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Sea trout face growth-mortality trade-offs when entering the sea to feed. Salmon lice epizootics resulting from aquaculture have shifted these trade-offs, as salmon lice might both increase mortality and reduce growth of sea trout. We studied mortality and behavioural adaptations of wild sea trout in a large-scale experiment with acoustic telemetry in an aquaculture intensive area that was fallowed (emptied of fish) synchronically biannually, creating large variations in salmon lice concentrations. We tagged 310 wild sea trout during 3 years, and gave half of the individuals a prophylaxis against further salmon lice infestation. There was no difference in survival among years or between treatments. In years of high infestation pressure, however, sea trout remained closer to the river outlet, used freshwater (FW) habitats for longer periods and returned earlier to the river than in the low infestation year. This indicates that sea trout adapt their migratory behaviour by actively choosing FW refuges from salmon lice to escape from immediate mortality risk. Nevertheless, simulations show that these adaptations can lead to lost growth opportunities. Reduced growth can increase long-term mortality of sea trout due to prolonged exposure to size-dependent predation risk, lead to lower fecundity and, ultimately, reduce the likelihood of sea migration. [ABSTRACT FROM AUTHOR]

Despite satisfactory reactions to seawater challenge tests indicative of appropriate physiological state, hatchery-reared Atlantic salmon Salmo salar smolts stocked in the Eira River in Norway between 2001 and 2011 performed less well at sea in terms of growth, age at maturity and survival than smolts of natural origin. The mean rates of return to the river for hatchery-reared and naturally produced S. salar were 0·98 and 2·35%. In the Eira River, c. 50 000 hatchery-reared S. salar smolts of local origin were stocked annually to compensate for reduced natural smolt production following regulation for hydroelectric purposes, while a mean of 17 262 smolts were produced naturally in the river. This study demonstrates that, although captive S. salar perform well in seawater challenge tests, hatchery-reared smolts are not necessarily as adaptable to marine life as their naturally produced counterparts. These findings suggest that production of hatchery-reared smolts more similar to naturally produced individuals in morphology, physiology and behaviour will be necessary to improve success of hatchery releases. Where possible, supplementary or alternative measures, including habitat restoration, could be implemented to ensure the long-term viability of wild stocks. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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; 10–15μgL All−1) for 2 to 14days whereupon they were transferred to a water quality assumed to be satisfactory for smolt (good water; pH 6.8–7.2 and <6μgL All−1) for 2 to 14days to monitor recovery from the prior exposure. Control fish had gill–Al concentrations in the range of 5 to 10μg Alg−1 gill dry weight (dw) while Al-exposed fish had gill–Al concentrations exceeding 30μg Alg−1 gill dw. Following transfer to good water, gill–Al did not return to control levels within a time span of 14days. The physiological responses measured as blood ions (Na+, Cl−), blood acid-base balance (pH, pCO2 and HCO3 −), 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 (34ppt) and gill Na+,K+-ATPase was reduced. The results suggest that smolts had not fully recovered 14days 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. [Copyright &y& Elsevier]

Abstract: 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 14days of recovery after a 2day 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. [Copyright &y& Elsevier]

The anadromous life cycle of Atlantic salmon Salmo salar involves long migrations to novel environments and challenging physiological transformations when moving between salt-free and salt-rich waters. In this article, (1) environmental factors affecting the migration behaviour and survival of smolts and post-smolts during the river, estuarine and early marine phases, (2) how behavioural patterns are linked to survival and (3) how anthropogenic factors affect migration and survival are synthesized and reviewed based on published literature. The timing of the smolt migration is important in determining marine survival. The timing varies among rivers, most likely as a consequence of local adaptations, to ensure sea entry during optimal periods. Smolts and post-smolts swim actively and fast during migration, but in areas with strong currents, their own movements may be overridden by current-induced transport. Progression rates during the early marine migration vary between 0·4 and 3·0 body lengths s−1 relative to the ground. Reported mortality is 0·3-7·0% (median 2·3) km−1 during downriver migration, 0·6-36% (median 6·0) km−1 in estuaries and 0·3-3·4% (median 1·4) km−1 in coastal areas. Estuaries and river mouths are the sites of the highest mortalities, with predation being a common cause. The mortality rates varied more among studies in estuaries than in rivers and marine areas, which probably reflects the huge variation among estuaries in their characteristics. Behaviour and survival during migration may also be affected by pollution, fish farming, sea lice Lepeophtheirus salmonis, hydropower development and other anthropogenic activities that may be directly lethal, delay migration or have indirect effects by inhibiting migration. Total mortality reported during early marine migration (up to 5-230 km from the river mouths) in the studies available to date varies between 8 and 71%. Hence, the early marine migration is a life stage with high mortalities, due to both natural and human influences. Factors affecting mortality during the smolt and post-smolt stages contribute to determine the abundance of spawner returns. With many S. salar populations in decline, increased mortality at these stages may considerably contribute to limit S. salar production, and the consequences of human-induced mortality at this stage may be severe. Development of management actions to increase survival and fitness at the smolt and post-smolt stages is crucial to re-establish or conserve wild populations. [ABSTRACT FROM AUTHOR]

