Quantitative trait loci for body weight, condition factor and age at sexual maturation in Arctic charr (Salvelinus alpinus): comparative analysis with rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar)

In salmonid fishes, life-history changes may often be coupled to early individual growth trajectories. We identified quantitative trait loci (QTL) for body weight (BW), condition factor (K) and age at sexual maturation (MT) in two full-sib families of Arctic charr (Salvelinus alpinus) to ascertain if QTL for MT were confounded with BW QTL intervals. Three significant QTL for BW, three QTL for MT and one significant QTL for K were identified. A BW QTL with major effect was localized to linkage group 8 (AC-8) and explained more than 34% of the phenotypic variation. Markers on AC-8 have previously been identified as being associated with variation in fork length and BW in this species. Similarly, a major QTL (PEV = 23%) with an influence on the female MT was localized to AC-23. Some of these regions are homologous to those in the genomes of rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar), where similar QTL effects have been detected. Our results also suggest the conservation of MT QTL on the homeologous linkage group pair AC-3/24 in Arctic charr. We further identified chromosomal regions that harbor QTL for multiple traits. In particular, markers on AC-4, -20 and -36 had detectable QTL for all traits studied. Significant MT QTL detected on AC-23, -24, and -27 were autonomous of any BW QTL regions, suggesting that the regulation of MT may be more independent of BW control within this species than in other species of salmonids.     

Genomic arrangement of salinity tolerance QTLs in salmonids: A comparative analysis of Atlantic salmon (Salmo salar) with Arctic charr (Salvelinus alpinus) and rainbow trout (

ABSTRACT: BACKGROUND: Quantitative trait locus (QTL) studies show that variation in salinity tolerance in Arctic charr and rainbow trout has a genetic basis, even though both these species have low to moderate salinity tolerance capacities. QTL were observed to localize to homologous linkage group segments within putative chromosomal regions possessing multiple candidate genes. We compared salinity tolerance QTL in rainbow trout and Arctic charr to those detected in a higher salinity tolerant species, Atlantic salmon. The highly derived karyotype of Atlantic salmon allows for the assessment of whether disparity in salinity tolerance in salmonids is associated with differences in genetic architecture. To facilitate these comparisons, we examined the genomic synteny patterns of key candidate genes in the other model teleost fishes that have experienced three whole-genome duplication (3R) events which preceded a fourth (4R) whole genome duplication event common to all salmonid species. RESULTS: Nine linkage groups contained chromosome-wide significant QTL (AS-2, -4p, -4q, -5, -9, -12p, -12q, -14q -17q, -22, and [MINUS SIGN]23), while a single genome-wide significant QTL was located on AS-4q. Salmonid genomes shared the greatest marker homology with the genome of three-spined stickleback. All linkage group arms in Atlantic salmon were syntenic with at least one stickleback chromosome, while 18 arms had multiple affinities. Arm fusions in Atlantic salmon were often between multiple regions bearing salinity tolerance QTL. Nine linkage groups in Arctic charr and six linkage group arms in rainbow trout currently have no synteny alignments with stickleback chromosomes, while eight rainbow trout linkage group arms were syntenic with multiple stickleback chromosomes. Rearrangements in the stickleback lineage involving fusions of ancestral arm segments could account for the 21 chromosome pairs observed in the stickleback karyotype. CONCLUSIONS: Salinity tolerance in salmonids from three genera is to some extent controlled by the same loci. Synteny between QTL in salmonids and candidate genes in stickleback suggests genetic variation at candidate gene loci could affect salinity tolerance in all three salmonids investigated. Candidate genes often occur in pairs on chromosomes, and synteny patterns indicate these pairs are generally conserved in 2R, 3R, and 4R genomes. Synteny maps also suggest that the Atlantic salmon genome contains three larger syntenic combinations of candidate genes that are not evident in any of the other 2R, 3R, or 4R genomes examined. These larger synteny tracts appear to have resulted from ancestral arm fusions that occurred in the Atlantic salmon ancestor. We hypothesize that the superior hypo-osmoregulatory efficiency that is characteristic of Atlantic salmon may be related to these clusters.     

