Foraging mode variation in three stream‐dwelling salmonid fishes

Despite long‐standing interest in foraging modes as an important element of animal space use, few studies document and compare individual foraging mode differences among species and ecological conditions in the wild. We observed and compared foraging modes of 61 wild Arctic charr, Salvelinus alpinus, 42 brown trout, Salmo trutta, and 50 Atlantic salmon, Salmo salar, in their first growing season over a range of habitats in 10 Icelandic streams. We found that although stream salmonids typically sit‐and‐wait to ambush prey from short distances, Arctic charr were more mobile during prey search and prior to prey attack than Atlantic salmon, whereas brown trout were intermediate. In all three species, individuals that were mobile during search were more likely to be moving when initiating attacks on prey, although the strength and the slope of this relationship differed among species. Arctic charr also differed from salmon and trout as more mobile individuals travelled longer distances during prey pursuits. Finally, coupled with published data from the literature, salmonid foraging mobility (both during search and prior to attack) clearly decreased from still water habitats (e.g., brook charr), to slow‐running waters (e.g., Arctic charr) to fast‐running waters (e.g., Atlantic salmon). Hence, our study suggests that foraging mode of young salmonids can vary distinctly among related species and furthers our understanding of the behavioural mechanisms shaping the geographical distribution of wild salmonids.     

Patterns of Tetracapsuloides bryosalmonae infection of three salmonid species in large,deep Norwegian lakes

Proliferative kidney disease (PKD), caused by the myxozoan endoparasite Tetracapsuloides bryosalmonae, is of serious ecological and economical concern to wild and farmed salmonids. Wild salmonid populations have declined due to PKD, primarily in rivers, in Europe and North America. Deep lakes are also important habitats for salmonids, and this work aimed to investigate parasite presence in five deep Norwegian lakes. Kidney samples from three salmonid species from deep lakes were collected and tested using real-time PCR to detect PKD parasite presence. We present the first detection of T. bryosalmonae in European whitefish in Norway for the first time, as well as the first published documentation of the parasite in kidneys of Arctic charr, brown trout and whitefish in four lakes. The observed prevalence of the parasite was higher in populations of brown trout than of Arctic charr and whitefish. The parasite was detected in farmed, but not in wild, charr in one lake. This suggests a possible link with a depth of fish habitat and fewer T. bryosalmonae-infected and PKD-affected fish. Towards a warmer climate, cold hypolimnion in deep lakes may act as a refuge for wild salmonids, while cold deep water may be used to control PKD in farmed salmonids.     

Territorial and agonistic interactions between farmed and wild cod (Gadus morhua)

Studies of contest competition between wild and farmed fish have mostly focused on fish with strongly territorial behaviour. Little is known about species with more plastic social behaviour, such as Atlantic cod (Gadus morhua L.), a species that can either aggressively defend territories or shoal. There is also concern that cod that escape from farms will compete with wild populations. We examined dyadic contest competition between wild and farmed juvenile cod using an intruder–resident experimental set‐up in the laboratory. No prior residency advantage was observed, but the differences between farmed and wild cod were clear. Farmed cod were more submissive than wild cod and fled earlier during contests, which suggests that wild fish often out‐compete farmed intruders. Both fish types initiated aggression earlier against fish of the same background. A multivariate analysis of 11 different behavioural traits indicated that a group of 59% of farmed fish were behaviourally very similar to the 55% most submissive wild fish. These results suggest that wild juvenile cod may be quite robust towards competition for food and shelter from juvenile farmed cod, but further research is needed to verify this pattern.     

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

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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.     

Putative virulence factors of atypical Aeromonas salmonicida isolated from Arctic charr,Salvelinus alpinus (L.), and European grayling,Thymallus thymallus (L.)

Atypical Aeromonas salmonicida (AAS) causes generalized lethal infections in farmed Arctic charr, Salvelinus alpinus (L.), and European grayling, Thymallus thymallus (L.), and is thus a serious threat for culture of these fish species. Virulence factors were studied among isolates of AAS from Arctic charr (n = 20), European grayling (n = 19) and other fish species (n = 20), of which 48 were of Finnish and 11 of Swedish origin. All isolates produced an A-layer. Extracellular products (ECP) of the AAS isolates did not produce detectable gelatinase and caseinase activity in test assays. Analysis of the same ECP preparations with substrate sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed weak proteolytic activity, indicating the different sensitivity of the detection methods used. The ECP from AAS isolates showed low cytotoxic activity against cultured cells. However, the ECP did not induce mortality in challenged Arctic charr. The results suggest that toxic components, like ECP, secreted by the bacterium may not be the major virulence factor in AAS-infection in Arctic charr and European grayling, and hence the pathogenesis also differs from the pathogenesis of AAS-infection in Atlantic salmon, Salmo salar L.     

