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Simple Summary: Winter-associated mortalities at Atlantic salmon marine aquaculture sites have been reported in the last decade in Atlantic Canada and Iceland. Some fish species develop a condition known as 'Winter Syndrome' or 'Winter Disease' (WS/WD) when held at cold temperatures that is associated with enlarged and pale livers (fatty liver disease; FLD). To determine if salmon can develop FLD, we measured liver fat content and composition, and the expression of 34 molecular biomarkers of FLD in fish held long-term at 3 °C. Total fat and triacylglycerol levels were 50% higher in fish that showed symptoms of FLD, compared with healthy fish. Also, the expression level of 32 of the 34 selected biomarkers changed in fish with symptoms of FLD. Our results show that prolonged exposure to cold temperatures can lead to the development of FLD in some Atlantic salmon, and suggest that the development of new diets may improve the health and welfare of salmon during the winter. Large-scale mortality events have occurred during the winter in Atlantic salmon sea cages in Eastern Canada and Iceland. Thus, in salmon held at 3 °C that were apparently healthy (i.e., asymptomatic) and that had 'early' and 'advanced' symptoms of 'winter syndrome'/'winter disease' (WS/WD), we measured hepatic lipid classes and fatty acid levels, and the transcript expression of 34 molecular markers of fatty liver disease (FLD; a clinical sign of WS/WD). In addition, we correlated our results with previously reported characteristics associated with this disease's progression in these same individuals. Total lipid and triacylglycerol (TAG) levels increased by ~50%, and the expression of 32 of the 34 genes was dysregulated, in fish with symptoms of FLD. TAG was positively correlated with markers of inflammation (5loxa, saa5), hepatosomatic index (HSI), and plasma aspartate aminotransferase levels, but negatively correlated with genes related to lipid metabolism (elovl5b, fabp3a, cd36c), oxidative stress (catc), and growth (igf1). Multivariate analyses clearly showed that the three groups of fish were different, and that saa5 was the largest contributor to differences. Our results provide a number of biomarkers for FLD in salmon, and very strong evidence that prolonged cold exposure can trigger FLD in this ecologically and economically important species. [ABSTRACT FROM AUTHOR]

Surprisingly, the impacts of environmental changes on the physiology of tropical/subtropical marine fishes have received limited attention. Given that (i) temperature is considered to be a key factor controlling the biology of fishes; (ii) no published data are available on the swimming performance, metabolic capacity or cardiac function of any of the ~165 grouper species worldwide; and (iii) the Nassau grouper is an endangered species of great ecological and socioeconomic significance in The Bahamas, we investigated how current summer/early fall (30°C) and winter (22°C) temperatures in South Eleuthera affected the aerobic metabolism and heart function of wild Nassau grouper when swum to exhaustion (i.e. to their critical swimming speed, U crit). The Nassau grouper had a very low U crit at 30°C (i.e. <1 body lengths s−1), and a 30% lower swimming performance during the winter (at 22°C), and this was that was indicative of a reduced absolute aerobic scope (~185 vs. 290 mg O2 kg−1 h−1) and values of maximum heart rate (⁠|$f$| HMax) and scope for |$f$| H that were only one-half of that achieved at 30°C (~60 vs. 120 and 29 vs. 61 beats min−1, respectively). Overall, these data reveal that the Nassau grouper's aerobic and swimming capacity are well below values reported for other tropical/subtropical fishes and suggest that, despite a compensatory (~30–40%) increase in stroke volume, constraints on |$f$| H near this species' lower thermal limit negatively affect its cardiac output and swimming performance. These findings have considerable ecological implications as Bahamian grouper populations migrate over long distances to spawn during the winter months, and given the predicted increase in temperature variability with climate change. [ABSTRACT FROM AUTHOR]