Temperature requirements of Atlantic salmon Salmo salar, brown trout Salmo trutta and Arctic charr Salvelinus alpinus: predicting the effects of climate change

Atlantic salmon Salmo salar, brown trout Salmo trutta (including the anadromous form, sea trout) and Arctic charr Salvelinus alpinus (including anadromous fish) provide important commercial and sports fisheries in Western Europe. As water temperature increases as a result of climate change, quantitative information on the thermal requirements of these three species is essential so that potential problems can be anticipated by those responsible for the conservation and sustainable management of the fisheries and the maintenance of biodiversity in freshwater ecosystems. Part I compares the temperature limits for survival, feeding and growth. Salmo salar has the highest temperature tolerance, followed by S. trutta and finally S. alpinus. For all three species, the temperature tolerance for alevins is slightly lower than that for parr and smolts, and the eggs have the lowest tolerance; this being the most vulnerable life stage to any temperature increase, especially for eggs of S. alpinus in shallow water. There was little evidence to support local thermal adaptation, except in very cold rivers (mean annual temperature <6·5° C). Part II illustrates the importance of developing predictive models, using data from a long-term study (1967-2000) of a juvenile anadromous S. trutta population. Individual-based models predicted the emergence period for the fry. Mean values over 34 years revealed a large variation in the timing of emergence with c. 2 months between extreme values. The emergence time correlated significantly with the North Atlantic Oscillation Index, indicating that interannual variations in emergence were linked to more general changes in climate. Mean stream temperatures increased significantly in winter and spring at a rate of 0·37° C per decade, but not in summer and autumn, and led to an increase in the mean mass of pre-smolts. A growth model for S. trutta was validated by growth data from the long-term study and predicted growth under possible future conditions. Small increases (<2·5° C) in winter and spring would be beneficial for growth with 1 year-old smolts being more common. Water temperatures would have to increase by c. 4° C in winter and spring, and 3° C in summer and autumn before they had a marked negative effect on trout growth.     

Tissue astaxanthin and canthaxanthin distribution in rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar)

A comparative investigation of tissue carotenoid distribution between rainbow trout, Oncorhynchus mykiss, and Atlantic salmon, Salmo salar, was undertaken to identify the relative efficiency of utilization of astaxanthin and canthaxanthin. Higher apparent digestibility coefficients (ADCs) (96% in trout vs. 28-31% in salmon; P<0.05), and pigment retention efficiencies (11.5-12.5% in trout vs. 5.5% in salmon; P<0.05), for both astaxanthin and canthaxanthin, were observed for rainbow trout. Astaxanthin deposition was higher than canthaxanthin in rainbow trout, while the reverse was true for Atlantic salmon, suggesting species-specificity in carotenoid utilization. The white muscle (95% in trout vs. 93% in salmon) and kidneys (0.5% in trout vs. 0.2% in salmon) represented higher proportions of the total body carotenoid pool in rainbow trout than in Atlantic salmon (P<0.05), whereas the liver was a more important storage organ in Atlantic salmon (2-6% in salmon vs. 0.2% in trout; P<0.05). The liver and kidney appeared to be important sites of carotenoid catabolism based on the relative proportion of the peak chromatogram of the fed carotenoid in both species, with the pyloric caecae and hind gut being more important in Atlantic salmon than in the rainbow trout. Liver catabolism is suspected to be a critical determinant in carotenoid clearance, with higher catabolism expected in Atlantic salmon than in rainbow trout.     

