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.     

Long-term variation in the population structure of Arctic charr, Salvelinus alpinus, and brown trout, Salmo trutta

The effects of induced water level fluctuations and introduction of the mysid Mysis relicta Lovén on population structure of brown trout, Salmo trutta L., and Arctic charr, Salvelinus alpinus (L.), were studied during 1953–1995 in Limingen hydroelectric reservoir, Norway. The main response was a marked reduction in catch‐per‐unit‐effort (CPUE) for trout and charr, probably caused by reduced recruitment following increased variation in water level. For both species, mean length decreased until 1967 and increased thereafter, whereas mean mass‐at‐length increased for the whole period. Both length and mass‐at‐length were negatively correlated with CPUE. The increases in mean length and mass‐at‐length were probably because of reduced competition following the reduced recruitment. Mysis relicta has become an important food item for charr but not for brown trout, but the increases in mean length and mass‐at‐length of charr started prior to the appearance of M. relicta in the charr diet.     

Changes in the density and growth of brown trout (Salmo trutta) after intensive removal of sympatric Arctic charr (Salvelinus alpinus) in the sub‐Arctic lake Møkkelandsvatn,Norway

This study tests the basic hypothesis that the removal of charr, Salvelinus alpinus (L.), would cause an increase in both the growth and density of a sympatric trout population, Salmo trutta L. The charr population was characterised by slow‐growing individuals, with a high proportion of mature fish, that is typical for so‐called overpopulated populations. A total of 31,000 charr was removed from the lake in the period 1990–1992, and the density of younger trout (1+, 2+), but not older trout (3+, 4+), increased. The growth of older trout (3+, 4+) increased, but the evidence for similar growth increases of younger trout (1+, 2+) was limited. From 1989 to 1990, the proportion of trout increased from 30 to only 40% of the total catch, but from 1991 to 1994, it was significantly higher (60–80%) than that of charr. Total trout biomass increased to a maximum in 1992 and then decreased so that the biomass of 1994 was nearly similar to that of 1989, that is before the start of the charr removal. Back‐calculated lengths of trout from otoliths showed that 2+ and 3+ trout caught in the pelagic were growing consistently faster over previous years than those caught in the littoral, while this was not the case for the 4+ fish. Therefore, the hypothesis was partially supported; the growth rate of trout increased (age groups 1+ to 4+), while the density of juvenile trout (1+, 2+), but not the older trout (3+, 4+), increased after the removal of charr.     

Brown trout (Salmo trutta L.) and Arctic charr (Salvelinus alpinus (L.)) as predators on three sympatric whitefish (Coregonus lavaretus (L.)) forms in the subarctic Lake Muddusjärvi

Abstract  – Brown trout ( Salmo trutta L.) and Arctic charr ( Salvelinus alpinus (L.)) use whitefish ( Coregonus lavaretus (L.)) as their main prey in the subarctic Lake Muddusjärvi. Brown trout dwelled in littoral and pelagic habitat, whereas Arctic charr lived only in epibenthic habitat. Both species shifted to whitefish predation at a length of 20–30 cm. At this size, brown trout fed on larger whitefish than Arctic charr. Whitefish occur in three sympatric forms, differing in their habitat, ecology and morphology. Both the predators preyed primarily upon the small-sized, densely rakered whitefish form (DR), which was the most numerous whitefish form in the lake. DR used both epibenthic and pelagic habitat, whereas two sparsely rakered whitefish forms dwelled (LSR and SSR) only in epibenthic habitat: LSR in littoral and SSR in profundal areas. Sparsely rakered whitefish forms had minor importance in predator diet.     

Atlantic salmon Salmo salar L., brown trout Salmo trutta L. and Arctic charr Salvelinus alpinus (L.): a review of aspects of their life histories

