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Sulfate (SO42‐) regulation is challenging for euryhaline species as they deal with large fluctuations of SO42‐ during migratory transitions between freshwater (FW) and seawater (SW), while maintaining a stable plasma SO42‐ concentration. Here, we investigated the regulation and potential role of sulfate transporters in Atlantic salmon during the preparative switch from SO42‐ uptake to secretion. A preparatory increase in kidney and gill sodium/potassium ATPase (Nka) enzyme activity during smolt development indicate preparative osmoregulatory changes. In contrast to gill Nka activity a transient decrease in kidney Nka after direct SW exposure was observed and may be a result of reduced glomerular filtration rates and tubular flow through the kidney. In silico analyses revealed that Atlantic salmon genome comprises a single slc13a1 gene and additional salmonid‐specific duplications of slc26a1 and slc26a6a, leading to new paralogs, namely the slc26a1a and ‐b, and slc26a6a1 and ‐a2. A kidney‐specific increase in slc26a6a1 and slc26a1a during smoltification and SW transfer, suggests an important role of these sulfate transporters in the regulatory shift from absorption to secretion in the kidney. Plasma SO42‐ in FW smolts was 0.70 mM, followed by a transient increase to 1.14 ± 0.33 mM 2 days post‐SW transfer, further decreasing to 0.69 ± 0.041 mM after 1 month in SW. Our findings support the vital role of the kidney in SO42‐ excretion through the upregulated slc26a6a1, the most likely secretory transport candidate in fish, which together with the slc26a1a transporter likely removes excess SO42‐, and ultimately enable the regulation of normal plasma SO42‐ levels in SW., This study address Sulfate regulation in Atlantic salmon as they deal with large fluctuations of sulfate during migratory transitions between freshwater and seawater. Our findings support the vital role of the kidney in sulfate excretion through the upregulated slc26a6a1, the most likely secretory transport candidate in fish, which together with the slc26a1a transporter likely removes excess sulfate, and ultimately enable the regulation of normal plasma SO4 2– levels in SW.

Sterile triploid Atlantic salmon (Salmo salar) show inconsistent seawater grow-out, but the reason why remains unclear. The purpose of this study was to determine the salinity optima of triploid post-smolts. Diploids and triploids were assessed for smoltification status during an underyearling smolt regime before being transferred to one of four different salinities, 0, 11, 23 and 35 ppt at 12 °C and under 24 h continuous light for 83 days. During this period, fish growth, plasma biochemistry, and production traits (vertebral deformities, ocular cataracts, sexual maturation) were monitored. Molecular biomarkers in the gill (nkaα1a, nkaα1b, nkcc1a) suggested triploids reached peak smolt earlier than diploids and began the desmoltification process before the start of the salinity treatments, however this was not reflected in gill Na+/K+-ATPase enzyme activity. At the initiation of the salinity treatments triploids were significantly larger than diploids (mean weight g ± SE: 71 ± 0.7 and 87.2 ± 0.8 for diploids and triploids, respectively) and there was a ploidy effect on post-smolt growth, with body weight showing a clearer positive trend with salinity in diploids (0 < 11 = 23 = 35 ppt) than in triploids (0 < 11 < 35 = 23 ppt) (final mean weight g ± SE: 255.2 ± 7.4, 303.9 ± 9, 313.9 ± 9 and 342.4 ± 12 for diploids and 322.9 ± 9.7, 361.7 ± 10.7, 425.9 ± 12.1, 415.2 ± 12.2 for triploids at 0, 11, 23, and 35 ppt, respectively). Plasma Na+ and Cl− increased, but plasma pH decreased, with increasing salinity in both ploidy. However, ploidy only had transient effects on plasma biochemistry depending on the salinity treatment. There was no ploidy effect on vertebral deformities (21% of both ploidy had one or more deformed vertebra). In contrast, triploids had a significantly higher prevalence of ocular cataracts (84 vs 98% in diploids and triploids, respectively) with a higher mean cataract score (mean ± SE: 1.93 ± 0.1 and 2.78 ± 0.1 for diploids and triploids, respectively), but a significantly lower prevalence of pubertal male post-smolts (15 vs 2% in diploids and triploids, respectively). Salinity treatment had no effect on vertebral deformities, cataracts, or post-smolt sexual maturation. In summary, there was a ploidy mismatch for smoltification biomarkers in the gill and salinity had a strong effect on post-smolt growth, but the effects were ploidy dependent. publishedVersion

Telemetry 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. publishedVersion

The use of reproductively sterile triploid salmonids would enhance the environmental sustainability of the aquaculture industry by preventing genetic exchange between escapees and wild conspecifics. To this end, we assessed smoltification and early seawater performance (241 days) following a yearling production cycle (i.e. spring smolts) in diploid and triploid female Atlantic salmon (Salmo salar) × male brown trout (Salmo trutta) hybrids compared to purebred diploid and triploid salmon. During freshwater rearing (n = 180/group), hybrids demonstrated a degree of bimodality in body size, significantly (p < 0.05) more so in diploid than triploid hybrids (11 and 37% in the lower mode, respectively) that was not seen in purebred salmon of either ploidy. This resulted in diploid hybrids being 66% smaller on average at sea transfer, whereas no hybridisation effect was seen in triploids, and both triploid groups were significantly heavier (16–43%) than diploid salmon. Irrespective of ploidy, lower mode hybrids grew poorly and showed low survival in seawater, suggesting they had failed to undergo smoltification. However, the upper mode diploid hybrids showed a similar Na+/K+-ATPase (NKA) enzyme activity surge during the spring as in diploid and triploid salmon, despite a higher ratio of the freshwater to seawater mRNA abundance of the NKA subunits (nkaα1a and nkaα1b) and a reduced plasma cortisol surge. At the end of the experimental period, both hybrids weighed significantly less than their salmon counterparts although the hybrid effect was again greater in diploids (71% smaller) than triploids (6% smaller). In addition, both triploid groups were on average heavier (15–22%) than diploid salmon. As such, both triploid Atlantic salmon and triploid hybrids can show enhanced growth performance from juveniles up to post-smolts compared to diploid salmon in an aquaculture setting. publishedVersion