Heart morphology in wild and farmed Atlantic salmon Salmo salar and rainbow trout Oncorhynchus mykiss

The normal shape of the salmonid ventricle is a triangular pyramid with the apex pointing caudoventrally. A strong positive correlation has been established between this shape and optimum cardiac output and function. Domesticated salmonids appear to have developed a more rounded ventricle with misaligned bulbus arteriosus. Several reports from fish health veterinarians indicate that fish with abnormal heart morphology have a high mortality rate during stress-inducing situations like grading, transportation and bath treatments. The present paper compares and describes the ventricle morphology of wild vs. farmed Atlantic salmon, and wild steelhead (anadromous rainbow trout) vs. farmed rainbow trout. Several parameters were measured to provide numerical measurement of the differences in shape, i.e. height:width ratio and the angle between the longitudinal ventricular axis and the axis of the bulbus arteriosus. We conclude that the hearts of farmed fish are rounder than those in corresponding wild fish, and that the angle between the ventricular axis and the axis of the bulbus arteriosus is more acute in wild fish than in their farmed counterparts. Further studies are necessary to reveal the prevalence, functional significance and possible causes of these abnormal hearts.     

Effects of acute iron overload on Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss)

Distribution of radioiron to various tissues after intraperitoneal injections was examined in Atlantic salmon and rainbow trout. Liver and spleen were found to be the major iron storage tissues. Injections of 1 or 5 mg iron as ferric ammonium citrate led to a fall in hemoglobin levels in both species after 2 d. Hemoglobin levels returned to normal levels in rainbow trout after 8 d, but Atlantic salmon had not recovered, and Hb levels fell below 3 g/100 mL. In both species, the fall in Hb was associated with a raise in iron levels in spleen and liver, suggesting damage to erythrocytes. Atlantic salmon liver ferritin showed a two- to threefold increase, while rainbow trout showed a sixfold increase, and a more rapid response. The toxic effect of iron in fish appears to be different from the effect in other vertebrates.     

Distribution of temperature tolerance quantitative trait loci in Arctic charr (Salvelinus alpinus) and inferred homologies in rainbow trout (Oncorhynchus mykiss)

We searched for quantitative trait loci (QTL) affecting upper temperature tolerance (UTT) in crosses between the Nauyuk Lake and Fraser River strains of Arctic charr (Salvelinus alpinus) using survival analysis. Two QTL were detected by using two microsatellite markers after correcting for experiment-wide error. A comparative mapping approach localized these two QTL to homologous linkage groups containing UTT QTL in rainbow trout (Oncorhynchus mykiss). Additional marginal associations were detected in several families in regions homologous to those with QTL in rainbow trout. Thus, the genes underlying UTT QTL may antedate the divergence of these two species, which occurred by approximately 16 MYA. The data also indicate that one pair of homeologs (ancestrally duplicated chromosomal segments) have contained QTL in Arctic charr since the evolution of salmonids from a tetraploid ancestor 25-100 MYA. This study represents one of the first examples of comparative QTL mapping in an animal polyploid group and illustrates the fate of QTL after genome duplication and reorganization.     

A new type of cysteine proteinase inhibitor--the salarin gene from Atlantic salmon (Salmo salar L.) and Arctic charr (Salvelinus alpinus)

Salarin is a 43 kDa glycoprotein which is found so far only in salmonid fish species. It is a strong inhibitor of cysteine proteinases. Here we characterised the salarin gene from Atlantic salmon and cDNA from Arctic charr. The salarin gene has 13 exons and 1026 bp long coding sequence. The translated amino acid sequence has four similar domains. The sequence resembles the proregion of cathepsins, known to inhibit cysteine proteinases. Salarin can be a new type of cysteine proteinase inhibitor.     

