The largest one-year smolts in each family had the greatest chance of maturing as grilse. The growth rate of fish maturing as grilse differed significantly from immature multi-sea-winter fish; it increased in early spring in the year of spawning and was sharply reduced in early autumn. One-year smolts gave the highest proportion of grilse.