The Relevance of Individual Size to Management of Arctic Charr, Salvelinus Alpinus, Populations

Arctic charr, Salvelinus alpinus, tend to form stunted populations presumably due to competition for limited resources. In this context a long-term intensive fishery programme aiming at reducing charr density, and thereby increasing growth and harvestable sizes, was initiated in the early 1980s in a Norwegian alpine lake. Here we present long-term data on catch statistics and changes in mean weights of charr caught with gill nets on spawning grounds, as well as changes in mean weight of juvenile charr caught with funnel traps during the 1990s. Furthermore, we present results from shorter-term studies on growth, size- and age-distribution, and size-related habitat and resource utilisation of Arctic charr and brown trout, Salmo trutta, in this lake. Mean weight (± SD) of charr caught on the spawning grounds increased significantly from 129.8g (±11.9) in the years 1982–1990 to 213.1g (±37.8) in the years 1996–1999, whereas catch per unit of effort decreased significantly. Mean weight of juveniles caught in funnel traps increased significantly from 21.2g (±6.9) in 1993 to 41.9g (±14.8) in 1999. Apparently the increase in weight of spawning charr coincided with the onset of trap fishing for juvenile charr. Compared to generally shallow dwelling trout, charr grew rapidly and were generally found in deeper areas of the lake. Charr went through a distinct size-related niche shift from mainly consuming small zooplankton in the pelagic to consuming large benthic prey in shallower waters. Resource and habitat utilisation in different size-groups of charr and trout are discussed with respect to possible competitive and predatory intra- and interspecific interactions, and with regard to management of charr populations.     

The growth of charr, Salvelinus willughbii Günther, in Windermere

The growth of charr ( Salvelinus willughbii Günther ) caught in Windermere from 1941–1952 has been studied. Scales were used for determination of age and back-calculation of length for age. Autumn and spring spawners, males and females, and charr of normal and dwarf growth were treated separately. In fish of normal growth, the spring spawners were significantly smaller than the autumn spawners at ages 1 and 2 years, and significantly larger from age 4 years onwards. There was little difference in growth between males and females within the two spawning populations. Charr of lengths of less than 200 mm at age 4 years were considered to be dwarfs. Mean lengths at capture of male charr were: autumn spawners normal growth 272 mm, dwarf 218 mm; spring spawners normal growth 327 mm, dwarf 194 mm. The oldest recorded age was 8 years.     

Density,length distribution,and diet of age-0 arctic charrSalvelinus alpinus in the surf zone of Thingvallavatn,Iceland

Synopsis Population densities of age-0 arctic chaff in the surf zone averaged 1.83 and 4.70 fish m^-2 in August 1984 and June 1985, respectively. Length variation of the littoral fish was low in early summer, increasing in late summer and autumn. Newly emerged charr, ∼ 20 mm long, appeared in the stony shallow water zone during both May and June. From length variation and variation in mouth position of the young charr, it is concluded that at least two of the four chaff morphs in the lake are present in the surf zone during spring and summer. In August, some of the larger age-0 charr had moved out from the surf zone, into the pelagic and the deeper epibenthic waters. The food of young littoral charr was dominated by large chironomid larvae (instar 3 and 4) and pupae Contribution from the Thingvallavatn project     

Mark-recapture experiments on fish in Windermere, 1943–1982

Marking experiments on perch, Perca fluviatilis L., pike, Esox lucius L., and charr, Salvelinus alpinus L., have been carried out in Windermere. Numbered tags were used for individual identification, except in some short term experiments on charr. A total of 13 182 perch and 4696 pike were tagged; 2066 charr were tagged subcutaneously and marked with a fin clip. In the short-term experiments 2015 charr were marked with a fin clip or punched hole. Perch were recaptured up to 8 years after tagging, pike 12 years and charr 5 years. Estimates of numbers and mortality were not satisfactory, except for limited sections of the populations of pike and charr. The main reasons for the unsatisfactory results were: for perch, unknown mortality at time of tagging; for pike, selectivity of the gillnets and in some years low numbers tagged; for charr, restriction to few spawning sites and in some places low numbers recaptured. The experiments provided useful information on movements and growth, which could not have been obtained in any other way.     