The early marine migratory behaviour of two populations of hatchery-reared Atlantic salmon Salmo salar was compared in a common-garden experiment. Post-smolts from a river in a long fjord (Lærdal River, 144 km from the open coastline, n = 79) and a short fjord (Flekke River, 20 km from the open coastline, n = 80) in western Norway were tagged with acoustic transmitters and released during the spring of 2005 and 2006 in the inner part of the Hardangerfjord system (Opo River mouth, 179 km from the open coastline). The migratory behaviour of the tagged fish was monitored by acoustic listening stations in the fjord system up to 167 km from the release site. The Lærdal fish began migrating before the Flekke fish and had higher progression rates in the middle part of the fjord system. A greater number of Lærdal fish was detected along the most direct migratory route and in the outermost part of the Hardangerfjord system, which is indicative of a higher survival. The results from this study demonstrate differences in early marine migratory behaviour between S. salar from two different stocks and suggest that the distance a S. salar population travels to reach the open coastline may influence its early marine migratory behaviour and performance. The selective pressures of marine predation and arrival time at feeding areas in the ocean may be stronger for stocks with a longer inshore migration, creating more efficient migrants over time. [ABSTRACT FROM AUTHOR]

The abundance of returning adult Atlantic salmon Salmo salar, in the River Orkla in mid-norway (1 sea-winter, SW, fish) and River Hals in north Norway (1–3 SW fish), was tested against the early marine feeding and the seawater temperature experienced by their corresponding year classes of post-smolts immediately after entry into the Trondheimsfjord (Orkla smolts, 22 years of data) and Altafjord (Hals smolts, 17 years of data). In both river–fjord systems, there was a significant positive correlation between the abundance of returning S. salar and the mean seawater temperature at the time of smolts descending to the sea. The number of 1SW fish reported caught in River Orkla was positively correlated to the proportion of fish larvae in the post-smolt stomachs in Trondheimsfjord. The abundance of returning S. salar was, however, neither correlated to forage ratio ( RF) nor other prey groups in post-smolt stomachs in the two fjord systems. In the Altafjord, the post-smolts fed mainly on pelagic fish larva (70–98%) and had a stable RF (0·009–0·023) over the 6 years analysed. In the Trondheimsfjord, however, there was a higher variation in RF (0·003–0·036), and pelagic fish larvae were dominant prey in only two (50 and 91%) of the 8 years analysed. These 2 years also showed the highest return rates of S. salar in River Orkla. These results demonstrate that the thermal conditions experienced by post-smolts during their early sea migration may be crucial for the subsequent return rate of adults after 1–3 years at sea. Pelagic marine fish larvae seem to be the preferred initial prey for S. salar post-smolts. As the annual variation in abundance of fish larvae is related to seawater temperature, it is proposed that seawater temperature at sea entry and the subsequent abundance of returning adult S. salar may be indirectly linked through variation in annual availability of pelagic fish larvae or other suitable food items in the early post-smolt phase. [ABSTRACT FROM AUTHOR]

Eight hatchery-reared Atlantic salmon Salmo salar post-smolts, implanted with acoustic depth sensing transmitters and manually tracked for 5–12 h in the Hardangerfjord (Norway), spent most of their time (49–99%) at 1–3 m depth during the day, whereas four of seven fish tracked were found close (<0·5 m) to the surface at night, with a strong negative cross-correlation between general swimming depth and surface light intensity. Hence, the actual swimming depth of post-smolts during their early marine migration may depend on the light conditions, although the individual variation in vertical movement pattern was large. No cross-correlations were found between light intensity and swimming depth during daytime periods with rapid changes in light intensity, indicating that other factors than light intensity were important in initiating the irregular dives that were recorded down to 6·5 m depth. [ABSTRACT FROM AUTHOR]

