Changes in polyamines during embryonic development of Atlantic salmon, Salmo salar.

1. Polyamine composition of eggs and alevins from cultured and wild Atlantic salmon, Salmo salar, was compared during early development. 2. Eggs collected from wild stock had higher amounts of bound and free putrescine, spermidine and spermine than those collected from cultured stock. 3. Cadaverine was present in bound form in cultured and wild stocks before and after fertilization, respectively. However, free cadaverine was detected in both stocks throughout development. 4. Atlantic salmon embryos and alevins can synthesize polyamines which could be associated with their growth and development.     

Feeding ecology of early marine phase Atlantic salmon Salmo salar post-smolts

Dietary analyses of Atlantic salmon Salmo salar post-smolt stomachs collected from 2001 to 2005 in Penobscot Bay, Maine, U.S.A., have yielded insights into the feeding ecology of early marine phase post-smolts from different rearing origins. Most stomachs contained only one or two prey types, suggesting active prey selection. Post-smolts that lived in the river longer (i.e. from naturally reared and parr-stocked origins) were smaller and consumed more fishes than invertebrates compared to larger post-smolts that emigrated immediately post-stocking (i.e. from smolt-stocked origins). Naturally reared S. salar consumed c. 84% fishes and 16% crustaceans and parr-stocked S. salar consumed 64% fishes and 34% crustaceans. Stocked smolts consumed 48% fishes and 40% crustaceans. Differences in the type and quantity of consumed prey may be indicative of behavioural differences among rearing origins that influence post-smolt survival.     

Triploidy alters brain morphology in pre‐smolt Atlantic salmon Salmo salar: possible implications for behaviour

Total brain mass and the volumes of five specific brain regions in diploid and triploid Atlantic salmon Salmo salar pre‐smolts were measured using digital images. There were no significant differences (P > 0·05) in total brain mass when corrected for fork length, or the volumes of the optic tecta or hypothalamus when corrected for brain mass, between diploids and triploids. There was a significant effect (P < 0·01) of ploidy on the volume of the olfactory bulb, with it being 9·0% larger in diploids compared with triploids. The cerebellum and telencephalon, however, were significantly larger, 17 and 8% respectively, in triploids compared with diploids. Sex had no significant effect (P > 0·05) on total brain mass or the volumes of any measured brain region. As the olfactory bulbs, cerebellum and telencephalon are implicated in a number of functions, including foraging ability, aggression and spatial cognition, these results may explain some of the behavioural differences previously reported between diploids and triploids.     

Energetics in Atlantic salmon (Salmo salar L.) parr fed elevated dietary cadmium

Atlantic salmon (Salmo salar L.) parr were reared for 4 months on experimental diets supplemented with Cd (0.5, 5, 25, 125, or 250 mg Cd x kg(-1)) to assess the long-term energetic changes based on the digestibility and biochemical deposition of the major dietary nutrients and to evaluate a maximum tolerable dietary toxicant concentration. Growth did not differ significantly (P>0.05) from the control groups. The biochemical composition of the carcass, but not the viscera, was negatively affected by dietary Cd exposure. The significant decreases in protein, lipid, and glycogen concentrations in the carcass (P<0.05, 25 mg x kg(-1) compared to control groups) caused a reduction in calculated whole-body energy content in fish fed 125 mg x kg(-1)compared to control groups. This reduction in calculated whole-body energy content was explained by a concurrent significant disturbance to the gastrointestinal function (measured as reduced digestibility). Only at the highest dietary Cd exposure (250 mg x kg(-1)), increased metabolic costs to cope with Cd toxicity was thought to contribute significantly to the reduction in carcass energy content. The most important factor effecting calculated total energetics was nutrient digestibility. Based on the logarithmic effective median concentration for reduced calculated energy digestibility (dietary Cd concentration corresponding to 50% reduction, EC50), the maximum tolerable dietary Cd concentration is 11 mg x kg(-1) diet.     