Most Atlantic salmon (Salmo salar L.) populations follow an anadromous life cycle, spending early life in freshwater, migrating to the sea for feeding, and returning to rivers to spawn. At the end of the last ice age ~10,000 years ago, several populations of Atlantic salmon became landlocked. Comparing their genomes to their anadromous counterparts can help identify genetic variation related to either freshwater residency or anadromy. The objective of this study was to identify consistently divergent loci between anadromous and landlocked Atlantic salmon strains throughout their geographical distribution, with the long‐term aim of identifying traits relevant for salmon aquaculture, including fresh and seawater growth, omega‐3 metabolism, smoltification, and disease resistance. We used a Pool‐seq approach (n = 10–40 individuals per population) to sequence the genomes of twelve anadromous and six landlocked Atlantic salmon populations covering a large part of the Northern Hemisphere and conducted a genomewide association study to identify genomic regions having been under different selection pressure in landlocked and anadromous strains. A total of 28 genomic regions were identified and included cadm1 on Chr 13 and ppargc1a on Chr 18. Seven of the regions additionally displayed consistently reduced heterozygosity in fish obtained from landlocked populations, including the genes gpr132, cdca4, and sertad2 on Chr 15. We also found 16 regions, including igf1 on Chr 17, which consistently display reduced heterozygosity in the anadromous populations compared to the freshwater populations, indicating relaxed selection on traits associated with anadromy in landlocked salmon. In conclusion, we have identified 37 regions which may harbor genetic variation relevant for improving fish welfare and quality in the salmon farming industry and for understanding life‐history traits in fish. publishedVersion

First paragraph: Most freshwater fish would not be able to cope with the osmotic stress intrinsic to seawater, quickly dehydrating and dying, and so are bound to freshwater ecosystems (Quinn et al., 2016). However, anadromous salmonids have developed a strategy to minimize osmotic counter effects, thus allowing them to live in both freshwater, ideal for safe spawning, hatching and early growth stages, and seawater environments, rich in energy resources for fast growth (Hendry et al., 2004; Lima and Dill, 1990).

Optimal water quality is vital for the growth of Atlantic salmon aquaculture production. Recent data showed that Atlantic salmon feed intake and growth reduce linearly with increasing water carbon dioxide (CO2) concentrations, suggesting that even relatively low concentrations may impact fish performance. This study evaluated the molecular and physiological responses of Atlantic salmon (Salmo salar) to long-term CO2 exposure. For this purpose, Atlantic salmon post-smolts (N = 900; 67 ± 8 g) were exposed to six CO2 treatments (5, 12, 19, 26, 33 and 40 mg/L) for 12-weeks (RAS phase) followed by non-CO2 exposure for a (

Producing a larger post-smolt in recirculating aquaculture systems (RAS) could shorten the production time in sea cages and potentially reduce mortality. Knowledge of the biological requirements of post-smolts in closed-containment systems is however lacking. In the present study, the effects of salinity and water velocity on growth, survival, health, and welfare of Atlantic salmon reared in RAS were examined. Salmon smolts were stocked in three separate RAS with salinities of 12, 22, and 32‰ and subjected to high (1.0 body lengths per s−1) or low (0.3 body lengths second−1) water velocity. Growth performance, survival, welfare, and physiological stress responses were monitored until the fish reached a bodyweight of around 450 g. Growth rate was higher at lower salinity and higher water velocity generally had a positive effect on growth in all salinities. Feed conversion ratio was lower at 12‰ compared to the 22 and 32‰ when the fish were between 250 and 450 g. Higher mortality, elevated plasma cortisol levels, higher incidence of cataract, and a higher expression of stress-induced genes in the skin (iNOS, Muc5ac-like) indicated a negative effect of higher salinity on fish welfare. Male maturation was low (

Insulin-like growth factor binding protein (IGFBP)-1a is one of three major circulating forms in salmon and induced under catabolic conditions. However, there is currently no immunoassay available for this form because of a lack of standard and specific antibodies. We developed a time-resolved fluoroimmunoassay (TR-FIA) for salmon IGFBP-1a using recombinant protein for labeling, an assay standard, and production of antiserum. The TR-FIA had a low cross-reactivity (3.6%) with IGFBP-1b, another major form in the circulation. Fasting for 4 wk had no effect on serum immunoreactive (total) IGFBP-1a levels in yearling masu salmon, whereas 6-wk fasting significantly increased it. There was a significant, but weak, negative relationship between serum total IGFBP-1a level and individual growth rate ( r2 = 0.12, P = 0.01). We next developed a ligand immuno-functional assay (LIFA) using europium-labeled IGF-I to quantify intact IGFBP-1a. In contrast to total IGFBP-1a, serum intact IGFBP-1a levels increased after 4 wk of fasting, and refeeding for 2 wk restored it to levels similar to those of the fed control. Serum intact IGFBP-1a levels showed a significant negative correlation with individual growth rate ( r2 = 0.52, P < 0.001), which was as good as that of IGFBP-1b. Our findings using newly developed TR-FIA and LIFA suggest that regulation of intact IGFBP-1a levels has an important effect on growth in salmon and that intact IGFBP-1a is a negative index of salmon growth.

The use of closed containment (CCS) or semi-closed containment systems (S-CCS) for Atlantic salmon Salmo salar aquaculture is under evaluation in Norway. One such system is the Preline S-CCS, a floating raceway system that pumps water from 35 m depth creating a constant current through the system. Exposing fish to moderate water currents is considered aerobic exercise and it is often perceived as positive for fish welfare, growth, food utilization, muscle development and cardiac health. The present study compared fish reared in the Preline S-CCS and in a reference open pen. Samples were taken in fresh water before being transferred to the seawater systems and after 1, 2 and 4 months in seawater and analysed for growth, mortality, muscle development and plasma insulin-like growth factor I (IGF-I) levels. Moreover, gene transcription were determined in the skeletal muscle [igf-I, insulin-like growth factor 1 receptor a (igf1ra) and insulin-like growth factor 1 binding protein 1a (igf1bp1a)] and cardiac transcription factors [myocyte-specific enhancer factor 2C (mef2c), gata4 and vascular endothelial growth factor (vegf)]. While the results suggest that post-smolts in Preline S-CCS were smaller than reference fish, fish from Preline S-CCS have less accumulated mortality at the end of the experiment and showed 2.44 times more small muscle fibres than the reference group fish after 4 months in seawater. These results confirmed what was previously observed in the second generation of Preline. Similar levels of big muscle fibres between Preline S-CCS and reference suggest a similar hypertrophy of muscle fibres even with lower IGF-I expression in the Preline S-CCS. Cardiac gene transcription suggests cardiac hypertrophy was observed after 4 months in seawater in the Preline S-CCS group. Altogether, Preline S-CCS is a promising technology able to produce more robust S. salar with a faster growth and lower mortality in the subsequent standard open cage system growth period.