Three tagged Atlantic salmon Salmo salar were recaptured as subadults or adults (1·4–3 kg) between 70·5 and 78° N in the western Barents Sea, two of which originated from the Alta Fjord region in northern Norway and one from the Drammen River, south-eastern Norway. An additional tag was recovered from the stomach of a Greenland halibut Reinhardtius hippoglossoides captured south-west of Bear Island at >600 m depth; this tag was from a smolt released in the River Alta 1 month earlier. These are the northernmost tag recoveries reported for Atlantic salmon, and indicate that Norwegian Atlantic salmon, especially the fish from northern populations, may use the northern Barents Sea as a feeding area during part of their life cycle. [ABSTRACT FROM AUTHOR]

Acidification has caused the loss or reduction of numerous Atlantic salmon (Salmo salar L.) populations on both sides of the North Atlantic. Acid deposition peaked in the 1980's and resulted in both chronically and episodically acidified rivers. At present, water quality is improving in all affected rivers due to reduced acid deposition. However, spring snow melt, heavy rainfall and sea salt episodes can still cause short term drops in pH and elevated concentrations of bioavailable aluminum. Technical malfunction in lime dozers will cause short termed episodic spates in the limed rivers. The current situation has prompted a need for dose-response relationships based on short term exposures of Atlantic salmon to assess the potential population effects of episodic acidification. Water quality guidelines for salmon have been lacking, despite a large number of experiments, all demonstrating dose-response relationships between water chemistry and fish health. We have summarized results from 347 short-term (<14 days) exposures of salmon parr and smolt performed between 1990 and 2003 in Norway. The experiments have been performed as bioassays, where fish have been exposed in tanks fed river water, in tanks where the river water quality has been manipulated (added H+ and Al) and as Carlin-tagged smolt releases after preexposure to moderately acidic waters. The results from the various bioassays are compared to water quality limits proposed on basis of the relationship between water quality and population status/health in Norwegian rivers. The focus of this article is placed on chemical-biological interactions that can be drawn across experiments and exposure protocols. We propose dose-response relationships for acid neutralizing capacity (ANC), pH, cationic Al and gill accumulated Al, versus mortality in freshwater, effects on hypo-osmoregulatory capacity in seawater challenge tests and on smolt to adult survival in release experiments. The "no effect" dose depends on the life history stage tested and on the sensitivity of the biomarkers. Parr are more tolerant than smolt. Concentrations of Al that have no significant impact on freshwater life history stages can still have major population effects if they occur prior to smolt migration. While smolt can survive in freshwater for a prolonged period of time (>10 days) at an Al dose resulting in a gill Al concentration of up to 300 μg Alg-1 dw, a 3 day exposure resulting in a gill Al accumulation in the range of 25 to 60 μg Alg-1 dw reduces smolt to adult survival in a dose related manner by 20 to 50%. For smolt to adult survival, the biological significant response is delayed relative to the dose and occurs first after the fish enters the marine environment. In addition to exposure intensity and timing, exposure duration is important for the setting of critical limits. [ABSTRACT FROM AUTHOR]

Abstract: Salmon populations on the western coast of Norway may experience both moderately acidified rivers and salmon lice (Lepeophtheirus salmonis Krøyer) attacks. The present study addresses the question of interactive effects of acidification and salmon lice infestation on postsmolt survival. Three groups each of approximately 1500 one-year old Atlantic salmon smolts of the Imsa strain, South-Western Norway, were exposed to one of three suboptimal water qualities (high acid, moderate acid and episodic acid) and experienced acidic water (pH 5.6–5.9 and 7–45 μg Ali/l) with different exposure duration (3 to 10 days). A fourth group exposed to pH>6.6 and<9 μg Ali/l acted as control (reference group). After freshwater exposure, smolts (n =150) from each group were moved into tanks containing brackish water (16‰) and after 8 h they were given full strength seawater (33‰) and given 1 day of recovery before being infected with salmon lice copepodids. Four non-infected groups (n =100) from the same exposures acted as controls. Over a 42 day period, postsmolts were regularly inspected and sampled for mortality, lice density and physiological status in seawater. The lice per smolt density were highest in the episodic acid group, followed by the high acid, moderate acid and the reference groups. Mortality was low in the four non-infected control groups, and significantly elevated in the lice infected groups (high acid>moderate acid>episodic acid>reference). Plasma chloride levels were within the normal range in the non-infected groups, while fish in the infected high acid and moderate acid groups had elevated plasma chloride levels. High gill aluminium was seen in the three exposure groups in freshwater. Year to year variations in acidification pressure and salmon lice densities can singularly and in combination explain some of the year to year variations in postsmolt survival and hence the variations in Atlantic salmon year–class strength in Norwegian rivers. [Copyright &y& Elsevier]

