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.     

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.     

Survival, growth and dressing traits of triploid hybrids between rainbow trout and three charr species

Triploid hybrids between female rainbow trout Oncorhynchus mykiss and male brook charr Salvelinus fontinalis, Arctic charr S. alpinus and lake charr S. namaycush, together with diploid and triploid rainbow trout controls from the same dams, were tested in freshwater farming up to their fourth year of life. All hybrids displayed lower survival rates than the controls, the weakest genotype being the Arctic charr hybrid. Mortalities were mostly observed at the embryonic and larval stages and at the adult stage as a consequence of male sexual maturation. Growth of all hybrids was hindered (compared with controls) during the first year, but only moderate differences remained after 3 years. Sexual maturation resulted in a weight inferiority of males in all genotypes. As to carcass traits, female hybrids displayed a slightly higher dressing percentage than female triploid rainbow trout, as a result of lower visceral losses. These results are discussed with reference to hybrid resistance to rhabdoviruses from the viewpoint of fish farming improvement.     

Assessing the food web impacts of an anadromous Arctic charr introduction to a sub‐Arctic watershed using stable isotopes

Anadromous Arctic charr, Salvelinus alpinus (L.), was introduced to a sub‐Arctic river–lake system near the village of Kujjuuaq, Nunavik, and the stable isotope values and diets of key resident fish species were used to assess changes in feeding patterns. Stable isotope values for most species did not differ significantly between the pre‐ and post‐introduction periods, with observed shifts being within the bounds of expected natural variation. Lake chub, Couesius plumbeus (Agassiz), were the single species to show a difference between study periods, with a small but significant increase in δ¹⁵N. No significant post‐introduction changes were seen in lake trout, Salvelinus namaycush (Walbaum), omnivory or in any of the assessed quantitative food web metrics. Gut contents of major fish species similarly showed significant temporal overlap between the pre‐ and post‐introduction periods, and there was no significant change in species' weight–length relationships. The minor ecological impact was interpreted in relation to the availability of open niches exploitable by ecological generalists such as Arctic charr. The explanation accords with the known habitat and feeding flexibility of Arctic charr and the ecological immaturity of sub‐Arctic lakes known to have driven adaptive variation among Arctic charr. Findings suggest that anadromous Arctic charr may be introduced at moderate densities to other sub‐Arctic watersheds without major negative food web consequences for other resident fish species.     

Survival of salmon lice, Lepeophtheirus salmonis Krøyer, on Arctic charr, Salvelinus alpinus (L.), in fresh water

The survival of salmon lice. Lepeophtheirus salmonis (Krøyer) on Arctic charr. Salvelinus alpinus (L.). in fresh water was studied. The results showed that salmon lice were able to survive for up to 3 weeks, in contrast with previous published information that the parasite is quickly shed when the fish enters fresh water.     

Dietary plasticity of Arctic charr (Salvelinus alpinus) facilitates coexistence with competitively superior European whitefish (Coregonus lavaretus)

Abstract – Habitat use and diet of Arctic charr (Salvelinus alpinus) coexisting with European whitefish (Coregonus lavaretus) and grayling (Thymallus thymallus) were studied in one deep and two relatively shallow subarctic lakes in northern Norway. Stomach content and stable isotope analyses revealed clear and temporally stable resource partitioning between the species in all three lakes. Arctic charr had a wide and flexible trophic niche and was the only piscivorous species. In contrast, whitefish and grayling had remarkably stable planktivorous and benthivorous niches, respectively. In the deepest lake, Arctic charr together with grayling mainly utilised littoral benthos, while piscivory was more prevalent in Arctic charr in the two shallower lakes. In one of the shallow lakes, whitefish was apparently relegated to the inferior profundal niche because of dominance of the littoral by grayling. Our results suggest that Arctic charr may not necessarily need an extensive profundal zone as a refuge, but can coexist with whitefish if a third competing fish species like grayling occurs in the littoral habitat or if profitable small prey fish are available. The study demonstrates that strong dietary plasticity of Arctic charr is instrumental in the observed coexistence with the commonly competitively superior whitefish.     

Domestication effects on fin nipping, survival and growth in hatchery-reared arctic charr, Salvelinus alpinus (L.), in competition with wild conspecifics

When reared in captivity, first-generation hatchery-reared Arctic charr, Salvelinus alpinus (L.), from Møkkeland lake, northern Norway, differed from wild-caught conspecifics from the same source. Incidence of caudal fin damage (used as an indirect assessment of aggressive interactions) was high amongst the wild-caught charr, and there was considerable mortality amongst these fish. By contrast, hatchery-reared fish displayed little evidence of fin damage, and mortalities were low. When the charr were held in mixed groups of wild-caught and hatchery-reared fish, the hatchery-reared fish showed the most extensive caudal fin damage. Weight losses and decreases in condition were also greatest amongst the hatchery-reared charr that were exposed to competition with wild fish. The hatchery-reared charr held together with the wild fish also suffered high mortality. Thus, aggression may have been selectively directed towards hatchery-reared fish in mixed groups. Taken together, the results suggest that the energetic costs may be high for the recipients of aggression.     

