Differential time budgets of two forms of juvenile brook charr in the open-water zone

No significant differences were found in the time budget (time spent in feeding, moving and stationary), attack rate (number of feeding bouts min⁻¹), and microhabitat use of juvenile (1+ years) littoral and pelagic brook charr Salvelinus fontinalis at 2 and 4 m depth, when restricted to feeding in pelagic enclosures. In contrast, fish of the littoral form allocated significantly more time than pelagic ones to feeding, moving and in total activity at 3 m depth. No significant differences were found in attack rate between the two forms at any given depth. Based on the mean for the water column (all depths pooled), however, fish of the littoral form executed a significantly higher attack rate than fish of the pelagic one. In multiple regressions analyses, the best predictors of time allocated to feeding and attack rate were the dummy variable 'form' (littoral individuals spend significantly more time in feeding than pelagic ones), light intensity at the surface (negative) and water temperature (positive), and explained 48 and 55% of these variations, respectively. Time allocated to moving was only explained by water temperature (negative) and explained 43% of the variation. Time in a stationary position was best explained by water temperature (negative) and light intensity at the surface (positive), explaining 52% of the variation. The results of this study support the hypothesis that littoral brook charr spend more energy in foraging than pelagic ones when restricted to feeding in the pelagic habitat, and thus that trophic diversification is adaptive in this species.     

Competitive interactions for foraging microhabitat among introduced brook charr, Salvelinus fontinalis, and native bull charr, S. confluentus, and westslope cutthroat trout, Oncorhynchus clarki lewisi, in a Montana stream

Competitive interactions for foraging microhabitat among introduced brook charr, Salvelinus fontinalis, and native bull charr, S. confluentus, and westslope cutthroat trout, Oncorhynchus clarki lewisi, were studied by species removal experiments in a tributary of the Flathead Lake and River system, northwestern Montana, focusing on brook charr influences on bull charr. When the three species were in sympatry, they interacted with each other, forming a size-structured, mixed-species dominance hierarchy in two stream pools. The influences of interference interactions were examined by measuring changes in five characteristics of foraging microhabitat and behavior, focal point height and velocity, cover use, and foraging rate and distance, after the successive removal of two species. Cutthroat trout removal resulted in increased foraging rates and distances, and decreased cover use for brook charr, but no changes for bull charr. After removal of brook charr from the two-species system, bull charr also increased foraging rates and distances and occupied more exposed positions. Moreover, total fish densities, which had initially decreased owing to the removal experiments, were partly compensated for by subsequent bull charr immigration, implying that competitive interactions with brook charr are an important factor in the mechanisms responsible for the regulation of bull charr densities, at least on a local scale.     

Phenotypic responses of lacustrine brook charr in relation to the intensity of interspecific competition

We investigated the structure of interindividual variations in the diet of brook charr (Salvelinus fontinalis) based on stomach contents data of 3776 charr captured in 69 lakes of the Canadian Shield (Québec, Canada); 29 of these contained allopatric brook charr populations, 24 contained brook charr and creek chub (Semotilus atromaculatus) and 16 contained brook charr and white sucker (Catostomus commersoni). In any given lake, some of the charr fed almost exclusively on benthic organisms (benthic specialists, i.e., mean percent weight of benthic prey >90%), others, almost exclusively on pelagic prey (pelagic specialists, i.e., mean percent weight of benthic prey <10%), and a lesser proportion were generalist feeders (i.e. mean percent weight of benthic prey between 10 and 90%). The proportion of benthic and pelagic specialists were respectively 41.3 and 18% in allopatric brook charr populations. These proportions fit remarkably well with those based on interindividual preferences in spatial distribution, identified through radio-telemetry in another study done in two lakes of the same area. The proportion of benthic specialists was related to competition for benthic organisms with creek chub and white sucker. The effect of white sucker on brook charr diet was more pronounced than that of creek chub: the proportion of benthic specialists among brook charr decreased from 41.3% in allopatry to 19.7% in sympatry with creek chub, and to 9.9% in sympatry with white sucker. Other response variables of brook charr populations also indicate that white sucker is a stronger competitor than creek chub in this system. Because sucker and chub were introduced in these lakes during the last century, phenotypic responses of brook charr to interspecific competition appear to be rapid. Furthermore, in addition to providing a strong field support to the current hypothesis that polymorphism is promoted by arelaxation of interspecific competition, our results also indicate that phenotypic response of brook charr (i.e. the proportion of each form in a given lake) is related to the intensity of this competition.     

