The suitability of anadromous Arctic charr as host and vector of the monogenean Gyrodactylus salaris

Abstract –  In 2000, the Atlantic salmon ( Salmo salar L.) stock in the river Signaldalselva, North Norway was found to be infected with the monogenean Gyrodactylus salaris (Malmberg). This river system has a multispecies fish community of nine species in total. Low densities of Atlantic salmon parr were found during electrofishing in October (2003) with a prevalence of 94% and a mean abundance of 848 G. salaris. This watercourse also holds a riverine stock of anadromous Arctic charr ( Salvelinus alpinus ), which is not common in Norway. Among the parr of Arctic charr, 64% were infested with an abundance of 23 parasites per fish. The most heavily infected 0+ and 1+ juvenile Arctic charr had 469 and 534 parasites, respectively, indicating that the parasites are able to reproduce on Arctic charr in the wild. There was little variation in the rates of infestation of Arctic charr along the studied stretch of the river, even in areas with very low densities of Atlantic salmon parr. The parr of anadromous Arctic charr seem to be suitable as long-term hosts for G. salaris . Moreover, a few (22%) infested adult sea-running Arctic charr were captured after ascending the river in the autumn, which shows that large fish may also act as carriers of G. salaris . However, no parasites were recorded on potential anadromous Arctic charr before descending during early spring (April 2003 and April 2004). Thus, it is still not clear whether anadromous Arctic charr are able to disperse the parasite between watercourses. So far, it is uncertain whether G. salaris may increase the mortality rates of Arctic charr and thereby be a threat to these unusual occurring riverine anadromous Arctic charr stocks.     

Modelling the life-history variation of Arctic charr

Abstract –  A model based on proximate considerations of life histories of Atlantic salmon, Salmo salar , was examined for its applicability to fit the variation in life-history of wild Arctic charr, Salvelinus alpinus , based on a qualitative assessment of information related to growth and lipid dynamics of Arctic charr. The original salmon model is discussed in context of modifications required to account for added complexities in the life history of Arctic charr in relation to anadromy versus residency. A study from North Norway shows that individual charr that emigrate from the lakes to the sea, maintain a high growth rate in the lake in late summer and early autumn compared with resident fish. Their relatively low lipid level in autumn combined with a high rate of change of lipid during winter was associated with postponement of maturation in the anadromous individuals. Individuals that remain resident in the lake arrested growth in autumn. Their high lipid level in autumn combined with a low rate of change of lipid during winter was associated with maturation the following summer, without emigration from freshwater. Results from this and other related studies show similarities with the model derived from lipid and growth dynamics of Atlantic salmon. The adjusted charr model illustrates possible proximate explanations for the high variation in life-history strategies of Arctic charr. However, the model does not account for the characteristic return migration of immature charr into freshwater several weeks after their entry to the sea. The proximate physiological stimulus for this movement of immature fish is not entirely clear.     

Stream spawning of Arctic charr in Scotland

Abstract –  Scotland is a stronghold for Arctic charr, with about 200 freshwater, wholly loch-resident populations, most of which have yet to be studied. To date, no anadromous populations or individual sea-run charr, have been reported. In Scotland, most Arctic charr spawn in stillwater, during autumn and early winter (September to January), with only one population spawning in spring. Spawning in running water does occur, but has been regarded as rare. This paper examines the status of stream spawning of charr in Scotland, describing instances from 10 populations, although two of these are now extinct. Most stream-spawning charr migrate relatively short distances to running water and construct redds in gently flowing water. The longest recorded migration occurs in Loch Insh, Speyside, with fish travelling 15 km within the River Spey. Information on the extent of stream spawning is essential to help conserve the rich phenotypic and genetic diversity of our remaining charr populations.     

Resource partitioning between lake-dwelling Atlantic salmon (Salmo salar L.) parr, brown trout (Salmo trutta L.) and Arctic charr (Salvelinus alpinus (L.))

