Tolerance and resistance to thermal stress in juvenile Atlantic salmon, Salmo salar

SUMMARY. 1. The chief objective was to construct a thermal tolerance polygon for juvenile Atlantic salmon, Salmo salar L., using fish from four groups and two populations: two age groups from one population (0+, 1+ parr from River Leven), two size groups from the other population (slow and Fast growing 1+ parr from River Lune). 2. Fish were acclimated to constant temperatures of 5, 10, 15, 20, 25 and 27°C; then the temperature was raised or lowered at 1°C h^?1 to determine the upper and lower limits for feeding and survival over 10 min, 100 min, 1000 min and 7 days. As they were not significantly different between the four groups of fish, values at each acclimation temperature were pooled to provide arithmetic means (with SE) for the thermal tolerance polygon. 3. Incipient lethal levels (survival over 7 days) defined a tolerance zone within which salmon lived for a considerable time; upper mean incipient values increased with increasing acclimation temperature to reach a maximum of 27.8±0.2°C, lower mean incipient values were below 0°C and were therefore undetermined at acclimation temperatures <20°C but increased at higher acclimation temperatures to 2.2±0.4°C. Resistance to thermal stress outside the tolerance zone was a function of time; the ultimate lethal level (survival for 10 min) increased with acclimation temperature to a maximum of 33°C whilst the minimum value remained close to 0°C. Temperature limits for feeding increased slightly with acclimation temperature to upper and lower mean values of 22.5±0.3°C and 7.0±0.3°C. 4. In spite of different methodologies, values in the present investigation are similar to those obtained in previous, less comprehensive studies in the laboratory. They also agree with field observations on the temperature limits for feeding and survival. Thermal tolerance polygons are now available for eight species of salmonids and show that the highest temperature limits for feeding and survival are those recorded for juvenile Atlantic salmon.     

Spatial distribution correspondence of a juvenile Atlantic salmon Salmo salar cohort from age 0+ to 1+ years

The spatial distribution of Atlantic salmon Salmo salar young-of-the-year (0+) and 1 year old parr (1+) from the 2006 spawning cohort in a 5125 m reach of the River Skauga in central Norway was documented. A high degree of similarity was found between the distribution of 0+ and 1+ parr based on catches at 205 transects sampled in both years. Cross-correlations and partial cross-correlations (correcting for habitat variables) confirmed significant positive association between the two distributions on a small spatial scale (within 100 m) and a clear pattern of decreasing correlation with distance.     

Carotenoid and lipid content in muscle of Atlantic salmon, Salmo salar, transferred to seawater as 0+ or 1+ smolts

Accumulation of lipids and carotenoids, including 4'-hydroxyechinenone (4'-hydroxy-beta,beta-carotene-4-one), growth and condition factor were investigated in Atlantic salmon (Salmo salar) transferred to seawater as 0+ and 1+ smolts. Salmon were fed a diet with 30 mg/kg astaxanthin (3,3'-dihydroxy-beta,beta-carotene-4,4'-dione) and 30 mg/kg canthaxanthin (beta,beta-carotene-4,4'-dione) for 35 weeks. The 0+ smolt contained more carotenoids than the 1+ smolt when mass differences were corrected for (P<0.0001), a difference also reflected by the tristimulus colour measurements (C1E a*- and b*-values). Astaxanthin and canthaxanthin comprised more than 93% of the total carotenoids, but small differences were observed in carotenoid composition. The condition factor was significantly higher in 0+ than 1+ smolts after correction for mass differences (P<0.01). There was a high correlation between ln-transformed muscle lipid (%) and ln-transformed body mass for 0+(R2=0.94) and 1+smolts (R2=0.97). The canthaxanthin metabolite 4'-hydroxyechinenone was isolated from muscle of Atlantic salmon fed a diet supplemented with canthaxanthin. It was characterised and identified by its absorption maximum (lambda(max)=458 nm in n-hexane), mass spectrometry (M+=566) and co-chromatography with authentic standard obtained by NaBH4-reduction of canthaxanthin on thin-layer chromatography and HPLC. HPLC of the camphanates of 4'-hydroxyechinenone revealed a stereoselective transformation in favour of the (4'S)-isomer, the (4'S) and (4'R)-isomers comprising approximately 81 and 19% of the total 4'-hydroxyechinenone, respectively. The percentage of 4'-hydroxyechineone of total carotenoids ranged from 1.3 to 3.1% and declined with fish size (P<0.001). We conclude that effects of time of seawater transfer of Atlantic salmon smolts have significant effect on carotenoid accumulation and other quality traits. The detailed biochemical and physiological basis for these differences require further elucidation.     

