Allozyme heterozygosity and development in Atlantic salmon, Salmo salar

The development of Atlantic salmon (Salmo salar) parr was monitored during the first year in a fresh-water hatchery environment to test whether multilocus heterozygosity (computed for six allozyme loci: _s AAT-4^*, IDDH-1^*, IDDH-2^*, IDPH-3^*, _s MDH-3,4^* and_m MEP-2^*) is associated with growth (body mass, fork length) and maturity status (mature male parr). The length frequency distribution of population was unimodal in September, but was clearly bimodal by December. UMG fish tended to be more heterozygous and had higher growth rates than LMG fish. The heterozygosity of UMG fish and mature male parr (MMP) did not differ significantly but MMP showed lower growth rates than UMG. There were positive correlations between the number of heterozygotic loci and the fork length, weight or growth rates of individuals.     

Protein metabolism during sexual maturation in female Atlantic salmon (Salmo salar L)

Body composition and fractional rates of protein synthesis (percentage of the protein mass synthesized per day) were determined in female Atlantic salmon returning to the River Tay, Scotland in July and in October after a 95 day period without food, during which time the animals became sexually mature. During the 95 day period of starvation/sexual maturation the ventricle and red muscle remained as a constant proportion of fresh weight whereas the liver, gill and ovary increased and the stomach and white muscle decreased. Fractional rates of protein synthesis increased markedly in the liver, stomach and ovary during the period of starvation/sexual maturation. In the gill, ventricle and white muscle fractional protein synthesis rates increased slightly or remained constant. From the estimated rates of protein loss or gain in the various tissues it is concluded that there is considerable protein turnover and repartitioning of amino acids during the period of starvation and sexual maturation. The absolute rate of protein synthesis rates in the ovary indicates that this tissue made the largest contribution to the energy and amino acid demands of the fish, whilst most of the amino acids required for maturation of the ovary were derived from white muscle, principally as the result of increased muscle protein degradation.     

Retrospective analysis of Atlantic salmon (Salmo salar) marine growth and condition in the northwest Atlantic based on tag‐recovery data

The life history of North American Atlantic salmon (Salmo salar) is characterized by extensive round‐trip migrations between freshwater rearing habitats and marine feeding grounds off the coasts of Canada and Greenland. Growth is rapid during the marine migration, and growth rate and condition factor may be indicators of salmon health during this period. Growth data were evaluated from a tag‐recovery program conducted from 1969 to 1991 using hatchery‐reared Atlantic salmon smolts released in the Penobscot River, Maine, U.S.A. Information from recaptures of 3167 salmon that were at large in the marine environment for 1 month to 3 yr was analyzed. Length–weight measurements coupled with time‐at‐large data were used to estimate von Bertalanffy and allometric growth parameters specific to the marine phase. Variations in growth and condition factor in relation to smolt age, release date, and temperature conditions in the northwest Atlantic were also examined. The von Bertalanffy k parameter declined with ordinal release date, indicating faster growth rates during the first year of smolts released earlier in the spring. The 2‐yr‐old smolts had a larger k than 1‐yr‐old smolts, although 1‐yr‐old smolts grew to a larger asymptotic size. Sea surface temperature had variable effects on growth parameters and condition factor, with temperature at the beginning of the migration and in overwintering habitat during the first year at sea having the greatest influence on length–weight relationships. Determining the mechanisms that influence growth of individuals during the marine phase will help elucidate the factors responsible for historic growth trends, establishing a baseline for current research.     

Changes in condition factor and gonadosomatic index in maturing and non-maturing Atlantic salmon (Salmo salar L.) in Bay of Fundy sea cages, and the effectiveness of photoperiod manipulation in reducing early maturation