Susceptibility of rainbow trout Oncorhynchus mykiss,Atlantic salmon Salmo salar and coho salmon Oncorhynchus kisutch to experimental infection with sea lice Lepeophtheirus salmonis

Physiological, immunological and biochemical parameters of blood and mucus, as well as skin histology, were compared in 3 salmonid species (rainbow trout Oncorhynchus mykiss, Atlantic salmon Salmo salar and coho salmon O. kisutch) following experimental infection with sea lice Lepeophtheirus salmonis. The 3 salmonid species were cohabited in order to standardize initial infection conditions. Lice density was significantly reduced on coho salmon within 7 to 14 d, while lice persisted in higher numbers on rainbow trout and Atlantic salmon. Lice matured more slowly on coho salmon than on the other 2 species, and maturation was slightly slower on rainbow trout than on Atlantic salmon. Head kidney macrophages from infected Atlantic salmon had diminished respiratory burst and phagocytic capacity at 14 and 21 d post-infection (dpi), while infected rainbow trout macrophages had reduced respiratory burst and phagocytic capacities at 21 dpi, compared to controls. The slower development of lice, coupled with delayed suppression of immune parameters, suggests that rainbow trout are slightly more resistant to lice than Atlantic salmon. Infected rainbow trout and Atlantic salmon showed increases in mucus lysozyme activities at 1 dpi, which decreased over the rest of the study. Mucus lysozyme activities of infected rainbow trout, however, remained higher than controls over the entire period. Coho salmon lysozyme activities did not increase in infected fish until 21 dpi. Mucus alkaline phosphatase levels were also higher in infected Atlantic salmon compared to controls at 3 and 21 dpi. Low molecular weight (LMW) proteases increased in infected rainbow trout and Atlantic salmon between 14 and 21 dpi. Histological analysis of the outer epithelium revealed mucus cell hypertrophy in rainbow trout and Atlantic salmon following infection. Plasma cortisol, glucose, electrolyte and protein concentrations and hematocrit all remained within physiological limits for each species, with no differences occurring between infected and control fish. Our results demonstrate that significant differences in mucus biochemistry and numbers of L. salmonis occur between these species.     

Morphologic description of infectious salmon anaemia virus (ISAV)-induced lesions in rainbow trout Oncorhynchus mykiss compared to Atlantic salmon Salmo salar

In the present study the pathogenesis of experimental infectious salmon anaemia virus (ISAV) infection in rainbow trout Oncorhynchus mykiss (Walbaum, 1972) and Atlantic salmon Salmo salar was compared. The virus infection in the 2 species demonstrated different mortality patterns and pathology characteristics. Atlantic salmon showed a typical acute mortality pattern peaking at 8 to 16 d post-infection (dpi) depending on virus dose, whereas in rainbow trout, only the highest virus dose (10(7.13-7.8) TCID50/200 microl) showed a similar pattern. The middle (10(4.13) TCID50/200 microl) and lowest virus doses (10(2.13) TCID50/200 microl) in rainbow trout induced only sporadic protracted mortality, lasting up to 46 dpi. Infected rainbow trout that were live-sampled and those that died demonstrated increased erythrophagia, clusters of cellular degeneration in the haematopoietic portion of the kidney, and occasionally epicarditis, endocarditis and myocarditis. These lesions are very different from the typical necrosis in liver and kidney that occur in infected Atlantic salmon, and some of them may be indicative of an antiviral response by a resistant host to the ISAV infection. Virus was detected in the endothelium of the rainbow trout tissues using in situ hybridization, supporting our conclusions of the ISAV-induced lesions in this report.     

Seasonal changes in hypoosmoregulatory ability in landlocked and anadromous populations of Arctic charr,Salvelinus alpinus,and Atlantic salmon,Salmo salar