Abstract – Resource partitioning between Atlantic salmon parr, brown trout and Arctic charr was studied throughout the ice-free season in a north Norwegian lake. Juvenile salmon and trout (≤160 mm) utilized the littoral zone and juvenile charr the profundal, while adult trout and charr (>160 mm) were found in both. Juvenile salmon and trout had a similar diet, although trichopteran larvae were more important for the trout and chironomid pupae and three-spined sticklebacks for the salmon parr. Small salmon and trout parr (≤120 mm) had a higher diet overlap than larger parr (121–160 mm). The feeding habits of adult trout were similar to that of juvenile trout, but the former took larger prey items. At the population level, both salmon and trout were generalistic feeders with a broad diet, but at the individual level, both species had specialized on a single or a few prey categories. Juvenile charr were segregated from salmon and trout in both habitat and food utilization; they had a narrow diet consisting of chironomids and zooplankton, possibly reflecting their confinement to the profundal habitat which have a low diversity of potential prey. Larger charr also took zoobenthos and sticklebacks in the littoral zone. ^Note     

The marine temperature and depth preferences of Arctic charr (Salvelinus alpinus) and sea trout (Salmo trutta), as recorded by data storage tags

Eleven Arctic charr (Salvelinus alpinus) (370–512 mm) and eight sea trout (Salmo trutta) (370–585 mm in length) were tagged externally or internally with depth‐ and temperature‐measuring data‐storage tags (DST) before they were released into the sea in the Alta Fjord in north Norway in June 2002. All sea trout were recaptured after they spent 1–40 days at sea, while all Arctic charr were recaptured after 0.5–33 days at sea. On average, trout preferred water about 0.6 m deeper and 1.3°C warmer than Arctic charr. Arctic charr spent >50% of their time between 0 and 1 m depth, while trout spent >50% of their time between 1 and 2 m depth. Both species spent >90% of their time in water no deeper than 3 m from the water surface. However, sea trout dove more frequently and to greater depths (max. 28 m) than Arctic charr (max. 16 m), and these deep dives were most frequently performed at the end of the sea migration. Arctic charr demonstrated a diel diving pattern, staying on average about 0.5 m deeper between 08:00 hours and about 15:00 hours than during the rest of the 24 h, even though there was continuous daylight during the experiments. When comparing data obtained from the DSTs with temperature measurements within the fjord system, the two species were observed to select different feeding areas during their sea migration, the sea trout choosing the inner and warmer parts of the fjord, in contrast to the Arctic charr that preferred the outer, colder parts of the fjord. The observed differences in migration behaviour between the two species are discussed in relation to species preferences for prey and habitat selection, and their optimal temperatures for growth.     

Feed intake,growth and osmoregulation in Arctic charr,Salvelinus alpinus (L.), transferred from freshwater to saltwater at 8°C during summer and winter

The purpose of the current study was to examine seasonal changes in seawater tolerance and growth performance of anadromous Arctic charr (Salvelinus alpinus L.) held at the same temperature (8°C) during winter and summer. Charr (20–27 cm), previously reared in freshwater under natural photoperiod, were transferred either directly (DT) from freshwater to seawater (35 ppt), from freshwater to brackish water (20 ppt), or were gradually adapted (GT) to seawater over a period of 10 days. Control fish were held in freshwater. Feed intake and osmoregulatory ability were then monitored on three occasions during the following 59 days. Two experiments were carried out, one during winter (December–January) and the other during summer (June–July). In both experiments fish mortality was low. Plasma osmolalities recorded in fish transferred to seawater were within normal ranges, but osmolalities on day 10, were significantly lower in summer (313 mOsm/kg (DT), 328 mOsm/kg (GT)) than in winter (323 mOsm/kg (DT), 352 mOsm/kg (GT)). In winter, feed intake and growth rates were high in fish kept in fresh and brackish water, but charr transferred directly to seawater ate little and lost weight. Fish that were gradually adapted to seawater occupied an intermediate position. During summer the observed differences in feed intake were small and all fish had relatively high growth rates. These results suggest that Arctic charr display seasonal changes in feed intake and growth performance that parallel seasonal changes in hypoosmoregulatory capacity. The ability to survive and hypoosmoregulate in full strength seawater does not, however, seem to be a particularly good indicator of successful seawater adaptation with respect to the ability to display high rates of feed intake and growth. During winter, a gradual transfer to seawater appeared to lead to improved feeding and growth compared to direct transfer.     