Influence of spring and summer water temperature on brook charr, Salvelinus fontinalis, growth and age structure in the Ford River, Michigan

Synopsis The influence of late spring and summer water temperatures on brook charr, Salvelinus fontinalis, growth and age structure was evaluated from 1984 to 1991 in the Ford River, Michigan. Temperature was monitored and brook charr sampled for vital statistics from late May through September using fyke nets and weirs at four locations within a 25.8 km section of stream. Scale analysis was used to determine captured brook charr age, past length at age and relative annual growth rates. Late spring and summer water temperature patterns varied between years with the greatest variability occurring in May and June. Age and size structure also varied between years and was significantly related to temperature. Years with cooler late spring and summer temperature patterns were dominated by older (age 2 and 3), larger brook charr, while years with warmer spring and summer temperature patterns were dominated by younger (age 1), smaller brook charr. Spring and summer temperature did not appear to have a significant effect on the growth of age 0 or age 1 brook charr. However, temperature was negatively related to brook charr growth from age 2 on. As spring and summer water temperatures are critical to brook charr growth and survival, it is important that a streams thermal regime be considered when establishing management goals for this species.     

Seawater tolerance in captive high-Arctic Svalbard charr (Salvelinus alpinus): effect of photoperiod and body size

Anadromous Arctic charr (Salvelinus alpinus) returning after spending summer at sea were captured in a fish trap in the Dieset River on Spitsbergen (79°10'N), Svalbard. Fish selected for breeding were transported to Trondheim in mainland Norway. Eggs obtained from the charr were fertilized and incubated in total darkness. First-fed alevins and resulting parr were kept under continuous light until an age of 0+ and 1+ years, respectively. Some 1+ charr were kept as controls under a continuous short-day photoperiod (6L:18D) from autumn until the end of the experiment the following July. Charr aged 0+ and 1+ years old were exposed to a short-day photoperiod from October until January and a simulated natural photoperiod for 80°N from January until the end of the experiments. Challenge tests demonstrated a size-dependent seawater tolerance for charr with a body length less than 18 cm. Fish smaller than 12 cm did not survive the 96-h test period. The larger charr kept under simulated natural photoperiod developed increased hypoosmoregulatory capacity. Charr kept under short-day treatment showed a slight, short-lived increase in seawater tolerance. A 7-days seawater challenge test at the end of the experiment (July) demonstrated that the anticipatory seawater preparation in charr is influenced by photoperiod. We conclude that offspring from anadromous high-Arctic charr must achieve a threshold body size (>25 cm) before they can respond to photoperiod signals which trigger the development of the hypoosmoregulatory capacity typical for smoltifying salmonids.     

Microprobe analyses of anadromous Arctic charr, Salvelinus alpinus, otoliths to infer life history migration events

Otoliths of Arctic charr, Salvelinus alpinus, from north Labrador were examined to gather life history information pertinent to anadromous migrations. Wavelength dispersive electron microprobe measurements confirm the presence of strontium in otolith tissue. Trace element analyses of charr otoliths imply that otolith strontium/calcium concentration ratios are related to external environmental factors with salinity having a great influence. Otoliths clearly record habitat shifts consistent with migration across major salinity boundaries. Most charr first migrate to sea following at least 2 years in freshwater. Generally, when anadromy begins, it continues annually. Robust relationships in otolith chemistry, combined with macrostructure patterns in field-captured specimens, provide time-series data describing individual migrational histories. Validation studies are still required in order to couple information contained within daily increments with chemical analyses to infer additional temporal events in diadromous migrations. Received: 6 November 1996 / Accepted: 18 May 1997     

Daily rations,diel feeding activity and distribution of age-0 brook charr,Salvelinus fontinalis,in two subarctic streams

Synopsis Diel variations in the stomach content weight of age-0 brook charr indicate that they fed primarily during the day in Matamek River and Rivière à la Truite, Québec. Age-0 charr in both rivers moved close to the shore at night. The mean weight of stomach contents in Rivière à la Truite was highest at the end of the daylight period, whereas in the Matamek River stomach content weights maintained maximum level during most of the day, except in August when high water temperatures reduced food intake. Daily rations were estimated at 5.09 and 4.09% body weight for charr from Rivière à la Truite in July and August, respectively. In the Matamek River, estimates were 7.13, 9.75, and 1.27%, in June, July, and August, respectively. Charr from Matamek River had a higher instantaneous growth rate than charr in Rivière à la Truite (3.96 versus 2.72% body weight per day) and were significantly heavier by mid-August. Differences observed between populations in daily rations and growth rates were attributed mainly to differences in water temperature regimes.     

