Winter ecology of Arctic charr (<Emphasis Type="Italic">Salvelinus alpinus</Emphasis>) and brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>) in a subarctic lake,Norway

We studied habitat choice, diet, food consumption and somatic growth of Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) during the ice-covered winter period of a subarctic lake in northern Norway. Both Arctic charr and brown trout predominantly used the littoral zone during winter time. Despite very cold winter conditions (water temperature <1°C) and poor light conditions, both fish species fed continuously during the ice-covered period, although at a much lower rate than during the summer season. No somatic growth could be detected during the ice-covered winter period and the condition factor of both species significantly declined, suggesting that the winter feeding rates were similar to or below the maintenance requirements. Also, the species richness and diversity of ingested prey largely decreased from summer to winter for both fish species. The winter diet of Arctic charr <20 cm was dominated by benthic insect larvae, chironomids in particular, and Gammarus lacustris, but zooplankton was also important in December. G. lacustris was the dominant prey of charr >20 cm. The winter diet of brown trout <20 cm was dominated by insect larvae, whereas large-sized trout mainly was piscivorous, feeding on juvenile Arctic charr. Piscivorous feeding behaviour of trout was in contrast rarely seen during the summer months when their encounter with potential fish prey was rare as the small-sized charr mainly inhabited the profundal. The study demonstrated large differences in the ecology and interactions of Arctic charr and brown trout between the winter and summer seasons.     

Food borne parasites as indicators of trophic segregation between Arctic charr and brown trout

The habitat and diet choice and the infection (prevalence and abundance) of trophically transmitted parasites were compared in Arctic charr and brown trout living sympatrically in two lakes in northern Norway. Arctic charr were found in all main lake habitats, whereas the brown trout were almost exclusively found in the littoral zone. In both lakes the parasite fauna reflected the niche segregation between trout and charr. Surface insects were most common in the diet of trout, but transmit few parasites, and accordingly the brown trout had a relatively low diversity and abundance of parasites. Parasites transmitted by benthic prey such as Gammarus and insect larva, were common in both salmonid host species. Copepod transmitted parasites were much more common in Arctic charr, as brown trout did not include zooplankton in their diets. Parasite species that may use small fish as transport hosts, were far more abundant in piscivorous fish, especially brown trout. The seasonal dynamics in parasite infection were also consistent with the developments in the diet throughout the year. The study demonstrates that the structure of parasite communities of charr and the trout is highly dependent on shifts in habitat and diet of their hosts both on an annual base and through the ontogeny, in addition to the observed niche segregation between the two salmonid species.     

The Relationship Between Spatial Distribution and Diet of Arctic Charr, Salvelinus Alpinus, In Loch Ness, U.K.

The pelagic fish community of Loch Ness, U.K., is dominated by Arctic charr, Salvelinus alpinus. Previous studies have shown that the distribution of zooplankton along the south-west to north-east axis of this elongate loch is very dynamic and determined largely by prevailing winds, but Arctic charr are consistently more abundant in the southern half of the loch. In July 1993, the diet compositions of 161 Arctic charr from 53 to 330mm in length were determined and related to their spatial distribution and those of microcrustacean zooplankton and pelagic chironomid larvae and pupae. Diets were dominated by chironomid larvae, with chironomid pupae, Bythotrephes longimanus, Bosmina coregoni and Daphnia hyalina also frequently taken. Over the whole study period, B. longimanus were more important in the diet of fish from the northern half of the loch, while chironomid larvae were more important for fish from the southern half. As a result, per capita prey weight of fish from the south was greater than that of fish from the north. However, wind-induced changes in the distribution of zooplankton along the loch were accompanied by a change in the diet composition of Arctic charr from the south.     