Changes in condition factor (cf) and gonadosomatic index (GSI) in maturing and non‐maturing Atlantic salmon (Salmo salar L.) cultured in Bay of Fundy sea cages was investigated under control (natural) and continuous light conditions. All experimental cages contained salmon stocked as smolts in April 2001. Six control cages received only natural illumination, three received continuous light from 3 November, 2001 until 31 May, 2002, and three received continuous illumination from 15 February, 2001 until 31 May, 2002. In mid‐July 2002, each cage was sampled, and sex, round weight, fork length (FL), mean muscle lipid content and gonad weight were recorded from each sampled fish. At harvest (August 2002 to February 2003), sex, maturation state, round weight and FL were measured from a sample of fish from each cage. The frequency distributions of GSIs from fish sampled in July indicated for both sexes a GSI>0.3 was indicative of early maturation. The relationship between cf and GSI suggested that salmon of both sexes had to have a cf greater than 1.3 in early summer for early maturation to develop. Continuous illumination from November greatly reduced the number of males with GSI>0.3 in mid‐July (0.8%, compared with 50% for control males); 17% of males from cages lit in February had GSIs exceeding 0.3. The percentage of control females exceeding 0.3 was 7%, compared with 0% and 4% for females from cages receiving continuous illumination from November and February respectively. For salmon with GSI of 0.3 or less, the percent muscle lipid increased linearly with cf. A multiple regression of cf and FL on GSI (GSI=?0.973+0.35 cf+0.013 FL) correctly identified 88% of maturing vs. non‐maturing salmon from control cages in July.     

Reduced sexual maturation in male post-smolt 1+ Atlantic salmon (Salmo salar L.) by dietary tetradecylthioacetic acid

In the present study, the possible effect of dietary treatment on early sexual maturation in post-smolt Atlantic salmon, without any negative effect regarding growth, was investigated. The experiment was performed using 4400 individually marked (Pit tag) 1+ salmon, fed either a control diet or a diet supplemented with 0.5% tetradecylthioacetic acid (TTA) in duplicates for 3, 6 or 12 weeks after sea transfer. Compared with the control, dietary supplementation of TTA resulted in a threefold reduction in incidence of sexual mature males (0.6% vs. 1.8%). A curve-linear relationship between relative reduction in maturation and weeks of feeding TTA was found, indicating that the effect is most marked as a result of the first weeks of feeding and then levelling off. No negative dietary impact on growth was observed. As the level of fat in the muscle was reduced by dietary TTA, it seems that post-smolt supplemented dietary TTA do not accumulate high enough energy stores to start the maturation process, whereas the energy-enhancing effect of TTA due to increased fatty acid oxidation capacity may maintain the growth potential. Compared with immature salmon, sexually maturing fish revealed increased spring growth before the onset of maturation.     

Effects of dietary testosterone on growth and sex ratio in juvenile Atlantic salmon (Salmo salar)

Diets to which testosterone (1 or 10 ng/g diet) had been added were fed to juvenile Atlantic salmon (Salmo salar) for nine months beginning for months after hatching (Experiment 1) and upon completion of yolk absorption (Experiment 2) to determine the effects on growth, gonadal development, and sex ratio. Dietary testosterone at 10 ng/g fed to juvenile salmon at four months after hatching (Exp. 1) induced significant changes in condition factor (0.69±0.01) compared to controls (0.79±0.01) at the end of the test period. In both experiments, salmon treated with 10 ng/g diet induced a significantly higher percentage of male fish compared to controls. Dietary testosterone at 1 ng/g fed to juvenile salmon beginning four months after hatching induced significant increases in weight (18.95±0.99) and length (13.58±0.23) compared to controls (14.55±1.50 and 11.94±0.43, respectively). In Experiment 1 or 2, there was no apparent influence of dietary testosterone on precocious male sexual development. Dietary testosterone at 1 or 10 ng/g fed to juvenile salmon upon completion of yolk absorption (Exp. 2) induced no consistent changes in growth in juvenile Atlantic salmon. These studies indicate that low levels of dietary testosterone may influence physiological responses in juvenile Atlantic salmon dependent upon timing of treatment.     

Density‐dependent diet composition of juvenile Atlantic salmon (Salmo salar)

Abstract – Decreases in body growth with increasing population density may be caused by reduced prey delivery rate. However, changes in food quality because of an increasing inclusion of suboptimal prey in the diet may also contribute to such effects. Here, we test for density‐dependent diet composition by creating spatial variation in Atlantic salmon young‐of‐the‐year (YOY) density in three replicate streams and obtain detailed information on individual positions (±1 m) and diet. Diet breadth with respect to prey size increased with increasing local density for the two most common prey types (Chironomidae and Ephemeroptera). For the largest prey type (Ephemeroptera), there was also an increase in mean prey size with increasing density, suggesting that YOY salmon preferentially utilise the smaller prey among those available. According to optimal foraging theory, changes in diet with increasing local density are likely to come at an energetic cost and hence may contribute to the commonly observed density‐dependent growth of juvenile salmonids.     