Background: Climate change is leading to increased water temperatures and reduced oxygen levels at sea-cage sites, and this is a challenge that the Atlantic salmon aquaculture industry must adapt to it if it needs to grow sustainably. However, to do this, the industry must better understand how sea-cage conditions influence the physiology and behavior of the fish. Method: We fitted ~2.5 kg Atlantic salmon on the south coast of Newfoundland with Star-Oddi milli-HRT ACT and Milli-TD data loggers (data storage tags, DSTs) in the summer of 2019 that allowed us to simultaneously record the fish's 3D acceleration (i.e., activity/behavior), electrocardiograms (and thus, heart rate and heart rate variability), depth, and temperature from early July to mid-October. Results: Over the course of the summer/fall, surface water temperatures went from ~10–12 to 18–19.5°C, and then fell to 8°C. The data provide valuable information on how cage-site conditions affected the salmon and their determining factors. For example, although the fish typically selected a temperature of 14–18°C when available (i.e., this is their preferred temperature in culture), and thus were found deeper in the cage as surface water temperatures peaked, they continued to use the full range of depths available during the warmest part of the summer. The depth occupied by the fish and heart rate were greater during the day, but the latter effect was not temperature-related. Finally, while the fish generally swam at 0.4–1.0 body lengths per second (25–60 cm s−1), their activity and the proportion of time spent using non-steady swimming (i.e., burst-and-coast swimming) increased when feeding was stopped at high temperatures. Conclusion: Data storage tags that record multiple parameters are an effective tool to understand how cage-site conditions and management influence salmon (fish) behavior, physiology, and welfare in culture, and can even be used to provide fine-scale mapping of environmental conditions. The data collected here, and that in recent publications, strongly suggest that pathogen (biotic) challenges in combination with high temperatures, not high temperatures + moderate hypoxia (~70% air saturation) by themselves, are the biggest climate-related challenge facing the salmon aquaculture industry outside of Tasmania. [ABSTRACT FROM AUTHOR]

Mitochondrial function can provide key insights into how fish will respond to climate change, due to its important role in heart performance, energy metabolism and oxidative stress. However, whether warm acclimation can maintain or improve the energetic status of the fish heart when exposed to short-term heat stress is not well understood. We acclimated Atlantic salmon, a highly aerobic eurythermal species, to 12 and 20 °C, then measured cardiac mitochondrial functionality and integrity at 20 °C and at 24, 26 and 28 °C (this species' critical thermal maximum ± 2 °C). Acclimation to 20 °C vs. 12 °C enhanced many aspects of mitochondrial respiratory capacity and efficiency up to 24 °C, and preserved outer mitochondrial membrane integrity up to 26 °C. Further, reactive oxygen species (ROS) production was dramatically decreased at all temperatures. These data suggest that salmon acclimated to 'normal' maximum summer temperatures are capable of surviving all but the most extreme ocean heat waves, and that there is no 'tradeoff' in heart mitochondrial function when Atlantic salmon are acclimated to high temperatures (i.e., increased oxidative phosphorylation does not result in heightened ROS production). This study suggests that fish species may show quite different acclimatory responses when exposed to prolonged high temperatures, and thus, susceptibility to climate warming. [ABSTRACT FROM AUTHOR]

Cardiac stroke volume (SV) is compromised in Atlantic cod and rainbow trout following acclimation to hypoxia (i.e., 40% air saturation; ~8 kPa O2) at 10–12°C, and this is not due to changes in heart morphometrics or maximum achievable in vitro end-diastolic volume. To examine if this diminished SV may be related to compromised myocardial contractility, we used the work-loop method to measure work and power in spongy myocardial strips from normoxic- and hypoxic-acclimated steelhead trout when exposed to decreasing Po2 levels (21 to 1.5 kPa) at several frequencies (30–90 contractions/min) at 14°C (their acclimation temperature). Work required to lengthen the muscle, as during filling of the heart, was strongly frequency dependent (i.e., increased with contraction rate) but was not affected by hypoxic acclimation or test Po2. In contrast, although shortening work was less frequency dependent, this parameter and network (and power) 1) were consistently lower (by ~30–50 and ~15%, respectively) in strips from hypoxic-acclimated fish and 2) fell by ~40–50% in both groups from 20 to 1.5 kPa Po2, despite the already-reduced myocardial performance in the hypoxic-acclimated group. In addition, strips from hypoxic-acclimated trout showed a poorer recovery of net power (by ~15%) when returned to normoxia. These results strongly suggest that hypoxic acclimation reduces myocardial contractility, and in turn, may limit SV (possibly by increasing end-systolic volume), but that this diminished performance does not improve the capacity to maintain myocardial performance under oxygen limiting conditions. [ABSTRACT FROM AUTHOR]