Evidence of repeated long-distance movements by lake charr <Emphasis Type="Italic">Salvelinus namaycush</Emphasis> in Lake Huron

Movements and dispersal distances of acoustically-tagged adult lake charr Salvelinus namaycush were estimated based on detections at acoustic receivers in Lake Huron during 2010–2014. Most lake charr were detected only at receivers proximate to their release location or were not detected at all, but 3–9% of tagged lake charr were detected at receivers located over 100 km from their release location. Several fish made extensive repeated migrations within the lake, some at the scale of the entire main basin. Our observations show that some lake charr individuals repeat a similar pattern each year of moving long distances, and some fish were observed to show annual fidelity to presumed foraging sites in the spring at a spatial scale of approximately 200 km. Our telemetry-based estimates were minimum estimates of dispersal, as the placement of receivers within Lake Huron was not optimal for detection of lake charr and did not cover the majority of the lake. Further study of long-distance movement in lake charr is necessary to fully understand the implications of this behavior to lake charr ecology, population dynamics, and management in the Great Lakes.     

Maturation,growth and fecundity of Arctic charr, <Emphasis Type="Italic">Salvelinus alpinus</Emphasis> (L.), life-history variants co-existing in lake systems of Southern Baffin Island,Nunavut, Canada

Two life-history variants of Arctic charr (Salvelinus alpinus), anadromous and lake-resident, have been previously identified in lakes of Southern Baffin Island, Nunavut, Canada. In accordance with classical life-history theory, it is hypothesised that anadromous charr will delay maturation in both size and age, and have increased fecundity (per spawning event) in comparison with lake-resident charr. Sagittal otoliths and biological data were collected for both life-history variants within the three studied lakes: Iqalugaarjuit, Qasigiat and Qinngu. Sagittal otoliths were embedded in epoxy resin, cross-sectioned for age determination, and imaged for back-calculation (size and age). Back-calculated data in each lake were fit to von Bertalanffy growth models for each life-history variant and compared via analysis of residual sums of squares. Anadromous charr had greater mean size at maturity and experienced a delay in mean age at maturity in comparison with lake-residents. The relationship between size and fecundity or egg diameter did not differ between the two life-history variants. Growth models indicate that the overall growth coefficients of lake-resident and anadromous charr were different in all three studied lakes. The Brody growth coefficient for all lake-resident charr populations was greater than anadromous individuals indicating that maximum length was reached at a rapid rate, resulting in a smaller asymptotic length. Indirect evidence suggests that anadromous and lake-resident charr belong to one reproductive population. Future genetic analysis is necessary to further determine the degree of divergence between the life-history variants.     

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.     

Diel Periodicity and Environmental Influence on the Smolt Migration of Arctic charr, Salvelinus alpinus, Atlantic salmon, Salmo salar, and Brown Trout, Salmo trutta, in Northern Norway

We caught smolts of Arctic charr, Salvelinus alpinus (L.), Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in a trap situated at the mouth of the river Halselva (70° N 23° E), northern Norway during a 5-year period. Salmon and charr were the first to leave freshwater at the end of May, while most trout left freshwater about 14 days later. Whereas the midnight sun shines continuously during the downstream migration period, the light intensity has a diel intensity pattern. The majority of the descending migrants were recorded during the night. The number of descending fish was relatively low at water temperatures below 3°C. The increase in water level was largely caused by snowmelt and thus correlated with lower water temperatures. The number of migrants of all three species increased with increasing water level and decreased with increasing water temperature, with the exception of trout, which increased with water temperature. Notably, the increase in number of migrants was also correlated with the increase in water level the following day, indicating that fish movements represent an early response to a later spate. There was no significant relationship between the number of migrants and the daily change in water level or temperature. The three species were highly synchronised in their daily number of migrants. The strongest synchronisation was found between Arctic charr and Atlantic salmon, followed by Arctic charr and brown trout.     

Emergence chronology of brook charr,Salvenus fontinalis,alevins in an acidic stream

Synopsis The emergence chronology of brook charr alevins in the Chikanishing River, Ontario, Canada and the concurrent changes in stream chemistry resulting from acidic snowmelt runoff were examined during the spring, 1986–1988. Emergence patterns were similar among years with the most intense emergence (> 70%) occurring during the declining stream discharge following the spring flood. This period coincided with stream pH depressions (minimum pH 5.2) in 2 of 3 years, suggesting emergence behaviour frequently subjects the earliest free-swimming life interval of brook charr to episodic acidification events.     