Chemical Alarm Signals Enhance Survival of Brook Charr (Salvelinus fontinalis) During Encounters with Predatory Chain Pickerel (Esox niger)

A diversity of aquatic organisms release chemical alarm signals when attacked or captured by a predator. These alarm signals are thought to warn other conspecifics of danger and, consequently, may benefit receivers by increasing their survival. Here we experimentally investigated the differences in behaviour and survival of hatchery-reared juvenile brook charr Salvelinus fontinalis that had been exposed to either brook charr skin extract (experimental treatment) or a control of swordtail skin extract (control treatment). Charr exposed to conspecific skin extract exhibited a significant reduction in movement and/or altered their foraging behaviour in the laboratory when compared with charr exposed to swordtail skin extract. We also exposed charr to either water conditioned by a single brook charr disturbed by a predatory bird model or water conditioned by a single undisturbed brook charr. Charr exposed to disturbance signals reduced activity significantly more than charr exposed to chemical stimuli from undisturbed charr. These results demonstrate the existence of both damage-released alarm signals and disturbance signals in brook charr. Wild brook charr also responded to damage-released alarm cues under natural conditions. Charr avoided areas of a stream with minnow traps labelled with conspecific alarm cues vs. control cues. During staged encounters with chain pickerel Esox niger in the laboratory, predator-naive charr fry were better able to evade the predator if they were previously warned by an alarm signal, thus suggesting a survival benefit to receivers. Collectively, these results demonstrate that the presence of alarm signals in brook charr has important implications for understanding predator–prey interactions.     

Ecosystem size predicts eco‐morphological variability in a postglacial diversification

Identifying the processes by which new phenotypes and species emerge has been a long‐standing effort in evolutionary biology. Young adaptive radiations provide a model to study patterns of morphological and ecological diversification in environmental context. Here, we use the recent radiation (ca. 12k years old) of the freshwater fish Arctic charr (Salvelinus alpinus) to identify abiotic and biotic environmental factors associated with adaptive morphological variation. Arctic charr are exceptionally diverse, and in postglacial lakes there is strong evidence of repeated parallel evolution of similar morphologies associated with foraging. We measured head depth (a trait reflecting general eco‐morphology and foraging ecology) of 1,091 individuals across 30 lake populations to test whether fish morphological variation was associated with lake bathymetry and/or ecological parameters. Across populations, we found a significant relationship between the variation in head depth of the charr and abiotic environmental characteristics: positively with ecosystem size (i.e., lake volume, surface area, depth) and negatively with the amount of littoral zone. In addition, extremely robust‐headed phenotypes tended to be associated with larger and deeper lakes. We identified no influence of co‐existing biotic community on Arctic charr trophic morphology. This study evidences the role of the extrinsic environment as a facilitator of rapid eco‐morphological diversification.     

How does the presence of a conspecific individual change the behavioral game that a predator plays with its prey?

Behavioral games predators play among themselves may have profound effects on behavioral games predators play with their prey. We studied the behavioral game between predators and prey within the framework of social foraging among predators. We tested how conspecific interactions among predators (little egret) change the predator–prey behavioral game and foraging success. To do so, we examined foraging behavior of egrets alone and in pairs (male and female) in a specially designed aviary consisting of three equally spaced pools with identical initial prey (comet goldfish) densities. Each pool was comprised of a risky microhabitat, rich with food, and a safe microhabitat with no food, forcing the fish to trade off food and safety. When faced with two versus one egret, we found that fish significantly reduced activity in the risky habitat. Egrets in pairs suffered reduced foraging success (negative intraspecific density dependence) and responded to fish behavior and to their conspecific by changing their visiting regime at the different pools—having shorter, more frequent visits. The time egret spent on each visit allowed them to match their long-term capture success rate across the environment to their capture success rate in the pool, which satisfies one aspect of optimality. Overall, egrets in pairs allocated more time for foraging and changed their foraging tactics to focus more on fish under cover and fish ‘peeping’ out from their shelter. These results suggest that both prey and predator show behavioral flexibility and can adjust to changing conditions as needed in this foraging game.     