A successful spawning migration in salmon depends on their athletic ability, and thus on efficient cardiovascular oxygen (O2) transport. Most teleost fishes have highly pH-sensitive haemoglobins (Hb) that can release large amounts of O2 when the blood is acidified at the tissues. We hypothesized that plasma-accessible carbonic anhydrase (paCA; the enzyme that catalyses proton production from CO2) is required to acidify the blood at the tissues and promote tissue O2 extraction. Previous studies have reported an elevated tissue O2 extraction in hypoxia-acclimated teleosts that may also be facilitated by paCA. Thus, to create experimental contrasts in tissue O2 extraction, Atlantic salmon were acclimated to normoxia or hypoxia (40% air saturation for more than six weeks), and the role of paCA in enhancing tissue O2 extraction was tested by inhibiting paCA at rest and during submaximal exercise. Our results show that: (i) in both acclimation groups, the inhibition of paCA increased cardiac output by one-third, indicating a role of paCA in promoting tissue O2 extraction during exercise, recovery and at rest; (ii) the recruitment of paCA was plastic and increased following hypoxic acclimation; and (iii) maximal exercise performance in salmon, and thus a successful spawning migration, may not be possible without paCA. [ABSTRACT FROM AUTHOR]

The microsporidian Loma morhua infects Atlantic cod ( Gadus morhua) in the wild and in culture and results in the formation of xenomas within the gill filaments, heart and spleen. Given the importance of the two former organs to metabolic capacity and thermal tolerance, the cardiorespiratory performance of cod with a naturally acquired infection of Loma was measured during an acute temperature increase (2 °C h−1) from 10 °C to the fish's critical thermal maximum ( CTMax). In addition, oxygen consumption and swimming performance were measured during two successive critical swimming speed ( Ucrit) tests at 10 °C. While Loma infection had a negative impact on cod cardiac function at warm temperatures, and on metabolic capacity in both the CTMax and Ucrit tests (i.e. a reduction of 30-40%), it appears that the Atlantic cod can largely compensate for these Loma-induced cardiorespiratory limitations. For example, (i) CTMax (21.0 ± 0.3 °C) and Ucrit (~1.75 BL s−1) were very comparable to those reported in previous studies using uninfected fish from the same founder population; and (ii) our data suggest that tissue oxygen extraction, and potentially the capacity for anaerobic metabolism, is enhanced in fish infected with this microsporidian. [ABSTRACT FROM AUTHOR]

The oxygen consumption [ABSTRACT FROM AUTHOR]