Influence of dietary conjugated linoleic acid (CLA) on lipid and fatty acid composition in liver and flesh of Atlantic salmon (Salmo salar)

The aim of the present study was to determine the effects of conjugated linoleic acid (CLA) on lipid and fatty acid metabolism in Atlantic salmon. The overall objective being to test the hypotheses that CLA has beneficial effects in salmon including growth enhancement, improved flesh quality through decreased adiposity and lipid deposition thereby minimising detrimental effects of feeding high fat diets, and increased nutritional quality through increased levels of beneficial fatty acids including n-3 highly unsaturated fatty acids (HUFA) and CLA itself. Salmon smolts were fed diets containing two levels of fish oil (low, approximately 18% and high, approximately 34%) containing three levels of CLA (a 1:1 mixture of 9-cis,trans-11 and trans-10,cis-12. at 0, 1 and 2% of diet) for 3 months and the effects on growth performance, liver and muscle (flesh) lipid contents and class compositions, and fatty acid compositions determined. The diets were also specifically formulated to investigate whether the effects of CLA, if any, were more dependent upon absolute content of CLA in the diet (as percentage of total diet) or the relative level of CLA to other fatty acids. Dietary CLA in salmon smolts had no effect on growth parameters or biometric parameters. However, there was a clear trend of increased total lipid and triacylglycerol contents in both liver and flesh in fish fed CLA, particularly in fish fed the high oil diets. Finally, CLA was incorporated into tissue lipids, with levels in flesh being 2-fold higher than in liver, but importantly, incorporation in liver was at the expense of saturated and monounsaturated fatty acids whereas in flesh it was at the expense of n-3HUFA.     

Differences in fat accumulation between immature male and female Atlantic salmon Salmo salar after dietary administration of tetradecylthioacetic acid

This study provoked sex‐specific differences in fat metabolism in Atlantic salmon Salmo salar, by dietary administration of tetradecylthioacetic acid (TTA) during their first spring and winter in the sea. The effects of TTA were evaluated in June of the first spring and May of the second spring in the sea, by analysing white muscle‐fat content. Muscle fat in males and females differed significantly as a result of TTA in their diet and diet interacted with the sex of the fish. The fat content during the first spring after dietary TTA was lowered by a greater amount in females than in males, 3·1–4·3%, respectively (P < 0·05). In contrast, during the second spring, fat content was lowered by a greater amount in males than in females, 15·8–16·7%, respectively (P < 0·01). Condition factor followed a similar pattern to the muscle fat. The results indicate that the difference in male and female fat accumulation dynamics is related to sex‐specific reproduction biology of S. salar. In addition, the findings show that it is important to consider the sex of the fish and the season of the year when studying fat dynamics and reproductive biology of S. salar.     

Avoidance responses to low frequency sound in downstream migrating Atlantic salmon smolt, Salmo salar

The possibility of using intense sound as an acoustic barrier for downstream migrating smolt of the Atlantic salmon ( Salmo salar ) was studied by observing, the reactions of smolt to 10 and 150 Hz sounds in a small river. At the observation site the river branched into a main course and a minor channel, the latter rejoining the main stream after 30 m. The sound sources were positioned at the lower end of the channel. The number of smolt re-entering the mam stream at the lower end of the channel was recorded during alternating periods with and without sound. Intense 150 Hz sound had no observable effects on the smolt, even at intensities 114 dB above the hearing threshold at this frequency. At intensities above 1.0. 10⁻²ms⁻² the 10 Hz sound was an effective deterrent for the smolt, which turned and left the channel at the upstream branching point.     