Resource partitioning and spatial segregation in native and stocked brown trout, Salmo trutta L., and Arctic charr, Salvelinus alpinus (L.), in a hydroelectric reservoir

Abstract. Habitat use, food and spatial segregation in native and stocked brown trout, Salmo trutta L., and Arctic charr, Salvelinus alpinus (L.), were studied during summer 1989 and 1990 in the hydroelectric reservoir Lake Tunhovdfjorden. There was no difference in habitat use and feeding habits between wild and stocked brown trout. In epibenthic areas brown trout lived chiefly down to 2 Secchi disc units, whereas Arctic charr were most abundant between 1 and 4 Secchi disc units. In pelagic areas the catches were low for both species, and they were chiefly confined to surface waters down to 1 Secchi disc unit. The food segregation between brown trout and Arctic charr was almost complete. Both pelagic and epibenthic Arctic charr fed mainly on cladocerans ( Bosmina longispina and Daphnia galeata ), whereas surface insects of terrestrial origin and Arctic charr were the dominant food items for brown trout. Pelagic Arctic charr were significantly older, larger and more homogeneous in size than epibenthic charr. During calm weather schools of Arctic charr were observed cruising with the dorsal fin above the surface.     

Impact of a new artificial shelter on Arctic charr (Salvelinus alpinus, L.) behaviour and culture performance during the endogenous feeding period

Shelter is of major importance in many species of fish both in the wild and in aquaculture. Sheltering behaviour of Arctic charr has been poorly studied in aquaculture. A new type of shelter made of PVC agricultural drain cut in half was tested on culture performance and behaviour during the endogenous feeding period. This device offered grooves where alevins could position themselves and lie in a vertical position. A first experiment compared fish with and without shelter in incubator compartments. All fish were measured at 122, 158, and 190 days post fertilization (dpf) and observed at 126, 139 and 157 dpf. Fish provided with shelter showed better growth performances, lower mortality and started first exogenous feeding about six days later compared to fish without shelter. These effects from shelter provision were also associated with much less mobility in fish provided with shelter. At 126 dpf, all fish provided with shelter were immobile while other fish displayed horizontal stationary body movement (86%) or swam against the water current (7%). At 157 dpf, 85% of fish without shelter were immobile compared to 95% in fish provided with shelter. Both categories displayed stationary body movements. In most cases, there was one single fish per groove and a stable average value of 2.4 grooves was maintained between two successive fish throughout the study. The second experiment was a preference test where the fish were placed in similar compartments but occupied by a half shelter. We found that 61% of the fish were located under the shelter at 122 dpf while at 157 dpf, 58% were on the shelter and 42% out of the shelter. The present results revealed Arctic charr behavioural characteristics in the presence of a new type of artificial shelter at very early stages of development. This shelter enables the fish to stabilize in a vertical position without producing any movement and to choose by itself its favorite location throughout development. This device could be used to improve Arctic charr growth, performance and fish welfare during the endogenous feeding period.     

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.     

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.     

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.     

Comparative growth study of wild- and hatchery-produced Arctic charr (Salvelinus alpinus L.) in a coldwater recirculation system

The growth performance of Arctic charr of wild (W) and hatchery (H) origin was compared in a commercial coldwater recirculation system (Villmarksfisk, Bardu, 68°N, 19°E, Norway). The initial individual body mass was 115 g and similar between groups. The rearing temperature was 9.2 °C and the fish were held under continuous light (24:0 L:D). At the end of the experiment (day 240), the average body mass of the H fish was 451 g compared to 231 g in the W fish. The accumulated mortality of wild Arctic charr was about 40%; 10 times higher than the mortality of hatchery-produced Arctic charr (4%). The difference in growth performance and survival rate impose a great disadvantage of using this wild caught fish as compared to commercially available hatchery-produced Arctic charr in coldwater recirculation system. However, further improvements in the production chain (catching, live transport, quarantine, size grading, etc.) may still make production of wild caught Arctic charr profitable, especially as it demands a higher price in niche markets.     

