Lumpfish are currently used as a part of the overall strategy for the prevention and control of sea lice in Atlantic salmon farming in Norway. Their use as biological delousers is advantageous as it exposes salmon to very little stress and injuries. However, there is a growing concern and criticism about the welfare and survival of lumpfish in salmon farms. This study monitored the welfare and survival of lumpfish from four commercial salmon farms for 6 to 12 months in 2019???2021. Environmental and production conditions were also registered. Lumpfish welfare decreased during the study period, mainly related to eye and skin injuries. Mortality rate was low in the first few months after transfer to sea, but increased thereafter. Mechanical delousing affected the welfare and survival most negatively of all monitored external parameters. Even though diseases are often pointed out as the main factor for mortality of lumpfish in sea cages, it was not the case in the present study. However, there were indications that diseases are more likely to occur when lumpfish is already weakened by other factors such as mechanical delousing. One to two thirds of lumpfish were assessed as underweighted or emaciated at all time during the monitoring period, suggesting that the nutritional requirements of lumpfish are not fully met in sea cages.

Lumpfish are currently produced and utilized as cleaner fish, to control sea lice infestation rates in salmon net pens, but information on environmental requirements is still limited. This study aimed to determine the zone of environmental hypoxia for two relevant fish sizes (15 and 60 g) and temperatures (5 and 12 degrees C), using intermittent flow respirometry (referred to as 15:5, 15:12, 60:5, 60:12), and to investigate parameters of stress in response to acute changes in dissolved oxygen (DO, % air saturation) from normoxia to 47, 63, 98 (control), 148 and 194% O-2 at 10 degrees C. The standard and maximal metabolic rates (SMR and MMR) were measured in normoxia (n = 8), and MMR was measured at 5-6 DO levels ranging from 20_160% O-2 (n = 8 per DO) to define the upper and lower boundaries of the hypoxic zone (DOlim and DOcrit). SMR, MMR and the aerobic metabolic scope (AS) increased with temperature and decreased with fish size. Similar effects of temperature and size were found on DOcrit - DOlim ranges: 20-55 (15:5), 35-147 (15:12), 21-53 (60:5) and 22-89 (60:12) % O-2 air saturation. Results from acute exposure tests resulted in elevated cortisol levels at 63 and 47% O-2, although not statistically significant at 47% O-2. Other parameters of hypoxic or hyperoxic stress (lactate, pH, osmolality, lipid peroxidation rates, catalase activity) were not affected. Results from the present study suggest that lumpfish may experience oxygen levels in sea cages that restricts metabolism, performance and induce hypoxic stress.

In this mini review, we systematized current knowledge about the number of Caligus elongatus on farmed Atlantic salmon in relation to the use of lumpfish as cleaner fish. The review was prompted by reports of an unusually large number of sea lice (C. elongatus) infestation of farmed salmon in northern Norway, Faroese Islands and Iceland and the urgent need to determine if common lumpfish can be used to reduce the number on farmed Atlantic salmon by actively grazing on sea lice. Available data from Norway clearly indicate that lumpfish grazes on C. elongatus, and it is possible to enhance this grazing with the assistances of live-feed conditioning prior to sea pen transfer and selective breeding. Observations from Iceland, Faroese Islands and Scotland also in-dicate that lumpfish can effectively lower infestations of C. elongatus on salmon. Overall, this mini review expresses that lumpfish can actively lower the number of C. elongatus on farmed Atlantic salmon.

This review was prompted by reports of unusually large numbers of sea lice tentatively identified as Caligus elongatus infesting farmed salmon in northern Norway. Following a brief introduction to the sea lice problem in salmonid aquaculture, the review is divided into a further eight sections. The first is a review of existing information on the life cycle and behaviour of Caligus spp. The second is a description of the morphology of different stages in the life cycle of C. elongatus. The third describes the effects of caligid infestations on salmonid hosts. The fourth reviews information on the geographical distributions and host preferences of the six species of Caligus reported from farmed salmonids in different parts of the world: C. elongatus, C. curtus, C. clemensi, C. rogercresseyi, C. teres and C. orientalis. The fifth section describes interactions between farmed and wild fish and the sixth presents information on the genetics of C. elongatus. A section reviewing the different methods used to control sea lice infestations follows. The eighth section discusses the predicted effects of climate change and invasive host species on the distribution and occurrence of caligid copepods, and the ninth gives conclusions and recommendations on how to further investigate the infestation that prompted this review. These include the confirmation of the identity of the caligid causing the problem, confirmation of the genotype involved and a study of the vertical distribution in the water column of the infective stages.