Effects of dietary reduced glutathione on the growth and antioxidant capacity of juvenile Atlantic salmon (Salmo salar)

To investigate the effects of dietary reduced glutathione (GSH) on the growth performance and antioxidant capacity of juvenile Atlantic salmon (Salmo salar), 396 juvenile fish with initial body weight of 143.07 ± 6.56 g were randomly distributed into four groups fed four diets with graded supplementation levels of GSH (0, 100, 200 and 400 mg/kg diet) for 83 days. The results showed that the appropriate GSH supplementation (100 and 200 mg/kg diet) significantly increased the growth performance, activities and gene mRNA expression levels of glutathione peroxidase (GPx) and glutathione transferase (GST), and the content of GSH and total antioxidant capacity (TAOC), whereas it significantly decreased activities and gene mRNA expression levels of superoxide dismutase (SOD) and catalase (CAT), and the content of malondialdehyde (MDA; p < 0.05). However, the excess dietary GSH (400 mg/kg diet) had an adverse effect on the all above indexes. Interestingly, the dietary GSH had the opposite effect on GSH‐related antioxidant enzymes (GPx and GST) and other antioxidant enzymes (SOD and CAT). The results showed that the diet with 200 mg/kg GSH supplementation was optimal for the juvenile Atlantic salmon, which had a measured GSH content of 209.54 mg/kg.     

Selective segregation in intraspecific competition between juvenile Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

Interactive segregation has been suggested as the ruling competition mechanism determining niche and niche segregation between juvenile Atlantic salmon (Salmo salar) and brown trout (Salmo trutta). Results from allopatry–sympatry observations of habitat use in both nature and in experiments were contrary to predictions derived from the interactive segregation hypothesis. Habitat use parameters under natural conditions such as distance to shore for Atlantic salmon parr were nearly identical in allopatric (mean ± SD; 3.2 ± 1.4 m) and sympatric (3.3 ± 1.4 m) situations. Occupied water depths largely reflected available water, but water depths <15 cm were avoided by salmon parr. Under experimental conditions, habitat use of allopatric salmon was density independent and salmon size had only minor effects, with smaller fish being more likely to occur in the shallow. Habitat use of salmon in sympatry with trout did not differ from allopatric salmon habitat use, and only salmon size had minor effects on depth choice – occurrence of trout or fish density had no effect. Allopatric trout was in general more frequent in the shallow habitat than salmon. Habitat use of sympatric trout was affected by the occurrence of salmon and trout size, resulting in a higher use of the shallow habitats for small trout. To conclude, selective segregation has a dominant role in salmon habitat use (not affected by trout occurrence), whereas a mixed situation occurs in trout habitat use with elements of interactive segregation when competing with Atlantic salmon (affected by salmon occurrence).     

Variation of juvenile Atlantic salmon (Salmo salar) body composition along sedimentary links*

Johnston P, Bergeron NE. Variation of juvenile Atlantic salmon (Salmo salar) body composition along sedimentary links.
Ecology of Freshwater Fish 2010: 19: 187–196. © 2010 John Wiley & Sons A/S Abstract – The objective of this study was to determine the proximate body composition of juvenile Atlantic salmon along the sequence of aquatic habitat types created by longitudinal changes in the riverbed substrate of two rivers (i.e., sedimentary links units). Interesting trends in the body composition were observed but our initial hypothesis, that fish of the upstream sections have higher energy content, was nevertheless not verified. No common longitudinal pattern was detected in the body composition (water, lipid, energy density) along the studied rivers. Trends in the body constituents were different between age‐classes and rivers, whereas they were highly variable within‐ and among‐samples. There was however a common trend in the pattern of variations, with the coefficient of variations increasing in the downstream direction for almost all constituents, age‐classes and rivers. Potential abiotic and biotic factors that might have contributed to these observations are discussed.     

Aeromonas salmonicida infects Atlantic salmon (Salmo salar) erythrocytes

Aeromonas salmonicida subsp. salmonicida (hereafter A. salmonicida) is the aetiological agent of furunculosis in marine and freshwater fish. Once A. salmonicida invade the fish host through skin, gut or gills, it spreads and colonizes the head kidney, liver, spleen and brain. A. salmonicida infects leucocytes and exhibits an extracellular phase in the blood of the host; however, it is unknown whether A. salmonicida have an intraerythrocytic phase. Here, we evaluate whether A. salmonicida infects Atlantic salmon (Salmo salar) erythrocytes in vitro and in vivo. A. salmonicida did not kill primary S. salar erythrocytes, even in the presence of high bacterial loads, but A. salmonicida invaded the S. salar erythrocytes in the absence of evident haemolysis. Naïve Atlantic salmon smolts intraperitoneally infected with A. salmonicida showed bacteraemia 5 days post‐infection and the presence of intraerythrocytic A. salmonicida. Our results reveal a novel intraerythrocytic phase during A. salmonicida infection.     