A test of the hypothesis of pheromone attraction in salmonid migration

Synopsis We tested the hypothesis that anadromous salmonids are guided on their homeward migration by population-specific pheromones. Our findings do not support the hypothesis. Wild migrant Arctic charr,Salvelinus alpinus, from Ikarut River, Labrador were transferred and held in a tributary previously uninhabited by anadromous fish. None of the charr migrating up Ikarut River entered the tributary after fish were transferred. Similarly, migrant charr, which were caught in Ikarut River and released in the tributary below the captive fish, did not remain in the tributary. We re-evaluated the data which have been used to uphold the concept of pheromone attraction in salmonid migration and concluded that support for the hypothesis is unsubstantiated.     

Effects of the eye fluke Diplostomum spathaceum on the energetics and feeding of Arctic charr Salvelinus alpinus

In the present study, effects of the trematode eye fluke Diplostomum spathaceum infection and parasite‐induced cataract on energetic costs and feeding of hatchery‐bred one‐summer‐old Lake Saimaa Arctic charr Salvelinus alpinus were examined. The energetic effects (measured as oxygen uptake) of acute eye fluke infection on S. alpinus were negligible, but in the chronic infections mean oxygen consumption of infected fish was significantly higher than that of controls. Feeding capability of S. alpinus was studied in aquarium experiments, and uninfected healthy‐eyed fish reacted to zooplankton more rapidly and caught zooplankton with better success compared to individuals with cataracts in the same school. The results suggested that to compensate energetic costs and impaired feeding capability caused by eye fluke, cataract‐bearing fish spend more time foraging, which may result in increased predation risk.     

Takvatn Through 20 Years: Long-term Effects of an Experimental Mass Removal of Arctic Charr, Salvelinus Alpinus, From a Subarctic Lake

Between 1984 and 1989, the experimental removal of 31 tons (666000 fish) of stunted Arctic charr, Salvelinus alpinus, from Takvatn in northern Norway, had strong effects on the populations of Arctic charr, brown trout, Salmo trutta, and three-spined sticklebacks, Gasterosteus aculeatus. The littoral catch per unit effort (CPUE) of charr had decreased by 90% in 1990 and then increased to about 50% of the initial level by 1994 while the pelagic CPUE had decreased to zero. Growth in both charr and trout greatly improved when the charr density had decreased, and large fish of both species appeared in the catches. These large fish became predators on small charr in the littoral zone. The incidence of trout increased from below 1% to 15% from 1988 to 1999 after a brief peak at 30% in 1992 and 1993. The charr population attained a bimodal size distribution and did not return to the stunted state during the 10 years following the intensive fishing period. The mass removal experiment showed that it is possible to change the structure of a charr population by intensive fishing. Predation on small charr from cannibals and large trout was probably essential for maintaining the new population structure. An increase in the growth of young charr from 1995 to 1997 was related to a high consumption of Daphnia and Eurycercus. Rapid changes in the growth of charr followed the density fluctuations in sticklebacks, which show large annual variations in this system; the rapid changes in charr growth were probably caused by variations in the competition intensity for cladoceran prey between young charr and sticklebacks. Twenty years of data has provided important information, but even more time is needed to follow the long-term trends in northern lakes such as Takvatn.     

Seasonal dynamics and persistence of Gyrodactylus salaris in two riverine anadromous Arctic charr populations

The seasonal occurrence of the monogenean ectoparasite Gyrodactylus salaris Malmberg infecting Arctic charr, Salvelinus alpinus (L.) in the two rivers Skibotnelva and Signaldalselva in northern Norway was studied in the period from autumn 2003 to autumn 2005. Skibotnelva has been infected with the parasite since 1979, and treated with rotenone twice. Most likely resident Arctic charr avoided the rotenone treatment in small tributary streams, and thus was the source of the repeated re-infection of this river. G. salaris was first recorded in Signaldalselva in the year 2000 and it is still untreated. Unlike Atlantic salmon, which is highly susceptible to G. salaris, Arctic charr can display a wide range of host-responses to G. salaris infections. Arctic charr were sampled by electro fishing with a total sample of 681 Arctic charr. The results from this study demonstrate an evident seasonal dynamic in G. salaris infection in charr in both rivers. Parasite intensities fluctuated with the rise and fall in temperature through the year, with an autumn high and spring low. There was a significantly lower prevalence and mean intensity of G. salaris in Skibotnelva than in Signaldalselva. There were also a lower prevalence and intensity of G. salaris in the older than in the youngest charr. The different history of infection and treatment in the two rivers might be the underlying cause of these observed dissimilarities. The current study indicates that Arctic charr is a good natural host for G. salaris.     