The regulation of vascular resistance in fishes has largely been studied using isolated large conductance vessels, yet changes in tissue perfusion/vascular resistance are primarily mediated by the dilation/constriction of small arterioles. Thus, we adapted mammalian isolated microvessel techniques for use in fish, and examined how several agents affected the tone/resistance of isolated coronary arterioles (< 150 µm) from steelhead trout (O. mykiss) acclimated to 10, 5 and 1°C. At 10°C, the vessels showed a concentration-dependent dilation to adenosine (ADE; 61 ± 8%), sodium nitroprusside (SNP; 35 ± 10%) and serotonin (SER; 27 ± 2%) (all values maximum responses). A biphasic response (mild contraction then dilation) was observed in vessels exposed to increasing concentrations of epinephrine (EPI; 34 ± 9% dilation) and norepinephrine (NE; 32 ± 7% dilation), whereas the effect was less pronounced with bradykinin (BK; 12.5 ± 3.5% constriction vs. 6 ± 6% dilation). Finally, a mild constriction was observed after exposure to acetylcholine (ACh; 6.5 ± 1.4%), while endothelin (ET)-1 caused a strong dose-dependent increase in tone (79 ± 5% constriction). Acclimation temperature had varying effects on the responsiveness of vessels. The dilations induced by EPI, ADE, SER and SNP were reduced/eliminated at 5°C and/or 1°C as compared to 10°C. In contrast, acclimation to 5 and 1°C increased the maximum constriction induced by ACh and the sensitivity of vessels to ET-1 (but not the maximum response) at 1°C was greater. Acclimation temperature had no effect on the response to NE, and responsiveness to BK was variable. [ABSTRACT FROM AUTHOR]

Recent in vivo experiments on Atlantic cod (Gadus morhua) acclimated to chronic hypoxia (6-12weeks at 10°C; Pwo2 -8-9kPa) revealed a considerable decrease in the pumping capacity of the heart. To examine whether this diminished cardiac performance was due to the direct effects of chronic moderate hypoxia on the myocardium (as opposed to alterations in neural and/or hormonal control), we measured the resting and maximum in situ function of hearts from normoxiaand hypoxia-acclimated cod: (1) when initially perfused with oxygenated saline; (2) at the end of a 15min exposure to severe hypoxia (Po2 ∼0.6kPa); and (3) 30min after the hearts had been reperfused with oxygenated saline. Acclimation to hypoxia did not influence resting (basal) in situ cardiac performance during oxygenated or hypoxic conditions. However, it caused a decrease in maximum cardiac Output (Omax) under oxygenated conditions (from 49.5 to 40.3mlmin-1kg-1 by 19%), that was due to diminished values for maximum stroke volume (Vs) and scope for Vs. Severe hypoxia reduced in both groups to -20mlmin-1 kg-1, yet, the hearts of hypoxiaacclimated fish were better able to sustain this level of under hypoxia, and the recovery of Omax (as compared with initial values under oxygenated conditions) was significantly improved (94% vs 83%). These data show that acclimation to hypoxia has a direct effect on cod myocardial function and/or physiology, and suggest that the cod heart shows some adaptations to prolonged hypoxia. [ABSTRACT FROM AUTHOR]

Low water oxygen content (hypoxia) is a common feature of many freshwater and marine environments. However, we have a poor understanding of the degree to which diminished cardiac function contributes to the reduction in fish swimming performance concomitant with acute exposure to hypoxia, or how fish cardiorespiratory physiology is altered by, or adapts to, chronic hypoxia. Thus, we acclimated adult Atlantic cod (Gadus morhua) to either ∼8-9kPa O2 (40-45% air saturation) or ∼21 kPaO2 (100% air saturation; normoxia) for 6-12 weeks at 10°C, and subsequently measured metabolic variables [routine oxygen consumption (Mo2), maximum Mo2, metabolic scope] and cardiac function (cardiac output, Q heart rate, fH; and stroke volume, Ucrit) in these fish during critical swimming speed (Ucrit) tests performed at both levels of water oxygenation. Although surgery (flow probe implantation) reduced the Ucrit of normoxia-acclimated cod by 14% (from 1.74 to 1.50 BLs-1) under normoxic conditions, exposure to acute hypoxia lowered the Ucrit of both groups (surgery and non-surgery) by ∼30% (to 1.23 and 1.02 BLs-1, respectively). This reduction in swimming performance was associated with large decreases in maximum Mo2 and metabolic scope (⩾50%), and maximum fH and O (by 16 and 22%), but not Ucrit. Long-term acclimation to hypoxia resulted in a significant elevation in normoxic metabolic rate as compared with normoxia-acclimated fish (by 27%), but did not influence normoxic or hypoxic values for Llcrit, maximum Mo2 or metabolic scope. This was surprising given that resting and maximum values for Q were significantly lower in hypoxia-acclimated cod at both levels of oxygenation, because of lower values for Ucrit However, hypoxia-acclimated cod were able to consume more oxygen for a given cardiac output. These results provide important insights into how fish cardiorespiratory physiology is impacted by short-term and prolonged exposure to hypoxia, and further highlight the tremendous capacity of the fish cardiorespiratory system to deal with environmental challenges. [ABSTRACT FROM AUTHOR]