The potential of Arctic charr, Salvelinus alpinus (L.), for mariculture

There would appear to be considerable potential for Arctic charr, Salvelinus alpinus (L.), to be cultured in marine conditions in countries where coastal winter salinity is below oceanic, and temperatures remain above freezing. Sea lochs on the west coast of Scotland represent one such environment where freshwater run-off leads to depressed salinities (20-30 practical salinity units) and the North Atlantic Drift leads to winter sea water temperatures typically around 6-8^oC.     

Dominance hierarchy formation in Arctic charr Salvelinus alpinus (L.): nutrient digestibility of subordinate and dominant fish

Arctic charr Salvelinus alpinus (L.) were individually tagged and maintained in circular tanks for 102 days in order to allow the development of dominance hierarchies. At the end of the trial period, the charr were anaesthetized in benzocaine and identified as dominant, beta (β) and subordinate according to a set of established criteria including size, coloration and bite marks. The gut contents were then collected and analysed for apparent digestibility coefficient of nutrients using the chromic oxide method. Subordinate fish had significantly lower specific growth rates and apparent nutrient digestibility coefficients of both dry matter and lipid compared with dominant fish. Although specific growth rate was significantly lower in the β fish compared with dominant charr, this did not influence the apparent digestibility coefficient of nutrients to any major extent.     

Growth in Arctic charr and rainbow trout fed temporally concentrated or spaced daily meals

The effect on growth of distributing feed over a few hours compared tomore frequent meals was tested on 1+ Arctic charr (Salvelinus alpinus L.) and 1+ rainbow trout (Oncorhynchus mykiss Walbaum). Triplicate hatchery groups for each treatment were fed at a ration level of 1%/dayeither with few meals (8 times per day divided into morning and evening)or with frequent meals (32 meals equally distributed during the day). Wefound an opposite effect of meal frequency on growth in the two species.Low feeding intensity (8 meals per day) had a significantly positive effecton growth in rainbow trout but a significantly negative effect on growth inArctic charr when compared to feeding the fish frequent meals. Theopposite response to meal frequency is likely to be an effect of thedifferences in activity during feeding. Rainbow trout feed much moreaggressively than charr which can result in feeding being a more stressfulevent. In this experiment, the specific growth rate was lower and the feedconversion ratio higher for Arctic charr compared to rainbow trout.     

Changes in demand feeding behaviour in Arctic charr, Salvelinus alpinus L., caused by differences in dietary energy content and reward level

The objective of this study was to determine whether Arctic charr, Salvelinus alpinus L., are able to adjust their demand feeding behaviour in accordance to differences in dietary energy content (experiment 1) and reward level (amount of food received in response to one trigger actuation) (experiment 2). Fish (initial size 215 g in experiment 1 and 88 g in experiment 2) were reared in 0.8-m³ indoor tanks at commercial stocking densities and fed using demand feeders for 57 and 180 days, respectively. Demand feeding activity did not differ significantly between groups of charr fed diets with a low (19.8 MJ kg^-1) gross energy content and those given high-energy (22.0 MJ kg^-1) feed. As a result, fish offered the high-energy diets grew significantly faster. The results show that charr held under culture conditions are unable to adjust their demand feeding activity based on the energy content of the food. On the contrary, Arctic charr are able to adjust their demand feeding activity to either low (0.33 g), medium (0.87 g) or high (1.52 g) rewards, and thereby, regulate their food supply to fit their needs. However, it took about 90 days before charr in the low-reward treatment released a daily food ration as high as that released in the high-reward groups. Consequently, there was a significant positive relationship between the size of the reward and final weights. To avoid any depression of initial growth rates, the optimal size of the reward should be 0.1 g per kg fish and trigger actuation.     

Effect of rearing temperatures on the growth and maturation of Arctic charr (Salvelinus alpinus) during juvenile and on‐growing periods