Lipid‐rich zooplankton subsidise the winter diet of benthivorous Arctic charr (Salvelinus alpinus) in a subarctic lake

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Seasonal changes in sea-water tolerance of Arctic charr (Salvelinus alpinus)

Summary Groups of Arctic charr,Salvelinus alpinus, which had been acclimated to water with a salinity of 7 g·l^–1 and natural temperature and photoperiod, were exposed to water with different salinities and temperatures in June, September and February. At a salinity of 15 g·l^–1, plasma osmolality, plasma Na⁺, Cl^–, Mg²⁺ concentrations and the activity of gill Na-K-ATPase were stable, irrespective of temperature and season. In June, the charr were able to regulate blood plasma ionic levels within narrow limits when exposed to a salinity of 34 g·l^–1 (sea water) and a temperature of 8°C. The hypo-osmoregulatory capacity was less, but sufficient if the temperature was only 1°C during the seawater exposure. At the start of the experiment, the gill Na-K-ATPase activity was significantly higher in June than corresponding enzyme activities in September and February. Furthermore, an increase in gill Na-K-ATPase activity during the seawater exposure (8°C) was seen in June. Irrespective of ambient temperature and salinity, no fish died during the June experiments. In September and February, exposure to sea water produced marked increases in plasma osmolality and plasma ion concentrations. There were no changes in gill Na-K-ATPase activity. Consequently, the fish became dehydrated and were moribund after a short period of seawater exposure. Highest mortality was recorded when charr were exposed to winter sea conditions (34 g·l^–1 and 1°C) in February. The results indicate that an increase in daylength induce a hypo-osmoregulatory capacity in the Arctic charr during summer. In fall and winter, however, reduced daylength are accompanied by poor hypo-osmoregulatory capacity. This leads to high mortality as a result of increased electrolyte levels and dehydration.     

Adaptations to Stochastic Environmental Variations: The Effects of Seasonal Temperatures on the Migratory Window of Svalbard Arctic Charr

The circumpolar Arctic charr, Salvelinus alpinus, is ideal for studying how environmental factors affect life history in fishes. Charr populations demonstrate a tremendous ecological plasticity and adaptations to harsh environments. Arctic charr is the only freshwater fish on Svalbard, including anadromous, resident, and landlocked stocks. Freshwater lake systems on Svalbard are characterized by very low water temperatures, long-term or even permanent ice cover, and low levels of nutrients. Food is thus limited and may lead to growth stagnation and early maturity in Arctic charr. The individual growth pattern may alternatively follow a sigmoid-shaped curve, caused by a shift to either cannibalistic or anadromous (migration to sea) behaviour. In lake systems that include migratory charr, the population may consist of a mixture of parr, postsmolt, and adult migratory individuals, as well as small-sized resident, large-sized resident (cannibals), and large formerly resident individuals transformed to anadromy. Our study in the Lake Dieset watercourse (79°N), Svalbard, demonstrates that the annual water flow in the outlet river is strongly correlated to air temperatures and provides a passage to the sea, allowing the charr access to the nutrient-rich seawater environment, during at most two months each year. During one of the years studied, the youngest and small-sized part of the sea-going stock was prevented from ascending the river and probably suffered mortality during winter. The migratory window of the Arctic charr in Lake Dieset is therefore highly variable among years and thus unpredictable. We hypothesize that in worst case scenarios (cold years, low water discharge), climatic variations may occasionally prevent charr from migrating upstream in Svalbard lake systems in late autumn, resulting in high mortality in the population.     