Behavioural responses in wild cod (Gadus morhua L.) exposed to fish holding water

Local fishermen in several areas of Norway assert that salmon farms have caused the wild migrating cod to change their migratory behaviour, so that they no longer enter their natural spawning grounds in the fjords. If the asserted changes in behaviour of wild cod populations can be linked to establishment of salmon farms, water-soluble odorants are then possible candidates to explain such a connection. Chemical stimulants are important to the individual fish's conception of the surrounding environment. High density stocks of fish in a farm are expected to release large amounts of waterborne information. The present laboratory experiments were conducted to test behavioural responses in Atlantic cod exposed to water containing metabolites and waste from farmed salmon. The trials were conducted on single fish in a multiple chamber preference system. The results show that migrating wild Atlantic cod chose to spend more time in chambers without addition of water from the salmon tank, regardless of their maturation status, and even at very low concentrations (0.2%). The avoidance is probably due to presence of chemical compounds with olfactory properties from salmon, since anosmic cod did not elicit such response. Farmed cod, on the other hand, does not avoid water from the salmon tank, and stationary wild cod caught nearby a fish farm had a less pronounced response as compared to wild migrating cod. The response seen is not species specific, as wild migrating cod responded similar to water from a tank holding farmed cod as to water from salmon. These results do not preclude fishermen's observations that cod change their behaviour in areas with fish farming activity. Such a change in behaviour could be a response to water-soluble odorants, but needs to be validated and detailed in further laboratory experiments as well as in nature before any conclusions can be made.     

Dietary plasticity in Arctic charr (Salvelinus alpinus) in response to long‐term environmental change

Corrigan LJ, Winfield IJ, Hoelzel AR, Lucas MC. Dietary plasticity in Arctic charr (Salvelinus alpinus) in response to long‐term environmental change.
Ecology of Freshwater Fish 2011: 20: 5–13. © 2010 John Wiley & Sons A/S Abstract –  In the face of widespread environmental change impacts, there is a need to better understand mechanisms promoting flexibility and resilience of ecosystem components to such change to inform strategies for conservation. Glacial relict species are especially vulnerable to such changes. We investigated the behavioural responses of a native, glacial relict species, Arctic charr (Salvelinus alpinus) to long‐term environmental changes. It was hypothesised that changes in feeding behaviour would occur as a key intermediary to reduction in habitat availability (through climate change and eutrophication) or competitive interactions [with introduced roach (Rutilus rutilus)]. Stomach content analysis was used to assess the diet of 199 charr caught from Windermere, United Kingdom, in the months of March, June, September and November 2003–2007. The results were compared to data from 1940 to 1951 prior to the environmental changes and revealed a marked increase in the contribution of benthic invertebrates in the present diet. Stable isotope analysis confirms the results of the stomach analysis, suggesting that the charr have switched their diet from zooplanktivory towards benthivory. We discuss the possibility that habitat modification and roach population expansion in Windermere have contributed to changes in charr diet. Complementary studies suggest that this diet shift is more likely to be a response to the increasing roach population than to habitat modification; however, further study in Windermere would be required to elucidate the exact mechanism. Long‐term data sets such as these provide information that is useful for determining the directivity of ecological change and the capability of species to respond to change.     

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.     

Loss of genetic variability in the captive stocks of tambaqui,Colossoma macropomum (Cuvier, 1818), at breeding centres in Brazil,and their divergence from wild populations

The loss of variability in farmed populations and the risks of interactions with wild populations support the need for the genetic monitoring of species farmed throughout the world. In Brazil, the tambaqui is the most widely farmed native fish species. Despite this, there are no data on the pedigree of the farmed stocks, and the potential for interactions with wild populations in the Amazon basin has raised concerns with regard to the genetic variability of these stocks. The present study analysed sequences of the mitochondrial Control Region and 12 microsatellites to characterize the genetic variability of seven historically important commercial tambaqui breeding centres located in four different regions of Brazil, and compared these sequences with those obtained from individuals collected from a wild population. High levels of genetic diversity were found in the wild population, whereas genetic diversity was reduced in both markers in most captive populations, except for the broodstock located near the Amazon River. High F_ST and D_EST indices were recorded between the wild population and most of the captive stocks analysed. The drastic reduction in genetic diversity found in most captive stocks and the difference between these stocks and the wild population may have been the result of the small size of the founding populations and the absence of breeding management. The renewal of the broodstocks and the application of breeding management techniques are recommended. In the Amazon region, in addition, the use of broodstocks that are genetically very different from local wild populations should be avoided.     

Movements and spatiotemporal distribution of escaped farmed and local wild Atlantic cod (Gadus morhua L.)