Heart rate as an indicator of metabolic rate and activity in adult Atlantic salmon, Salmo salar

Telemetered heart rate (f_H) was examined as an indicator of activity and oxygen consumption rate (VO₂) in adult, cultivated, Atlantic salmon, Salmo salar L. Heart rate was measured during sustained swimming in a flume for six fish at 10° C [mean weight, 1114 g; mean fork length (f. l.), 50·6 cm] and seven fish at 15° C (mean weight, 1119 g; mean f. l., 50·7 cm) at speeds of up to 2·2 body lengths/s. Semi–logarithmic relationships between heart rate and swimming speed were obtained at both temperatures. Spontaneously swimming fish in still water exhibited characteristic heart rate increases associated with activity. Heart rate and Vo₂ were monitored simultaneously in a 575–1 circular respirometer for six fish (three male, three female) at 4° C (mean weight, 1804 g; mean F. L., 62· cm) and six fish (three male, three female) at 10° C (mean weight, 2045 g; mean f. l., 63·2 cm) during spontaneous but unquantified activity. Linear regressions were obtained by transforming data for both f_H and Vo₂ to log values. At each temperature, slopes of the regressions between f_H and Vo₂ for individual fishes were not significantly different, but in some cases elevations were. All differences in elevation were between male and female fish. There were no significant differences in regression slope or elevation for fish of the same sex at the two temperatures and so regressions were calculated for the sexes, pooling data from 4 and 10° C. There was no significant difference in the mean ± S. D. Vo₂ between the sexes at 4° C (male, 66·0 ± 59·6 mgO₂ kg^?1 h^?1; female, 88·0 ± 60·1 mgO₂ kg^?1 h^?1) or 10° C (male, 166·2 ± 115·4 mgO₂ kg^?1 h^?1; female, 169·2 ± 111–1 mgO₂ kg^?1h^?1). Resting Vo₂ (x?± s. d.) at 4°C was 36·7 ± 8.4 mgO₂ kg^?1 h^?1, and 10° C was 72·8 ± 11·9 mgO₂ kg^?1 h^?1. Maximum Vo₂ (x?± S. D.) at 4° C was 250·6 ± 40·2 mgO₂ kg^?1 h^?1, and at 10° C was 423·6 ± 25·2 mgO₂ kg^?1 h^?1. Heart rate appears to be a useful indicator of metabolic rate over the temperature range examined, for the cultivated fish studied, but it is possible that the relationship for wild fish may differ.     

Environmental and dietary influences on highly unsaturated fatty acid biosynthesis and expression of fatty acyl desaturase and elongase genes in liver of Atlantic salmon (Salmo salar)

Highly unsaturated fatty acid (HUFA) synthesis in Atlantic salmon (Salmo salar) was known to be influenced by both nutritional and environmental factors. Here we aimed to test the hypothesis that both these effectors involved similar molecular mechanisms. Thus, HUFA biosynthetic activity and the expression of fatty acyl desaturase and elongase genes were determined at various points during an entire 2 year production cycle in salmon fed diets containing either 100% fish oil or diets in which a high proportion (75% and 100%) of fish oil was replaced by C18 polyunsaturated fatty acid-rich vegetable oil. The results showed that HUFA biosynthesis in Atlantic salmon varied during the growth cycle with peak activity around seawater transfer and subsequent low activities in seawater. Consistent with this, the gene expression of Delta6 desaturase, the rate-limiting step in the HUFA biosynthetic pathway, was highest around the point of seawater transfer and lowest during the seawater phase. In addition, the expression of both Delta6 and Delta5 desaturase genes was generally higher in fish fed the vegetable oil-substituted diets compared to fish fed fish oil, particularly in the seawater phase. Again, generally consistent with this, the activity of the HUFA biosynthetic pathway was invariably higher in fish fed diets in which fish oil was substituted by vegetable oil compared to fish fed only fish oil. In conclusion, these studies showed that both nutritional and environmental modulation of HUFA biosynthesis in Atlantic salmon involved the regulation of fatty acid desaturase gene expression.     