Effects of Partially Anadromous Arctic Charr (Salvelinus alpinus) Populations on Ecology of Coastal Arctic Lakes

Little research has been conducted on effects of iteroparous anadromous fishes on Arctic lakes. We investigated trophic ecology, fish growth, and food web structure in six lakes located in Nunavut, Canada; three lakes contained anadromous Arctic charr (Salvelinus alpinus) whereas three lakes did not contain Arctic charr. All lakes contained forage fishes and lake trout (Salvelinus namaycush; top predator). Isotope ratios (δ¹³C, δ¹⁵N) of fishes and invertebrates did not differ between lakes with and without anadromous Arctic charr; if anadromous Arctic charr deliver marine-derived nutrients and/or organic matter to freshwater lakes, these inputs could not be detected with δ¹³C and/or δ¹⁵N. Lake trout carbon (C):nitrogen (N) and condition were significantly higher in lakes with Arctic charr (C:N = 3.42, K = 1.1) than in lakes without Arctic charr (C:N = 3.17, K = 0.99), however, and ninespine stickleback (Pungitius pungitius) condition was significantly lower in lakes with Arctic charr (K = 0.58) than in lakes without Arctic charr (K = 0.64). Isotope data indicated that pre-smolt and resident Arctic charr may be prey for lake trout and compete with ninespine stickleback. Linear distance metrics applied to isotope data showed that food webs were more compact and isotopically redundant in lakes where Arctic charr were present. Despite this, lake trout populations in lakes with Arctic charr occupied a larger isotope space and showed greater inter-individual isotope differences. Anadromous Arctic charr appear to affect ecology and feeding of sympatric freshwater species, but effects are more subtle than those seen for semelparous anadromous species.     

Characterisation and expression of the paired box protein 7 (Pax7) gene in polymorphic Arctic charr (Salvelinus alpinus)

Arctic charr (Salvelinus alpinus L.) from Lake Thingvallavatn, Iceland occur as four distinct morphs: large benthivorous (LB), dwarf benthivorous (DB), piscivorous (PI) and planktonivorous (PL). The morphs differ with respect to body size, head morphology, growth rate, and life history. The aim of this study was to investigate the paired box protein 7 (Pax7) gene as a candidate for such polymorphisms due to its importance in cranio-facial, skeletal muscle, and central nervous system development. No variation in coding and intronic sequences was found between morphs. We identified 10 alternate Pax7 isoforms with insertions/deletions: a four-residue (GNRT) deletion, a GEASS insertion truncated by the first serine residue (GEAS), and a thirteen-residue insertion (GQYA/TGPEYVYCGT). The latter insertion with a threonine (T) contains a putative casein kinase II (CK-2) phosphorylation site. Pax7 spatial expression patterns were identical in embryos of DB-, LB-, and PL-morphs, and were similar to those described for zebrafish Pax7c, but a difference in temporal expression for segmentation was observed between DB and LB morphs. At the end of segmentation, novel expression was observed in the mandibular region as two bilateral domains. The potential role of multiple alternative splicing of the Pax7 gene for the generation of different Arctic charr morphs is briefly discussed.     

Parasite-induced Host Mortality: Indirect Evidence From a Long-term Study

A long-term field study of a perturbed host–helminth system provides indirect evidence that a long-lived swimbladder nematode, Cystidicola farionis, induces mortality of Arctic charr, Salvelinus alpinus. The prevalence and abundance of this parasite has changed little over the period from 1987 to 1999. The cumulative numbers of L₃-stage larvae steadily increased with increasing host age, indicating a continuous exposure to infection throughout the life of the target fish host. Indirect methods, which used data pooled over years and long-term cohort analyses, indicate that parasite-induced host mortality (PIHM) occurs in hosts older than 10 years. Furthermore, using a short-term cohort method adjusted for worm recruitment, we found indications of PIHM occurrence even in younger age groups. These patterns do not seem to be caused by high parasite mortality rates since dead worms are rarely observed inside the swimbladder. Age-related changes in infection rates or in resistance to infection seem to play only a minor role as there were only slight changes in the preference of charr for feeding on amphipods (which are intermediate hosts) and in the acquisition rate of L₃ larvae in older hosts. Mortality of the most heavily infected hosts is the most probable explanation for the observed patterns.     

