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.     

Local Movement as a Measure of Habitat Quality in Stream Salmonids

Habitat assessments are often based on the premise that spatial variation in population density arises from, and accurately reflects, underlying differences in quality among habitats. Nonetheless, this premise has been criticized on both theoretical and empirical grounds. Habitat quality perhaps is best evaluated by examining behavioural processes which directly influence habitat use. We present an approach based on the assumption that measures of local movement, such as habitat-specific immigration and loss rates, provide useful indicators of habitat quality. A dynamic turnover model was used in conjunction with mark-recapture techniques to estimate movement parameters for brook charr, Salvelinus fontinalis, and Atlantic salmon, Salmo salar, in different stream habitats during the summer. Immigration and loss rates were derived from mark-recapture experiments covering short periods of time (6 days). Movement-based rankings of habitat quality for both charr (pools glides > riffles) and salmon (riffles > glides > pools) were in agreement with results from earlier studies. Over evaluation periods of up to 65 days, observed abundances were highly variable in time and fluctuated about the equilibrium abundances calculated from movement parameters in the short-term experiments, suggesting that movement-based parameters may be more stable than measures of abundance for evaluating salmonid habitat. Because the approach based on movement behaviour focuses on immigration and loss, two processes that directly generate density differentials between habitats, it provides a more reliable mechanistic basis for understanding habitat selection than do traditional approaches based on density–quality relationships.     

Density‐dependent habitat selection predicts fitness and abundance in a small lizard

Density‐dependent habitat selection has been used to predict and explain patterns of abundance of species between habitats. Thermal quality, a density‐independent component of habitat suitability, is often the most important factor for habitat selection in ectotherms which comprise the vast majority of animal species. Ectotherms may reach high densities such that individual fitness is reduced in a habitat due to increased competition for finite resources. Therefore, density and thermal quality may present conflicting information about which habitat will provide the highest fitness reward and ectotherm habitat selection may be density‐independent. Using ornate tree lizards Urosaurus ornatus at 10 sites each straddling two adjacent habitats (wash and upland), we tested the hypothesis that habitat selection is density‐dependent even when thermal quality differs between habitats. We first tested that fitness proxies decline with density in each habitat, indicating density‐dependent effects on habitat suitability. We also confirmed that the two habitats vary in suitability (quantified by food abundance and thermal quality). Next, we tested the predictions that habitat selection depends on density with isodar analyses and that fitness proxies are equal in the two habitats within a site. We found that monthly survival rates decreased with density, and that the wash habitat had more prey and higher thermal quality than the upland habitat. Lizards preferred the habitat with more food and higher thermal quality, lizard densities in the two habitats were positively correlated, and fitness proxies of lizards did not differ between habitats. These patterns are consistent with density‐dependent habitat selection, despite differences in thermal quality between habitats. We expect that density‐dependent habitat selection is widespread in terrestrial ectotherms when densities are high and temperatures are close to their optimal performance range. In areas where thermal quality is low, however, we expect that depletable resources, such as food, become less limiting because assimilating resources is more difficult.     

The effects of depth and salinity on juvenile Chinook salmon Oncorhynchus tshawytscha (Walbaum) habitat choice in an artificial estuary

The energetic cost for juvenile Chinook salmon Oncorhynchus tshawytscha to forage in habitats of different salinity and depth was quantified using a behavioural titration based on ideal free distribution theory. When given a choice between freshwater habitats of different depths (>0·83 or <0·83 m), a greater proportion of fish used the deeper habitat. When the deeper habitat was saltwater, the proportion of fish using it increased. When food was added to both the shallow freshwater and deep saline habitats, however, fish distribution returned to that observed when both habitats were fresh water. This indicates that the preference for deep saline habitats during the stratified phase was driven by some benefit associated with residency in deeper water, rather than salinity. The low perceived cost of low salinity might be in part due to the fish's ability to minimize this cost by only making brief forays into the alternate freshwater habitat. When the food ration delivered to the more costly, shallow habitat was 50% greater than that delivered to the less costly, deep habitat, fish distributed themselves equally between the two habitats, presumably because of equal net benefits. This study demonstrates that juvenile Chinook salmon prefer deep saline habitat to shallow freshwater habitats but will make brief forays into the freshwater habitat if food availability is sufficiently high.     