Abstract: Acidification is acknowledged as the cause for extinction or catch reductions in numerous Atlantic salmon (Salmo salar L.) populations in Norway. In freshwater, labile (cationic/inorganic) forms of Al (LAl) accumulate in fish gills, where high concentrations result in mortality due to respiratory and ionoregulatory dysfunction. At lower concentrations, Al may still have population effects by inhibiting gill Na+,K+-ATPase activity, thereby reducing hypoosmoregulatory capacity and marine survival. Over the years 1999 to 2003 we exposed groups of 1150 to 1200 one-year old hatchery reared, Carlin tagged Atlantic salmon smolts of the Imsa strain (South-Western Norway) to moderately acidified water (pH 5.8; 5–15 μg LAl L−1) from 3 (short term exposure) to 60 (long term exposure) days. Fish exposed to Lake Imsa water (pH>6.5 and <5 μg LAl L−1) acted as controls. 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 20 μg Al g−1 gill dw prior to seawater release. The physiological responses measured as plasma Cl− and glucose were related to the LAl concentration in water and to the accumulation of Al onto the gills. Gill Na+,K+-ATPase activity was depressed in all groups having >25 μg Al g−1 gill dw. Following exposure, the smolts were released into River Imsa to monitor downstream migration and ocean return rates. Acid exposed smolts migrated out of the river together with controls. Adult return rates were reduced by 20 to 50% in all Al-exposed groups relative to the control groups, although marine growth was unaffected. The results suggest that even moderately and/or episodically acidified rivers containing 5–15 μg LAl L−1 can cause substantial reductions in returns of Atlantic salmon. [Copyright &y& Elsevier]

A synthesis of results from two projects was assessed to analyse possible influence of sea lice Lepeophtheirus salmonis on marine Atlantic salmon Salmo salar survival. During the years 1992–2004, trawling for wild migrating post-smolts was performed in Trondheimsfjord, a fjord in which no Atlantic salmon aquaculture activity is permitted. Prevalence and intensity of sea lice infections on migrating wild post-smolts differed between years. A correlation analysis between 1 sea-winter (SW) Atlantic salmon catch statistics from the River Orkla (a Trondheimsfjord river) and sea lice infections on the migrating smolts in the Trondheimsfjord was not significant. Up to 2% reduction in adult returns due to sea-lice infection was expected. In addition, experimental releases from 1996 to 1998 with individually tagged groups of hatchery-reared Atlantic salmon smolts given protection against sea-lice infection was performed. Higher recaptures of adult Atlantic salmon from 1998 treated smolts compared to the control group may correspond to high abundance of sea lice found on the wild smolt, and may indicate influence on post-smolt mortality. These studies indicate that post-smolt mortality in Trondheimsfjord is marginally influenced by sea lice infection; however, the methods for assessing wild smolt mortality might be insufficient. Higher infections of sea lice farther out in the fjord may indicate more loss in Atlantic salmon returns in some years. [ABSTRACT FROM AUTHOR]

The article features a study on the Atlantic salmon. The development of small-sized acoustic transmitters that could be applied to wild Atlantic salmon smolts makes it possible to access information such as swimming speeds, migration efficiency and orientation that is essential for understanding the factors affecting salmons. The study also focuses in recording the observed and actual swimming speeds and directions of wild Atlantic salmon post-smolts based on simultaneous recordings of fish movements and water current, and to compare these with results from larger hatchery-reared post-smolts.