Pancreas disease (PD) caused by salmonid alphavirus (SAV) is the most serious viral disease in Norwegian aquaculture. Study of the immune response to SAV will aid preventative measures including vaccine development. The innate immune response was studied in Atlantic salmon infected by either bath immersion (BI) or by intra-muscular (i.m.) injection (IM) with SAV subtype 3, two and nine weeks after seawater transfer (Phases A and B respectively). Phase A results have been previously published (Moore et al., 2017) and Phase B results are presented here together with a comparison of results achieved in Phase A. There was a rapid accumulation of infected fish in the IM-B (IM Phase B) group and all fish sampled were SAV RNA positive by 7 dpi (days post infection). In contrast, only a few SAV RNA positive (infected) fish were identified at 14, 21 and 28 dpi in the BI-B (BI Phase B) group. Differences in the transcription of several immune genes were apparent when compared between the infected fish in the IM-B and BI-B groups. Transcription of the analysed genes peaked at 7 dpi in the IM-B group and at 14 dpi in the BI-B group. However, this latter finding was difficult to interpret due to the low prevalence of SAV positive fish in this group. Additionally, fish positive for SAV RNA in the BI-B group showed higher transcription of IL-1β, IFNγ and CXCL11_L1, all genes associated with the inflammatory response, compared to the IM-B group. Histopathological changes in the heart were restricted to the IM-B group, while (immune) cell filtration into the pancreas was observed in both groups. Compared to the Phase A fish that were exposed to SAV3 two weeks after seawater transfer, the Phase B fish in the current paper, showed a higher and more sustained innate immune gene transcription in response to the SAV3 infection. In addition, the basal transcription of several innate immune genes in non-infected control fish in Phase B (CT-B) was also significantly different when compared to Phase A control fish (CT-A).

The growth hormone (Gh)/insulin-like growth-factor (Igf) system is a key regulator of growth in teleost fishes, including salmonids. Igfs interact with a suite of binding proteins, termed Igf binding proteins (Igfbps), which modify their biological activities. Atlantic salmon (Salmo salar) express an expanded array of igfbp gene transcripts in liver, the primary site of Igfbp production. While gonadal steroids are known to impact the growth of fishes, there is little information on how androgens and estrogens affect the expression of various igfbps. In the current study, we characterized how 14-day implants of adrenosterone (OA), testosterone (T) or 17β-estradiol (E2) affected the Gh/Igf/Igfbp network in Atlantic salmon smolts. OA did not affect any of the parameters studied. There were no effects of T or E2 on body mass, plasma levels of Igf1, or hepatic igf1, igf2 or gh receptor mRNA levels. E2 stimulated hepatic estrogen receptor α mRNA levels by 17-fold, while E2 and T stimulated hepatic vitellogenin ...

In order to provide year round spawning broodstock, lumpfish (initial size 746 g and 24.9 cm) were reared under four different photoperiod regimes from January 2017 to July 2018. One group was reared under simulated natural photoperiod (LDN, control group) for Tromsø (70°N). The second group was transferred to continuous light (LD240) on 30 January 2017 and reared at LD24:0 throughout the trial period. Two compressed and phase advanced photoperiods were also established. Both groups were moved from LDN to LD24:0 on 30 January 2017, and after that reared at compressed natural photoperiods where the annual photoperiod was compressed down to six months (L6) or nine months (L9) for the duration of the study. Spawning time was shifted in both compressed groups during both years of the study. Spawning activity in the second year of the study was higher and followed more closely the expected spawning period in the compressed and the LDN groups. Spawning in the LD240 group was spread out over the experimental period with no distinct peak in spawning. A seasonal and pronounced drop in condition factor was found for females in the L9, L6 and the LDN groups. This post-spawning loss in condition was closely related to the spawning activity of each group. The current findings suggest that photoperiod has a strong influence on the timing of lumpfish maturation and can be used as an efficient and inexpensive tool to secure lumpfish reproduction operations i.e. year-round supply of egg and milt and/or timing with optimal temperature regimes. acceptedVersion

Photoperiod is thought to be the main zeitgeber that induces smoltification in salmonids. However, its effects on the smoltification of rainbow trout (Oncorhynchus mykiss) are not fully understood and no published data documents the effects of the photoperiod regime currently used commercially, continuous light (LL). The present study compared the effect of four different photoperiod regimes (i.e. advanced phase photoperiod (APP), delayed phase photoperiod (DPP), LL and simulated natural photoperiod (SNP)) on the smoltification and growth of juvenile rainbow trout during their freshwater phase of winter-spring and the following summer post smolt phase. Smoltification was evaluated by monitoring gill Na+,K+–ATPase (NKA) activity and transcription of NKA α-subunit isoforms 1a and 1b, and Na+,K+,2Cl‐ cotransporter 1a. Growth was measured as specific growth rate of both length and weight, and through molecular growth proxies such as the levels of circulating insulin-like growth factor 1 (IGF-I) in plasma and transcription of igf-I, igf binding protein 1b (igfbp1b), growth hormone receptor 1 (ghr1) and cathepsin L (ctsl) in the liver. Results indicate that APP induces a longer smolt window and higher levels of plasma IGF-I in both freshwater and seawater (two months post transfer), while DPP led to a shorter smolt window, lower plasma IGF-I levels in freshwater and seawater, an earlier decrease in liver igf-I and ctsl transcription in freshwater (as seen by modelling over time) and lower specific growth rate in freshwater. The transcription analysis of osmoregulatory genes complemented NKA activity and allowed for the detection of a transient response to light and of differences between the osmoregulatory capacity of parr and desmolted fish. Furthermore, an upregulation of the liver transcription of igf-I, ghr1 and ctsl was found in all treatments during the smolt window, which corresponded to the periods with highest growth. Finally, both plasma IGF-I and liver igf-I in seawater were found to be significantly correlated to fish growth in seawater. However, our data did not show that plasma IGF-I prior to seawater transfer could be used as a reliable predictor of growth in seawater. Overall, and especially when compared with other salmonid species, photoperiod seems to be a weaker inducer of smoltification in rainbow trout, according to the parameters that were tested, suggesting that other environmental cues might be more important drivers of this process. publishedVersion