Habitat use of arctic charrSalvelinus alpinus in Thingvallavatn,Iceland

Synopsis Habitat use by four morphs of arctic charr,Salvelinus alpinus, was investigated in Thingvallavatn, Iceland, by sampling with pelagic and benthic gill nets. Sampling was done in May/June and August/September. Greatest abundance of fish was recorded in the littoral and epipelagic zone in early autumn. Catches were low in early summer. The four morphs are partly segregated in habitat. Small (SB-) and large benthivorous (LB-) charr have a more restricted spatial distribution than piscivorous (PI-), and especially planktivorous (PL-) charr. Both benthivorous morphs are mainly found in the littoral zone, and occur in largest numbers in stony shallows at depths between 0 and 10 m. PL-charr usually dominates in numbers in all habitats. PI-charr is most abundant in epibenthic habitats, although numbers are always low. All morphs are caught in higher numbers at night than during the day, but the diurnal activity difference is highest among SB-charr. The habitat use by different morphs is as may be expected from their morphology and diets. Within the population of PL-charr, young and small fish are more abundant on the bottom than in the pelagic zone, and there is a surplus of females in the pelagic zone. Along the benthic profile, young, small and immature PL-charr are more abundant in deep than in shallow waters. The results are discussed in relation to food supply, competition and predation. Possible reasons for the occurrence of four arctic charr morphs are also discussed.Contribution from the Thingvallavatn project.     

新疆准噶尔盆地东部波斑鸨秋季栖息地选择

2000年9月,利用粪便示踪和野外直接观察法对准噶尔盆地东部波斑鸨（Chlamydotis undulata)秋季栖息地进行了采样调查。结果表明：波斑鸨秋季栖息地地热平坦,视野开阔,广泛分布旱生和盐生荒膜植物,植被稀疏、低矮,并镶嵌有相对较高的灌丛块;影响波斑鸨秋季栖息地选择的环境因子是植被密度、物种丰富度和物候;秋季栖息地内的植被密度、草木植物种数、叉毛蓬密度、结实植物密度和物种数都极显著高于对照地内的相应成分;生长植物密度显著高于对照地;而假木贼密度和琵琶柴密度显著低于对照地。     

The functional significance of inherited differences in feeding morphology in a sympatric polymorphic population of Arctic charr

Artificially fertilised eggs from wild-caught Arctic charr parents of two sympatric morphs (benthivorous and planktivorous) from Loch Rannoch, Scotland were reared in the laboratory under identical conditions. During the subsequent 2 years, aspects of their trophic anatomy and feeding behaviour were compared. As previously described for wild-caught fish, charr derived from the benthivorous morph had an increasingly wider mouth gape for a given body length than those derived from the planktivorous morph. The functional significance of these differences in gape was tested by comparing the maximum size of prey that could be handled by each of the two morphs. In both forms, a larger gape enabled larger food particles to be eaten, but the elevation of the regression of maximum prey size on gape was higher in the benthivorous form, indicating the existence of additional morphological and/or behavioural differences influencing the size of prey consumed. When offered a choice between a typical benthic prey item and a typical pelagic food item, charr of benthivorous origin were more likely to feed on the former, whereas those of planktivorous origin were more likely to feed on the latter. Thus inherited differences in gape place constraints on foraging ability and are associated with inherited differences in dietary preference. We conclude that the functional significance of the foraging specialisations indicate a strong selection pressure for the evolution of the divergence and propose that heterochronic growth is the mechanism resulting in the divergence of tropic anatomy.     

Effects of Fish-farm Activity on the Limnetic Community Structure of Brown Trout, Salmo trutta, and Arctic Charr, Salvelinus alpinus

Establishment of four fish-farms during the period 1971 to 1994 in the oligotrophic lake Skogseidvatnet affected Arctic charr, Salvelinus alpinus, but not brown trout, Salmo trutta. From 1971 to 1987, an increase in mean individual size of Arctic charr was recorded, while the mean individual size of brown trout remained stable. Arctic charr were found to use deeper benthic areas than brown trout. Approximately 8% of the Arctic charr population (>26cm), were found to switch to waste food from fish-farms, resulting in a novel feeding habitat for the species. They were, however, found in gillnets distant from the fish farm cages, indicating high mobility. The habitat segregation between the two species can most likely be explained by selective differences and asymmetric competition with brown trout as the dominant species. Based on the present results, changes in the Arctic charr population may be due to increased food availability and due to a new habitat use as a waste food feeder. The reason for the brown trout population to have remained stable with respect to mean size, growth pattern and habitat use, may be due to a different diet choice than Arctic charr in this lake. Brown trout were found to feed mainly on terrestrial insects, while Arctic charr fed mainly on zooplankton and on waste food.     