Abstract – Among the species in the family Salmonidae, those represented by the genera Salmo, Salvelinus, and Oncorhynchus (subfamily Salmoninae) are the most studied. Here, various aspects of phenotypic and life‐history variation of Atlantic salmon Salmo salar L., brown trout Salmo trutta L., and Arctic charr Salvelinus alpinus (L.) are reviewed. While many strategies and tactics are commonly used by these species, there are also differences in their ecology and population dynamics that result in a variety of interesting and diverse topics that are challenging for future research. Atlantic salmon display considerable phenotypic plasticity and variability in life‐history characters ranging from fully freshwater resident forms, where females can mature at approximately 10 cm in length, to anadromous populations characterised by 3–5 sea‐winter (5SW) salmon. Even within simple 1SW populations, 20 or more spawning life‐history types can be identified. Juveniles in freshwater can use both fluvial and lacustrine habitats for rearing, and while most smolts migrate to sea during the spring, fall migrations occur in some populations. At sea, some salmon undertake extensive oceanic migrations while other populations stay within the geographical confines of areas such as the Baltic Sea. At the other extreme are those that reside in estuaries and return to freshwater to spawn after spending only a few months at sea. The review of information on the diversity of life‐history forms is related to conservation aspects associated with Atlantic salmon populations and current trends in abundance and survival. Brown trout is indigenous to Europe, North Africa and western Asia, but was introduced into at least 24 countries outside Europe and now has a world‐wide distribution. It exploits both fresh and salt waters for feeding and spawning (brackish), and populations are often partially migratory. One part of the population leaves and feeds elsewhere, while another part stays as residents. In large, complex systems, the species is polymorphic with different size morphs in the various parts of the habitat. Brown trout feed close to the surface and near shore, but large individuals may move far offshore. The species exhibits ontogenetic niche shifts partly related to size and partly to developmental rate. They switch when the amount of surplus energy available for growth becomes small with fast growers being younger and smaller fish than slow growers. Brown trout is an opportunistic carnivore, but individuals specialise at least temporarily on particular food items; insect larvae are important for the young in streams, while littoral epibenthos in lakes and fish are most important for large trout. The sexes differ in resource use and size. Females are more inclined than males to become migratory and feed in pelagic waters. Males exploit running water, near‐shore and surface waters more than females. Therefore, females feed more on zooplankton and exhibit a more uniform phenotype than males. The Arctic charr is the northernmost freshwater fish on earth, with a circumpolar distribution in the Holarctic that matches the last glaciation. Recent mtDNA studies indicate that there are five phylogeographic lineages (Atlantic, Arctic, Bering, Siberian and Acadian) that may be of Pleistocene origin. Phenotypic expression and ecology are more variable in charr than in most fish. Weights at maturation range from 3 g to 12 kg. Population differences in morphology and coloration are large and can have some genetic basis. Charr live in streams, at sea and in all habitats of oligotrophic lakes, including very deep areas. Ontogenetic habitat shifts between lacustrine habitats are common. The charr feed on all major prey types of streams, lakes and near‐shore marine habitats, but has high niche flexibility in competition. Cannibalism is expressed in several cases, and can be important for developing and maintaining bimodal size distributions. Anadromy is found in the northern part of its range and involves about 40, but sometimes more days in the sea. All charr overwinter in freshwater. Partial migration is common, but the degree of anadromy varies greatly among populations. The food at sea includes zooplankton and pelagic fish, but also epibenthos. Polymorphism and sympatric morphs are much studied. As a prominent fish of glaciated lakes, charr is an important species for studying ecological speciation by the combination of field studies and experiments, particularly in the fields of morphometric heterochrony and comparative behaviour.     

Homing experiments with parr, smolt and residents of anadromous Arctic char Salvelinus alpinus and brown trout Salmo trutta: transplantation between neighbouring river systems

Abstract –  Olfaction is decisive for the homeward migration of anadromous salmonids. Two different olfactory hypotheses for explaining how this mechanism works have been proposed (the imprinting and the pheromone hypothesis), and they differ with regard to the origin of the odours that define home. The pheromone hypothesis assumes that fish respond to strain-specific pheromones and that no imprinting is necessary. The imprinting hypothesis assumes that juvenile fish become imprinted to odours of abiotic or biotic origin in natural rivers; the imprinting may be a single event or may occur sequentially during downstream migration as smolts. The two hypotheses were challenged by reciprocal transplantation of parr, residents and smolt of Arctic char ( Salvelinus alpinus ) and brown trout ( Salmo trutta ) between the Salangen and the Løksebotten rivers. Both rivers empty into the Salangen Fjord, Norway, about 10 km apart. Salangen Arctic char released in the Løksebotten River, and Løksebotten brown trout released in the Salangen River (parr, resident and smolt) initiated spontaneous homeward migration without preceding contact with home stream water. This indicates that site imprinting (single or sequential) could not have been involved. In the sea, the released fish apparently integrated into the migratory system of relatives from their river of origin. Recapture rates did not differ for fish allowed to receive single or sequential imprinting.     