Synopsis Seasonal changes in hypoosmoregulatory ability were compared in landlocked and anadromous strains of Arctic charr and Atlantic salmon. Seawater adaptability was assessed using periodic 48 h seawater challenge tests with 25. seawater. The landlocked strains of Arctic charr, two from northern Sweden and one from Southern Norway, displayed similar seasonal changes in seawater adaptability as the anadromous strain. Seawater tolerance increased during spring and remained high until the end of July — early August after which it declined. The two strains of Atlantic salmon displayed different seasonal patterns in hypoosmoregulatory ability. The anadromous strain showed a pronounced seasonal pattern with maximal seawater adaptability in early June. In contrast, seawater tolerance in the landlocked strain improved steadily during spring and remained high until late autumn. During the period of enhanced seawater tolerance, hypoosmoregulatory ability increased significantly with body size in both Arctic charr and anadromous Atlantic salmon. The minimum size at which fish were able to regulate plasma sodium following seawater transfer at a level comparable to freshwater levels (<170 mmol I^–1) differed significantly between anadromous Atlantic salmon (ca. 14 cm) and Arctic charr (ca. 22 cm). The results show that seasonal changes in hypoosmoregulatory ability are present in both Atlantic salmon and Arctic charr, and that these physiological traits are retained in the corresponding landlocked strains. However, the seasonal pattern of seawater adaptability as well as the minimum size at which seawater tolerance occurs differs between the two species.     

Identification of the sex chromosomes of brown trout (Salmo trutta) and their comparison with the corresponding chromosomes in Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss)

Males are the heterogametic sex in salmonid fishes. In brown trout (Salmo trutta) the sex-determining locus, SEX, has been mapped to the end of linkage group BT-28, which corresponds to linkage group AS-8 and chromosome SSA15 in Atlantic salmon (Salmo salar). We set out to identify the sex chromosomes in brown trout. We isolated Atlantic salmon BAC clones containing microsatellite markers that are on BT-28 and also on AS-8, and used these BACs as probes for fluorescent in situ hybridization (FISH) analysis. SEX is located on the short arm of a small subtelocentric/acrocentric chromosome in brown trout, which is consistent with linkage analysis. The acrocentric chromosome SSA15 in Atlantic salmon appears to have arisen by a centric fusion of 2 small acrocentric chromosomes in the common ancestor of Salmo sp. We speculate that the fusion process that produced Atlantic salmon chromosome SSA15 disrupted the ancestral sex-determining locus in the Atlantic salmon lineage, providing the impetus either for the relocation of SEX or selection pressure for a novel sex-determining gene to arise in this species. Thus, the sex-determining genes may differ in Atlantic salmon and brown trout.     

New subtype of salmonid alphavirus (SAV), Togaviridae, from Atlantic salmon Salmo salar and rainbow trout Oncorhynchus mykiss in Norway

In Europe, 2 closely related alphaviruses (Togaviridae) are regarded as the causative agents of sleeping disease (SD) and salmon pancreas disease (SPD): SD virus (SDV) has been isolated from rainbow trout Oncorhynchus mykiss in France and the UK, while SPD virus (SPDV) has been isolated from salmon Salmo salar in Ireland and the UK. Farmed salmonids in western Norway also suffer from a disease called pancreas disease (PD), and this disease is also believed to be caused by an alphavirus. However, this virus has not yet been characterised at the molecular level. We have cultured a Norwegian salmonid alphavirus from moribund fishes diagnosed with cardiac myopathy syndrome (CMS) and fishes diagnosed with PD. The virus has also been found in salmon suffering from haemorrhagic smolt syndrome in the fresh water phase. The genomic organisation of the Norwegian salmonid alphavirus is identical to that in SPDV and SDV, and the nucleotide sequence similarity to the other 2 alphaviruses is 91.6 and 92.9%, respectively. Based on the pathological changes, host species and the nucleotide sequence, we suggest naming this virus Norwegian salmonid alphavirus (NSAV). Together with SPDV and SDV it constitutes a third subtype of salmonid alphavirus (SAV) species within the genus Alphavirus, family Togaviridae.     