Observations of a wild polar bear (<Emphasis Type="Italic">Ursus maritimus</Emphasis>) successfully fishing Arctic charr (<Emphasis Type="Italic">Salvelinus alpinus</Emphasis>) and Fourhorn sculpin (<Emphasis Type="Italic">Myoxocephalus quadricornis</Emphasis>)

Polar bears, Ursus maritimus, throughout their range, are nutritionally dependent on ringed (Phoca hispida) and bearded seals (Erignathus barbatus), which are predominantly caught on the sea ice. Other marine prey species are caught and consumed, but less frequently. As the annual sea ice retreats, polar bears throughout their range are forced ashore, where they mostly live off their stored adipose tissue. However, while land-bound they have been observed catching birds and terrestrial mammals. Although polar bears evolved from brown bears (U. arctos), direct observations of polar bears diving for and catching fish have not been reported. Here, we document observations of a young male polar bear catching Arctic charr (Salvelinus alpinus) and Fourhorn sculpin (Myoxocephalus quadricornis) by diving in Creswell Bay, Nunavut. We recorded six search bouts, where six fish were caught during dives, which were preceded by a snorkel. The average dive and snorkel length was (mean ± SD) 13 ± 5 and 6 ± 2 s, respectively.     

The Arctic charr (<Emphasis Type="Italic">Salvelinus alpinus</Emphasis>) populations of Windermere,UK: population trends associated with eutrophication,climate change and increased abundance of roach (<Emphasis Type="Italic">Rutilus rutilus</Emphasis>)

The north and south basins of Windermere in the English Lake District, UK, support autumn- and spring-spawning populations of Arctic charr, Salvelinus alpinus, which have been studied since the 1930s. Continuous investigations of the population dynamics of Arctic charr at this lake have involved gill netting since 1939, collection of fishery catch-per-unit-effort data since 1966, and hydroacoustic surveys since 1990. Analysis of these and associated long-term data on the abiotic environment and other components of the fish communities revealed recently contrasting fortunes of the Arctic charr populations of the north and south basins, the latter of which has been significantly impacted by eutrophication while both basins have shown elevated water temperatures and increasing roach, Rutilus rutilus, populations. Despite the introduction of phosphate stripping in 1992 and some subsequent initial improvement, the hypolimnion of the south basin still remains significantly eutrophicated and the fishery catch-per-unit-effort in this basin is now at record low levels. In addition, the spatial distribution of roach has expanded to form significant components of the fish communities of inshore and offshore surface habitats, where this cyprinid may compete with Arctic charr for zooplanktonic prey. It is concluded that the Arctic charr populations of Windermere, particularly those of the south basin, currently face a number of significant environmental pressures and continued management action is required to ensure their survival.     