The aryl hydrocarbon receptor (Ahr) is a ligand-activated transcription factor that mediates the toxicity of dioxins and dioxin-like compounds (DLCs) in vertebrates. Two clades of the Ahr family exist in teleosts (Ahr1 and Ahr2), and it has been demonstrated that Ahr2 is the main protein involved in mediating the toxicity of dioxins and DLCs in most teleost species. Recently, we characterized the Atlantic cod (Gadus morhua) Ahr1a and Ahr2a receptors. To further explore a possible subfunction partitioning of Ahr1a and Ahr2a in Atlantic cod we have mapped the expression and localization of ahr1a and ahr2a in early developmental stages. Atlantic cod embryos were continuously exposed in a passive-dosing exposure system to the Ahr agonist, benzo[a]pyrene (B[a]P), from five days post fertilization (dpf) until three days post hatching (dph). Expression of ahr1a, ahr2a, and the Ahr-target genes, cyp1a and ahrrb, was assessed in embryos (8 dpf and 10 dpf) and larvae (3 dph) with quantitative real-time PCR analyses (qPCR), while in situ hybridization was used to assess the localization of expression of ahr1a, ahr2a and cyp1a. Quantitative measurements showed an increased cyp1a expression in B[a]P-exposed samples at all sampling points, and for ahr2a at 10 dpf, confirming the activation of the Ahr-signalling pathway. Furthermore, B[a]P strongly induced ahr2a and cyp1a expression in the cardiovascular system and skin, respectively, of embryos and larvae. Induced expression of both ahr2a and cyp1a was also revealed in the liver of B[a]P-exposed larvae. Our results suggest that Ahr2a is the major subtype involved in mediating responses to B[a]P in early developmental stages of Atlantic cod, which involves transcriptional regulation of biotransformation genes, such as cyp1a. The focused expression of ahr1a in the eye of embryos and larvae, and the presence of ahr2a transcripts in the jaws and fin nodes, further indicate evolved specialized roles of the two Ahrs in ontogenesis. publishedVersion

We induced season independent smolts in masu salmon (Oncorhynchus masou) by rearing juveniles under an advanced photoperiod (AP) in which LD15:9 was interrupted by LD9:15 from June for 4 months. AP fish showed higher degree of skin slivering from November than fish reared under a simulated natural photoperiod (SNP) and became larger in length and lower in condition factor in January. Gill Na+,K+-ATPase in AP fish started increasing in November and peaked in December to typical spring smolt levels while it remained low in SNP fish. In addition, AP fish had higher hypo-osmoregulatory ability than SNP fish, as judged by the lower serum sodium and chloride concentrations 48 h after seawater transfer. Serum levels of insulin-like growth factor (IGF)-I and IGF-binding protein (IGFBP)-1b, components of the somatotropic axis, in AP fish peaked in November and January, respectively. Serum IGFBP-1b levels were increased 48 h after transfer to 70% seawater in both groups, suggesting that it is not a quantitative index of hypo-osmoregulatory ability in the short-term but a marker of osmotic stress. The present study showed that off-season smolts could be induced in underyearling masu salmon by a square-wave photoperiod, and off-season smolts showed morphological and physiological changes comparable to yearling spring smolts. Increased levels of circulating IGF-I and IGFBP-1b in smolting fish suggest that these endocrine parameters are involved in the smoltification process.

The fish gill is subject to an osmorespiratory compromise in physiologically demanding situations where conditions that favor gas exchange may compromise osmotic balance, especially when large gradients between the blood and the aquatic environment are present. Fish in isosmotic water should therefore be less restricted by an osmorespiratory compromise, which should improve aerobic performance. To investigate this hypothesis, Atlantic salmon were acclimated for minimum 3 weeks to freshwater, near isosmotic brackish water or sea water, and tested in groups of 10 in a large swim tunnel respirometer to assess metabolic rates, swimming capacity and hematological parameters. Oxygen uptake rates and the critical swimming speed were similar between treatments. However, osmolality and plasma [ions] before and after swim trials, and subsequent recovery differed. Fish in sea water experienced a substantially larger osmotic disturbance in the swim trials, which had increased further 3 h post-fatigue, while fish in lower salinities were approaching full recovery. Swim trials increased plasma cortisol levels, which may modulate increased gas transfer and facilitate beneficial ion regulation in both low and high salinities. Swimming also increased hematocrit and hemoglobin concentration that returned to control levels after recovery, suggesting recruitment of erythrocytes via splenic contraction. These results show that Atlantic salmon do not elicit a clear salinity optimum in terms of metabolic and locomotory advantages. Although, swimming in sea water imposes larger osmoregulatory challenges which may have implications for repeated swim challenges. Hence, Atlantic salmon are well-equipped to minimize the potential restrictions of an osmorespiratory compromise on aerobic performance, and more so in brackish and freshwater. Oxygen Uptake and Osmotic Balance of Atlantic Salmon in Relation to Exercise and Salinity Acclimation

High carbon dioxide (CO2) concentrations negatively impact fish, which makes data on its tolerance especially relevant for production systems that can accumulate CO2 such as recirculating aquaculture system (RAS). The current study evaluates the effect of CO2 on the growth performance, welfare, and health of Atlantic salmon post-smolts in RAS. This study consisted of two phases. The first was a CO2 exposure phase, where eighteen tanks were used with six treatments in triplicate: 5, 12, 19, 26, 33 and 40 mg/L of CO2 during 12 weeks in a 12 ppt salinity RAS (hereafter RAS phase). In the second phase, PIT-tagged fish were transferred to a 34 ppt salinity single flow-through tank at CO2 < 5 mg/L (hereafter seawater phase) for an additional 6-week experimental period mimicking a seawater phase. Overall, mortality of fish exposed to CO2 was low and not related to treatments. The mean final body weight was significantly higher in the 5 mg/L treatment compared to CO2 treatments ≥12 mg/L at the end of RAS phase and to CO2 treatments ≥33 mg/L at the end of seawater phase. Moreover, regressions showed that growth significantly decreased linearly with increasing CO2 in the water. Eye cataracts and visible external damage on skin, operculum, and fins were inexistent and similar among CO2 treatments. Kidneys showed no signs of mineral deposits in any of the structures of the tissue. However, skin analysis showed that fish exposed to high CO2 concentrations had a significantly thinner dermis layer (both at the end of RAS and seawater phase) and a significantly thinner epidermis layer and lower mucus cells count (at the end of seawater phase). In conclusion, Atlantic salmon post-smolts cultured in brackish water RAS showed a maximum growth performance at CO2 concentrations below 12 mg/L. Except skin, no major effects of health and welfare were observed, including cataracts and nephrocalcinosis. Further studies should evaluate the molecular and physiological responses to both short-term and long-term carbon dioxide exposure.