The effect of rearing temperature on the growth and maturation of Arctic charr (Salvelinus alpinus) was investigated. Arctic charr juveniles were reared for 6 months (phase I, October–April, size range 20–500 g) at constant temperatures of 9, 12 and 15 °C and according to two temperature‐step groups (T_step) i.e. fish transferred from 15 to 12 °C or from 12 to 9 °C. All the previous treatments were then reared either at 7 °C or at 12 °C for an additional 4 months (phase II, size range 300–1000 g) and then slaughtered in August 2008. The overall growth rate was the highest at a constant temperature of 15 °C for the first 6 months of the trial, with the fish in this group being 44% and 78% heavier than the fish reared at a constant temperature of 12 or 9 °C respectively. Arctic charr showed a negative response in terms of the growth rate when transferred from higher to lower temperatures, especially for groups previously reared at 15 °C. There was a trend for higher gonadosomatic index values at the end of the experiment for groups of fish that were exposed to higher rearing temperatures during the juvenile phase i.e. 4.18% (±0.79) and 7.29% (±0.89), for the temperature groups of 12 and 15 °C, respectively, compared with 2.49% (±0.74) for the 9 °C group. Our results suggest that for the production of fish >1000 g, moderate or low temperatures (here 9 °C) should be applied during the juvenile phase in order to reduce the negative effects arising from maturation. Farmers with access to heat sources should accordingly choose more moderate rearing temperatures during the juvenile stage, especially if the fish is to be moved down in the temperature regime during the on‐growing period.     

Feeding,growth and environmental requirements of Arctic charr: a review of aquaculture potential

Interest in the cultivation of Arctic charr arose during the 1970s, and research into charr farming was instigated in the Nordic countries and in Canada. Most work has been conducted on fish from anadromous populations, although land-locked freshwater populations of Arctic charr have also received attention. Research has also been carried out in the British Isles and in the alpine regions of central Europe, where land-locked populations of charr. Small-scale commercial farming is now carried out in several countries of northern Europe and North America, and charr are reared for restocking purposes in a number of countries.Growth of charr is rapid during the early freshwater rearing stages, and quite good rates of growth can be achieved at low water temperatures. Growth may be submaximal if charr are reared in systems designed for other salmonids, and problems may arise when charr are held at low stocking densities. Growth and food conversion can be improved by exposing the fish to water currents, forcing them to swim at moderate speeds. Growth in seawater has been reported as being highly variable, probably as a result of the use of inappropriate rearing techniques and owing to the seasonal changes in the hypo-osmoregulatory ability of the charr.Prospects for aquaculture development and areas requiring further research effort are briefly discussed.     

Effect of sorting on size-frequency distributions and growth of Arctic charr,Salvelinus alpinus L.

Arctic charr, Salvelinus alpinus, were reared in either unsorted groups or in groups that had been sorted into ‘small’ and ‘large’ fish. At the start of the experiment the size-frequency distributions of unsorted and pooled ‘small’ and ‘large’ fish were similar, but after 5 months' rearing distributions were markedly different. There was evidence that growth of ‘small’ fish improved in the absence of larger conspecifics, but size-sorting also led to reduced growth of some large fish, with the consequence that size-sorting did not lead to overall increases in biomass gain and rates of production.     

Evaluation of growth performance and intestinal barrier function in Arctic Charr (Salvelinus alpinus) fed yeast (Saccharomyces cerevisiae), fungi (Rhizopus oryzae) and blue mussel (Mytilus edulis)

Arctic charr (Salvelinus alpinus) were fed for 99 days on experimental diets with 40% of fish meal replaced, on a crude protein basis, with intact yeast (Saccharomyces cerevisiae) (ISC), extracted yeast (ESC), Rhizopus oryzae fungus (RHO) or de‐shelled blue mussels (Mytilus edulis) (MYE). The fish were evaluated for growth performance, nutrient digestibility and fish intestinal function. Growth performance, retention of crude protein and sum of amino acids were not affected in fish fed diets ISC or MYE compared with those fed the reference (REF) diet. However, fish fed diet ISC displayed decreased digestibility of crude protein and indispensable amino acids and decreased intestinal barrier function compared with fish fed the REF diet. Fish fed diet ESC exhibited decreased growth performance and protein retention, but had comparable digestibility to fish fed the REF diet. Fish fed diets MYE and RHO showed similar performance in terms of growth, nutrient digestibility and intestinal barrier function. Overall, the results indicated that blue mussel and intact S. cerevisiae yeast are promising protein sources for Arctic charr.     

Influence of sea ranching on hatchery-reared anadromous Arctic charr, Salvelinus alpinus (L.)

Anadromous Arctic charr, Salvelinus alpinus (L.), from the River Halselva in Finnmark, northern Norway, were used as broodstock for the production of smolts. During the smolting period the fish were either shielded from normal working routines, subjected to normal work routines, or exposed to excessive work routines (stress) within the hatchery. None of the handling routines affected the gill-Na-K-ATPase activity, or the hypoosmoregulatory capacity of the charr. Compared with normal handling, neither stress nor shielding caused any significant difference in the migratory behaviour of the charr, in the duration of the sea residence, or in homing success. However, the stressed group did show a lymphocytopenia at end of the rearing period.     