Local habitat complexity correlates with song complexity in a vocally elaborate honeyeater

Song complexity is an important behavioural trait in songbirds, subject to sexual selection. Elucidation of intraspecific variation in song complexity can provide insights into its evolution. In this study, we investigated song complexity variation in tūī (Prosthemadera novaeseelandiae), a vocally complex songbird endemic to New Zealand. At two separate nature reserves, we recorded male songs in two habitat types: forest remnants with high habitat complexity, and open habitats with lower habitat complexity. Analyses indicated strong evidence that song complexity was higher in forest habitats. Possible explanations for this divergence include: (i) competition between individuals results in higher quality, dominant males with more complex songs occupying forest habitats, and less competitive males occupying open habitat zones; (ii) forest habitats provide more abundant resources therefore higher tūī density, resulting in more complex songs; and (iii) a higher abundance of food in dense forest habitats may reduce nutritional stress during development resulting in full development of song nuclei. However, these hypotheses on the drivers of habitat effects on tūī song complexity remain to be tested.     

Asymmetric competition drives lake use of coexisting salmonids

To what degree are population differences in resource use caused by competition and the occupation of adjacent positions along environmental gradients evidence of competition? Habitat use may be the result of a competitive lottery, or restricted by competition. We tested to what extent population differences in habitat use of two salmonids, cutthroat trout (Oncorhynchus clarki) and Dolly Varden charr (Salvelinus malma) were influenced by interspecific competition. We hypothesized that the depth distribution of Dolly Varden charr would be affected by competition from the more littoral and surface-oriented cutthroat trout, and that the depth distribution of cutthroat trout would be little affected by competition from Dolly Varden charr. Sympatric populations of cutthroat trout and Dolly Varden charr were created by reciprocal transfers of previously allopatric populations in two experimental lakes. We found evidence of asymmetric competition, as Dolly Varden charr were displaced from littoral habitats when sympatric with cutthroat trout, whereas cutthroat trout remained unaffected by the presence of Dolly Varden charr. Evolved differences between the species, and differences between experimental lakes, also contributed to population differences in habitat use, but asymmetric competition remained as the main driver of different depth distributions in sympatry.     

Lean birds in the city: body size and condition of house sparrows along the urbanization gradient

1. Urbanized habitats differ from natural ones in several ecological features, including climate, food availability, strength of predation and competition. Although the effects of urbanization on avian community composition are well known, there is much less information about how individual birds are affected by these human-generated habitat differences. 2. In this study we investigated the relationships between the morphological characteristics and the degree of habitat urbanization in house sparrows, Passer domesticus (Linne 1758) . We collected data for more than 1000 non-breeding adult birds in Hungary between 1997 and 2006, from seven sites including farmlands, suburban areas and city centres. 3. We found that the body mass, tarsus length and body condition of free-living sparrows differed among the sites: birds in more urbanized habitats were consistently smaller and in worse condition than birds in more rural habitats. A composite measure of habitat urbanization (based on building density, road density and vegetation cover) explained over 75% of variance between sites in the studied traits, after we controlled for the effects of sex, year, season and time of capture. 4. The difference in body mass between rural and urban sparrows was significant when birds were kept in aviaries under identical conditions, with constant ad libitum food availability. It is therefore unlikely that the reduced body size and condition of urban sparrows are a consequence of reduced access to food for adults (e.g. due to strong competition), or their short-term responses to high food predictability (e.g. by strategic mass regulation). 5. We suggest that habitat differences in nestling development or adaptive divergence of sparrow populations due to distinct environmental conditions (such as differing predation pressure) may account for the differences along the urbanization gradient.     

Temporal stability of individual feeding specialization may promote speciation

1.  Inter-individual differences in trophic behaviour are considered important in the disruptive selection process for resource specialization and may represent an early phase in the evolution of polymorphism and adaptive radiation. Here, we provide evidence of high stability of individual trophic niches of a fish predator from a 15-year study.
2.  Individual resource specialization was investigated by combining data from analyses of stomach contents (recent trophic niche), trophically transmitted parasites (long-term niche) and trophic morphology (niche adaptations) from single specimens of a postglacial fish (Arctic charr) population sampled from contrasting pelagic and littoral habitats.
3.  Based on the relationships between morphology, parasites and diet, high inter-individual temporal consistency of narrow niches (zooplanktivorous vs . benthivorous) was evident through the ontogeny of the charr, indicating low degree of switching both in habitat utilization and feeding strategy of individual fish. Co-occurrence of differently specialized behavioural phenotypes was sustained over multiple generations.
4.  The stable long-term habitat and feeding specializations may represent an important initial step in an adaptive radiation process, and our findings suggest a case of sympatric speciation into two incipient forms diverging along the littoral–pelagic resource axis.     