Salmonid alphavirus (SAV) causes pancreas disease (PD) in Atlantic salmon (Salmo salar L.) and disease outbreaks are mainly detected after seawater transfer. The influence of the smoltification process on the immune responses, specifically the adaptive response of Atlantic salmon after SAV infection, is not fully understood. In this study, Atlantic salmon post-smolts were infected by either bath immersion (BI) or intramuscular injection (IM) with SAV subtype 3, 2 weeks (Phase A) or 9 weeks (Phase B) after seawater transfer. The transcript levels of genes related to cellular, humoral and inflammatory responses were evaluated on head kidney samples collected at 3, 7, 14, 21, and 28 days post-infection (dpi). Corresponding negative control groups (CT) were established accordingly. Significant differences were found between both phases and between the IM and BI groups. The anti-inflammatory cytokine IL-10 was up-regulated in Phase A at a higher level than in Phase B. High mRNA levels of the genes RIG-1, SOCS1 and STAT1 were observed in all groups except the BI-B group (BI-Phase B). Moreover, the IM-B group showed a higher regulation of genes related to cellular responses, such as CD40, MHCII, and IL-15, that indicated the activation of a strong cell-mediated immune response. CD40 mRNA levels were elevated one week earlier in the BI-B group than in the BI-A group (BI-Phase A). A significant up-regulation of IgM and IgT genes was seen in both IM groups, but the presence of neutralizing antibodies to SAV was detected only in Phase B fish at 21 and 28 dpi. In addition, we found differences in the basal levels of some of the analysed genes between non-infected control groups of both phases. Findings suggest that Atlantic salmon post-smolts adapted for a longer time to seawater before they come into contact with SAV, developed a stronger humoral and cell-mediated immune response during a SAV infection. acceptedVersion

Despite the use of fish models to study human mental disorders and dysfunctions, knowledge of regional telencephalic responses in non-mammalian vertebrates expressing alternative stress coping styles is poor. As perception of salient stimuli associated with stress coping in mammals is mainly under forebrain limbic control, we tested region-specific forebrain neural (i.e. mRNA abundance and monoamine neurochemistry) and endocrine responses under basal and acute stress conditions for previously characterised proactive and reactive Atlantic salmon. Reactive fish showed a higher degree of the neurogenesis marker proliferating cell nuclear antigen ( pcna ) and dopamine activity under basal conditions in the proposed hippocampus homologue (Dl) and higher post-stress plasma cortisol levels. Proactive fish displayed higher post-stress serotonergic signalling (i.e. higher serotonergic activity and expression of the 5-HT 1A receptor) in the proposed amygdala homologue (Dm), increased expression of the neuroplasticity marker brain-derived neurotropic factor ( bdnf ) in both Dl and the lateral septum homologue (Vv), as well as increased expression of the corticotropin releasing factor 1 ( crf 1 ) receptor in the Dl, in line with active coping neuro-profiles reported in the mammalian literature. We present novel evidence of proposed functional equivalences in the fish forebrain with mammalian limbic structures.

The aim of the present study was to investigate cataract development in diploid (2N) and triploid (3N) Atlantic salmon smolts and post-smolts at two water temperatures (10 and 16 °C) given diets with different histidine supplementation (LH, 10.4 and HH, 13.1 g kg−1) before and after seawater transfer. In freshwater, a severe cataract outbreak was recorded in both ploidies reared at 16 °C. The cataract score was significantly higher in triploids compared to diploids, and the severity was lower in both ploidies fed the HH diet. The cataract development at 10 °C was minor. Low gill Na+, K+-ATPase activity in fish reared at 16 °C before seawater transfer was followed by osmoregulatory stress with elevated plasma electrolyte concentrations and high mortality in sea water. Both diploids and triploids reared at 10 °C developed cataracts during the seawater period, with higher severities in triploids than diploids and a reduced severity in the fish fed the HH diet. The findings of this study demonstrate the importance of environmental conditions in the husbandry of Atlantic salmon, and particularly triploids, with regard to smoltification and adjusted diets to mitigate cataract development in fresh and sea water. publishedVersion

This study investigated the expression of ion transporters involved in intestinal fluid absorption and presents evidence for developmental changes in abundance and tissue distribution of these transporters during smoltification and seawater (SW) acclimation of Atlantic salmon Salmo salar. Emphasis was placed on Na(+) , K(+) -ATPase (NKA) and Na(+) , K(+) , Cl(-) co-transporter (NKCC) isoforms, at both transcriptional and protein levels, together with transcription of chloride channel genes. The nka α1c was the dominant isoform at the transcript level in both proximal and distal intestines; also, it was the most abundant isoform expressed in the basolateral membrane of enterocytes in the proximal intestine. This isoform was also abundantly expressed in the distal intestine in the lower part of the mucosal folds. The protein expression of intestinal Nkaα1c increased during smoltification. Immunostaining was localized to the basal membrane of the enterocytes in freshwater (FW) fish, and re-distributed to a lateral position after SW entry. Two other Nka isoforms, α1a and α1b, were expressed in the intestine but were not regulated to the same extent during smoltification and subsequent SW transfer. Their localization in the intestinal wall indicates a house-keeping function in excitatory tissues. The absorptive form of the NKCC-like isoform (sub-apically located NKCC2 and/or Na(+) , Cl(-) co-transporter) increased during smoltification and further after SW transfer. The cellular distribution changed from a diffuse expression in the sub-apical regions during smoltification to clustering of the transporters closer to the apical membrane after entry to SW. Furthermore, transcript abundance indicates that the mechanisms necessary for exit of chloride ions across the basolateral membrane and into the lateral intercellular space are present in the form of one or more of three different chloride channels: cystic fibrosis transmembrane conductance regulator I and II and chloride channel 3.

The duration of hypo-osmoregulatory capacity in downstream migrating Atlantic salmon Salmo salar L smolts previously stocked as startfed young-of-the year (YOY) parr was tested in the River Dalaa from mid-May to late-June 1999. Hypo-osmoregulatory capacity, measured as plasma osmolality and chloride, was assessed after seawater (SW) challenge tests (168 h, salinity = 35). All S. salar exhibited sufficient hypo-osmoregulatory capacity at the initiation of downstream migration in mid-May. Migrating S. salar smolts caught in mid-May and retained in fresh water displayed no signs of de-smoltification as they maintained hypo-osmoregulatory capacity through June. This indicates a physiological smolt window that lasts a minimum of 6 weeks (330 degree days; D°) for hatchery-produced S. salar smolts stocked as YOY parr. Based on the observed river migration speeds, it can be assumed that the S. salar smolts entered SW 2-4 weeks after initiation of migration in the upper parts of the River Dalaa. Hence, based on smolt migration and SW tolerance, it is suggested that stocking of YOY parr is a viable enhancement strategy in the River Dalaa.