Transcriptional control mechanisms of genes of lipid and fatty acid metabolism in the Atlantic salmon (Salmo salar L.) established cell line, SHK-1

The regulatory control mechanisms of lipid and fatty acid metabolism were investigated in Atlantic salmon. We identified sterol regulatory element binding protein (SREBP) genes in salmon and characterised their response, and the response of potential target and other regulatory genes including liver X receptor (LXR), to cholesterol and long-chain polyunsaturated fatty acids (LC-PUFA) in the salmon established cell line, SHK-1. Two cDNAs for SREBPs homologous to mammalian SREBP-1 and SREBP-2 were characterised. We identified three groups of genes whose expression responded differently to the treatments. One group of genes, including cholesterol biosynthetic genes, showed increased expression in response to lipid depletion but supplementary cholesterol or LC-PUFA had no further effect. The expression of a second group of genes belonging to fatty acid biosynthetic pathways, included fatty acid synthase, Δ6 and Δ5 fatty acyl desaturases, also increased after lipid depletion but this was negated by cholesterol or by LC-PUFA supplementation. The expression of a third group of genes including acyl-CoA oxidase, HMG-CoA reductase and Elovl5 elongase was increased by cholesterol treatment but was not affected by lipid depletion or by LC-PUFA. This same pattern of expression was also shown by liver X receptor (LXR), indicating that acyl-CoA oxidase, HMG-CoA reductase and Elovl5 are possible direct targets of LXR. This suggests that salmon Elovl5 may be regulated differently from mammalian Elovl5, which is an indirect target of LXR, responding to LXR-dependent increases in SREBP-1.     

Awareness reactions and avoidance responses to sound in juvenile Atlantic salmon, Salmo salar L.

The possibility of using intense sound as a deterrent for juvenile Atlantic salmon ( Sulmo salar L. ) was studied by recording both physiological awareness reactions in an acoustic tube and behavioural avoidance responses in a pool. The measured awareness reactions consisted of decreased heart rate and breathing movements. Three criteria for the awareness reaction were used to compare the effect of different frequencies between 5 and 150 Hz: (i) threshold for spontaneous awareness reactions relative to the hearing thresholds, (ii) magnitude of the change in heart rate, (iii) degree of habituation to sound. After these criteria the lowest frequencies (5–10 Hz) were most effective in eliciting an awareness reaction from the fish. The responses of freely swimming fish to 10 and 150 Hz sounds were studied in an artificial pool. Juvenile salmon showed avoidance responses to 10 Hz stimulation at intensities 10–15 dB above the threshold for spontaneous awareness reactions measured in the acoustic tube. The 150 Hz sound failed to evoke avoidance responses, even at a level 30 dB above the threshold for spontaneous awareness reactions.     

Food consumption and growth of Atlantic salmon Salmo salar parr in sub-Arctic rivers: empirical support for food limitation and competition

The quantitative food consumption and somatic growth of Atlantic salmon Salmo salar parr were compared between three sub-Arctic rivers in northern Norway and Finland, addressing the potential occurrence of resource limitation and interspecific competition. In one of the rivers, previous resource partitioning studies have suggested severe dietary competition between juvenile S. salar and a dense population of alpine bullheads Cottus poecilopus . It was hypothesized that S. salar parr in this river would have restricted food consumption and growth rates compared to the S. salar populations in the other two rivers where interspecific competition was less likely to occur. The feeding and growth performance differed significantly between the S. salar populations. The lowest food acquisition and growth rates were in the S. salar parr population living in sympatry with C. poecilopus , confirming a restricted food supply for the S. salar parr and providing empirical support for the presence of resource limitation and interspecific food competition in this river system. The study reveals that S. salar parr in sub-Arctic rivers may experience food limitations resulting in diminished growth rates.     

Ova fecundity in Scottish Atlantic salmon Salmo salar: predictions, selective forces and causal mechanisms

Ova fecundities of Scottish Atlantic salmon Salmo salar, predicted from log(10) regression of ova numbers and female fork length (L(F) ), differed widely between upland and lowland stocks within the same river, whereas sea-age, river and year factors had insignificant effects on fecundity once L(F) was accounted for. For upland fish, the relationship between log(10) L(F) and log(10) ova mass (M(O) ) was stable between two datasets collected 40 years apart. Although upland and lowland females both produced comparable log(10) M(O) (log(10) L(F) )(-1) , lowland females partitioned this into 45% more, but smaller ova, whereas upland females produced fewer, but larger, eggs. The possible causes and implications of this are discussed for evolutionary perspectives (lifetime production), population structure (local tributary v. large catchments; environmental effects), population dynamics and stability (density-dependent control mechanisms) and fisheries management (stock-recruitment; short and long-term stock sustainability).     