Size-scaling of zooplankton foraging in Arctic charr

Prey capture rate (number of prey s⁻¹) and the mode of feeding of Arctic charr Salvelinus alpinus were studied by performing foraging experiments with two sizes (1·1 and 1·8 mm) of Daphnia longispina prey. Arctic charr were particulate feeders at all densities tested. Adjusted for the effect of prey density, the capture rate showed a hump-shaped relationship with Arctic charr size for both sizes of D. longispina . Estimated attack rates ( a ) also tended to show a hump-shaped relationship with fish size. The estimated size-scaling exponent of the attack rate function, however, was relatively small, implying small changes in attack rate over fish sizes. Simultaneous estimations of a and handling time were used in combination with published data on fish metabolism and dry mass rations of prey to estimate maintenance resource density of prey as a function of Arctic charr mass. Maintenance resource densities increased monotonically with Arctic charr size, and rapidly as optimum fish size relative to attack rate on prey was passed.     

Genetic Diversity and Hybridisation between Native and Introduced Salmonidae Fishes in a Swedish Alpine Lake

Understanding the processes underlying diversification can aid in formulating appropriate conservation management plans that help maintain the evolutionary potential of taxa, particularly under human-induced activities and climate change. Here we assessed the microsatellite genetic diversity and structure of three salmonid species, two native (Arctic charr, Salvelinus alpinus and brown trout, Salmo trutta) and one introduced (brook charr, Salvelinus fontinalis), from an alpine lake in sub-arctic Sweden, Lake Ånn. The genetic diversity of the three species was similar and sufficiently high from a conservation genetics perspective: corrected total heterozygosity, H’_T = 0.54, 0.66, 0.60 and allelic richness, A_R = 4.93, 5.53 and 5.26 for Arctic charr, brown trout and brook charr, respectively. There were indications of elevated inbreeding coefficients in brown trout (G_IS = 0.144) and brook charr (G_IS = 0.129) although sibling relationships were likely a confounding factor, as a high proportion of siblings were observed in all species within and among sampling locations. Overall genetic structure differed between species, Fst = 0.01, 0.02 and 0.04 in Arctic charr, brown trout and brook charr respectively, and there was differentiation at only a few specific locations. There was clear evidence of hybridisation between the native Arctic charr and the introduced brook charr, with 6% of individuals being hybrids, all of which were sampled in tributary streams. The ecological and evolutionary consequences of the observed hybridisation are priorities for further research and the conservation of the evolutionary potential of native salmonid species.     

Water temperature, light intensity and zooplankton density and the feeding activity of juvenile brook charr (Salvelinus fontinalis)

SUMMARY 1. The objective of this study was to evaluate the effects of zooplankton biomass (as a measure of density), fish biomass, light intensity and water temperature on the attack rate and swimming characteristics (i.e. swimming speed and angle of turn) of juvenile (1+) brook charr (Salvelinus fontinalis) in field enclosures. We used a portable underwater camera system in a series of pelagic enclosures to quantify the feeding behaviour of brook charr over a gradient of natural conditions. 2. In simple linear or non‐linear regression models we found (i) that attack rate and angle of turn were positively related to water temperature, (ii) that attack rate and swimming speed were positively related to zooplankton biomass and light intensity and (iii) that attack rate was positively related to swimming speed. In multiple regression models, fish biomass, light intensity and variance of the angle of turn accounted for 87% of the variation in attack rate. Light intensity and water temperature accounted for 86% of the variation in swimming speed. Fish gut fullness and attack rate accounted for 83% of the variation in the variance of the angle of turn executed by fish. 3. The increase in the number of attacks as zooplankton biomass increases conforms to the general positive functional response observed in other fish species. Our results also support the hypothesis that swimming speed increases with prey biomass. We did not observe a plateau in attack rate as zooplankton biomass increased. As our experiments were performed under natural biotic and abiotic conditions, factors other than zooplankton biomass might affect or limit this response, such as water temperature and light intensity. 4. Because zooplankton biomass was correlated with water temperature and light intensity, it was not possible to evaluate the independent contribution of these factors on the attack rate and swimming characteristics (swimming speeds and angle of turn) of brook charr. However, this study highlighted the impact of these factors on the feeding behaviour of juvenile brook charr when feeding in the pelagic habitat under natural conditions, and their importance in future models of optimal foraging and fish habitat quality.     