The community structure of trophically transmitted intestinal helminths of Atlantic salmonSalmo salarpost-smolts was highly variable among four fjords in Norway. There were no severely pathogenic parasite species. Post-smolts from the southernmost Trondheimsfjord had a higher diversity of freshwater parasite species compared to the three northern fjords(Tanafjord, Altafjord and Malangen). In contrast, the highest diversity and proportion of marine species was found in the three northern fjords. Post-smolts were generally more infected with marine parasites in the outer rather than inner parts of all of the fjords. The prevalence of the acanthocephalanEchinorynchus gadi(range: 13–42%) and marine trematodes(range: 14–47%) was higher in post-smolts in outer zones of the northern fjords than in fish from Trondheimsfjord(0 and 6%, respectively). The within-fjord variability and north-south geographical gradient in parasite infection patterns reflected differences in marine feeding of the post-smolts on potential intermediate hosts such as amphipods( E. gadi) and fish larvae(trematodes), which were higher in the northern fjords(range: 27–28 and 67–85%, respectively) than in Trondheimsfjord(5 and 19%, respectively). High intensities of marine parasites suggest that some post-smolts from northern fjords may have a prolonged fjord-feeding compared to those from Trondheimsfjord. Parasites of both freshwater and marine origin appear to be suitable as bio-indicators of feeding and migratory pattern of Atlantic salmon post-smolts and preadults during their seaward migration. [ABSTRACT FROM AUTHOR]

Hatchery-reared Atlantic salmonSalmo salar( n = 25) and wild anadromous brown trout(sea trout) Salmo trutta( n = 15) smolts were tagged with coded acoustic transmitters and released at the mouth of the River Eira on the west coast of Norway. Data logging receivers recorded the fish during their outward migration at 9, 32, 48 and 77 km from the release site. Seventeen Atlantic salmon(68%) and eight sea trout(53%) were recorded after release. Mean migratory speeds between different receiver sites ranged from 0·49 to 1·82 body lengths(total length) per second(bl s−1) for Atlantic salmon and 0·11–2·60 bl s−1 for sea trout. Atlantic salmon were recorded 9, 48 and 77 km from the river mouth on average 28, 65 and 83 h after release, respectively. Sea trout were recorded 9 km from the release site 438 h after release. Only four(23%) sea trout were detected in the outer part of the fjord system, while the rest of the fish seemed to stay in the inner fjord system. The Atlantic salmon stayed for a longer time in the inner part than in the outer parts of the fjord system, but distinct from sea trout, migrated through the whole fjord system into the ocean. [ABSTRACT FROM AUTHOR]

Atlantic salmon Salmo salar were infected with sea lice Lepeophtheirus salmonis(0·08 ± 0·007 sea lice g−1) over a period of 4 h. Both infected and non-infected fish were swim tested in sea water (SW) and fresh water (FW). The ventral aorta of each fish was fitted with a Doppler cuff in order to measure cardiac output, stroke volume and heart rate during swim testing. Blood samples were taken at rest and after exercise. Critical swimming speed of infected fish in SW (2·14 ± 0·08 body lengths, bl s−1) was significantly lower ( P < 0·05) than infected fish switched to FW (2·81 ± 0·08 bl s−1) and non-infected fish in SW (2·42 ± 0·04 bl s−1) and FW (2·61 ± 0·08 bl s−1). Cardiac and blood results indicated infected fish exposed to FW did experience stress, but relief from osmotic and ionic distress probably reduces energy expenditure, allowing the increase in performance. As the performance of sea lice-infected fish improved upon transfer to FW, it is likely that heavily infected salmonids do return to FW to restore compromised osmotic and ionic balance, and remove sea lice in the process. [ABSTRACT FROM AUTHOR]

Atlantic salmon Salmo salar were infected with two levels of sea lice Lepeophtheirus salmonis (0.13 ± 0.02 and 0.02 ± 0.00 sea lice g[SUP-1]). Once sea lice became adults, the ventral aorta of each fish was fitted with a Doppler cuff to measure cardiac output (Q), heart rate (f[SUBH]) and stroke volume (V[SUBS]) during swimming. Critical swimming speeds (U[SUBcrit]) of fish with higher sea lice numbers [2.1 ± 0.1 BL(body lengths) s[SUP-1]] were significantly lower (P <0.05) than fish with lower numbers (2.4 ± 0.1 BL s[SUP-1]) and controls (sham infected, 2.6 ± 0.1 BL s[SUP-1]). After swimming, chloride levels in fish with higher sea lice numbers (184.4 ± 11.3 mmol l[SUP-1]) increased significantly (54%) from levels at rest and were significantly higher than fish with fewer lice (142.0 ± 3.7 mmol l[SUP-1]) or control fish (159.5 ± 3.5 mmol l[SUP-1]). The f[SUBH] of fish with more lice was 9% slower than the other two groups at U[SUBcrit]. This decrease resulted in Q not increasing from resting levels. Sublethal infection by sea lice compromised the overall fitness of Atlantic salmon. The level of sea lice infection used in the present study was lower than has previously been reported to be detrimental to wild Atlantic salmon. [ABSTRACT FROM AUTHOR]