Remodeling of the notochord during development of vertebral fusions in Atlantic salmon (Salmo salar)

Histological characterization of spinal fusions in Atlantic salmon (Salmo salar) has demonstrated shape alterations of vertebral body endplates, a reduced intervertebral space, and replacement of intervertebral cells by ectopic bone. However, the significance of the notochord during the fusion process has not been addressed. We have therefore investigated structural and cellular events in the notochord during the development of vertebral fusions. In order to induce vertebral fusions, Atlantic salmon were exposed to elevated temperatures from fertilization until they attained a size of 15 g. Based on results from radiography, intermediate and terminal stages of the fusion process were investigated by immunohistochemistry and real-time quantitative polymerase chain reaction. Examination of structural extracellular matrix proteins such as Perlecan, Aggrecan, Elastin, and Laminin revealed reduced activity and reorganization at early stages in the pathology. Staining for elastic fibers visualized a thinner elastic membrane surrounding the notochord of developing fusions, and immunohistochemistry for Perlecan showed that the notochordal sheath was stretched during fusion. These findings in the outer notochord correlated with the loss of Aggrecan- and Substance-P-positive signals and the further loss of vacuoles from the chordocytes in the central notochord. At more progressed stages of fusion, chordocytes condensed, and the expression of Aggrecan and Substance P reappeared. The hyperdense regions seem to be of importance for the formation of notochordal tissue into bone. Thus, the remodeling of notochord integrity by reduced elasticity, structural alterations, and cellular changes is probably involved in the development of vertebral fusions.     

Evaluation of a new semi-natural incubation technique for Atlantic salmon Salmo salar fry

The incubation environment had a significant effect on fork length ( L _F) and body mass ( M ) of Atlantic salmon Salmo salar fry at the time of emergence. Fry that were incubated in a new incubator design, that mimics the conditions in a natural redd (the Bamberger-Box), achieved significantly greater and attained a significantly higher M than those reared in conventional hatchery troughs for control. Fewer fry from the Bamberger-Boxes had visible deformities compared with those from the hatchery troughs. Results were consistent for five consecutive seasons using both wild and domesticated broodstock from genetically different origins. Survival from the eyed embryo stage in the Bamberger-Boxes and hatchery troughs was >93% during normal climatic conditions. Only larvae reared in Bamberger-Boxes, however, survived abnormally high water temperatures during one test season. The results demonstrate that the Bamberger-Box is an effective alternative to the conventional incubation technology.     

A microsatellite linkage map for Atlantic salmon (Salmo salar)

A linkage map of the Atlantic salmon is described here consisting of 15 linkage groups containing 50 microsatellite loci with a 14 additional unlinked markers (including three allozymes). The map shows the largest sex-specific recombination rate differences so far found in any vertebrate species (3.92:1 female:male). Homologies with previous linkage mapping studies of Atlantic salmon and rainbow trout are described. An in silico search of the Genbank database carried out using the microsatellites used in the mapping process identified significant matches between the flanking regions of the microsatellite SS11 and the calcium-binding mitochondrial carrier protein, 'Aralar1'.     

Dynamics of fatty acid composition of total lipids during embryonic development of Atlantic salmon Salmo salar L

Dynamics of fatty acid composition of total lipids was studied for freshwater salmon Salmo salar L. during its embryonic development from blastula (3 hours) up to hatching (108 days) as well as in unfertilized eggs. Stable amount of total and some saturated, monounsaturated and polyunsaturated fatty acids (PUFA) of total lipids was observed during embryonic development. Considerable changes in fatty acid composition were observed at the stage of prelarvae hatching, i.e., significant decrease of (n-6) PUFA (18:2(n-6) and 20:4(n-6)) and (n-3) PUFA and increase of total and some saturated and monounsaturated fatty acids was registered. Change in saturation ratio of membrane lipids justifies the presence of the biochemical mechanism forwarded on regulation of cell membrane enzymes in accordance with the changes of internal physiological processes taking place in the organism and fluctuations of external environmental conditions or the preparation period (as reproduction). Data on peculiarities of transformation and utilization of fatty acids during salmon embryonic development may be used for understanding of their functional role in the developing organism as well as for assessing the quality of the caviar.     

Thermal habitat of adult Atlantic salmon Salmo salar in a warming ocean

The year-round thermal habitat at sea for adult Atlantic salmon Salmo salar (n = 49) from northern Norway was investigated using archival tags over a 10 year study period. During their ocean feeding migration, the fish spent 90% of the time in waters with temperatures from 1.6–8.4°C. Daily mean temperatures ranged from −0.5 to 12.9°C, with daily temperature variation up to 9.6°C. Fish experienced the coldest water during winter (November–March) and the greatest thermal range during the first summer at sea (July–August). Trends in sea-surface temperatures influenced the thermal habitat of salmon during late summer and autumn (August–October), with fish experiencing warmer temperatures in warmer years. This pattern was absent during winter (November–March), when daily mean temperatures ranged from 3.4–5.0°C, in both colder and warmer years. The observations of a constant thermal habitat during winter in both warmer and colder years, may suggest that the ocean distribution of salmon is flexible and that individual migration routes could shift as a response to spatiotemporal alterations of favourable prey fields and ocean temperatures.     