Interactive effects of habitat selection, food supply and predation on recruitment of an estuarine fish

 Seagrass meadows are often important habitats for newly recruited juvenile fishes. Although substantial effort has gone into documenting patterns of association of fishes with attributes of seagrass beds, experimental investigations of why fish use seagrass habitats are rare. We performed two short-term manipulative field experiments to test (1) the effects of food supply on growth and densities of fish, and (2) effects of predation on the density and size distribution of fish recruits, and how this varies among habitat types. Experiments were conducted in Galveston Bay, Texas, and we focused on the common estuarine fish, pinfish Lagodon rhomboides. In the first experiment, replicate artifical seagrass and sand plots were either supplemented with food or left as controls. Recruitment of pinfish was significantly greater to seagrass than sand habitats; however, we detected no effect of food supplementation on the abundance of recruits in either habitat. Pinfish recruits in artifical seagrass grew at a significantly faster rate than those in sand habitats, and fish supplemented with food exhibited a greater growth rate than controls in both sand and artifical grass habitats. In our second experiment, we provided artificial seagrass and sand habitats with and without predator access. Predator access was manipulated with cages, and two-sided cages served as controls. Recruitment was significantly greater to the cage versus cage-control treatment, and this effect did not vary between habitats. In addition, the standard length of pinfish recruits was significantly larger in the predator access than in the predator exclusion treatment, suggesting size-selective predation on smaller settlers or density-dependent growth. Our results indicate that the impact of predation on pinfish recruits is equivalent in both sand and vegetated habitats, and thus differential predation does not explain the higher recruitment of pinfish to vegetated than to nonvegetated habitats. Since predators may disproportionately affect smaller fish, and a limited food resource appears to be more effectively utilized by fish in vegetated than in unvegetated habitats, we hypothesize that pinfish recruits may select vegetated habitats because high growth rates allow them to achieve a size that is relatively safe from predation more quickly. Received: 10 October 1996 / Accepted: 5 April 1997     

Lipid‐rich zooplankton subsidise the winter diet of benthivorous Arctic charr (Salvelinus alpinus) in a subarctic lake

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Effect of reward level on individual variability in demand feeding activity and growth rate in Arctic charr and rainbow trout

Groups of Arctic charr and rainbow trout were fed by using demand feeders and their individual trigger actuations registered with a PIT-tag (Passive Integrated Transponder) system. Food was supplied at two reward levels, low and high, to five replicate groups of each species for 21 to 27 days. The reward level was defined as the amount of food obtained in response to a single trigger actuation. The effects of reward level on individual demand feeding activity and growth rale were assessed.
As a result of high demand feeding activity, the daily food rations for trout were in excess of their needs at both reward levels. This can be ascribed to the fact that they compensated a low reward level by increasing their bite activity. In contrast, demand feeding activity in charr did not differ significantly between reward levels. Instead, resulting food rations were limiting and excessive, at low and high reward levels, respectively. The variation in bite activity between individuals (measured as their proportional contribution to the total number of trigger actuations within a group) for charr was significantly higher in the low-reward treatment than in the high-reward level. For trout, the variation in bite activity did not differ significantly between treatments. Differences in response to reward level are suggested to be due to the fact that the social hierarchy is weaker in trout than in charr; i.e. the differences in bite activity between dominant and non-dominant individuals are smaller in trout. At both reward levels, the benefit of being dominant, measured in terms of growth rate was significant for charr but non-significant for trout.     

The effect of stocking density on the behaviour of Arctic charr (Salvelinus alpinus L.)

Recent studies have demonstrated that Arctic charr ( Salvelinus alpinus ) exhibit higher growth rates when held at high stocking densities. It has been argued that these increased growth rates are a result of decreased social interactions at the increased densities. The current study was designed to test this hypothesis by examining: (1) the frequency of agonistic interactions, (2) the proportion of time spent swimming and (3) growth rates among groups of young of the year Arctic charr held at different stocking densities. It was found that charr reared at higher densities exhibited significantly higher mean weights and lengths than those at the lower densities. The behavioural observations revealed that charr initiated significantly fewer agonistic interactions and spent a significantly greater amount of time shoaling at high densities than charr held at the medium or low densities. It is argued that these behavioural responses may serve to lower energy expenditures, resulting in the increased growth rates.     