Specific growth hormone (GH)-binding protein (Ghbp) was purified from Atlantic salmon Salmo salar and rainbow trout Oncorhynchus mykiss plasma with immunoprecipitation and characterized in cross-linking studies using autoradiography and western blots. The size of the Ghbp was estimated to be c. 53 kDa. A radioimmunoassay was established to measure Ghbp in salmonids, using antibodies specific against the extracellular segment of the S. salar growth hormone receptor 1 (grh1; GenBank AY462105). Plasma Ghbp levels were measured in S. salar smolts in fresh water and after transfer to seawater (SW; experiments 1 and 2), and in post-smolts kept at different salinities (0, 12, 22 and 34) for 3 months (experiment 3). A transient increase in plasma Ghbp, which lasted for 1 month or less, was noted in smolts after transfer to SW. Concomitantly, plasma GH and gill Na(+) -K(+) -ATPase activity increased during smoltification (in experiment 2). No difference in plasma Ghbp was evident between post-smolts kept at different salinities, although the fish kept at salinity 34 had higher plasma GH than the group kept at salinity 22 and higher hepatic ghr1 expression than post-smolts kept at salinity 12. This suggests that plasma Ghbp regulation may respond to salinity changes in the short term. The lack of correlation between Ghbp, plasma GH and hepatic ghr1 expression in the long-term post-smolt experiment indicates that Ghbp levels may be regulated independently of other components of the endocrine GH system in salmonids.

In order to investigate how changes in gill Na(+) , K(+) -ATPase (NKA) α1a and α1b subunits, Na(+) , K(+) , 2Cl(-) co-transporter (NKCC1) and the apical cystic fibrosis trans-membrane conductance regulator-I (CFTR-I) transcripts in wild strain of Atlantic salmon, Salmo salar, smolts are affected by temperature during spring, hatchery-reared parr (mean ± s.e. fork length = 14·1 ± 0·5; mean ± s.e. body mass = 28·5 ± 4·5 g) originating from broodstock from the Vosso river (western Norway) were acclimated to three temperature regimes (4·1, 8·1 and 12·9° C) in May and reared under gradually increasing salinity between May and June. Changes in plasma Na(+) , haematocrit (Hct) and PCO2 were monitored in order to assess and compare key physiological changes with the transcriptional changes in key ion transporters. The temperatures reflect the natural temperature range in the River Vosso during late spring. Overall, higher gill NKA α1b mRNA levels, gill NKCC1a levels and CFTR-I levels were observed in the 4·1° C group compared to the 11·9° C group. This coincided with a 2-3 week period with decreased Hct and PCO2 and may indicate a critical window when smolts suffer from reduced physical performance during migration. Further research is needed to confirm the potential interaction between ecological and physiological conditions on mortality of hatchery-reared smolts from River Vosso during their natural migration.

This study investigated the development of hypo-osmoregulatory capacity and timing of downstream migration in wild Atlantic salmon Salmo salar smolts from the River Stjordalselva and stocked young-of-the-year (YOY), derived S. salar smolts from the tributary River Dalaa. Both wild and stocked S. salar smolts developed seawater (SW) tolerance in early May, persisting through June, measured as their ability to regulate plasma osmolality and chloride following 24 h SW (salinity = 35) exposure. Although the majority of downstream migration among the stocked S. salar smolts occurred later than observed in their wild counterparts, the development of SW tolerance occurred concurrently. The wild S. salar from Stjordalselva and stocked YOY smolts from the River Dalaa started to migrate on the same cumulative day-degrees (D°). The study revealed no downstream migration before development of SW tolerance. This emphasizes the importance of incorporating physiological status when studying environmental triggers for downstream migration of S. salar smolts. Overall, these findings suggest that the onset of smolt migration in stocked S. salar smolts was within the smolt window from an osmoregulatory point of view.

In order to investigate how changes in gill Na+, K+-ATPase (NKA) α1a, α1b subunits, Na+, K+, 2Cl− cotransporter (NKCC1) and the apical cyctic fibrosis trans-membrane conductance regulator (CFTR-I) transcripts in wild Atlantic salmon, Salmo salar, smolts are affected by temperature during spring Atlantic salmon parr (initial mean length 12.8 cm, s.d. = 0.6, mean mass 22.1 g, s.d. = 3.2) originating from the Vosso river (western Norway) were reared under three temperature regimes (5.1, 8.1 and 10.8 °C) from March to May. The temperatures reflect the natural temperature range in the study river during late spring. Findings from the present study indicate that smolt development differs within same strains held at different temperatures. Hence an absolute lower temperature limit for smolt development in wild salmon may be difficult to define. Overall present findings indicate that the smolt window will be narrower in warmer water. A temperature controlled smolt window may signify serious implications for smolt survival in wild (i.e., if migrants are delayed by obstacles or if hatchery produced smolts are released to late).

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.

Salmonid alphavirus (SAV) causes pancreatic disease (PD) in salmonids in Northern Europe which results in large economic losses within the aquaculture industry. In order to better understand the underlying immune mechanisms during a SAV3 infection Atlantic salmon post-smolts were infected by either i.m.-injection or bath immersion and their immune responses compared. Analysis of viral loads showed that by 14 dpi i.m.-injected and bath immersion groups had 95.6% and 100% prevalence respectively and that both groups had developed the severe pathology typical of PD. The immune response was evaluated by using RT-qPCR to measure the transcription of innate immune genes involved in the interferon (IFN) response as well as genes associated with inflammation. Our results showed that IFNa transcription was only weakly upregulated, especially in the bath immersion group. Despite this, high levels of the IFN-stimulated genes (ISGs) such as Mx and viperin were observed. The immune response in the i.m.-injected group as measured by immune gene transcription was generally faster, and more pronounced than the response in the bath immersion group, especially at earlier time-points. The response in the bath immersion group started later as expected and appeared to last longer often exceeding the response in the i.m-injected fish at later time-points. High levels of transcription of many genes indicative of an active innate immune response were present in both groups.

Unlike in mammals, persistent postembryonic retinal growth is a characteristic feature of fish, which includes major remodeling events that affect all cell types including photoreceptors. Consequently, visual capabilities change during development, where retinal sensitivity to different wavelengths of light (photopic vision), -and to limited photons (scotopic vision) are central capabilities for survival. Differently from well-established model fish, Atlantic cod has a prolonged larval stage where only cone photoreceptors are present. Rods do not appear until juvenile transition (metamorphosis), a hallmark of indirect developing species. Previously we showed that whole gene families of lws (red-sensitive) and sws1 (UV-sensitive) opsins have been lost in cod, while rh2a (green-sensitive) and sws2 (blue-sensitive) genes have tandem duplicated. Here, we provide a comprehensive characterization of a two-step developing duplex retina in Atlantic cod. The study focuses on cone subtype dynamics and delayed rod neurogenesis and differentiation in all cod life stages. Using transcriptomic and histological approaches we show that different opsins disappear in a topographic manner during development where central to peripheral retina is a key axis of expressional change. Early cone differentiation was initiated in dorso-temporal retina different from previously described in fish. Rods first appeared during initiation of metamorphosis and expression of the nuclear receptor transcription factor nr2e3-1, suggest involvement in rod specification. The indirect developmental strategy thus allows for separate studies of cones and rods development, which in nature correlates with visual changes linked to habitat shifts. The clustering of key retinal genes according to life stage, suggests that Atlantic cod with its sequenced genome may be an important resource for identification of underlying factors required for development and function of photopic and scotopic vision. publishedVersion