In salmon farming, the use of sterile triploids (3N) can mitigate the problem of escapees interbreeding with wild salmon. However, triploid salmon appear less tolerant to high water temperatures and low oxygen levels compared to diploids (2N). To investigate how the thermal performance and physiology of large (2.5 kg) triploid Atlantic salmon Salmo salar L. differs from those of diploids, both ploidies were subjected to water temperatures between 3 and 18 degrees C. The fish were exposed to reduced oxygen saturations (O-2 sat, 70%), termed hypoxia, at 6 and 18 degrees C. Triploids fed more than diploids between 3 and 9 degrees C and at similar levels at 12 degrees C. At 15 degrees C, the feed intake significantly dropped in both ploidies, although more in triploids. During hypoxia, feed intake was higher in triploids at 6 degrees C and equal to diploids at 18 degrees C. The overall feed conversion ratio was similar between ploidies. Muscle energy phosphates were generally lower in triploids than diploids, while muscle glucose, blood haemoglobin and haematocrit were lower in triploids than diploids at >= 12 degrees C. Plasma lactate levels tended to be higher in triploids and increased with increasing temperature and at hypoxia in both ploidies. Plasma cortisol increased in both ploidies at high temperatures and was highest in triploids under hypoxic conditions at 18 degrees C. Triploids had a higher cataract score at the start of the experiment and developed more cataracts throughout the experiment. The present findings show that large triploid Atlantic salmon perform better at colder water temperatures compared to diploids and differ in parts of their physiological expression at increasing and high temperature.

The use of sterile triploids in Atlantic salmon aquaculture would mitigate the environmental risks associated with introgressive hybridization between escaped farmed and wild Atlantic salmon. However, production of farmed triploid salmon is limited due to reports of poorer growth and higher mortality when compared to diploids, in particular under sub-optimal environmental conditions. To address these concerns, we monitored triploid and diploid Atlantic salmon post-smolts at temperatures between 3 and 18 degrees C and 100% oxygen saturation (O-2 (sat)), and additional periods of 60% O-2 (sat) (hypoxia) at 6 or 18 degrees C, respectively. Feed intake and oxygen consumption rate were monitored throughout the experimental period. Muscle and blood samples were collected at 100 and 60% O-2 (sat) at 6 and 18 degrees C for analysis of white muscle energy phosphates (creatine phosphate, adenosine triphosphate) and carbohydrate fuels (glucose, glycogen) as well as blood clinical chemistry (whole blood: hematocrit; plasma: Na+, K+, Cl-, glucose, lactate, pH, triacylglycerol). Mortality was similar between ploidies, but higher in triploids compared to diploids during reduced O-2 (sat) at 18 degrees C. Compared to diploids, triploids had higher feed intake (% biomass) at <= 9 degrees C, but lower feed intake at >= 15 degrees C. Feed intake peaked at 12 and 15 degrees C for triploids and diploids, respectively. Triploids progressively reduced feed intake with increasing temperature after peak feeding, indicating reduced scope for specific dynamic action with increasing water temperature. During hypoxia, triploids had lower feed intake than diploids at 6 and 18 degrees C. The difference in feed intake was not associated with any ploidy effect on body weight gain or feed conversion ratio, but triploids had greater body length growth compared to diploids. At >= 15 degrees C triploids consumed less oxygen than diploids. In the white musculature, the only observed difference between ploidies was a lower level of glycogen in triploids compared to diploids at 18 degrees C and 100% O-2 (sat). In the blood plasma, the concentration of ions was lower and glucose level higher in triploids compared to diploids at 18 degrees C and 60% O-2 (sat). The results of this study indicate that triploid Atlantic salmon post-smolts can substitute diploids, but are less tolerant to high seawater temperature and low O-2 (sat). For sea-cage farming of triploid salmon post-smolts, this would favour production areas with maximum temperatures of 15 degrees C and sufficient oxygen.Statement of Relevance: This study demonstrates that triploid Atlantic salmon post-smolts have lower temperature optima for feeding and growth than diploids, a similar physiological response to high temperatures, but higher mortality under suboptimal conditions. Consequently triploid Atlantic salmon post-smolts may be well suited for commercial farming in geographical regions with moderate water temperatures in the summer and autumn months. (C) 2017 Elsevier B. V. All rights reserved.