An experimental test of the genetic component of the ontogenetic habitat shift in Arctic charr (Salvelinus alpinus)

Abstract— Fry of the Arctic charr, Salvelinus alpinus , were experimentally stocked into a small fish-free lake to test the hypothesis that the size-dependent habitat shift from the epibenthic to the pelagic habitat is genetically determined. The charr originated from a nearby lake inhabiting predatory brown trout Salmo trutta. The cohort of stocked charr was investigated for three years. The Arctic charr started to exploit the pelagic habitat in their first summer at a size of 7–9 cm in contrast to about 15 cm in the donor lake. In the next two summers, the pelagic fraction of the cohort increased. The main fraction lived in epibenthic areas, utilizing the same prey as pelagic charr. Water temperature moderated the habitat use of juveniles such that they avoided warm (>16°C) waters and resided in cool, deep areas. The result was consistent with the hypothesis of a tradeoff between feeding benefit and the predation risk producing spatial segregation of Arctic charr and demonstrated that the fish can facultatively respond to predation risk and adjust the size at which they migrate to the pelagic zone to feed on zooplankton.     

Divergent, genetically based feeding behaviour of two sympatric Arctic charr, Salvelinus alpinus (L.), morphs

Abstract –  Arctic charr, Salvelinus alpinus (L.), is one of the several northern fishes that show resource polymorphisms in postglacial lakes. Two reproductively isolated morphs of Arctic charr coexist in distinct ecological niches in the subarctic lake Fjellfrøsvatn, North Norway. Offspring of the two morphs (termed littoral charr and profundal charr) were reared separately but under identical conditions. Their feeding behaviour was compared experimentally using different kinds of live prey. The fishes had no experience with natural prey before the experiments. The littoral morph were more effective in eating live plankton ( Daphnia ) and littoral benthos ( Gammarus ), and had a higher attack rate against pleuston (surface prey, Gerris ) compared with the profundal morph. The two morphs behaved in accordance with expectations from their in situ niche utilisation towards the three prey types. This indicates a case of incipient ecological speciation where divergence in resource utilisation in contrasting niches has evolved adaptations in feeding behaviour by natural selection.     

Resource use of native and stocked brown trout Salmo trutta L., in a subarctic lake

Habitat use, food composition and growth of stocked and native brown trout, Salmo trutta L., were studied in the subarctic Lake Muddusjärvi in northern Finland. Stocked brown trout and native brown trout preferred littoral and pelagic areas. Trout were stocked in October. In June stocked trout fed primarily on invertebrates while native fish were piscivorous. From July onwards the composition of the diet of both stocked and native trout was similar and consisted almost entirely of small‐sized whitefish. Brown trout were already piscivorous at a length of about 20 cm. The mean length of prey consumed was about 12 cm. Mean length‐at‐age was similar from the second year in the lake despite of the larger size of stocked fish during the first year in the lake.     

The influence of predation risk on habitat selection and food intake by Arctic charr, Salvelinus alpinus (L.)

Abstract— Large piscivorous fish are assumed to affect habitat selection and food intake of prey fish. To study the effects of cannibalistic Arctic charr, Salvelinus alpinus (L.), on smaller stunted charr, we sampled the prey fish in littoral and pelagic habitats using gill nets, before and shortly after the release of large charr in a small lake (0.52 km²). In the habitats where the risk of predation was highest, the catch per unit effort de creased from 13.3 to 4.8 fish per 100 m² of gillnet after release of pred ators. The large decrease in numbers of charr < 18 cm corresponded with the predicted vulnerable prey sizes, according to a model based on the size distribution of predators. The occurrence of planktivorous fish and weight-specific food intake decreased in the high risk habitat and remained unaffected in the low risk habitats. Changes in the food intake of prey fish could not be explained in terms of fish length, indicating that prey fish changed diet when the risk of predation was high.     

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.     

Habitat utilization of juvenile Atlantic salmon (Salmo salar L.), brown trout (Salmo trutta L.) and Arctic charr (Salvelinus alpinus (L.)) in two lakes in northern Norway

Abstract— Habitat utilization of juvenile Atlantic salmon, brown trout and Arctic charr was investigated in two lakes in northern Norway during the icefree season. Both the vertical distribution and the distribution among different habitat types were studied by gillnetting with small mesh sized gillnets (8-15 mm) in different habitats. Salmon and trout were predominantly caught in the littoral and sublittoral zones (0-6 m depth). Access to shelter seemed to be the most important factor determining the horizontal distribution of small salmon and trout. Most of these fish were caught in stony or vegetated habitats, while few salmon and trout were caught on sandy locations or in the pelagic zone. In one of the lakes, there were significantly higher catch rates of salmon than of trout in the stony littoral (0-3 m), while in the other lake there were no significant differences in spatial distribution between these two species. Charr were primarily found in the profundal, sublittoral or pelagic zones of the lakes.     