Kinetics of Mx expression in rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar L.) parr in response to VHS-DNA vaccination

The duration of the Mx mRNA response to an intramuscular injection of the viral haemorrhagic septicaemia virus (VHSV) glycoprotein (G) gene DNA vaccine as well as to the control plasmid was determined in rainbow trout at 14 degrees C over a period of 11 weeks. The Mx response was detectable on day 7, peaked on day 14 and returned to pretreatment levels on day 21 and thereafter. No increase in Mx expression was detectable to the control plasmid. In further experiments, the kinetics of the Mx response were compared in rainbow trout and Atlantic salmon parr kept at 10 degrees C and injected with the DNA vaccine or the synthetic double-stranded RNA, poly I:C. In both species there was a rapid response to poly I:C detectable from day 1, reaching maximum from days 3 to 9 and decreasing to background level by day 12. The peak level and return to background was reached slightly later in salmon. In both species the response to the VHS/DNA vaccine was slower to begin, not being detectable on days 1 and 3, but elevated levels were found on day 6. However, in the salmon parr, the peak level was on day 6 and the signal disappeared by day 12, while in the rainbow trout, the response peaked at day 12 and lasted until day 21. The kinetics of the Mx response to the VHS/DNA vaccine in rainbow trout correlate with the early non-specific protection against VHS in this species following vaccination. It is speculated that the more transient Mx response in Atlantic salmon parr to the DNA vaccine may be related to the innate resistance of salmon to VHS.     

Effects of soybean meal and salinity on intestinal transport of nutrients in Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss)

Groups of fresh- and seawater-adapted Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss) were fed diets with (SBM diet) or without (control diet) extracted soybean meal (30% of protein substituted with SBM) for 3 weeks. Average fish size per group ranged from 597 to 1763 g. One tank or net pen per species, dietary group and water salinity was used. In vitro nutrient transport (D-glucose, the L-amino acids aspartate, lysine, methionine, phenylalanine and proline, and the dipeptide glycyl-sarcosine) was measured using intact tissue (everted sleeve method) from the different postgastric intestinal regions. The dimensions of the different intestinal regions were also measured for each treatment group. Results indicate that SBM causes decreased carrier-mediated transport and increased permeability of distal intestinal epithelium for the nutrients, and the capacity of this region to absorb nutrient was diminished. Salinity may also affect the relative contribution of carrier-mediated and independent uptake to total nutrient absorption.     

Spatial arrangement of fish gill secondary lamellar cells in intact and dissociated tissues from rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar)

Gill structure of rainbow trout and Atlantic salmon was investigated using cell disaggregation and dry fracture techniques for scanning electron microscopy (SEM), allowing new interpreta-tions of the structure of the secondary lamella. The basement membrane underlying the lamellar epithelium (secondary epithelium) was shown to be a tough sheet with numerous depressions corresponding to underlying pillar cells. This membrane is probably the most important structural element of the secondary lamella, capable of withstanding considerable mechanical stress. For the first time the structure of the apical surface of the secondary lamella was shown by SEM to consist of an outer microridged coat overlying a fibrous coat which appears continuous with the extracellular matrix surrounding the rest of the cell. When cells were detached they rounded up and the external microridged coat became more vesicle like, indicating the labile nature of this coat. In cell suspension preparations, epithelial, mucus and chloride cells are present as well as many blood derived cells such as erythrocytes, presumptive leucocytes and thrombocytes.     

Evolution of the immunoglobulin M constant region genes of salmonid fish,rainbow trout (Oncorhynchus mykiss) and Arctic charr (Salvelinus alpinus): implications concerning divergence time of species

The nucleotide sequence data reported in this paper have been submitted to the EMBL nucleotide sequence database and have been assigned the accession numbers X83372 (Oncorhynchus mykiss) and X83373 (Salvelinus alpinus)     

A comparative study of the relationship between light intensity and feeding ability in brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus)