Hemoglobin (Hb) polymorphism in cod is associated with temperature-related differences in biogeographical distribution, and several authors have suggested that functional characteristics of the various hemoglobin isoforms (HbIs) directly influence phenotypic traits such as growth rate. However, no study has directly examined whether Hb genotype translates into physiological differences at the whole animal level. Thus, we generated a family of juvenile Atlantic cod consisting of all three main Hb genotypes (HbI-1/1, HbI-2/2, and HbI-1/2) by crossing a single pair of heterozygous parents, and we compared their metabolic and cortisol responses to an acute thermal challenge (10°C to their critical thermal maximum ECTM] or 22°C, respectively) and tolerance of graded hypoxia. There were no differences in routine metabolism (at 10°C), maximum metabolic rate, metabolic scope, CTM (overall mean 22.9° ± 0.2°C), or resting and poststress plasma cortisol levels among Hb genotypes. Further, although the HbI-1/1 fish grew more (by 15%-30% during the first 9 mo) when reared at 100 ± 1°C and had a slightly enhanced hypoxia tolerance at 10°C (e.g., the critical O2 levels for HbI-1/1, HbI-2/2, and HbI-1/2 cod were 35.56% ± 1.24%, 40.56% ± 1.99%, and 40.20% ± 1.19% air saturation, respectively), these results are contradictory to expectations based on HbI functional properties. Thus, our findings (1) do not support previous assumptions that growth rate differences among cod Hb genotypes result from a more efficient use of the oxygen supply--that is, reduced standard metabolic rates and/or increased metabolic capacity--and (2) suggest that in juvenile cod, there is no selective advantage to having a particular Hb genotype with regards to the capacity to withstand ecologically relevant environmental challenges. [ABSTRACT FROM AUTHOR]

Aeromonas salmonicida is a global distributed Gram-negative teleost pathogen, affecting mainly salmonids in fresh and marine environments. A. salmonicida strains are classified as typical or atypical depending on their origin of isolation and phenotype. Five subspecies have been described, where A. salmonicida subsp. salmonicida is the only typical subspecies, and the subsp. achromogenes, masoucida, smithia, and pectinolytica are considered atypical. Genomic differences between A. salmonicida subsp. salmonicida isolates and their relationship with the current classification have not been explored. Here, we sequenced and compared the complete closed genomes of four virulent strains to elucidate their molecular diversity and pathogenic evolution using the more accurate genomic information so far. Phenotypes, biochemical, and enzymatic profiles were determined. PacBio and MiSeq sequencing platforms were utilized for genome sequencing. Comparative genomics showed that atypical strains belong to the subsp. salmonicida, with 99.55% ± 0.25% identity with each other, and are closely related to typical strains. The typical strain A. salmonicida J223 is closely related to typical strains, with 99.17% identity with the A. salmonicida A449. Genomic differences between atypical and typical strains are strictly related to insertion sequences (ISs) activity. The absence and presence of genes encoding for virulence factors, transcriptional regulators, and non-coding RNAs are the most significant differences between typical and atypical strains that affect their phenotypes. Plasmidome plays an important role in A. salmonicida virulence and genome plasticity. Here, we determined that typical strains harbor a larger number of plasmids and virulence-related genes that contribute to its acute virulence. In contrast, atypical strains harbor a single, large plasmid and a smaller number of virulence genes, reflected by their less acute virulence and chronic infection. The relationship between phenotype and A. salmonicida subspecies' taxonomy is not evident. Comparative genomic analysis based on completed genomes revealed that the subspecies classification is more of a reflection of the ecological niche occupied by bacteria than their divergences at the genomic level except for their accessory genome. [ABSTRACT FROM AUTHOR]