Critical swimming speed (Ucrit) has traditionally been measured in relatively small swimming tunnels with 1 fish alone. However, both increased flume lengths and swimming in groups are known to improve performance. Atlantic salmon Salmo salar farming is currently expanding to more exposed locations, which necessitates guidelines for limits in water current peak velocity to secure animal welfare. A large swim tunnel system was therefore developed which allowed for swimming trials at relevant stocking densities of Atlantic salmon comparable to the conditions in exposed sea cages. Ucrit was measured in groups of 3 size classes (small post-smolts, large post-smolts, adults) in this new swim tunnel and compared to measurements from individual fish in a smaller swim tunnel. Ucrit (in cm s–1) increased with size and was significantly lower in the smaller setup. Our results suggest that Ucrit can be used as a maximum current velocity tolerance threshold in exposed aquaculture, where longer periods above this value would be detrimental to the welfare and physio logical function of the fish. This study represents a starting point in obtaining suitable current velocity profiles for farmed Atlantic salmon during the on-growing phase in sea cages.

In order to maximize the growth potential of Atlantic salmon (Salmo salar L.) in the on-growing phase in open sea cages, the dissolved oxygen (DO, % of air saturation) level must exceed the minimum DO required for maximal feed intake (DOmaxFI). The main aim of this study was to determine the effect of temperature on this important threshold DO, and secondly, to provide aquaculturists with a model that estimates feed intake as function of both temperature and DO. Quadruple tank groups of Atlantic salmon post-smolts (~0.3–0.5kg) were kept at 7, 11, 15 and 19°C, and subjected to seven DO levels per temperature, ranging from ≈32–92, 42–102, 52–112 and 62–122% O2, respectively, for 42days. DO levels were changed every second day, in random order. Fish were fed a known amount of feed in excess twice daily, waste feed was collected and daily feed intake (DFI, % of biomass, BM, per day) estimated. The routine oxygen consumption rate (MO2 rout, measured in partially fasted and active fish) and the DO below which MO2 rout started to decline (termed the routine limiting oxygen saturation, LOSrout) was determined in fasted fish at all four temperatures at the end of experimentation, and the latter was used to estimate the “zero feed intake DO” in the DFI model. The data demonstrate sustained appetite with decreasing DO until a threshold level (DOmaxFI) is reached, at which point appetite gradually decreases. Both the maximal level of feed intake (DFImax) and the DOmaxFI were highly dependent on temperature, increasing from 0.47 to 0.88% of BM/day, and from 42 to 76% O2, respectively, within the temperature range tested. The LOSrout ranged from 24 to 40% O2. The present study is the first to present the pronounced effect of temperature on DOmaxFI, and results suggest that the DO requirement of A. salmon has previously been considerably over-estimated, particularly at lower temperatures.

We investigated the effect of temperature on the limiting oxygen saturation (LOS) of gilthead sea bream Sparus aurata. This threshold was defined as the % O-2 saturation where fish no longer upheld their routine metabolic rate (RMR, the metabolic rate of fed and active fish) during a progressive decline in oxygen saturation. S. aurata (398 +/- 10 g, mean +/- SE) were kept in 3 replicate tanks and subjected to 3 changes in temperature: 16 to 20 degrees C, 20 to 16 degrees C and 16 to 12 degrees C. At each temperature, fish were left to acclimatize for 8 to 10 d, before daily feed intake (DFI), the routine oxygen consumption rate (routine MO2, mg kg(-1) min(-1)) and the LOS were measured. In addition, at 20 degrees C the swimming speed was measured in fish subjected to a decline in O-2 from full air saturation to levels below the LOS (minimum of 8-10% O-2). For the temperature range tested (12-20 degrees C), DFI, MO2 and LOS increased exponentially with temperature (7.5-, 3.6- and 2.2-fold, respectively) with mean (+/- SE) LOS being 17 +/- 1, 21 +/- 0 and 35 +/- 5% O-2 at 12, 16 and 20 degrees C, respectively. A gradual decline in swimming activity was observed as O-2 declined below the LOS, indicating increasing metabolic stress and/or a 'sit-out' coping strategy which may prolong survival time in severe hypoxia. The results show the importance of temperature as an influential variable over the environmental O-2 requirements of S. aurata.