Anadromous juvenile Arctic charr (Salvelinus alpinus) were caught in the Sila River (66°21′N, 13°10′E, Nordland County, Norway) by use of a fish trap during their migration towards the sea in May/June. The absence of tags (not captured before) and examination of otoliths from sacrificed fish revealed that none of the experimental charr had previously encountered seawater. During 4 days of exposure to seawater, only minor changes in blood plasma osmolality, and blood plasma concentrations of Na+, Cl– and Mg2+ were detected. An increase in Na+-K+-ATPase activity was measured during the exposure period. These results verified that first-time migrants exhibit hypoosmoregulatory capacity similar to that of smoltified Atlantic salmon (Salmo salar). Accordingly, the Arctic charr first-time migratory behaviour, including late descent and a possible stay within an estuary, cannot be taken as evidence of a poorly developed seawater tolerance. [ABSTRACT FROM AUTHOR]

Abstract The effects of metomidate anaesthesia on levels of plasma cortisol, glucose, haematocrit and chloride in Atlantic salmon (1[sup +]) (Salmo salar L.), after a 2-h transport and during a 48-h recovery period were investigated. The use of metomidate anaesthesia during transport led to a reduced release of cortisol and significantly lower levels of plasma cortisol after a 48-h recovery period. Plasma glucose did not return to basal level after a 48-h recovery period, indicating that even longer recovery may be needed for the fish to return to a pre-stress state. The results show that metomidate anaesthesia combined with a recovery period lessens the stress burden imposed by hauling and transport. [ABSTRACT FROM AUTHOR]

Abstract Hatchery-reared 1-year-old Atlantic salmon post-smolts (Salmo salar L.), artificially infected with salmon lice [Lepeophtheirus salmonis (Krøyer)] copepodids, were found to suffer from primary alterations (increased cortisol levels) at early lice stages. Secondary alterations, such as osmotic stress (increased chloride levels), first occurred after the preadult stages of the lice appeared. Fish with the highest salmon lice infections died throughout the experiment. Seven years of field investigation of Trondheimsfjorden showed that Atlantic salmon post-smolts descending coastal waters can become heavily infected with salmon lice. The migrating post-smolts were only infected with the chalimus stages, showing that the fish had only recently left the rivers. The infection level, however, varied considerably between the years, and, in 1998, the infection was higher than previous years. The experimental results have been combined with the field data to appraise the consequences of the infection. [ABSTRACT FROM AUTHOR]

Investigates the prevalence and mean intensity of salmon lice, Lepeophtheirus salmonis, infection on wild Atlantic salmon. Description and characteristics of salmon louse; Physiological impact of the lice on Atlantic salmon; Prevalence of salmon lice in sea water.

Endurance of farmed and sea-ranched Atlantic salmon Salmo salar L. males was analysed during spawning time. The fish were endurance tested at different swimming speeds (1.6–2.1 m s-1) in forced swim trials. The sea-ranched males (51–65 cm, n = 20) fatigued significantly earlier than the farmed males(59–72 cm, n = 20), although the sea-ranched males were significantly smaller than the farmed males. When the size difference between the two groups were corrected for, no significant difference in the endurance of the two groups was found. Farmed salmon had a significant higher fat content in white muscle (4.7%) than sea-ranched salmon (1.1%). [ABSTRACT FROM AUTHOR]

The authors comment on the article "Impact of Lepeophtheirus salmonis infestations on migrating Atlantic salmon, Salmo salar L., smolts at eight locations in Ireland with an analysis of lice-induced marine mortality," by D. Jackson and colleagues. Discussion and highlights are given on Jackson and colleagues' conclusion regarding the role of sea lice in salmon survival.