In recent years rapidly growing research has led to identification of several fish leptin orthologs and numerous duplicated paralogs possibly arisen from the third and fourth round whole genome duplication (3R and 4R WGD) events. In this study we identify in Atlantic salmon a duplicated LepRA gene, named LepRA2, that further extend possible evolutionary scenarios of the leptin and leptin receptor system. The 1121 amino acid sequence of the novel LepRA2 shares 80% sequence identity with the LepRA1 paralog, and contains the protein motifs typical of the functional (long form) leptin receptor in vertebrates. In silico predictions showed similar electrostatic properties of LepRA1 and LepRA2 and high sequence conservation at the leptin interaction surfaces within the CHR/leptin-binding and FNIII domains, suggesting conserved functional specificity between the two duplicates. Analysis of temporal expression profiles during pre-hatching stages indicate that both transcripts are involved in modulating leptin developmental functions, although the LepRA1 paralog may play a major role as the embryo complexity increases. There is ubiquitous distribution of LepRs underlying pleiotropism of leptin in all tissues investigated. LepRA1 and LepRA2 are differentially expressed with LepRA1 more abundant than LepRA2 in most of the tissues investigated, with the only exception of liver. Analysis of constitutive LepRA1 and LepRA2 expression in brain and liver at parr, post-smolt and adult stages reveal striking spatial divergence between the duplicates at all stages investigated. This suggests that, beside increased metabolic requirements, leptin sensitivity in the salmon brain might be linked to important variables such as habitat, ecology and life cycle. Furthermore, leptins and LepRs mRNAs in the brain showed gene-specific variability in response to long term fasting, suggesting that leptin's roles as modulator of nutritional status in Atlantic salmon might be governed by distinct genetic evolutionary processes and distinct functions between the paralogs.

Background Pancreas disease (PD), caused by salmonid alphavirus (SAV), is an important disease affecting salmonid aquaculture. It has been speculated that Atlantic salmon post-smolts are more prone to infections in the first few weeks following seawater- transfer. After this period of seawater acclimatization, the post-smolts are more robust and better able to resist infection by pathogens. Here we describe how we established a bath immersion (BI) model for SAV subtype 3 (SAV3) in seawater. We also report how this challenge model was used to study the susceptibility of post-smolts to SAV3 infection in two groups of post-smolts two weeks or nine weeks after seawater - transfer. Methods Post-smolts, two weeks (Phase-A) or nine weeks (Phase-B) after seawater- transfer, were infected with SAV3 by BI or intramuscular injection (IM) to evaluate their susceptibility to infection. A RT-qPCR assay targeting the non-structural protein (nsP1) gene was performed to detect SAV3-RNA in blood, heart tissue and electropositive-filtered tank-water. Histopathological changes were examined by light microscope, and the presence of SAV3 antigen in pancreas tissue was confirmed using immuno-histochemistry. Results Virus shedding from the Phase-B fish injected with SAV3 (IM Phase-B) was markedly lower than that from IM Phase-A fish. A lower percentage of viraemia in Phase-B fish compared with Phase-A fish was also observed. Viral RNA in hearts from IM Phase-A fish was higher than in IM Phase-B fish at all sampling points (p

Prolonged production of Atlantic salmon (Salmo salar) post-smolts in closed-containment systems has prompted research into biological requirements under higher production intensities. This study examined the effect of fish density and specific water flow on skin health in post post-smolts particularly focusing on epithelial cell morphology and gene expression. In the density experiment, post-smolts were kept at five different fish densities (25, 50, 75, 100 and 125 kg/m3) at a specific water flow rate of 0.6 l/kg/min. Microscopic examination of fluorescence stained whole-mount skin samples demonstrated differences in epithelial cell morphology with increased spacing between epithelial cells at 50 kg/m3 and 125 kg/m3. Gene expression analysis revealed increased transcription of mucin-like 2, cathepsins B, -D, -L, matrix metalloproteinase 9 and claudin 10 in fish reared at a density of 125 kg/m3, while only matrix metalloproteinase 9 and claudin 10 had increased transcription at a density of 100 kg/m3. Together, these results suggest structural alterations in the skin epithelium at densities ≥ 100 kg/m3. In the specific water flow experiment, four different water flow levels were established (0.2, 0.3, 0.4 and 0.5 l/kg/min) while the fish density was kept constant at 75 kg/m3. After eight weeks, transcription of mucin-like 2 and -5ac, inducible nitric oxide synthase, lysozyme and cathepsin B and -L increased in skin samples from fish reared in tanks with a specific water flow of ≤ 0.3 l/kg/min. Increased transcription of these genes implies activation of stress and immune responses in skin at low specific water flow. Results from this study suggests that skin is a sensitive organ for environmental changes, and suggests several molecular indicators that may be valuable in predicting the effects of varying rearing conditions on skin health. Further validation through long-term studies, combined with other health parameters is required for practical recommendations regarding critical fish density and water flow for optimal fish health and performance in semi-closed production systems. Statement of relevance This paper is of high importance for the industry as it investigates the effects of fish density and specific water flow on skin properties in semi-closed-containment-systems in Atlantic salmon post-smolt production. Increased fish densities and reduced specific water flow can increase production profitability. Thus, biological limits related to these two factors are important to identify to avoid reduced animal welfare.

High levels of hybridization between Atlantic salmon Salmo salar and brown trout Salmo trutta have been reported in the River Driva. This study presents the underlying mechanisms of development of seawater (SW) tolerance and marine migration pattern for S. salar × S. trutta hybrids. Migrating S. salar × S. trutta hybrid smolts caught in the River Driva, Norway (a river containing Gyrodactylus salaris), displayed freshwater (FW) gill Na + ,K + -ATPase (NKA) activity levels of 11·8 μmol ADP mg protein h −1 , which were equal to or higher than activity levels observed in S. salar and S. trutta smolts. Following 4 days of SW exposure (salinity 32·3), enzyme activity remained high and plasma ion levels were maintained within the normal physiological range observed in S. salar smolts, indicating no signs of ion perturbations in S. salar × S. trutta hybrids. SW exposure induced an increase in NKA α1b-subunit mRNA levels with a concurrent decrease in α1a levels. Salmo salar × S. trutta post-smolts migrated rapidly through the fjord system, with increasing speed with distance from the river, as is often seen in S. salar smolts. The present findings suggest that S. salar × S. trutta smolts, as judged by the activity and transcription of the NKA system, regulation of plasma ion levels and migration speed more closely resemble S. salar than S. trutta.