Density-dependent growth of young-of-the-year Atlantic salmon (Salmo salar) revisited

Imre I, Grant JWA, Cunjak RA. Density-dependent growth of young-of-the-year Atlantic salmon ( Salmo salar ) revisited. Ecology of Freshwater Fish 2010: 19: 1–6. © 2009 John Wiley & Sons A/S
Abstract –  The length of individual young-of-the-year (YOY) Atlantic salmon ( Salmo salar ) in Catamaran Brook decreases with increasing population density following a negative power curve. Because most of this decrease in growth rate occurs at low densities (<1 fish·m⁻²), ( Imre et al. 2005 ; Journal of Animal Ecology, 74: 508–516) suggested that exploitation competition for drifting prey rather than space limitation might be responsible for this pattern. Recently, ( Ward et al. 2007 ; Journal of Animal Ecology, 76: 135–138) showed that the negative power curve of growth rate versus density can be caused by other mechanisms and suggested that Imre et al.'s evidence for density-dependent growth would have been stronger if we had analysed final size versus initial density rather than final density. We examined (i) whether the negative power curve of size versus density was also apparent in an analysis of final size versus initial density and tested two predictions that emerge from Ward et al.'s model, (ii) the variance in body size increases with population density, and (iii) the maximum fish size at a site is density-independent. The final size of YOY salmon decreased with increasing initial density following a negative power curve. Our data did not provide strong support for the above predictions emerging from Ward et al.'s model. Our analyses of different years, sites and seasons were consistent with the hypothesis of density-dependent growth of YOY salmon.     

Reduction of early sexual maturation in male S0 Atlantic salmon (Salmo salar L.) by dietary supplementation of tetradecylthioacetic acid (TTA)

Effects of dietary tetradecylthioacetic fatty acid (TTA) on muscle fat, development of gonads and early sexual maturation in S0 Atlantic salmon during the first year in sea were investigated. TTA (0.5% w/w) was added to the feeds for 8 weeks in the spring. In May, at the end of the TTA‐feeding period, the fish in the TTA group had significantly (P < 0.05) less fat (10.1%) stored in muscle compared with the control group (10.8%). In September, mean male gonadosomatic index (GSI) in maturing fish in the TTA group was found to be lower compared with the maturing fish in the control group (P = 0.05). On the basis of GSI values, male sexual maturation in September was 10.0% vs. 14.4% for the TTA and the control group respectively. Thus, relative to the control group, the incidence of male sexual maturation in the TTA group was reduced by about 1/3 (P = 0.002). Production data was not affected by dietary supplementation of TTA. This study reveals that TTA significantly reduces the incidence of male sexual maturation in S0 Atlantic salmon. A significant elimination model of TTA in fish muscle that takes into account, the growth rate of the fish was further developed in this study.     

Dietary phosphorus requirement of juvenile Atlantic salmon, Salmo salar L.

The objective of this study was to determine the dietary phosphorus (P) requirement of juvenile Atlantic salmon, Salmon salar L. Triplicate groups of fish (mean initial weight 1.4 g) were fed semipurified, casein-gelatine-based diets containing one of five levels of P (4, 8, 10, 15 and 25 g kg⁻¹) from Ca(H₂PO₄)₂·H₂O, or a commercial feed (17 g kg⁻¹ P) for 9 weeks. Weight gains did not differ significantly among treatment groups fed the experimental diets but were slightly less than gains in fish fed the commercial feed. Feed efficiency (wet weight gain/dry feed consumed) was similar in all groups, averaging 1.45. Availability of dietary P, estimated from apparent retention and apparent digestibility, was 86%. Whole-body P concentrations declined in fish fed diets containing less than 10 g kg⁻¹ P. Fitting a logistic curve to dietary P vs. whole-body P concentrations indicated that a minimum of 11 g kg⁻¹ dietary P (9 g kg⁻¹ digestible P) was required by juvenile Atlantic salmon to maintain whole-body P concentrations at initial levels. Calculation of a dietary requirement using a simple factorial model which incorporated measurements of P availability, feed efficiency and normal whole-body P concentration indicated that the dietary requirement was approximately 10 g kg⁻¹. The dietary requirement established in this study (10–11 g kg⁻¹) is higher than previously reported for Atlantic salmon or other fishes. Possible reasons for the wide range of reported dietary P requirements in fishes are discussed.     