This report is based on a finding of spent males and overripe females of Atlantic salmon on 30 August 2013. The studied fish population had been reared in tanks from seawater transfer in May 2011 to April 2013, and then in a sea-cage under natural light. From May 2011 until August 2012, and then again from February until April 2013, the fish were under natural light and temperature. In between these periods, the fish were reared under continuous light and a temperature ranging between 3 and 18 °C in order to study temperature effects on food consumption and oxygen consumption rate. In February 2013, a dissection of 33 fish showed that 29% of the females and 79% of the males were maturing. In August 2013, all remaining fish (n = 80) were inspected by dissection and measured for gonad weight; males were either spent ex-spawners (50%), maturing (25%) or immature (25%), while females were either overripe (11%), maturing (29%) or immature (61%). The fish that were classified as maturing in August had gonads that resembled the gonads of the fish that were classified as maturing 5 months earlier. The females that were overripe and the spent males in August were estimated to have been ready for spawning sometime between April and June. This is the first indication of summer ovulation of female Atlantic salmon and shows that a relatively short period of temperature manipulation combined with continuous light may be used as a tool to prolong the spawning period in salmon to all year round. This would have a major impact on salmon farming.

Sterilization by triploid induction prevents interbreeding between escaped farmed salmon and wild stocks, but reduced performance of triploids at high seawater temperatures has been reported. As high temperature may be followed by limited oxygen (O2) supply in net cages, this study compared the effect of reducing O2 from 100% to 70% of air saturation (termed hypoxia) on parameters of production performance (feed intake, growth, feed conversion ratio, mortality), and physiological status (plasma K+, Cl−, Na+, osmolality, glucose, creatinine (Cr), bilirubin, triacylglycerol (TAG) and alkaline phosphatase (ALP) concentrations) in triploid versus diploid Atlantic salmon kept at high seawater temperature (19°C). Two triplicate groups of diploid and two triplicate groups of triploid Atlantic salmon post-smolts were acclimated to 10°C and 100% O2 before experiment start up. During the experiment, temperature was maintained at 10°C for 10days, increased to 19°C over 9days and kept stable at 19°C until the experiment ended (day 51). From day 22 to 51, the O2 level was reduced from 100% O2 to 70% O2 in one diploid and one triploid group. The abbreviated group names are 2N100, 2N70, 3N100 and 3N70. Triploidy led to reductions of feed intake and growth, and this effect was amplified by reducing O2 from 100% to 70% O2. Analyses from blood samples drawn on day 51 show that plasma levels of Cl−, TAG, ALP and bilirubin were lowered in triploids in general, and that plasma Cr levels trebled and plasma K+ levels dropped in triploids subjected to 70% O2 for 29days. Mortality was also significantly higher in the 3N70 group. According to these effects, the following order of production performance is suggested at high seawater temperature (best to worst): 2N100>2N70≥3N100>3N70. An interesting difference in the behavior between diploid and triploid fish was observed during the experiment: triploids generally moved against the tank water current, ram ventilating, as opposed to diploids, which displayed normal gill ventilation and were in part moving along with the current. The inability of triploid Atlantic salmon to withstand high temperature in combination with moderate hypoxia could set limitations to the geographical distribution of triploid salmon farming.

This study investigated the production performance of the Atlantic salmon postsmolt (Salmo salar L.) subjected to cyclic oxygen reductions (hypoxia) of varying severity. Triplicate groups (N = 955) were kept at constant 80% O-2 (control) or subjected to 1 h and 45 min of hypoxia (50, 60 or 70% O-2, termed 80: 70, 80: 60 and 80: 50 groups) every 6 h at 16 C for 69 days. Feed was provided in normoxia. One third of the fish were kept further for 30 days in normoxia to study possible compensatory growth. Cyclic hypoxia did not alter the oxygen uptake rates of fish, measured in night-time. Fish subjected to 50% and 60% O-2 reduced feeding by 13% and 6% compared with the controls, respectively, with corresponding reductions in specific growth rates. Feed utilization was not reduced. Compensatory growth was observed in fish from the 80: 50 group, but full compensation was not achieved. The main conclusions were that feeding in normoxia does not fully alleviate negative effects of cyclic hypoxia on feeding and growth, when oxygen is reduced to 60% or below in hypoxic periods, that feed utilization is maintained, and that compensatory growth may lessen negative effects.