Smolt characteristics of small first-time migrant, and resident Arctic charr, Salvelinus alpinus (L.), from a river system in northern Norway

Smolt characteristics were investigated in sympatric anadromous and resident Arctic charr. Salvelinus alpinus (L.), of a similar size (11-20 cm), A group of first-time migrant anadromous charr was caught while descending the Hals River and two groups of resident conspecifics were caught in Lake Storvatn. one before, and one after, the sea-run of anadromous fish had terminated. When sampled immediately after capture in fresh water the anadromous group had a higher proportion of fish visually classified as smolts. and these charr had higher gill Na⁺-K⁺ ATPase activity, higher densities of developed chloride cells in the gills, a lower condition factor, and lower plasma osmolality, than resident fish. When exposed to sea water (33%o S), only minor differences in mortality and plasma electrolyte levels were observed between anadromous and resident fish, average values of plasma osmolality and chloride concentrations being 377 mOsm and 169 mM and 387 mOsm and 174 mM, respectively. The results suggest that the first-time migrant Arctic charr had undergone some of the physiological changes that are considered typical for a parr-smolt transformation before they left fresh water. Such changes did not appear to have occurred in sympatric, resident, fish of a similar size. The parr-smolt transformation in the first-time migrants may, however, have been incomplete, because the fish did not appear to have the ability to rapidly re-establish osmo- and ionoregutatory homeostasis after direct transfer to sea water.     

Influence of feeding frequency on growth and size dispersion in Arctic charr Salvelinus alpinus (L.)

Abstract Dominance hierarchy formation in groups of fish is thought to be a major reason for the development of size heterogeneity. The objective of the current experiment was to examine the influence of different feeding frequencies on growth and changes in size distribution in groups of Arctic charr Salvelinus alpinus (L.). The fish were fed equivalent excess rations continuously for 24 h a day, for 2 h twice a day (early morning and evening), for 2 h once a day in the morning or for 2 h once every other day also in the morning. Growth was measured by individual weighing of fish four times during the 130‐day experiment. Coefficient of variation in size within rearing groups was used to assess size dispersion and the possible development of hierarchies within the groups. Feeding frequency did not influence variation in size within groups, and there were no signs of hierarchy formation.     

Survival, growth, sexual maturation and reproduction of brook charr, Salvelinus fontinalis (Mitchill), Arctic charr, Salvelinus alpinus L., and their hybrids

Survival of hybrids (FA, AF) between brook charr, Salvelinus fontinalis (Mitchill), (FF) and Arctic charr, Salvelinus alpinus L., (AA) were compared at three fish farms. Survival of hybrids and Arctic charr was impaired in warm water. In cold water, brook charr and hybrids showed similar survival percentages until the spawning period. Each spawning period was followed by an increase of mortality, but to a much higher degree in brook charr. Growth was studied in the best (cold water) fish farm. During the first 2 years of rearing, brook charr had the highest relative weight. The weight of the two hybrids approached that of brook charr in the third year because of the drastic decrease of growth rate in brook charr after the spawning period. No differences between genotypes of either females or males could be detected in the eviscerated weights, the dressing percentages, or the gonado-somatic indexes. The two hybrids matured sexually, but their sperm quality was inferior to that of the pure species. Crosses between the 4-year-old breeders from the F1 population (FF, FA, AF and AA) were achieved. Survival from the eyed stage to 6 weeks post-hatching was significantly lower in F2 progeny than in back-crosses or pure species crosses. Even though the survival following spawning periods and the total biomass produced were better in hybrids than in brook charr raised in cold water, we believe the farming of this hybrid should not be carried out. The full development of sexually mature hybrids does not resolve problems linked with precocious sexual maturation in brook charr farming carried out in eastern Canada.     

Trophic interactions between introduced lake trout (Salvelinus namaycush) and native Arctic charr (S. alpinus) in a large Fennoscandian subarctic lake

Introduced fishes may have major impacts on community structure and ecosystem function due to competitive and predatory interactions with native species. For example, introduced lake trout (Salvelinus namaycush) has been shown to replace native salmonids and induce major trophic cascades in some North American lakes, but few studies have investigated trophic interactions between lake trout and closely related native Arctic charr (S. alpinus) outside the natural distribution of the former species. We used stomach content and stable isotope analyses to investigate trophic interactions between introduced lake trout and native Arctic charr in large subarctic Lake Inarijärvi in northern Finland. Both salmonids had predominantly piscivorous diets at >280 mm total length and were mainly caught from the deep profundal zone. However, lake trout had a more generalist diet and showed higher reliance on littoral prey fish than Arctic charr, whose diet consisted mainly of pelagic planktivorous coregonids. According to length at age and condition data, lake trout showed slightly faster growth but lower condition than Arctic charr. The results indicate that introduced lake trout may to some extent compete with and prey upon native Arctic charr, but currently have only a minor if any impact on native fishes and food web structure in Inarijärvi. Future monitoring is essential to observe potential changes in trophic interactions between lake trout and Arctic charr in Inarijärvi, as well as in other European lakes where the two salmonids currently coexist.