Vegetation quality and habitat selection by European haresLepus europaeus in a pastural landscape

European haresLepus europaeus Pallas, 1778 have lower population densities and body condition in pastural landscapes than in arable landscapes, but reasons for this are not understood. The aim of this study was to determine whether forage quality is low in pastural landscapes during certain seasons. We carried out chemical analysis of the nutritional quality of 5 habitat types to determine whether hares select high quality habitats, and whether nutritional quality explains seasonal differences in range sizes of hares in pastural landscapes. Hares did not tend to select habitats of high nutritional quality (protein, fat or energy) over those of lower quality. Hares did not increase active range size as the overall energy content of forage at the study site decreased; seasonal differences in active range size were not explained by nutritional quality. Differences may be explained by behavioural changes related to breeding. Pastural habitat is fairly stable in terms of nutritional quality through the year, and results suggest that poor forage quality is unlikely to be responsible for the poor body condition of hares in pastural landscapes. Hares in these landscapes are more likely to be limited by habitat quality in terms of cover than by forage.     

Habitat-specific estimates of competition in stream salmonids: A field test of the isodar model of habitat selection

Summary The population densities of sympatric Atlantic salmon,Salmo salar and brook charr,Salvelinus fontinalis, were measured in riffle and pool stream habitats to test whether non-linear isodars, a multispecific model of habitat selection based on ideal distribution assumptions, could (1) predict the distribution of densities between habitats and (2) reproduce the processes postulated to underlie spatial segregation and species interactions in previous laboratory and field studies. The model provided a good fit to observed density patterns and indicated that habitat suitability declined non-linearly with increased heterospecific competitor densities. Competitive effects in riffles appeared to be due to exploitative resource use, with salmon always emerging as the superior competitor. No evidence was found for interference competition in riffles. In contrast, interspecific competition in pools seemed to occur through exploitation and interference. The specific identity of the superior competitor in pools depended on the density of both species; pools provided the charr with refuge from competition with the salmon, presumably through the adoption by the charr of density-dependent behaviours, such as schooling and group foraging, that mitigated the negative impact of the salmon. Charr were displaced from the riffles toward the pools as the total salmon density increased. The isodar analysis, based on limited density data, successfully reproduced the processes suggested to underlie spatial segregation in previous field and laboratory studies and provided new insights into how changes in competitor densities modify habitat suitability in this system.     

Occurrence of deformed fish and their fitness-related traits in Kirikuchi charr, Salvelinus leucomaenis japonicus, the southernmost population of the genus Salvelinus

Kirikuchi charr, Salvelinus leucomaenis japonicus, is the southernmost population of the genus Salvelinus. It is endemic to the Kii Peninsula, central Honshu Island, Japan. As a consequence of anthropogenic disturbances, a few populations of Kirikuchi charr with low genetic diversity now survive only in small, isolated habitats. This study investigated the occurrence of deformed individuals and assessed differences between deformed and nondeformed fish in fitness-related traits, i.e., body size, body condition, growth rate, reproductive traits, survival rate, and habitat use, for two small isolated populations of Kirikuchi charr in the upper drainage of the Totsu River system of the Kii Peninsula. The two populations contained deformed fish in the respective proportions of 8.0-17.4% and 3.0-5.8% between 2003 and 2005. Annual survival rates of deformed fish were approximately half those of nondeformed fish. Other traits were not significantly different between deformed and nondeformed fish. These results indicate that the occurrence of deformities is an important ecological indicator that reflects the decline in fitness of small, isolated populations.     

Using stable isotopes to confirm the trophic ecology of Arctic charr morphotypes from Lake Hazen, Nunavut, Canada

Stable isotopes of carbon and nitrogen were used to examine differences in the feeding ecology of sympatric morphotypes of Arctic charr Sahelinus alpinus from Lake Hazen, Ellesmere Island, in the Canadian High Arctic. Large and small morphotypes possessed significantly different carbon and nitrogen signatures with large-form Arctic charr being more depleted in ¹³C and more enriched in ¹⁵N than the small-form. Isotope and stomach content analyses yielded consistent results and indicated short- and long-term reliance on fish as a food for large Arctic charr. Large-form individuals predate on juveniles but do not predate on small-form individuals ≥ 250 mm. The observed cannibalism by large-form individuals, therefore, does not act to maintain the bimodal length-frequency distribution in Lake Hazen. Bimodality is argued to arise for ecological reasons connected with differing habitat use by the morphotypes and the associated differences in resource consumption opportunities.