Global warming is projected to lead to increased freshwater growth potential and changes in pace of life in Atlantic salmon Salmo salar

Global warming has been implicated in widespread demographic changes in Atlantic salmon Salmo salar populations, but projections of life-history responses to future climate change are lacking. Here, we first exploit multiple decades of climate and biological data from the Burrishoole catchment in the west of Ireland to model statistical relationships between atmospheric variables, water temperature, and freshwater growth of juvenile Atlantic salmon. We then use this information to project potential changes in juvenile growth and life-history scheduling under three shared socioeconomic pathway and representative concentration pathway scenarios from 1961 to 2100, based on an ensemble of five climate models. Historical water temperatures were well predicted with a recurrent neural network, using observation-based atmospheric forcing data. Length-at-age was in turn also well predicted by cumulative growing degree days calculated from these water temperatures. Most juveniles in the Burrishoole population migrated to sea as 2-year-old smolts, but our future projections indicate that the system should start producing a greater proportion of 1-year-old smolts, as increasingly more juveniles cross a size-based threshold in their first summer for smoltification the following spring. Those failing to cross the size-based threshold will instead become 2-year-old smolts, but at a larger length relative to 2-year-old smolts observed currently, owing to greater overall freshwater growth opportunity. These changes in age- and size-at-seaward migration could have cascading effects on age- and size-at-maturity and reproductive output. Consequently, the seemingly small changes that our results demonstrate have the potential to cause significant shifts in population dynamics over the full life cycle. This workflow is highly applicable across the range of the Atlantic salmon, as well as to other anadromous species, as it uses openly accessible climate data and a length-at-age model with minimal input requirements, fostering improved general understanding of phenotypic and demographic responses to climate change and management implications.     

Polyunsaturated fatty acid profiles of whole body phospholipids and triacylglycerols in anadromous and landlocked Atlantic salmon (Salmo salar L.) fry

We compared the fatty acid compositions and gains of whole body triacylglycerols (TAG) and phospholipids (PL) in anadromous and landlocked Atlantic salmon (Salmo salar) fry, of the same age, fed the same commercial marine oil-rich diet over a 42-day feeding trial. The landlocked strain exhibited significantly (P<0.05) higher growth rate and feed efficiency, due principally to a higher fat retention, particularly of monounsaturated and saturated fatty acids (SFA). n-3 and n-6 long-chain polyunsaturated fatty acid (PUFA) gains and retentions were significantly higher (P<0.05) in the landlocked fry. Great similarities were found in the fatty acid profiles of whole body TAG of both strains. However, marked genotypic differences were observed in the PUFA profiles of whole body PL fractions. The total PUFA, n-3 PUFA and docosahexaenoic acid (DHA) level in PL was significantly higher (P<0.05) while the SFA level, and the PUFA C18/C20 and eicosapentaenoic acid/arachidonic acid ratios were significantly lower (P<0.05) in the anadromous fry than in landlocked fry. Our results indicate that the level of DHA in salmon PL is under strong genetic control and that the capacity for incorporation, and possibly for the conversion of dietary n-3 and n-6 PUFA, is higher in the landlocked strain.     

In vivo and in vitro insulin and fasting control of the transmembrane fatty acid transport proteins in Atlantic salmon (Salmo salar)

We have examined the nutritional and insulin regulation of the mRNA expression of transmembrane fatty acid (FA) transporters [FA transport protein-1 (FATP1) and CD36] together with the lipoprotein lipase (LPL), the cytosolic FA carrier FA binding protein (FABP3), and mitochondrial FA-CoA and -carnitine palmitoyl transferase carriers (CPT)1 and -2 in Atlantic salmon tissues and myocyte cell culture. Two weeks of fasting diminished FATP1, CD36, and LPL in adipose tissue, suggesting a reduction in FA uptake, while FABP3 increased in liver, probably enhancing the transport of FA to the mitochondria. Insulin injection decreased FATP1 and CD36 in white and red muscles, while both transporters were upregulated in the adipose tissue in agreement with the role of insulin-inhibiting muscle FA oxidation and stimulating adipose fat stores. Serum deprivation of 48 h in Atlantic salmon myotubes increased FATP1, FABP3, and CPT-2, while CPT-1 was diminished. In myotubes, insulin induced FATP1 expression but decreased CD36, FABP3, and LPL, suggesting that FATP1 could be more involved in the insulin-stimulated FA uptake. Insulin increased the FA uptake in myotubes mediated, at least in part, through the relocation of FATP1 protein to the plasma membrane. Overall, Atlantic salmon FA transporters are regulated by fasting and insulin on in vivo and in vitro models.     