Arctic charr in sympatry with burbot: ecological and evolutionary consequences

The trophic niche and parasite infection of Arctic charr (Salvelinus alpinus) were explored in two lakes with sympatric burbot (Lota lota) and two lakes without burbot in subarctic Norway. The CPUE of burbot and charr were similar in one lake, but burbot had a low population density in the other. Burbot were benthivorous in both lakes. Other co-occurring species like brown trout (Salmo trutta), Atlantic salmon parr (Salmo salar), grayling (Thymallus thymallus) and minnow (Phoxinus phoxinus) were also benthivores. At high densities, benthivorous burbot forced the whole Arctic charr population to utilise mainly the limnetic trophic niche. In contrast, at low burbot density or without burbot present, Arctic charr were primarily benthivorous in the littoral zone. Thus, a clear interactive segregation in diet was observed between Arctic charr and burbot at high burbot densities. There was also a high predation pressure from burbot on young Arctic charr along the benthic zones. The extensive use of zooplankton as prey caused a high parasite infection pressure of copepod transmitted Diphyllobothrium spp. larvae, with the potential for high negative impact on the Arctic charr population. As the benthivore trophic niche was occupied by burbot, the ecological opportunities for polymorphism with benthivorous ecotypes or morphs of Arctic charr were probably prevented. Therefore, the sympatry with burbot seems to have large ecological and evolutionary consequences for this Arctic charr population compared with neighbouring lakes where burbot is absent.     

Reproduction,condition and food of adult arctic charr (Salvelinus alpinus,L.) in Lake Pavin (Massif Central,France)

RésuméTo determine the role and the importance of adult fish predation directly and indirectly in the food web of oligomesotrophic Lake Pavin, reproduction, condition, and feeding activity of the dominant adult chctic charr (Salvelinus alpinus, L.) were studied. Fish were caught by gillnets of different mesh sizes once a month from April to December 1992. The gonad cycle showed that the spawning period occurred between October to December. Relative fecundity is about 2509 ovules per kg of female fish and ovule mean diameter is 4.7 mm ( 0.6 mm). Seasonal variations of the sex-ratio (Max. in April = 2.67 and Min. in October=0.19) indicated the intralake migrations of females and males. The condition coefficients showed higher values for females than males; the maximum was observed in summer for both sexes and the minimum in April and in October for males and females, respectively. Low values of the condition were also observed during the spawning period. High feeding activity was recorded in spring and in December after reproduction; low feeding activity was observed in October during reproduction. Asellus aquaticus was quantitatively the dominant prey from April to September; during the sp awning period, S. alpinus consumed its own eggs, particularly in large quantity in December. Daphnia longispina was numerically the dominant prey all over the studied cycle and the electivity (Ivlev Index) of arctic charr on the latter prey ranged from +s+0.79 to +0.16. During April, more than 15 500 individuals of D. longispina per kg of fish were found in the gut contents. The selective predation of S. alpinus on benthic (A. aquatics) and zooplankton prey (D. longispina) indicated that the fish immigrate during their feeding activity from benthic/littoral to the pelagic area.Afin de d terminer le rôle et l'importance de la prédation directe et indirecte de l'omble chevalier adulte (Salvelinus alpinus, L.) dans le réseau trophique du lac Pavin, milieu oligo- mésotrophe, nous avons étudié la reproduction, deux facteurs de condition et l'activité alimentaire de ce poisson qui est l'espéce dominante dans ce lac. Les poissons ont été capturés par des filets maillants de différent vide de maille selon un pas d'échantillonnage mensuel d'avril á décembre 1992. Le cycle du développement des gonades montre que la période de reproduction se déroule entre octobre et décembre. La fécondité relative calculée est de 2509 ovules par kg de femelles et le diamétre moyen d'un ovule est de 4,7 mm ( 0,6). Les variations saisonniéres du sex-ratio (Max. en avril = 2,67 et Min. en octobre=0,19) indiquent le déplacement actif et successif des femelles et des mâles. Les variations saisonniéres des coefficients de condition indiquent une copulence plus importante des femelles par rapport aux mâles avec des valeurs maximales en été pour les deux sexes et des valeurs minimales en avril et en octobre pour les mâles et les femelles respectivement. De faibles valeurs des facteurs de condition ont été observées pendant la période de reproduction pour les deux sexes. L'activité alimentaire de l'omble chevalier est intense au printemps et au mois de décembre aprés la fraie et faible au mois d'octobre pendant la reproduction. Asellus aquaticus est, en termes de biomasse, la proie dominante d'avril á septembre. Pendant la période de reproduction, S. alpinus se nourrit de ses propres oeufs sur les zones de frayéres et en décembre la quantité d'oeufs consommée par le Salmonidé est importante. Daphnia longispina est, en termes d'abondance, la proie dominante tout au long de cette étude et l'indice d'électivité (Indice d'Ivlev) de l' ombre chevalier vis-á-vis de cette proie varie entre + 0,79 et + 0,16.De plus, au mois d'avril, plus de 15 500 D. longispina par kg de poisson ont été enregistrées dans les contenus stomacaux. La prédation sélective de S. alpinus sur des proies benthiques (A. aquaticus) et zooplanctoniques (D. longispina) indique que le poisson effectue des déplacements en fonction de son activité alimentaire de la zone littorale/benthique á la zone pélagique.     