Survival, oxygen consumption ( ), total plasma cortisol and glucose levels and gill heat-shock protein 70 (hsp70) expression were measured in 10 and 50 g juvenile Atlantic cod Gadus morhua during an acute temperature increase (2° C h−1) to their critical thermal maximum. Ninety three per cent of the fish in both size classes survived to 24° C; however, mortality was 100% within 15 min of reaching this temperature. The for both size classes increased significantly with temperature, reaching peak values at 22° C that were c. 2·8-fold those of control (10° C) fish. Resting plasma cortisol and glucose levels were lower in 10 g as compared to 50 g fish. Plasma glucose levels were highly variable in both size classes, and significant increases were only seen at >22° C for the 10 g fish. In contrast, plasma cortisol showed an exponential increase with temperature starting at 16° C in both size classes, and reached maximum levels at 22° C that were 19-fold (10 g fish) and 35-fold (50 g fish) higher than their respective control groups. Both the constitutive (73 kDa) and inducible (72 kDa) isoforms of hsp70 were detected in both size classes using the widely utilized mouse monoclonal antibody. Expression of these isoforms, however, did not change when Atlantic cod were exposed to elevated temperature, and the 72 kDa isoform was not detected using salmonid-specific antibodies. These results indicate that juvenile Atlantic cod are very sensitive to acute increases in water temperature. In addition, they (1) show that and plasma cortisol, but not plasma glucose or gill hsp 70 levels, are sensitive indicators of thermal stress in Atlantic cod and (2) support previous reports that the upper critical temperature for this species is 16° C. [ABSTRACT FROM AUTHOR]

The present study investigated the metabolic response of young ocean pout Zoarces americanus to temperature acclimation (3 v. 11° C), and to acute changes in water temperature from 3 to 17° C. The Q10 value for standard metabolic rate between acclimation temperatures was 5·3, warm-acclimated fish displayed higher rates of oxygen uptake at all temperatures during the acute thermal challenge, and changes in whole-body citrate synthase activity were qualitatively similar to those seen for metabolism. These results indicate that, in contrast to temperate species, young ocean pout from Newfoundland do not show thermal compensation in response to long-term temperature changes. [ABSTRACT FROM AUTHOR]

Juvenile haddock Melanogrammus aeglefinus ( c. 39 g) were exposed to either a handling stressor (1 min out of water) or heat shock (increase from 10 to 15° C for 1 h), and plasma cortisol, plasma glucose and gill hsp70 levels were determined before, and at 1, 3, 6, 12, 24 and 48 h post-stress. The pattern of cortisol increase was similar following both stressors, with levels increasing by 25-fold at 1 h post-stress, but returning to pre-stress levels (2–5 ng ml−1) by 3 h. In contrast, neither handling nor heat shock caused an increase in plasma glucose levels. Although gill hsp70 was detected, presumably constitutive levels, in both control and heat shocked groups, there were not significant changes in gill hsp70 levels after exposure to heat shock. The lack of glucose and hsp70 responses to these typical stressors is consistent with previous studies on Atlantic cod Gadus morhua, and suggests that the stress physiology of Gadidae differs from the ‘typical’ teleost. [ABSTRACT FROM AUTHOR]