Ultrastructural alterations in branchial chloride cells of Atlantic salmon, Salmo salar, during parr-smolt transformation and early development in sea water

The ultrastructure of the gill primary lamellae of juvenile Atlantic salmon was examined during the parr-smolt transformation and for 42 days after smolts were exposed to sea water. Scanning electron microscopy indicated that primary lamellae were characterized by rough convoluted surfaces that became rougher throughout the experimental period and that crypts did begin to form in freshwater fish. Crypt formation increased in sea water.
Transmission electron microscopy indicated that parr preadapt for life in sea water in part by changes in chloride cells. Chloride cells show an elaboration of rough endoplasmic reticulum in fresh water and a decline of rough endoplasmic reticulum after 42 days of sea water exposure. The tubular membrane system becomes well developed in fresh water, and apical vesicles become abundant only after seawater exposure. Mitochondria are both spherical and elongate through the period and contain well developed cristae. No evidence of mitochondrial rupture was observed. The junctions between chloride cells and adjacent cells were characterized in fresh water by long tight junctions with desmosomes. This type of junction continued in sea water and was the norm between chloride cells and accessory cells after 42 days of seawater exposure. While leaky junctions appeared to be forming, no evidence was found of membrane interdigitation between accessory cells and chloride cells after 42 days of seawater exposure. It also appeared that seawater exposure influenced the number of chloride cells exposed to the external milieu.
Pavement cells showed an elaboration in fresh water of free ribosomes and rough endoplasmic reticulum and these elements became less prominent after seawater exposure.     

Deposition of Atlantic salmon Salmo salar carcasses in a Scottish upland catchment

Radio telemetry was utilized to track 38 Atlantic salmon Salmo salar across space and time during and following their spawning run on the Conon system, Scotland. The data collected were used to assess the proportion of S. salar able to migrate successfully from an upland oligotrophic area of the catchment and the distribution of the carcasses of those fish that remained. Of these fish, 35% successfully migrated from the study area after the spawning period. The fish that remained were distributed approximately equally between riverine and lacustrine areas, but with a strong tendency to accumulate in regions of relatively slow water flows.     

Wild Atlantic salmon Salmo salar L. strains have greater growth potential than a domesticated strain selected for fast growth

A study was undertaken to examine the responses of three Atlantic salmon Salmo salar strains to growth hormone (GH) treatment. A positive growth response to sustained-release GH implants was found in two wild strains (Namsen and Imsa) as well as one domesticated strain (AquaGen). The data revealed that the growth-selected AquaGen strain has further growth potential, however, a stronger growth response was observed in the wild strains which outgrew the domesticated strain after GH treatment. These observations suggest that some growth potential may have been lost during the selection for rapid growth in the AquaGen strain. In September, the parr were GH implanted and in December sampled for plasma GH and insulin-like growth factor I (IGF-I) levels, liver, muscle and gill GH receptor, IGF-I mRNA levels, gill Na⁺,K⁺-ATPase activity, muscle and liver lipid content and body silvering. Low temperature and seasonal growth cessation probably explains the relatively limited GH effects found. Body silvering in all strains was positively correlated to size. GH increased IGF-I plasma levels in the Namsen strain inspite of liver IGF-I mRNA levels being lower in GH-treated fish.     

Farmed Atlantic salmon Salmo salar L. parr may reduce early survival of wild fish

The study examined the density‐mediated effects on growth, survival and dispersal of wild and farmed Atlantic salmon Salmo salar offspring in the period immediately following emergence, using a substitutive design. In small confined stream channels, wild parr coexisting with farmed parr had a significantly poorer survival, than wild parr alone. Density did not affect this relationship. In larger unconfined stream channels, wild parr coexisting with farmed parr entered a downstream trap in higher numbers than wild parr in allopatry. The results suggests that during the earliest life stages, farmed S. salar can outcompete wild S. salar, resulting in a reduced survival of wild S. salar.     