1. Field observations indicate that the ability to feed at different light intensities may differ between brown trout and Arctic charr, and this is the first study to test this experimentally. To establish a background level of feeding in daylight at midday, trout and charr in two size groups were kept in tanks (one fish per tank) at three constant temperatures (5.0, 10.8 and 13.0 °C) and each fish was offered, one at a time, 50 freshly killed shrimps (Gammarus pulex), the number eaten being recorded. Shrimps could only be taken in the water column because a metal mesh prevented access to dead shrimps on the tank bottom. In a first series of experiments, individual fish were kept at one of 10 natural light intensities (range 0.001–50 lx). In a second series, conditions were similar except that the fish tank was covered in black polyethylene and had a light‐tight lid with a shutter so that light levels could be kept constant, using artificial illumination. In a third series, the fish were fed in total darkness, but the false bottom was removed, allowing access to dead shrimps on the tank bottom as well as in the water column. 2. The results of the first and second series differed interspecifically but were very similar intraspecifically, with no significant differences between the food intake for the two size groups or in the experiments at 10.8 and 13.0 °C. Food intake remained fairly constant at light intensities between 50 lx (dusk or dawn) and 0.03 lx and was similar to that of fish feeding at midday. At 10.8 and 13.0 °C, food intake between 0.03 and 50 lx was higher for trout than for charr, mean values for shrimps eaten per fish being 39.9 for trout (range 36–44, n = 100 fish) and 32.0 for charr (range 28–38, n = 100), but at 5.0 °C, the situation was reversed with mean values of 15.1 for trout (range 11–18, n = 50 fish) and 19.8 for charr (range 17–22, n = 50). 3. As light intensity decreased from 0.04 to 0.001 lx, feeding rate decreased exponentially but was always higher for charr than for trout, with a mean number of shrimps eaten at 0.001 lx of 9.3 for trout (range 5–13, n = 20 fish) and 13.6 for charr (range 9–20, n = 20) at 10.8 and 13.0 °C, and 2.0 for trout (range 1–4, n = 10 fish) and 5.5 for charr (range 2–8, n = 10) at 5.0 °C. In total darkness (false bottom fitted), none of the 50 shrimps was taken by either species. When the false bottom was removed in the third series, the mean number of shrimps consumed over 24 h was eight for trout (range 3–11, n = 20 fish) and 14.9 for charr (range 9–20, n = 20) at 10.8 and 13.0 °C, and two for trout (range 0–4, n = 10 fish) and five for charr (range 3–8, n = 10) at 5.0 °C. 4. Therefore, the feeding ability of trout was superior to that of charr when using photopic vision in daylight and mesotopic vision at dusk and dawn, but inferior to that of charr when using scotopic vision at low light intensity. Charr were also superior at low temperatures and when foraging for food in total darkness. Therefore, as light intensity decreases after dusk in their natural habitat, the advantage in feeding will shift from trout to charr, with the reverse occurring as light intensity increases after dawn.     

Marine food and diet overlap of co-occurring Arctic charr Salvelinus alpinus (L.), brown trout Salmo trutta L. and Atlantic salmon S. salar L. off Senja,N. Norway

Summary Stomach contents analyses and other biological information of Arctic charr (Savelinus alpinus (L.)), brown trout (Salmo trutta L.) and small Atlantic salmon (S. salar L.) caught 1982–85 close to the Åelv estuary (69°N) on the island of Senja, N. Norway are presented, and extracts of a 1975–85 fishing log given. this appears to be the first case study of the feeding habits of all three European anadromous salmonids in marine sympatry, and also one of very few reports on the marine food of the Arctic charr from Europe. The general feeding habits of the charr were similar to that found in N. Canada. Pelagic fish (herring, sand-eel) seem to be preferred. Plankton (crab megalopae, krill) and hyperbenthos (amphipods, mysids) are also taken, especially when suitable fish are scarce. In 1985 high herring densities provided superabundant food, and diet overlap between charr, trout and salmon was high. Salmonid nursery rivers are abundant in N. Norway and during summer the three species coexist in a near-shore, surface-oriented pelagic guild of fishes. The salmon seems to be a relatively specialized piscivore, while the trout takes a wider range of fish and also invertebrate prey. The charr probably is the most euryphagous of the three, being able to exploit the more marginal parts of the prey resources of their common habitat.