Annual variability in the life-history characteristics of brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus) in a subalpine Norwegian lake

The annual variability in growth and life history traits of brown trout (Salmo trutta L.) and Arctic charr (Salvelinus alpinus (L.)) in Lake Atnsjøen, a Norwegian subalpine lake, was studied over a period of 13 years (1985–1997). The extent to which life-history characteristics recorded on one occasion can be regarded as representative for the population was explored. We found inter-cohort variation in growth for both species; estimates of asymptotic length (L _∞) in ten cohorts ranged between 225–305 mm (CV = 10.5%) for brown trout and 273–301 mm (CV = 4.1%) for Arctic charr. However, this variation was much lower than inter-population variation for brown trout based on single samples from 169 populations (CV = 24.6%). In Lake Atnsjøen, annual growth increment correlated highly with the number of days warmer than 7?°C (R ²=0.60–0.89) for brown trout, and days warmer than 10?°C (R ²=0.40–0.58) for Arctic charr. Females of Arctic charr were younger at sexual maturity than males, while no such difference was found in brown trout. Generally speaking, early maturing individuals of both species grew faster, particularly from age-2 and onwards, than immature individuals. Early maturing individuals, however, were smaller at maturity than those maturing one year older. Age and size at maturity were significantly correlated with asymptotic lengths only in Arctic charr females.     

Habitat utilization by sympatric arctic charr Salvelinus alpinus L. and brown trout Salmo trutta L. in Lake Atnsjø, south-east Norway

SUMMARY. 1. Habitat utilization, as well as inter- and intraspecific relations of different size groups of arctic charr (Salvelinus alpinus (L.)) and brown trout (Salmo trutta L.) in Lake Atnsjø, south-east Norway, were investigated by analysing food and spatial niches from monthly benthic and pelagic gillnet catches during June-October 1985.
2. Small individuals (150–230 mm) of both arctic charr and brown trout occurred in shallow benthic habitats. However, they were spatially segregated as arctic charr dominated at depths of 5–15 m and brown trout at depths of 0–5 m.
3. Larger (>230 mm) arctic charr and brown trout coexisted in the pelagic zone. Both species occurred mainly in the uppermost 2-3 m of the pelagic, except in August, when arctic charr occurred at high densities throughout the 0–12 m depth interval. On this occasion, arctic charr were segregated in depth according to size, with significantly larger fish in the top 6 m. This was probably due to increased intraspecific competition for food.
4. The two species differed in food choice in both habitats, Arctic charr fed almost exclusively on zooplankton, whereas brown trout had a more variable diet, consisting of surface insects, zooplankton. aquatic insects and fish.
5. The data suggest that the uppermost pelagic was the more favourable habitat for both species. Large individuals having high social position occupied this habitat, whereas small individuals lived in benthic habitat where they were less vulnerable to agonistic behaviour from larger individuals and less exposed to predators. The more aggressive and dominant brown trout occupied the more rewarding part of the benthic habitat.     

Comparative ontogeny of photobehavioural responses of charrs (Salvelinus species)

Synopsis The development of photobehavioural responses in brook (Salvelinus fontinalis) and lake (S. namaycush) charr was studied by monitoring the intrasubstrate movements and concurrent photoresponse behaviour of incubated embryos and alevins. Photoresponse behaviour of both F-1 hybrids of the parent species was also recorded. All embryos initially moved downward in the substrate, however brook charr descended farther and faster into the substrate than did lake charr. Photoresponse tests demonstrated a similar pattern of photoresponse transformation from a photonegative to a photopositive state in both species. However, photoresponse reversal was faster, more extensive and occurred later in brook charr than in lake charr. Patterns of photoresponse change in F-1 hybrids were intermediate between those of the parent species. Photoresponse shifts preceded the onset of alevin emergence in both species. occurring when differential development of various morphological characters existed. Developmental states of characters were synchronously maximal towards the end of alevin emergence. Intermediate measures of morphological development were observed for F-1 hybrids. Possible functions and mechanisms of photoresponse transitions are discussed in relation to ecological differences between the species.     