Seawater acclimation of diploid and triploid brook charr (Salvelinus fontinalis), diploid Arctic charr (Salvelinus alpinus), and their diploid and triploid hybrids

Seawater acclimation of diploid (FF) and triploid (F2F) brook charr Salvelinus fontinalis , diploid (AA) Arctic charr Salvelinus alpinus , and diploid (FA) and triploid (F2A) hybrids between female brook charr and male Arctic charr was investigated. Triploidization of brook charr and the hybrid did not have any effect on the acclimation. Seawater acclimation of the hybrid was achieved during the experimental period and was comparable to that observed in brook charr. Acclimation could not be ascertained in Arctic charr since the level of cortisol, a stress indicator, was still high at the end of the experiment. No relationship between either length or condition factors and plasma osmolality was observed. Elevated plasma cortisol concentrations in Arctic charr and in diploid or triploid hybrids, both in fresh water and sea water, indicate more favourable rearing conditions for brook charr.     

Trophic polymorphism in brook charr revealed by diet, parasites and morphometrics

Stomach contents, parasite assemblages and morphometrics were compared in brook charr Salvelinus fontinalis from the littoral and pelagic zone of two adjacent lakes on the Canadian Shield. In lac Baie des Onze Îles, fish from the littoral zone had greater abundance of benthic prey in their stomach and were more heavily infected by parasites that use intermediate hosts associated with the littoral zone than fish captured in the pelagic zone. Littoral and pelagic brook charr from this lake also differed in regard to body shape and fin length, with each group being anatomically adapted to exploit their respective habitats. The highly significant correlation between morphometric and parasite canonical scores supports the hypothesis of functional diversification of individuals within lac Baie des Onze Îles. While fish from littoral and pelagic zones of lac Caribou did not differ in terms of diet, parasite assemblages or morphometrics, they were different to fish from lac Baie des Onze Îles in that they were less frequently infected with parasites that use gastropods as intermediate hosts, and had shorter pectoral fins. The inter-lake comparisons suggested that parasite assemblages and morphometrics of brook charr reflected the dominance of the limnetic and littoral habitats in lacs Caribou and lac Baie des Onze Îles, respectively.     

More than a corridor: use of a main stem stream as supplemental foraging habitat by a brook trout metapopulation

Coldwater fishes in streams, such as brook trout (Salvelinus fontinalis), typically are headwater specialists that occasionally expand distributions downstream to larger water bodies. It is unclear, however, whether larger streams function simply as dispersal corridors connecting headwater subpopulations, or as critical foraging habitat needed to sustain large mobile brook trout. Stable isotopes (δ¹³C and δ¹⁵N) and a hierarchical Bayesian mixing model analysis was used to identify brook trout that foraged in main stem versus headwater streams of the Shavers Fork watershed, West Virginia. Headwater subpopulations were composed of headwater and to a lesser extent main stem foraging individuals. However, there was a strong relationship between brook trout size and main stem prey contributions. The average brook trout foraging on headwater prey were limited to 126 mm standard length. This size was identified by mixing models as a point where productivity support switched from headwater to main stem dependency. These results, similar to other studies conducted in this watershed, support the hypothesis that productive main stem habitat maintain large brook trout and potentially facilitates dispersal among headwater subpopulations. Consequently, loss of supplementary main stem foraging habitats may explain loss of large, mobile fish and subsequent isolation of headwater subpopulations in other central Appalachian watersheds.     

Why do Foraging Stream Salmonids Move During Summer?