Oxygen consumption, as an indicator of routine metabolic rate (RoMR), and tissue-specific changes in protein synthesis, as measured by ³H-labelled phenylalanine incorporation rates, were determined in Astronotus ocellatus to investigate the cellular mechanisms behind hypoxia-induced metabolic depression and recovery. RoMR was significantly depressed, by approximately 50%, when dissolved oxygen levels reached 10% saturation (0.67±0.01 mg l-1 at 28±1°C). This depression in RoMR was accompanied by a 50-60% decrease in liver, heart and gill protein synthesis, but only a 30% decrease in brain protein synthesis. During recovery from hypoxia, an overshoot in RoMR to 270% of the normoxic rate was observed, indicating the accumulation of an oxygen debt during hypoxia. This conclusion was consistent with significant increase in plasma lactate levels during the hypoxic exposure, and the fact that lactate levels rapidly returned to pre-hypoxic levels. In contrast, a hyperactivation of protein synthesis did not occur, suggesting the overshoot in oxygen consumption during recovery is attributed to an increase in cellular processes other than protein synthesis. [ABSTRACT FROM AUTHOR]

For fish to survive large acute temperature increases (i.e. >10.0°C) that may bring them close to their critical thermal maximum (CTM), oxygen uptake at the gills and distribution by the cardiovascular system must increase to match tissue oxygen demand. To examine the effects of an acute temperature increase (∼1.7°C h-1 to CTM) on the cardiorespiratory physiology of Atlantic cod, we (1) carried out respirometry on 10.0°C acclimated fish, while simultaneously measuring in vivo cardiac parameters using Transonic® probes, and (2) constructed in vitro oxygen binding curves on whole blood from 7.0°C acclimated cod at a range of temperatures. Both cardiac output (Q) and heart rate (ƒʜ) increased until near the fish's CTM (22.2±0.2°C), and then declined rapidly. Q10 values for Q and ƒH were 2.48 and 2.12, respectively, and increases in both parameters were tightly correlated with O2 consumption. The haemoglobin (Hb)-oxygen binding curve at 24.0°C showed pronounced downward and rightward shifts compared to 20.0°C and 7.0°C, indicating that both binding capacity and affinity decreased. Further, Hb levels were lower at 24.0°C than at 20.0°C and 7.0°C. This was likely to be due to cell swelling, as electrophoresis of Hb samples did not suggest protein denaturation, and at 24.0°C Hb samples showed peak absorbance at the expected wavelength (540 nm). Our results show that cardiac function is unlikely to limit metabolic rate in Atlantic cod from Newfoundland until close to their CTM, and we suggest that decreased blood oxygen binding capacity may contribute to the plateau in oxygen consumption. [ABSTRACT FROM AUTHOR]

In recent years, there has been a great deal of interest in how growth hormone (GH) transgenesis affects fish physiology. However, the results of these studies are often difficult to interpret because the transgenic and non-transgenic fish had very different environmental/rearing histories. This study used a stable line of size-matched GH Atlantic salmon (Salmo salar) that were reared in a shared tank with controls (at 10°C, for -9 months) to perform a comprehensive examination of the cardiorespiratory physiology of GH transgenic salmon, and serves as a novel test of the theory of symmorphosis. The GH transgenic salmon had a 3.6× faster growth rate, and 21 and 25% higher values for mass-specific routine and standard oxygen consumption (ṀO2), respectively. However, there was no concurrent increase in their maximum ṀO2, which resulted in them having an 18% lower metabolic scope and a 9% reduction in critical swimming speed. This decreased metabolic capacity/performance was surprising given that the transgenics had a 29% larger heart with an 18% greater mass-specific maximum in situ cardiac output, a 14% greater post-stress blood haemoglobin concentration, 5–10% higher red muscle and heart aerobic enzyme (citrate synthase or cytochrome oxidase) activities, and twofold higher resting and 1.7× higher post-stress, catecholamine levels. However, gill surface area was the only cardiorespiratory parameter that was not enhanced, and our data suggest that gill oxygen transfer may have been limiting. Overall, this research: (1) shows that there are significant metabolic costs associated with GH transgenesis in this line of Atlantic salmon; (2) provides the first direct evidence that cardiac function is enhanced by GH transgenesis; (3) shows that a universal upregulation of post-smolt (adult) GH transgenic salmon cardiorespiratory physiology, as suggested by symmorphosis, does not occur; and (4) supports the idea that whereas differences in arterial oxygen transport (i.e. cardiac output and blood oxygen carrying capacity) are important determinants of inter-specific differences in aerobicity, diffusion-limited processes must be enhanced to achieve substantial intra-specific improvements in metabolic and swimming performance. [ABSTRACT FROM AUTHOR]