Recaptures and resource use of native and non-native brown trout, Salmo trutta L., released in a Norwegian lake

Abstract Rate of recapture (gill netting), habitat use, and diet of three strains of stocked brown trout, Salmo trutta L., were compared with resident brown trout in a Norwegian lake. The strains originated from an alpine lake, from a boreal lake, and from the native brown trout population in the lake. Overall recapture rate was 5–8% for all strains. The low recapture rate could be due to the relatively small size at stocking; mean fish length varied between 13.1 and 14.5 cm with strain and stocking method. Two years after release, the frequency of the different strains decreased from about 12% in the first year to stabilize at about 1%. The alpine strain showed the highest overall recapture rate, whereas the native strain was recaptured at an intermediate rate. The overall recapture rate of scatter-planted brown trout was higher than that of spot-planted brown trout. Immediately after being stocked, introduced fish ate less and had a less-varied diet than resident trout; however, stocked fish adopted a natural diet within the first summer. The distribution of trout between the pelagic and the upper epibenthic habitat was similar for both the resident and the stocked brown trout. Results indicate that the habitat use of stocked brown trout is adaptive and becomes similar to that of indigenous fish.     

Char ecology. Natal homing in sympatric populations of anadromous Arctic char Salvelinus alpinus (L.): roles of pheromone recognition

Abstract –  This study revealed that progeny of sympatric Salangen Arctic char, omitted from any kind of imprinting to home water, returned with high precision to the home river and parental locality upon releases in their native marine environments. Fish did not select river randomly, and the homing precision was the same for progeny whether fertilized in home stream or in foreign water. The progeny studied originate from the lake Øvervatnet and the Salangen River population and were hatched and reared at a hatchery at Voss in southern Norway. Experimental material comprised freshwater resident and smolt released from parr directly. The freshwater resident is partly anadromous exhibiting about 74% smolt when released directly into the sea. Char at all stages (resident, transformed resident, and smolt released from parr directly) returned in significant numbers to the home river system and the site of the specific parental population.     

Long-term variation in diet of Arctic char, Salvelinus alpinus, and brown trout, Salmo trutta: effects of changes in fish density and food availability

Abstract  The diet of Arctic char, Salvelinus alpinus (L.), and brown trout, Salmo trutta L., was studied in a large Norwegian reservoir over the first four decades (1954–1995) of regulation. Significant changes in diet occurred concurrent with changes in fish density and prey availability. Reduced biomass of fish due to the effect of hydropower development probably led to reduced inter- and intra-specific competition. Consumption of zoobenthos increased for both species during the first decade, fish then became an important food item concurrent with an increase in small-sized Arctic char in the littoral zone. The diet of Arctic char, but not brown trout, then changed radically as the introduced mysid, Mysis relicta Lovén, became an important food item.     

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.     

Migrations and harvest rates of Arctic charr (Salvelinus alpinus) in a marine protected area

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Muscle carotenoid concentrations in sexually maturing and immature Arctic charr, Salvelinus alpinus (L.)

Muscle carotenoid concentrations were measured in immature and maturing 2+ Arctic charr, Salvelinus alpinus (L.), sampled from April until ovulation started in November. In immature fish, muscle carotenoid concentration increased steadily and had reached about 10 mg kg^?1 by the end of the experiment. In mature fish of both sexes, pigment concentrations decreased during the autumn and muscle carotenoid concentrations were significantly lower than in the immature fish during the latter stages of the trial. No significant difference in muscle pigmentation was observed between sexes. Carotenoid concentrations were lowest in the cranial region of the fillet, but decreased in all parts of the fillet when the charr became mature. In females, the decrease seemed to be most marked in the caudal region. At the termination of the experiment, fillets accounted for about one-half of the carotenoid content in mature fish of both sexes. High concentrations of pigment were recorded in the skin and fins of mature males. In mature female charr, pigment was almost equally distributed among skin, fins and ovaries     

Small-scale spatial variation in age and size at maturity of stream-dwelling brown trout, Salmo trutta

Abstract – This study documents substantial small-scale spatial variation in age and size at maturity of brown trout ( Salmo trutta ) found either in allopatry (above major waterfalls) or in sympatry (below waterfalls) with the Alpine bullhead ( Cottus poecilopus ) in forest streams in south-east Norway. Within two streams, female brown trout above waterfalls tended to delay the onset of sexual maturity, as compared with females from neighbouring sites below the waterfalls. Four additional streams were represented with either an allopatric or a sympatric site. There was considerable variation in age and size at maturity among these streams, but no consistent difference between allopatric and sympatric sites. It is suggested that the spatial variation in maturity responses is influenced by local opportunities for growth, and possibly also survival. Earlier studies in these streams have linked spatial variation in brown trout behaviour and demography to the presence or absence of the Alpine bullhead.