Inter-stage survival of wild juvenile Atlantic salmon, Salmo salar L.

A biological model was developed to calculate annual survival between life stages of juvenile Atlantic salmon, Salmo salar L., in Catamaran Brook, a small stream basin (52 km²) in the Miramichi River catchment in New Brunswick, Canada. Seven years’ data (1990–1996) were used in the model. Input variables included: daily fish counts and measurements of parr (3–4 age classes), smolts, and adult salmon at a fish-counting fence near the stream mouth; biennial quantification of all habitat types along the watercourse; fish density estimated by electric fishing at 30 sites; and estimates of young-of-the-year emigration via stream drift. Continuous recording of stream discharge provided data to assist in interpretation of survival estimates. Annual survival for juvenile salmon in their first 3 years of life in the stream averaged between 31% and 34%. The greatest annual variation (CV = 0.699) occurred at the egg to 0+ (summer) stage with a low of 9.2% survival recorded for a winter with an atypical midwinter flood event; parr and pre-smolt survival were similarly affected. Survival from egg deposition (after correction for losses caused by predation and retention/non-fertilization) to smolt emigration was between 0.16% and 0.52%, which is low relative to estimates from many other studies. Survival of smolts to returning 1-sea-winter adults (grilse) averaged 8.5%. Potential errors in the computation of the model are discussed, e.g. inaccurate counts of spawning adults during high autumn stream flow. A possible explanation for the low egg to smolt survival was the environmental conditions experienced during various winters. Mean egg survival was 1.3 times higher (39.3%) and egg to smolt survival increased to 1.03% when the two winters characterized by extremely low discharge or midwinter freshets were excluded from the calculation. Density-dependent factors related to a beaver dam, which limited spawning distribution, may also have contributed to poor survival and increased fry emigration in one year. Environmental factors, particularly winter conditions, in streams such as Catamaran Brook may act as bottlenecks to natural production of Atlantic salmon.     

The effect of physiological and environmental conditions on smolt migration in Atlantic salmon Salmo salar

Hydropower development has negatively influenced Atlantic salmon (Salmo salar) populations. Compensatory hatchery‐rearing programmes exist, but released fish suffer from high mortality that may be related to the lack of experience from natural environments in hatchery‐reared smolts and their large body size and high energetic state. Here, we used acoustic telemetry to test how body size, energetic state, and the environmental conditions of the river affect migration in hatchery‐reared smolts. The study was conducted in three consecutive years between 2011 and 2013 in the lower part of the River Umeälven, Sweden. For individual fish, there was no effect of body size but the energetic state of the fish had a negative effect on sea entry. The most important factor affecting sea entry rate was the water discharge in the old river bed that differed among years. Smolts were more likely to enter the sea in years when the discharge was high or when the discharge increased substantially shortly after release. Hatchery‐reared fish had higher migration speed at a slower flowing section compared with a faster flowing section, which was likely a result of large hesitation to enter the rapid section. The increase in water discharge led to an increase in fish migration speed disproportional to the increase in water velocity. Our results highlight the importance of water discharge for the smolts during smolt migration, and we argue that concern should be given to migrating fish when managing regulated rivers.     

High marine survival rates of sea‐ranched Atlantic salmon smolts (Salmo salar) that had previously matured as male parr

Abstract The growth and recapture rates of Atlantic salmon, Salmo salar L., smolts that had previously matured as parr were compared with those of immature siblings in a sea‐ranching experiment in western Norway. The smolts were the offspring of three river populations. They were individually tagged, transferred to cages in a small bay where estuarine salinity gradients had been established and released on four dates from mid‐May to early June after either 3 or 7 days of acclimatisation. The previously mature male parr (PMMP) were smaller than their siblings before release, but performed much better during their time in the sea. As grilse, they reached sizes comparable with the formerly immature fish, and their recapture rate was more than four times higher (8.6 vs 2.1%). This difference decreased somewhat during the following years because 0.85% of the immature fish and only 0.1% of PMMP were recaptured as multi‐sea‐winter salmon. The highly successful marine performance of the PMMP demonstrated that this life history strategy may be more flexible than previously believed.