Seasonal and ontogenetic shifts in the diet of Arctic charr Salvelinus alpinus in a subarctic lake

Seasonal and ontogenetic shifts in the diet of Arctic charr Salvelinus alpinus were studied in a deep, ultra‐oligotrophic lake in subarctic Finland from both stomach contents and the stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope compositions of muscle and liver tissues. Both diet and isotope results indicated that the S. alpinus population relied mainly on littoral benthic energy sources. The strong littoral reliance appeared largely independent of season or fish size, although the data lacked small (total length, L_T, <130 mm) and young (<3 years) S. alpinus. Liver isotope values of intermediate‐sized S. alpinus (200–350 mm), however, suggested exploitation of the increase in the abundance of pelagic zooplankton in the late open‐water season. The results suggest that, in general, a strong littoral reliance of fishes can be a feature in subarctic lakes throughout the year. Due to its faster isotopic turnover rate and thus higher resolution for temporal diet changes, liver could be more commonly used in stable‐isotope studies of fish trophic niche shifts instead of using only the less responsive muscle tissue.     

Mercury accumulation in five species of freshwater fish in Lake Tyrifjorden,south-east Norway,with emphasis on their suitability as test organisms

Synopsis Mercury concentration in axial muscle of brown trout, Salmo trutta, whitefish, Coregonus lavaretus, Arctic charr, Salvelinus alpinus, smelt, Osmerus eperlanus and pike, Esox lucius, were studied in Lake Tyrifjorden during 1978–1982. Our data demonstrate that older and bigger fish on an average have higher mercury concentration than smaller and younger. Further, complex life histories as in brown trout influence the pattern of mercury accumulation. During young stages accumulation in brown trout is moderate, while accumulation in older stages is highly correlated to lake residency time. Based on our data we suggest the following requirements for a test organism and the collecting procedure; (1) life history should be simple with small sexual differences, (2) ageing should be easy and reliable, and (3) large representative samples should be easily obtained during (4) a fixed biological period i.e. the spawning period. We consider smelt as an appropriate test organism based on these criteria.     

Isolation and cross‐familial amplification of 41 microsatellites for the brook charr (Salvelinus fontinalis)

The brook charr (Salvelinus fontinalis; Osteichthyes: Salmonidae) is a phenotypically diverse fish species inhabiting much of North America. But relatively few genetic diagnostic resources are available for this fish species. We isolated 41 microsatellites from S. fontinalis polymorphic in one or more species of salmonid fish. Thirty‐seven were polymorphic in brook charr, 15 in the congener Arctic charr (Salvelinus alpinus) and 14 in the lake charr (Salvelinus namaycush). Polymorphism was also relatively high in Oncorhynchus, where 21 loci were polymorphic in rainbow trout (Oncorhynchus mykiss) and 16 in cutthroat trout (Oncorhynchus clarkii) but only seven and four microsatellite loci were polymorphic in the more distantly related lake whitefish (Coregonus clupeaformis) and Atlantic salmon (Salmo salar), respectively. One duplicated locus (Sfo228Lav) was polymorphic at both duplicates in S. fontinalis.