Migratory behaviour of wild and farmed Atlantic salmon (Salmo salar) during spawning

Migratory behaviour at spawning of wild and newly-escaped farmed Atlantic salmon was analysed by radio telemetry in the River Alta, North Norway. Spawning areas were located by aerial surveys. Farmed females moved significantly more than wild females ( P <0.01). There was no such difference between the two groups of males. About 83% of the wild fish stayed within identified spawning areas for 1 day or longer. The corresponding figure for farmed salmon was only 43% ( P <0.05). Wild salmon stayed 8.1 days inside spawning areas and farmed salmon 5.2 days. The present results suggest that escaped farmed salmon had reduced spawning success compared with wild fish.     

Evidence for episodic acidification effects on migrating Atlantic salmon Salmo salar smolts

Field studies were conducted to determine levels of gill aluminium as an index of acidification effects on migrating Atlantic salmon Salmo salar smolts in the north‐eastern U.S.A. along mainstem river migration corridors in several major river basins. Smolts emigrating from the Connecticut River, where most (but not all) tributaries were well buffered, had low or undetectable levels of gill aluminium and high gill Na⁺/K⁺‐ATPase (NKA) activity. In contrast, smolts emigrating from the upper Merrimack River basin where most tributaries are characterized by low pH and high inorganic aluminium had consistently elevated gill aluminium and lower gill NKA activity, which may explain the low adult return rates of S. salar stocked into the upper Merrimack catchment. In the Sheepscot, Narraguagus and Penobscot Rivers in Maine, river and year‐specific effects on gill aluminium were detected that appeared to be driven by underlying geology and high spring discharge. The results indicate that episodic acidification is affecting S. salar smolts in poorly buffered streams in New England and may help explain variation in S. salar survival and abundance among rivers and among years, with implications for the conservation and recovery of S. salar in the north‐eastern U.S.A. These results suggest that the physiological condition of outmigrating smolts may serve as a large‐scale sentinel of landscape‐level recovery of atmospheric pollution in this and other parts of the North Atlantic region.     

Thiamin dynamics during the adult life cycle of Atlantic salmon (Salmo salar)

Thiamin is an essential water-soluble B vitamin known for its wide range of metabolic functions and antioxidant properties. Over the past decades, reproductive failures induced by thiamin deficiency have been observed in several salmonid species worldwide, but it is unclear why this micronutrient deficiency arises. Few studies have compared thiamin concentrations in systems of salmonid populations with or without documented thiamin deficiency. Moreover, it is not well known whether and how thiamin concentration changes during the marine feeding phase and the spawning migration. Therefore, samples of Atlantic salmon (Salmo salar) were collected when actively feeding in the open Baltic Sea, after the sea migration to natal rivers, after river migration, and during the spawning period. To compare populations of Baltic salmon with systems without documented thiamin deficiency, a population of landlocked salmon located in Lake Vänern (Sweden) was sampled as well as salmon from Norwegian rivers draining into the North Atlantic Ocean. Results showed the highest mean thiamin concentrations in Lake Vänern salmon, followed by North Atlantic, and the lowest in Baltic populations. Therefore, salmon in the Baltic Sea seem to be consistently more constrained by thiamin than those in other systems. Condition factor and body length had little to no effect on thiamin concentrations in all systems, suggesting that there is no relation between the body condition of salmon and thiamin deficiency. In our large spatiotemporal comparison of salmon populations, thiamin concentrations declined toward spawning in all studied systems, suggesting that the reduction in thiamin concentration arises as a natural consequence of starvation rather than to be related to thiamin deficiency in the system. These results suggest that factors affecting accumulation during the marine feeding phase are key for understanding the thiamin deficiency in salmonids.     

A primary phosphorus‐deficient skeletal phenotype in juvenile Atlantic salmon Salmo salar: the uncoupling of bone formation and mineralization

To understand the effect of low dietary phosphorus (P) intake on the vertebral column of Atlantic salmon Salmo salar, a primary P deficiency was induced in post‐smolts. The dietary P provision was reduced by 50% for a period of 10 weeks under controlled conditions. The animal's skeleton was subsequently analysed by radiology, histological examination, histochemical detection of minerals in bones and scales and chemical mineral analysis. This is the first account of how a primary P deficiency affects the skeleton in S. salar at the cellular and at the micro‐anatomical level. Animals that received the P‐deficient diet displayed known signs of P deficiency including reduced growth and soft, pliable opercula. Bone and scale mineral content decreased by c. 50%. On radiographs, vertebral bodies appear small, undersized and with enlarged intervertebral spaces. Contrary to the X‐ray‐based diagnosis, the histological examination revealed that vertebral bodies had a regular size and regular internal bone structures; intervertebral spaces were not enlarged. Bone matrix formation was continuous and uninterrupted, albeit without traces of mineralization. Likewise, scale growth continues with regular annuli formation, but new scale matrix remains without minerals. The 10 week long experiment generated a homogeneous osteomalacia of vertebral bodies without apparent induction of skeletal malformations. The experiment shows that bone formation and bone mineralization are, to a large degree, independent processes in the fish examined. Therefore, a deficit in mineralization must not be the only cause of the alterations of the vertebral bone structure observed in farmed S. salar. It is discussed how the observed uncoupling of bone formation and mineralization helps to better diagnose, understand and prevent P deficiency‐related malformations in farmed S. salar.     