Lake-use by juvenile Atlantic salmon (Salmo salar L.) and other salmonids in northern Norway

Abstract– The utilization of lakes, and inlet and outlet streams by juvenile Atlantic salmon ( Salmo salar L.), brown trout ( Salmo trutta L.) and Arctic charr ( Salvelinus alpinus (L.), were investigated in 16 watercourses northern Norway, all known to inhabit salmon stocks. In lakes, fish were caught by small mesh size gill nets, while in rivers fish were caught electrofishing. In the shallow littoral (0-3 m depth) there were juvenile salmon in 15 of 19 investigated lakes, juvenile trout in 17 and juvenile charr in seven. Trout dominated significantly in numbers in the shallow littoral of seven lakes, while salmon and charr dominated in three lakes each. When trout and salmon were frequent in the shallow littoral, charr was usually not present in this habitat, but were found in the profundal zone in most of the lakes. Atlantic salmon parr utilized both shallow and deep lakes, and used both stones and macrophytic vegetation as shelter. The utilization of lakes by salmon parr seemed to be closely related to utilization of small inlet streams for spawning. In most inlet and outlet streams salmon dominated over trout in numbers, while charr were absent. This is the first documentation of lake-use by naturally occurring salmon parr in Scandinavia.     

Temperature–growth patterns of individually tagged anadromous Arctic charr Salvelinus alpinus in Ungava and Labrador,Canada

Individual measurements of annual, or within‐season growth were determined from tag‐recaptured Arctic charr and examined in relation to summer sea surface temperatures and within‐season capture timing in the Ungava and Labrador regions of Eastern Canada. Differences between two years of growth (2010–2011) were significant for Ungava Bay Arctic charr, with growth being higher in the warmer year. Growth of Labrador Arctic charr did not vary significantly among years (1982–1985). Regional comparisons demonstrated that Ungava Arctic charr had significantly higher annual growth rates and experienced warmer temperatures than Labrador Arctic charr. The higher annual growth of Ungava Bay Arctic charr was attributed to the high sea surface temperatures experienced in 2010–2011 and the localised differences in nearshore productivity as compared to Labrador. Within‐season growth rates of Labrador Arctic charr peaked in June, declined towards August and were negatively correlated with the length of time spent at sea and mean experienced sea surface temperatures. A quadratic model relating growth rate to temperature best explained the pattern of within‐season growth. Collectively, results suggest that increases in water temperature may have profound consequences for Arctic charr growth in the Canadian sub‐Arctic, depending on the responses of local marine productivity to those same temperature increases.     

Effects of stocking of piscivorous brown trout, Salmo trutta L., on stunted Arctic charr, Salvelinus alpinus (L.)

To study the effects on a stunted freshwater population of Arctic charr, Salvelinus alpinus (L.), two groups of large (26–45 cm) individually tagged brown trout, Salmo trutta L., were released and recaptured with gillnets after 1, 7, 11 and 63 weeks. One group of trout was trained on a fish diet before release, and the other, reared on commercial dry pellets, served as a control. Specific growth rates in both groups were negative 1 week after release and approached zero after 63 weeks. Condition factor and internal fat content decreased during the experiment. Although only 11% of the trout stomachs examined contained fish prey, charr represented 79% of the total stomach weight content. Gillnet samples of charr before and 63 weeks after the release of trout indicated a decreasing population size of charr. Individual growth and mean length of charr increased after release of trout, especially for charr at age 4 years. After the release of trout, 35% of the charr were longer than 20 cm as compared with 6% before the release.     

Use of biological reference points for the conservation of Atlantic salmon, Salmo salar, in the River Lune, North West England

Abstract  The development and use of biological reference points (BRPs) for salmon, Salmo salar L., conservation on the River Lune, England were examined. The Lune supports recreational and net fisheries with annual catches averaging 1332 and 2146 salmon, respectively. Using models transported from other river systems, BRPs were developed that were exclusive to the Lune; specifically the number of eggs deposited and carrying capacity estimates for age 0+ and 1+ parr. The conservation limit was estimated at 11.9 million eggs, and to ensure that the conservation limit was exceeded 80% of the time, the management target was set at 14.4 million eggs (equivalent to ∼5000 adults). Between 1989 and 1998 the management target was exceeded in only 2 years. Comparison of juvenile salmon densities in 1991 and 1997 with estimates of carrying capacity indicated that 0+ and 1+ parr densities were at around 60% of carrying capacity and may relate to the number of eggs deposited in 1990 and 1996 being approximately 70% of the target value. From, and including, the 2000 fishing season, regulations to ensure that the conservation limit is met 4 years out of 5 were introduced. These consisted of a reduction from 26 to 12 haaf nets, from 10 to seven drift nets and a four-fish bag limit for the rod fishery. In the period between 2000 and 2004 there was a marked increase in the estimated number of salmon spawning and the management target value of ∼5000 spawning adults was exceeded in all years. There was also an increase in the juvenile salmon population. The estimated level of exploitation in the net and rod fisheries reduced from 29.9% to 13.8% and from 26.4% to 14.8% respectively, after the introduction of the regulations.