Otolith and somatic growth rates in Atlantic salmon parr, Salmo salar L: evidence against coupling

It is often assumed that otolith growth is in some way dependent on somatic growth (i.e. that the two processes are coupled). We examined the relationships between sagitta radius and fork length in 0+ Atlantic salmon parr that would subsequently smolt aged 1 + (UMG fish) or 2+ (LMG fish). Repeated measurements of fork lengths of individually marked parr, taken over a 211-day period from first feeding, were compared to sagitta radii on the same measuring dates (obtained by analysis of daily increments). The results showed that there was a linear relationship between fork length and otolith radius in UMG parr. However, this was not true for LMG parr. These fish enter a state of natural anorexia in their first autumn (despite excess food), but their otoliths continued to grow at the same rate despite the virtual cessation of somatic growth; they had therefore developed disproportionately large otoliths by the end of the study period. The relative growth rates of soma and otoliths first changed in LMG fish in late July/early August; this is the most precise estimate yet obtained of the timing of divergence in the developmental pathways of UMG and LMG parr. The rate of sagitta accretion was consistently lower in LMG parr, possibly indicating a lower metabolic rate in these fish. The results are discussed in relation to previous theories of the relationship between otolith and somatic growth.     

Effects of increasing dietary linoleic acid on phospholipid fatty acid composition and eicosanoid production in leucocytes and gill cells of Atlantic salmon (Salmo salar).

Diets containing linoleic acid at 10, 25 and 45% of total dietary fatty acids were fed to three groups of post-smolt Atlantic salmon (Salmo salar) for 18 weeks. Incorporation of linoleic acid into membrane phospholipids of leucocytes and gills increased in response to dietary intake. In general, there was an increase in arachidonic acid and a decrease in eicosapentaenoic acid in the individual phospholipids of both cell types in response to increasing dietary linoleic acid. These changes in eicosanoid precursors were reflected in significantly increased plasma concentrations of 6-keto-PGF1 alpha and TXB2 in salmon given the highest dietary linoleic acid. In whole blood stimulated with the calcium ionophore A23187, LTB4, 12-HETE and TXB2 were significantly increased and 12-HEPE significantly decreased in response to increasing dietary linoleic acid. In isolated gill cells stimulated with A23187, 12-HEPE, 12-HETE, 14-HDHE and TXB2 were all decreased in response to increasing dietary linoleic acid, although the ratio of 12-HEPE/12-HETE was also decreased.     

FABP3 and FABP10 in Atlantic salmon (Salmo salar L.)--general effects of dietary fatty acid composition and life cycle variations

The increased use of dietary plant oil supplementation combined with high dietary lipid loads challenges the lipid transport systems of cultivated fish species. Fatty acid binding proteins (FABPs) have been thoroughly studied as intracellular fatty acid transporters in vertebrates, but no data have been reported in Atlantic salmon. In the present study, comparative characterizations were performed, and dietary influence of plant oil supplementation on FABP3 and FABP10 expression was studied for several tissues in two separate dietary trials. In trial I, groups (6 fish each) were fed diets for 42 weeks (body mass 142+/-1 to 1463+/-83 g) (mean+/-S.D.), containing graded levels of rapeseed oil substituting for fish oil using a linear regression design. In trial II, groups (3 fish each) were fed 100% fish oil or 100% plant oil for 22 months (0.160+/-0.052 to 2523+/-590 g) (mean+/-S.D.) and sampled at regular intervals. Liver and muscle tissues appeared to express several FABPs possibly linked to different metabolic functions. FABPs mRNA expression did not change with dietary inclusion of 75% rapeseed oil, whereas FABP3 protein expression seemed to be affected by dietary rapeseed oil inclusion. Significant changes in red muscle FABP3 mRNA expression correlate to significant changes in total beta-oxidation capacity during the energy consuming process of smoltification.