Aplasia of the septum transversum (AST) is a malformation that results in alterations in ventricle morphology. The condition has been linked to increased mortality during periods of increased cardiac demand in Atlantic salmon Salmo salar. The blood plasma biochemical response to an acute hypoxic event (1 h at 31-39% O₂saturation) was investigated in fish with and without a septum transversum to assess levels of anaerobic respiration (lactate) and the stress response (cortisol, glucose, osmolality, Na⁺, Cl⁻ and K⁺). AST had no effect on body size parameters or relative ventricular mass. The hypoxic event increased the levels of anaerobic respiration and induced a typical stress response. Contrary to our expectations, AST had no effect on any plasma parameter in normoxia or following severe acute hypoxia. We conclude that in the current scenario, AST does not affect the levels of anaerobic respiration or the plasma stress response in Atlantic salmon.

Atlantic salmon (Salmo salar L.) experience periodic drops in dissolved oxygen (O2) inside aquaculture sea cages. In order to evaluate whether fish function or welfare is compromised during such drops in O2 (termed hypoxia), it is necessary to establish the limit for acceptable O2 reductions; the hypoxia tolerance threshold. In the present study, effects of temperature (6, 12 and 18°C) and hypoxia acclimation (33days of hypoxic periods down to 50% O2 occurring every 6h at 16°C) on the routine oxygen consumption rate (ṀO2) and the limiting oxygen saturation (LOS, defined as the hypoxia tolerance threshold) were investigated in undisturbed, fed fish kept in groups with the aim of resembling commercial aquaculture conditions. ṀO2 was measured using open respirometry where a progressive decline in O2 was caused by fish O2 consumption during a period of low water turnover. LOS was defined as the O2 below which fish were no longer able to uphold routine ṀO2. Both ṀO2 and LOS were found to increase exponentially with temperature (Q10=2.7 for ṀO2 and 1.8 for LOS), but no effect of hypoxia acclimation was found. The mean (±SE) LOS at 6, 12, 16 and 18°C was 30±1, 39±1, 47±1 and 55±2% of air saturation respectively. The variation in LOS within and across temperatures was to a large extent explained by variation in ṀO2 (R2=0.93), suggesting that LOS increases with any factor that raises the metabolic rate of Atlantic salmon. This study provides aquaculturists and legislators with a lower limit for acceptable drops in O2 at temperatures relevant for the on-growing phase in Atlantic salmon aquaculture, and discusses the variation in LOS that can be expected at a given temperature.

In order to investigate the effects of hypoxic periods on the feeding behaviour and physiology of Atlantic salmon (Salmo salar L.) post-smolts (237±7g), fish were kept at constant 90% O2 (control) or subjected to cyclic reductions in oxygen, from 90% O2 (termed normoxia) to 40, 50, 60 or 70% O2 (termed hypoxia) for 2h every 6h for 23days at 16°C (groups are denoted as 90:40, 90:50, 90:60, 90:70 and 90:90). Fish were fed to satiation three times per day, twice in hypoxia and once in normoxia. Blood samples were drawn during hypoxic periods on day 0, 7, 14 and 21 and analysed for haematocrit, haemoglobin concentration and plasma cortisol, chloride, lactate and glucose concentrations. During first exposure to hypoxia (day 0), plasma lactate was increased in 90:60, 90:50 and 90:40 groups, plasma cortisol was increased in 90:50 and 90:40 groups and plasma chloride was increased in the 90:40 group, indicating oxygen shortage and stress at oxygen saturations below 60% O2. On day 7, 14 and 21, there were no signs of stress, but plasma lactate levels remained increased in hypoxic periods in 90:40, 90:50 (all the days) and 90:60 groups (day 7 and 14), suggesting that the stress response was down-regulated in spite of a persisting oxygen shortage. Feed intake was reduced according to hypoxia severity in groups subjected to 40–60% O2 during the first meal of the day, and in all experimental groups (40–70% O2) during the third meal of the day. The effect of hypoxia on feed intake persisted throughout the experiment, but total daily feed intake was substantially increased in 90:40 and 90:50 groups during weeks 2 and 3 due to compensatory feeding in the normoxic feeding period. Present results suggest that 70% O2 may represent a threshold for reduced growth and that 60% O2 represents a minimum O2 saturation considering the welfare of Atlantic salmon post-smolts subjected to cyclic hypoxia at 16°C.