Trophic specialization in Arctic charr Salvelinus alpinus (Pisces; Salmonidae): morphological divergence and ontogenetic niche shifts

Lake Thingvallavatn supports four trophic morphs of Arctic charr, Salvelinus alpinus (L.); two of the morphs are benthic (small and large benthivorous charr) one exploits pelagic waters (planktivorous charr) and the fourth is found in both habitats (piscivorous charr). The morphological variation among these morphs was analysed by use of principal component analysis and canonical discriminant analysis. The benihic morphs have a short lower jaw and long pectoral fins. The benthic fish also have fewer gillrakers than the other morphs. Small and large benthivorous charrs attain sexual maturity from 2 and 6 years of age, and at fork lengths from 7 and 22 cm, respectively. Small benthivorous charr retain their juvenile parr marks as adults, have beige ventral colours, and are frequently melanized under the lower jaw. Planktivorous and piscivorous charr attain sexual maturity from 4 and 6 years of age, from fork lengths of 15 and 23 cm, respectively. This phenotypic polymorphism is associated with habitat utilization and diet of the fish, and has probably arisen within the lake system through diversification and niche specialization. The pelagic morphs apparently stem from a single population, and are possibly diversified through conditional niche shifts which affect ontogeny. Juveniles reaching a body length of 23 cm may change from zooplankton to fish feeding. Asymptotic length increases thereby from 20.5 cm in planktivorous charr to 30.2 cm in piscivorous charr. The benthic morphs appear to represent separate populations, although both feed chiefly on the gastropod Lymnaea peregra. Their co-existence seems to be facilitated by size dependent constraints on habitat use. The small morph (asymptotic length 13.3 cm) exploit the interstitial crevices in the lava block substratum, whereas the large morph (asymptotic length 55.4 cm) live epibenthically.     

Segregation in spawning and early life history among polymorphic Arctic charr, Salvelinus alpinus, in Thingvallavatn, Iceland

The relationships among time of spawning, incubation temperature, timing of first feeding and early growth were examined in four sympatric morphs of Arctic charr in Thingvallavatn, Iceland. Large benthivorous charr spawn in July-August at sites with cold ground-water flow. Planktivorous and piscivorous charr spawn in September-November and are not confined to ground-water sites. The spawning of small benthivorous charr overlaps with that of other morphs. Progeny of large benthivorous charr start feeding 2-3 months earlier than the progeny of autumn spawners. This results in differential size distribution and growth rates of young in the spring.     

Comparison of growth, diet and food consumption of sea-run and lake-dwelling Arctic charr

Sea-run post-smolt Arctic charr Salvelinus alpinus , (15–26 cm) from Storvatn, northern Norway (70°39'48"N) had significantly higher average specific growth rates in two years (1·64 and 1·66) than the corresponding lake-dwelling charr (0·53 and 1·20). The post-smolts displayed fast compensatory growth in the first 2–3 weeks of their sea residency, but then almost stopped growing prior to their return to fresh water. Lake-dwelling charr grew more evenly during the same time period. Thus, the anadromous charr may return to the lake after only 5–6 weeks in the sea, because the potential to maintain a high growth rate in the sea is reduced. The marine diet consisted mainly of the two crustacean plankton species Calanus finmarchicus , and Thysanoëssa , sp. (88%), and less of fish (6%), insects (4%) and benthos (2%). The diet of lake-dwelling charr consisted mainly of insects (58%, mostly chironomid pupae) and zoobenthos (29%), and less of zooplankton (13%) during the same time period. Although post-smolts had the highest growth rates, they had significantly lower food consumption rates and higher frequencies of empty stomachs than the corresponding lake-dwelling fish. Possible explanations for this paradox are discussed in relation to stomach evacuation rates, water temperature, feeding behaviour and the energy content of the food in the two environments.     

Genetic basis of life-history variation of dwarf and normal Arctic charr,Salvelinus alpinus (L.), in Stora Rösjön,central Sweden*

The genetic basis of life-history variation of dwarf and normal Arctic charr forms in Stora Rosjon was studied by means of two separate rearing experiments on the progeny of artificially bred wild fish. The concept of differential life-strategies gained support, since the progeny of the normal charr form had a higher growth rate and became sexually mature at a larger size and later age than the progeny of the dwarf charr form. The hybrid progeny (dwarf × normal parental charr) was recognized as intermediate in growth as well as in size and age at maturity, compared to the normal and dwarf progeny. However, a dominance effect of the growth and maturation pattern of the dwarf charr was revealed, since hybrid progeny were more similar to dwarf than to normal progeny. The rearing experiments also support the view that the dwarf and normal charr forms are inherited characteristics of one population, since any individual progeny cannot be assigned to a certain form according to its size and age at maturity.