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We tested the hypothesis that juvenile rainbow trout (Oncorhynchus mykiss) would select a temperature colder than their acclimation temperature (16° ± 1°C) to minimize postexhaustive exercise metabolic demands and enhance oxygen availability. After an initial 3-h exploratory period in a thermal gradient (6°-25°C), fish selected a temperature of approximately 14°C and had a baseline exploratory swimming activity of approximately 60 cm min¹. Subsequently, experimental (chased) fish were individually removed, exhaustively exercised for 1.5 mm, and replaced. Both control (unchased) and experimental fish were allowed to explore the thermal gradient for another 2 h. immediately after being chased, trout had a metabolic profile that was consistent with being exhausted; levels of plasma and muscle lactate were 4.38 ± 0.25 mmol L¹ and 28.0 ± 2.0 mmol kg-1, respectively, and levels of muscle glycogen, adenosine triphosphate, and phosphocreatine were 3.89 ± 0.95, 4.23 ± 0.62, and 3.07 ± 0.73 mmol kg-1, respectively. Although exploratory swimming activity of the chased fish was significantly lower (by 81%) as compared with control fish during the first 5 min postchase, differences in the mean, median, and mode values for selected temperatures during the next 2 h were neither large (<1°C) nor significant (P> 0.05). Contrary to our initial hypothesis, these findings suggest that juvenile rainbow trout do not select a colder temperature to decrease metabolic rate following exhaustive exercise. instead, rainbow trout selected a temperature marginally cooler than their acclimation temperature (16°C) regardless of whether they had been previously exhausted. [ABSTRACT FROM AUTHOR]

The dynamic interactions between nitric oxide (NO) and myoglobin (Mb) in the cardiovascular system have received considerable attention. The loss of Mb, the principal O2 carrier and a NO scavenger/producer, in the heart of some red-blooded fishes provides a unique opportunity for assessing this globin's role in NO homeostasis and mitochondrial function. We measured Mb content, activities of enzymes of NO and aerobic metabolism [NO Synthase (NOS) and citrate synthase, respectively] and mitochondrial parameters [Complex-I and -I+II respiration, coupling efficiency, reactive oxygen species production/release rates and mitochondrial sensitivity to inhibition by NO (i.e., NO IC50)] in the heart of three species of red-blooded fish. The expression of Mb correlated positively with NOS activity and NO IC50, with low NOS activity and a reduced NO IC50 in the Mb-lacking lumpfish (Cyclopterus lumpus) as compared to the Mb-expressing Atlantic salmon (Salmo salar) and short-horned sculpin (Myoxocephalus scorpius). Collectively, our data show that NO levels are fine-tuned so that NO homeostasis and mitochondrial function are preserved; indicate that compensatory mechanisms are in place to tightly regulate [NO] and mitochondrial function in a species without Mb; and strongly suggest that the NO IC50 for oxidative phosphorylation is closely related to a fish's hypoxia tolerance. [ABSTRACT FROM AUTHOR]

A correction to the article "Lipid Class and Nonesterified Fatty Acid Profiles in Plasma of North Atlantic Cod (Gadus morhua)," that was published in the previous issue is presented.