Commercial farming of Atlantic cod (Gadus morhua L.) is now being developed in several countries. The ecological consequences of cod culture are poorly understood, but recent research suggests that Atlantic cod are more prone to escape from net pens than Atlantic salmon. Here, we describe the movements and the spatiotemporal distribution of farmed cod after escape relative to wild cod, both during and outside the natural spawning season. The experimental design included simulating escape incidents of farmed cod tagged with acoustic transmitters and using an array of automatic listening stations to monitor their dispersal and distribution. For comparison, local wild cod were monitored using the same array of receivers. The farmed cod dispersed rapidly after a simulated escape, they randomly distributed over large areas and their distribution overlapped with local wild cod. Moreover, escaped farmed fish were found at local cod spawning areas during the spawning season. The study also indicated that the recapture rate of escaped farmed cod was high compared with that of escaped farmed salmon. Thus, while our results showed that there is a considerable potential for ecosystem effects caused by escaped farmed cod, mitigating actions such as an efficient recapture fishery for escapees may be possible.     

Real-time PCR detection of Parvicapsula pseudobranchicola (Myxozoa: Myxosporea) in wild salmonids in Norway

The myxozoan genus Parvicapsula contains 14 species infecting fish, some of which are known to cause severe disease in farmed and wild salmonids. Parvicapsula pseudobranchicola infections were first reported from seawater-reared Atlantic salmon, Salmo salar, in Norway in 2002 and have since then been an increasing problem. The present study describes a Taqman real-time PCR assay for specific detection of P. pseudobranchicola. The Taqman assay targets the 18S rRNA gene of P. pseudobranchicola and is able to detect as few as ten copies of the target sequence. Using the described assay, P. pseudobranchicola was detected in both farmed and wild salmonids, indicating that wild Atlantic salmon, sea trout, Salmo trutta, and Arctic char, Salvelinus alpinus, may be natural hosts of the parasite. Parvicapsula pseudobranchicola was found in samples from wild salmonids in the far south and the far north of Norway, displaying a wide geographic range of the parasite. Farmed salmonids showed P. pseudobranchicola infection levels many folds higher than that observed for wild sea trout, indicating that farmed Atlantic salmon are subjected to an elevated infection pressure compared with wild salmonids.     

Habitat use, size and age structure in sympatric brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus) stocks: resistance of populations to change following harvest

Abstract– Habitat use and population dynamics in brown trout Salmo trutta and Arctic charr Salvelinus alpinus were studied in an oligotrophic lake over a period of 10 years. Previous studies showed that the species segregated by habitat during summer. While brown trout occupied the surface water down to a depth of 10 m, Arctic charr were found deeper with a maximum occurrence at depth 10–15 m. Following the removal of a large number of intermediate sized fish in 1988–89, habitat segregation between the species broke down and Arctic charr were found in upper waters, while brown trout descended to deeper waters. The following year, both species were most frequently found in surface waters at depths of 0–5 m. During the last four years, the species reestablished their original habitat segregation despite another removal experiment of intermediate-sized fish in 1992–1994. The removal of fish resulted in an increased proportion of large (≥ 25 cm) fish in both species. Furthermore, the charr stock responded by reduced abundance and increased size-at-age. The results revealed plasticity and strong resistance to harvest populations of brown trout and Arctic charr. This is probably due to internal mechanisms of intraspecific competition within each population, which result in differential mortality among size classes.     

Comparison of the growth performance of offspring from cultured versus wild populations of arctic charr,Salvelinus alpinus (L.), kept at three different temperatures

The effects of three different temperatures on the growth and maturation of the offspring of cultured versus wild populations of juvenile arctic charr (Salvelinus alpinus L.) were investigated. The fish (start weight 17 g) were reared for 5 months at constant temperatures of 12, 14 and 16°C under a continuous light regime (LD24:0). Growth performance was significantly influenced by both temperature and source population. The offspring from the farmed fish displayed significantly higher mean weights at all temperatures compared with the offspring from the wild fish. The results indicate that the optimal temperature for growth (T_opt) decreases with increasing fish size in the offspring of both cultivated and wild fish. Significant differences in length–weight relationship were found, with the offspring of wild fish displaying higher condition factor throughout the experiment. The results demonstrate that the offspring from multiple generation farmed population of arctic charr are better adapted to high temperatures compared with the offspring (F₁ generation) from a wild population of arctic charr, and the former are also able to maintain growth at higher temperatures. There was a significant difference in maturation between the two populations, as the offspring from cultured fish displayed significantly lower level of maturation: 0%, 4% and 2% mature compared with 24%, 40% and 42% in the offspring from wild fish at temperatures of 12, 14 and 16°C respectively. The offspring from farmed strains of arctic charr thus appear to be the most suitable population for aquaculture.