We have examined physiological parameters associated with seawater adaptability, growth and energetics, as well as major endocrine regulators of these processes in wild migrating Atlantic salmon smolts and post-smolts from the river through the fjord, coastal areas and the open ocean. Muscle RNA/DNA ratio suggests that growth rate increases soon after entry into seawater and continues to increase after the post-smolts reach the offshore banks and the feeding grounds in the Norwegian Sea. Post-smolts prioritize rapid growth and protein deposition in spring and summer, and their energy intake during this period is so high that deposition of energy is possible in addition to muscle growth. An increase in thyroxine (T 4 ) and triiodothyronine (T 3 ) levels was observed, suggesting a major activation of hepatic conversion of T 4 to T 3 in post-smolts in seawater, probably related to the high metabolic activity and rapid growth and development of the post-smolts. Decreased plasma growth hormone (GH) levels were observed from the river through the fjord, with levels around 2 ng ml −1 in rapidly growing post-smolts, concurrent with an increase in circulating insulin-like growth factor I (IGF-I). An increase in pituitary GH expression levels and hepatic GH receptor (GH-R) and local IGF-I mRNA levels suggest a physiological basis for the changes in circulating GH and IGF-I levels. Receptor expression in brain and pituitary suggests that both hormones are actively involved in the growth and differentiation of these tissues during the critical early marine phase. Gill Na + ,K + -ATPase (NKA) activity increased to post-smolt levels above 20 μmol ADP mg prot. ÷ 1 h ÷ 1 , probably representing long-term NKA activity levels of Atlantic salmon in seawater. Concurrent with the changes in NKA activity the expression of the NKA α1b isoform remained high in post-smolts, while the expression of the NKA α1a decreased from smolts to post-smolts. Both cystic fibrosis transmembrane conductance regulator (CFTR) I and II showed a reduction in mRNA levels from smolts to post-smolts, and remained stable at low expression levels in seawater.

Acidification is acknowledged as a cause for extinction or catch reductions in numerous rivers supporting Atlantic salmon ( Salmo salar L.) populations in Norway. In freshwater, labile (cationic/inorganic) forms of Al (LAl) accumulate onto and in fish gills, where high concentrations can result in mortality due to respiratory and ionoregulatory dysfunction. At lower concentrations, Al may still have major population effects, mainly through the inhibition of gill Na + ,K + -ATPase activity, reducing hypo-osmoregulatory capacity and thereby affecting marine survival. Following episodic exposure, normal smolt properties are expected to be restored. In 2006 and 2007 we exposed groups of 1100 to 1200 one-year old hatchery reared, Carlin tagged Atlantic salmon smolts of the Imsa strain (South-Western Norway) to moderately acidified water (pH 5.6–5.7; 10–15 μg L Al l −1 ) for 2 to 14 days whereupon they were transferred to a water quality assumed to be satisfactory for smolt (good water; pH 6.8–7.2 and −1 ) for 2 to 14 days to monitor recovery from the prior exposure. Control fish had gill–Al concentrations in the range of 5 to 10 μg Al g − 1 gill dry weight (dw) while Al-exposed fish had gill–Al concentrations exceeding 30 μg Al g − 1 gill dw. Following transfer to good water, gill–Al did not return to control levels within a time span of 14 days. The physiological responses measured as blood ions (Na + , Cl − ), blood acid-base balance (pH, pCO 2 and HCO 3 − ), and blood glucose improved relative to the acid/aluminum (Al) exposed groups, but not to levels measured in the control fish. Mortality was elevated in all Al-exposed/recovery groups following transfer to seawater (34 ppt) and gill Na + ,K + -ATPase was reduced. The results suggest that smolts had not fully recovered 14 days after a short-term acid/Al exposure, where recovery depends on what traits are used as indicators of healthy fish. An Al-exposure experienced by presmolt several weeks prior to the smolt spring migration can have negative population effects, both by reducing hypo-osmoregulatory capacity and by making the fish more vulnerable to secondary stressors in the marine environment.

Acid rain has reduced several salmonid stocks in Norway and salmon lice have been identified as a major population regulation factor. The combination of these two factors has also been seen to reduce postsmolt survival. In the present study, we have addressed the effects of an episodic exposure to acidic water and later recovery in good freshwater quality (Acid/Recovery groups) followed by salmon lice infestation in seawater in more detail. The ecological perspective of this can be directly addressed to changes over the last decades from chronic acidification over Norway and Europe, to more episodic spring acidification of rivers prior to or during downstream migration of smolts. The results showed that salmon lice-induced mortality increased in all episodic Acid/Rec groups. However, the group given the longest recovery period experienced the lowest mortality compared to the other treatment groups. A period of recovery after acid exposure may eventually restore the fish back to normal physiological level, but in the present experiment a period of 14 days of recovery after a 2 day exposure to acid water was too short to fully restore the fish back to normal levels. Even short-time episodic acidification followed by recovery during springtime and the vulnerable smoltification process, may therefore have negative and often unnoticed effects in wild salmonids until the postsmolts meet other stressors in the marine phase such as salmon lice and other fish diseases.

The growth-promoting effects of in vivo growth hormone (GH) treatment were studied in relation to size and lipid content of energy stores including liver, mesentery, white muscle and belly flap in rainbow trout. In order to elucidate endocrine interactions and links to regulation of growth, adiposity and energy metabolism, plasma levels of GH, insulin-like growth factor I (IGF-I), leptin (Lep) and ghrelin, were assessed and correlated to growth and energy status. In addition tissue-specific expression of lepa1 mRNA was examined. Juvenile rainbow trout were implanted with sustained-release bovine GH implants and terminally sub-sampled at 1, 3 and 6 weeks. GH increased specific growth rate, reduced condition factor (CF) and increased feed conversion efficiency resulting in a redistribution of energy stores. Thus, GH decreased mesenteric (MSI) and liver somatic index (LSI). Lipid content of the belly flap increased following GH-treatment while liver and muscle lipid content decreased. Independent of GH substantial growth was accompanied by an increase in muscle lipids and a decrease in belly flap lipids. The data suggest that the belly flap may function as an energy buffering tissue during episodes of feeding and lean growth. Liver and muscle lipids were positively correlated to body weight, indicating a size-dependent change in adiposity. Hepatic lepa1 mRNA positively correlated to MSI and CF and its expression decreased following GH treatment, coinciding with decreased hepatic lipid content. Plasma Lep was positively correlated to MSI and belly flap lipid content, suggesting that Lep may communicate energy status. In summary, the observed GH tissue-specific effects on lipid metabolism in rainbow trout highlight the complex physiology of the energy reserves and their endocrine control.