We hypothesize that foraging stream salmonids move during summer because (1) they monitor habitat conditions at a reach scale (100s of m), and (2) dominant fish move when conditions in their present foraging location become sub-optimal relative to conditions at other locations in the reach. To test these ideas, we quantified temporal variation in foraging habitat quality between late spring and early fall in a reach of a small Rocky Mountain brook charr, Salvelinus fontinalis, stream, predicted optimal-foraging fish distributions within the reach, and experimentally manipulated access to foraging sites and measured fish responses. Our results show that high-quality foraging sites were located at certain places in the reach during one period, but at different places during others, consistent with the hypothesis that fish movement is required if dominant fish are to occupy high-quality foraging sites throughout summer. The optimal foraging model was able to predict foraging locations within study pools, but not the exact location of individual fish within the pools or the reach. However, empirical evidence suggests that fish were distributed in order to maximize energy intake at the reach scale. Finally, dominant fish excluded from their preferred foraging location either left the pools (three of six cases), or began to occupy focal points of the next largest fish which, in turn, exited the pool (two of six cases). If habitat selection was occurring only within habitat units, then large fish, when excluded from their preferred locations, would select the next best locations within the pool. Taken together, these results suggest that charr use summertime movements to both monitor habitat conditions at a large spatial scale, and to gain access to optimal foraging locations even as conditions change temporally.     

Diel and density-related changes in food consumption and prey selection by brook charr in a New Hampshire stream

Synopsis We report the results of a field study testing influences of both density and changes over the diel cycle on food consumption and prey selection by brook charr,Salvelinus fontinalis. Charr density in replicate 35 m long sections of a New Hampshire stream was adjusted to either medium or high levels (relative to natural densities). Diets of charr and the availability of drifting prey were then sampled every four hours for 24 hours. There were no significant diel changes in the weight of prey consumed by charr per four hours, though there was some indication of reduced feeding at night. Chary fed selectively on different prey taxa, showing most preference for cased caddis larvae. Several species of mayflies and stoneflies were selected more strongly during the day than at night. Charr fed selectively on larger prey during the daytime but showed no size-selection at night. The density of charr had no significant effects on either their rate of food consumption or on selection for prey of different taxa or sizes.     

Pyloric caecal morphology of brook charr,Salvelinus fontinalis,in relation to diet

Synopsis Brook charr, Salvelinus fontinalis, shifts its diet from zoobenthos to pelagic prey when living sympatrically with white sucker, Catostomus commersoni, in lakes of the Laurentian Shield. We tested the hypothesis that this diet difference would have a significant impact on their pyloric caecal morphology in 5 lakes containing allopatric brook charr populations and 6 other lakes containing both brook charr and white sucker. We observed that the mean length of the most posterior caecum of charr was significantly greater in sympatry than in allopatry (X ± 1 SD: 9.91 ± 1.12 mm versus 8.44 ± 0.67 mm). This is equivalent to an increase of 18% of total pyloric caecal mass (dry weight) in sympatric brook charr. These results indicate that this response to differences in diet, well known in birds, also occurs in fish.     

Morphological and behavioural differences among recently-emerged brook charr,Salvelinus fontinalis,foraging in slow- vs. fast-running water

Synopsis Morphology and resource use were compared among recently-emerged brook charr,Salvelinus fontinalis, sampled from field locations differing in current speed. Individuals from faster running water were slightly longer, and had more fusiform body shapes and larger caudal fin heights, than individuals from slower running water. In addition, individuals from faster running water also directed more foraging attempts toward the middle of the water column and fewer toward the benthos and water surface. They also ate more dipteran larvae, fewer aquatic crustaceans, and fewer insect pupae and adults. Individuals located in the slowest and fastest current speeds made fewer foraging attempts per min, on average, than individuals located in current speeds of intermediate magnitude. Dry weight of stomach contents did not vary significantly with current speed, however. The form of the relationship between body shape and current speed suggests that it is adaptive. Small-scale variation in the location of foraging sites may account for some of the individual variability in resource use often reported for stream salmonids. Variation in the locations of foraging sites may also entail a trade-off between an individual's swimming effort and the quality of prey it consumes.     