Heart rates of Atlantic salmon Salmo salar during a critical swim speed test and subsequent recovery

In this study, heart rate (HR) bio-loggers were implanted in the abdominal cavity of 12 post-smolt Atlantic salmon Salmo salar weighing 1024 ± 31 g and acclimated to 12°C sea water. One week after the surgical procedure, a critical swim speed (U_crit) test was performed on tagged and untagged conspecifics, whereafter tagged fish were maintained in their holding tanks for another week. The U_crit was statistically similar between tagged and untagged fish (2.67 ± 0.04 and 2.74 ± 0.05 body lengths s⁻¹, respectively) showing that the bio-logger did not compromise the swimming performance. In the pre-swim week, a diurnal cycle was apparent with HR peaking at 65 beats min⁻¹ during the day and approaching 40 beats min⁻¹ at night. In the U_crit test, HR increased approximately exponentially with swimming speed until a plateau was reached at the final speed before fatigue with a maximum of 85.2 ± 0.7 beats min⁻¹. During subsequent recovery tagged fish could be divided into a surviving group (N = 8) and a moribund group (N = 4). In surviving fish HR had fully recovered to pre-swim levels after 24 h, including reestablishment of a diurnal HR cycle. In moribund fish HR never recovered and remained elevated at c. 80 beats min⁻¹ for 4 days, whereafter they started dying. We did not identify a proximal cause of death in moribund fish, but possible explanations are discussed. Tail beat frequency (TBF) was also measured and showed a more consistent response to increased swimming speeds. As such, when exploring correlations between HR, TBF and metabolic rates at different swimming speeds, TBF provides better predictions. On the contrary, HR measurements in free swimming fish over extended periods of time are useful for other purposes such as assessing the accumulative burden of various stressors and recovery trajectories from exhaustive exercise.     

Dynamics of cortisol and the development of the glucocorticoid function in the early ontogenesis of Atlantic salmon Salmo salar

The dynamics of cortisol were studied during the embryonic and early postembryonic development of Atlantic salmon Salmo salar (in brood obtained from the same pair of spawners, but with differing hatching dates). The initial level of maternal cortisol in the eggs after fertilization comprised of 25.6 ± 2.2 ng/g, and decreased continuously in the development process to 1.1 ± 0.2 ng/g by the 95th day after fertilization (t1) or the 18th day after hatching (t2). The first increase in the concentration of endogenous cortisol to 6.1 ± 1.2 ng/g was recorded at t2 = 24 days. Then the level of cortisol increased constantly and reached 32.8 ng/g by t1 = 149 (t2 = 72). A comparison of the hormone dynamics in two offspring groups from the same pare of spawners, differing in the longevity of embryogenesis, demonstrated that the level of cortisol increased after hatching more intensely in fishes with shorter embryogenesis. An injection of DL-aminoglutethimide, a compound preventing the synthesis of cortisol, significantly decreased the concentration of cortisol for the first time at the day of t1 = 101 (t2 = 24), i.e. at the first moment of the increase of endogenous cortisol. The effect from the adrenaline injection as a stress hormone was observed only at the day of t1 = 149 (t2 = 72), i.e., during the transition to the fry stages of development. Problems concerning when steroidogenesis begins, of the formation of the hypothalamic-hypophyseal-interrenal axis as a single hormone-competent ensemble, and of the development the stressed response in salmon juveniles are discussed.     

Counterintuitive migration patterns by Atlantic salmon Salmo salar smolts in a large lake

What little is known about the seaward migration of Salmo salar smolt migration through standing waters indicates that it is both slow and results in high mortality rates, compared with riverine migration. This may be partly because smolts in lakes need to swim more actively and require more complex directional cues than they do in rivers. In this telemetry study of smolt migration through Loch Lomond, S. salar smolts made repeated movements in directions away from the outflowing river, which considerably increased migration time.