Release strategies of hatchery-reared Atlantic salmon, Salmo salar L., smolts were compared by studying survival and migration of smolts ( n = 99) and their predators (Atlantic cod, Gadus morhua L., n = 8; and saithe, Pollachius virens (L.), n = 2) during the first 37 km of the marine migration using acoustic telemetry. Survivorship was higher in smolts released at the river mouth (30%) compared with smolts released in the river (12%). This was likely due to mortality or reduced migratory behaviour in fresh water. The marine mortality was 37% during the first 2 km after leaving the river (at least 25% mortality because of predation from marine fishes), and total marine mortality over 37 km was 68%. Detection-depth data were useful for evaluating whether the tagged smolts were alive or predated; mortality during the first 2 km of outward migration would have been underestimated at 26% instead of 37% without the analysis of depth detection. Transmitters from consumed post-smolts remained in predators for up to 47 days (average 29 days). [ABSTRACT FROM AUTHOR]

Understanding the tolerance limits of fish is crucial for developing aquaculture operations that ensure good animal welfare. However, there exist little data describing the physiological responses in farmed Atlantic salmon, much because the technological tools for taking such measurements have not existed. Recent advances in electronic implants have enabled concurrent measurement of electrocardiogram (ECG) and photoplethysmograms (PPG) in salmon that can be used for robust estimation of HR and oxygen saturation in arterial blood (i.e., S p O 2 /pulse oximetry) if appropriate strategies for motion artifact and light scattering compensation can be realized. To enable pulse oximetry for farmed Atlantic salmon (and fish in general), two experiments have been conducted. In Experiment 1, PPGs were obtained from salmon induced to swim at two different water currents under normoxic conditions. By using two water currents, the resulting data provided a foundation for developing methods for motion artifact compensation. Data from this experiment were also used to calculate an average light scattering parameter using the modified Beer–Lambert law, under the assumption that S p O 2 was 100 % for individual fish. In Experiment 2, fish were placed in a swim tunnel and subjected to hypoxic conditions and corresponding changes in S p O 2 were estimated using the motion artifact and light scattering compensation approaches from Experiment 1. Results show that the suggested compensation approaches gives S p O 2 estimates within the expected range (95 % to 100 %) under normoxic conditions. Under hypoxic conditions, changes in S p O 2 that coincide with experiment events were observed, demonstrating that PPGs can be used to quantify such changes. The results from this pilot study therefore extend the selection of physiological parameters feasible to measure using electronic implants for Atlantic salmon. In doing so, the scope for physiological measurements is extended such that an improved understanding of physiological responses and tolerances in Atlantic salmon farming can be acquired, and ultimately be used to improve animal welfare in fish production. • Animal welfare is a challenge in Atlantic salmon farming. • Novel biologging implants can measure PPGs for pulse oximetry. • Light scattering can be compensated for in data from free swimming fish. • Motion artifacts can be compensated for in data from free swimming fish. • SpO2 can be estimated using a pulse oximetry implant in Atlantic salmon. [ABSTRACT FROM AUTHOR]

We investigated the relationship between telemetry measurements of heart rate and swimming activity and the physiological status in farmed Atlantic salmon (Salmo salar) to assess the potential to use telemetry measurements as proxies for stress. Sensor tags measuring heart rate and swimming activity were surgically implanted into the peritoneal cavity of Atlantic salmon individuals kept in tanks. Four tanks were stocked with three tagged fish and four untagged cohabitants, while two additional tanks containing 16 untagged fish were used as reference groups. Following surgery, tagged fish were kept undisturbed for 14 days as acclimation period. All fish were then subjected to physical stress by reducing the tank water level in 4 consecutive rounds, after which they were left undisturbed for another ten days before the experiment ended. Plasma cortisol, glucose, lactate and osmolality were measured to assess stress levels from fish in the reference groups before and after being subjected to stressing and from all fish at the end of the experiment. Both heart rate and swimming activity rose after the stress treatment, remaining elevated for 24.5 and 16.2 Hrs respectively. Glucose, plasma cortisol, lactate and osmolality levels were significantly greater when measured immediately after stress. Results from the experiment indicate that heart rate and swimming activity can be used as proxies for fish stress, thus opening the possibility for on-line stress monitoring in full scale production. • Atlantic salmon were implanted with electronic sensors. • Experiment animals were subject to reduced water level as repeating stressor. • Heart rate and activity increased. • Stress elevated plasma cortisol, glucose, lactate and osmolality. • Measurements from implants were linked to stress levels. [ABSTRACT FROM AUTHOR]