Fluctuations in prey density: effects on the foraging tactics of scolopendrid centipedes

Foraging animals are faced with the problem of acquiring information about prey populations and utilizing that information in making foraging decisions. In this paper the effect of variation in prey density on the search tactics and mechanisms of prey density assessment in the centipede Scolopendra polymorpha are examined. Centipedes exhibited a prey density-dependent repertoire of search tactics. After 50 min of exposure to high prey density, centipedes switched from active search to ambush-like tactics, while maintaining a high rate of search at the lowest prey density. An initial period of sampling of prey density was involved in the switch in search behaviour and it is suggested that the encounter rate with prey was the key element in density assessment. When the prey density was changed from low to high, centipedes switched from active search to ambush tactics and when prey density was changed from high to low centipedes switched from ambush to active search within 40 min. Such behaviour may decrease the unreliability of sampling information and the risk involved in foraging decisions in variable environments.     

Quantitative field investigations of feeding and territorial behaviour of young-of-the-year brook charr,Salvelinus fontinalis

Synopsis Direct observations of young-of-the-year brook charr, Salvelinus fontinalis, in a second-order woodland stream indicated that most of their feeding effort was directed toward sub-surface, drifting prey (83% of feeding time). Feeding from the substrate and water surface was much less frequent (17% of feeding time). Comparisons of gut contents to drift net and substrate fauna samples corroborated that the most commonly consumed prey (chironomid and trichopteran larvae, ostracods, and ephemeropteran nymphs) were captured primarily from sub-surface, invertebrate drift. The disproportionate numbers of some prey species in the guts of several fish indicate that some prey selection occurred. Territories appeared to be cardioid-shaped, and were often contiguous, with dominance hierarchies evident among the residents. Agonistic interactions were frequent. Charges and chases predominated (91% of interactions) while lateral displays were infrequent (9% of interactions). Overall, these fish spent most of the daylight hours station-holding (77%) and feeding (18%). While only 3% of total time was spent in aggression, this amounted to 14% of the time a fish spent away from its station. There was some indication that territories were defended at a cost of feeding time.     

Attraction of slimy sculpins to chemical cues of brook charr eggs

In a laboratory study to examine the responses of slimy sculpin Cottus cognatus to chemical cues of brook charr Salvelinus fontinalis eggs, water that held freshly laid eggs (<20 min old), water-hardened eggs (i.e. eggs >4 h old) or injured eggs was collected and used in a series of two-choice tests. Slimy sculpin were exposed to paired stimuli of (1) hard egg water v . control water, (2) fresh egg water v . control water, (3) hard egg water v . injured egg water or (4) hard egg water v . fresh egg water. Sculpin spent considerably more time on the side of the tank with hard egg water and fresh egg water v . control water and injured egg water v . hard egg water. Sculpin did not show a preference for hard egg water v . fresh egg water. In a field study, brook charr were attracted to chemical cues from brook charr eggs, suggesting that brook charr eggs produce sufficient odour to attract some species under natural conditions.     

Variation in predation risk and vole feeding behaviour: a field test of the risk allocation hypothesis

Many prey animals experience temporal variation in the risk of predation and therefore face the problem of allocating their time between antipredator efforts and other activities like feeding and breeding. We investigated time allocation of prey animals that balanced predation risk and feeding opportunities. The predation risk allocation hypothesis predicts that animals should forage more in low- than in high-risk situations and that this difference should increase with an increasing attack ratio (i.e. difference between low- and high-risk situations) and proportion of time spent at high risk. To test these predictions we conducted a field test using bank voles (Clethrionomys glareolus) as a prey and the least weasel (Mustela nivalis nivalis) as a predator. The temporal pattern and intensity of predation risk were manipulated in large outdoor enclosures and the foraging effort and patch use of voles were measured by recording giving-up densities. We did not observe any variation in feeding effort due to changes in the level of risk or the proportion of time spent under high-risk conditions. The only significant effect was found when the attack ratio was altered: the foraging effort of voles was higher in the treatment with a low attack ratio than in the treatment with a high attack ratio. Thus the results did not support the predation risk allocation hypothesis and we question the applicability of the hypothesis to our study system. We argue that the deviation between the observed pattern of feeding behaviour of bank voles and that predicted by the predation risk allocation hypothesis was mostly due to the inability of voles to accurately assess the changes in the level of risk. However, we also emphasise the difficulties of testing hypotheses under outdoor conditions and with mammals capable of flexible behavioural patterns.