Multiple central-place territories in wild young-of-the-year Atlantic salmon Salmo salar

1. The space use of central-place foragers, animals that forage from and return to a single central place such as a nest, burrow or sleeping site, is well documented. Limited data, however, exist for multiple central-place foragers that alternate among several central places. 2. The conventional view of stream-dwelling salmonids suggests that they conform to the central-place territorial model (CPTM) by (i) attacking prey and intruders from one primary foraging station, and defending (ii) small (iii) exclusive areas that (iv) increase with body size. 3. Recent studies suggest greater variability in salmonid space-use than would be expected by the CPTM, but tend to focus on the time allocated towards different activities rather than their distribution in space, especially for young-of-the-year (YOY) fish that are hard to tag and monitor in the wild. 4. In this study, the validity of CPTM was tested by mapping the daily space use of 50 YOY Atlantic salmon in a natural stream via repeated observations of tagged fish in diverse habitats, and by comparing these to earlier estimates of territory use in YOY salmonids. 5. The 50 YOY Atlantic salmon were multiple central-place foragers. All fish visited more than one foraging station (median = 12.5 stations), visited most stations (68.5%) repeatedly, showed limited fidelity to a particular station and typically attacked prey only while holding a position at a station. 6. The multiple central-place territories of the 50 fish were large (mean = 0.932 m(2)) compared to earlier territory size estimates (mean = 0.107 m(2)) for salmonids of similar size and, surprisingly, did not increase with body size. Focal fish attacked intruders from similar distances as reported earlier for much smaller territories, suggesting that large territories are less exclusively defended at any given time. 7. Overall, this study provides a new view on foraging and territoriality in stream salmonids, and on the small but diverse literature on multiple central-place foragers. Further studies, however, are needed to clarify the evolutionary benefits and population consequences of multiple central-place space-use in mobile animals.     

Prey capture and selection throughout the breeding season in a deep‐diving generalist seabird,the thick‐billed murre

Generalist seabirds forage on a variety of prey items providing the opportunity to monitor diverse aquatic fauna simultaneously. For example, the coupling of prey consumption rates and movement patterns of generalist seabirds might be used to create three‐dimensional prey distribution maps (‘preyscapes’) for multiple prey species in the same region. However, the complex interaction between generalist seabird foraging behaviour and the various prey types clouds the interpretation of such preyscapes, and the mechanisms underlying prey selection need to be understood before such an application can be realized. Central place foraging theory provides a theoretical model for understanding such selectivity by predicting that larger prey items should be 1) selected farther from the colony and 2) for chick‐feeding compared with self‐feeding, but these predictions remain untested on most seabird species. Furthermore, rarely do we know how foraging features such as handling time, capture methods or choice of foraging location varies among prey types. We used three types of animal‐borne biologgers (camera loggers, GPS and depth‐loggers) to examine how a generalist Arctic seabird, the thick‐billed murre Uria lomvia, selects and captures their prey throughout the breeding season. Murres captured small prey at all phases of a dive, including while descending and ascending, but captured large fish mostly while ascending, with considerably longer handling times. Birds captured larger prey and dove deeper during chick‐rearing. As central place foraging theory predicted, birds travelling further also brought bigger prey items for their chick. The location of a dive (distance from colony and distance to shore) best explained which prey type was the most likely to get caught in a dive, and we created a preyscape surrounding our study colony. We discuss how these findings might aid the use of generalist seabirds as bioindicators.     

Short‐term Costs and Benefits of Habitat Complexity for a Territorial Fish

An increase in habitat complexity is thought to decrease visibility and the territory size of visually oriented animals. Hence, the addition of physical structure has been viewed as a useful restoration technique to increase the density of territorial species, particularly in stream fishes. However, a decrease in territory size may have a negative effect on the fitness of individual organisms. We attempted to quantify some of the positive and negative effects of increasing habitat structure on the behaviour and growth rate of wild young‐of‐the‐year (YOY) Atlantic salmon. Fish were exposed to one of two habitat treatments in mesh enclosures in Catamaran Brook, New Brunswick: a gravel substrate (low complexity) or a gravel substrate with boulders added (high complexity). Wild‐caught individuals were tagged, weighed and measured before being stocked at densities of 1·m^?2 for 7‐d trials. While fish from high‐complexity treatments benefited from lower rates of aggression, they also had lower foraging rates and smaller territories compared to those in low‐complexity treatments. Specific growth rate, however, did not differ significantly between treatments. While the addition of structure to a habitat may be beneficial at the population level in terms of an increase in population density, our results suggested that individual fish may pay some short‐term costs in these environments. Further research is needed to evaluate the longer term costs and benefits of adding structure to improve the habitat quality for stream salmonids.     

Visual isolation furthers access to drift‐feeding positions for subordinate juvenile brown trout (Salmo trutta) in dominance hierarchies

Juvenile salmonids are visual predators that primarily feed on drifting invertebrates and compete for suitable feeding positions in swift water. We used an outdoor experimental stream to test the effect of visual isolation on agonistic interactions and habitat use by age‐1 brown trout ( Salmo trutta ) in riffle‐pool sections. We hypothesized that dominant fish defend suitable feeding positions in riffle and that visual obstruction between individuals enhance access to riffle for subordinates. Groups of juveniles, caught in the wild, were stocked in high and low visibility treatments. Visual isolation was manipulated by placing dark plastic ribbons or opaque Plexiglas boards onto the substrate of riffles. As expected, dominant fish held profitable positions in riffle and the proportion of fish in riffle significantly increased in presence of artificial structures. In high visibility treatment, the dominant fish despotically excluded subordinates from the riffle. In low visibility treatment, the proportion of fish that foraged on benthic prey in the pool and the number of major aggressive acts (chase, nip) decreased. Our results support the hypothesis that screening effect of physical structures in the water column loosen resource monopolization in dominance hierarchies of juvenile salmonids.     

Examining the link between dietary specialization and foraging modes of stream‐dwelling brown trout Salmo trutta

The aim of this study was to explore differences in dietary specialization across two foraging modes (benthic v. surface‐drift foraging) of stream‐dwelling brown trout Salmo trutta. The degree of inter‐individual niche variation within each foraging mode was high, but the dietary specialization was maintained between foraging modes. This study supports the view that if aquatic invertebrates are more abundant and accessible than surface prey, the individuals will not specialize on surface prey (surface‐drift foraging).     

Consistent size‐independent harvest selection on fish body shape in two recreationally exploited marine species

Harvesting wild animals may exert size‐independent selection pressures on a range of morphological, life history, and behavioral traits. Most work so far has focused on selection pressures on life history traits and body size as morphological trait. We studied here how recreational fishing selects for morphological traits related to body shape, which may correlate with underlying swimming behavior. Using landmark‐based geometric morphometrics, we found consistent recreational fishing‐induced selection pressures on body shape in two recreationally exploited marine fish species. We show that individuals with larger‐sized mouths and more streamlined and elongated bodies were more vulnerable to passively operated hook‐and‐line fishing independent of the individual's body size or condition. While the greater vulnerability of individuals with larger mouth gapes can be explained by the direct physical interaction with hooks, selection against streamlined and elongated individuals could either involve a specific foraging mode or relate to underlying elevated swimming behavior. Harvesting using passive gear is common around the globe, and thus, size‐independent selection on body shape is expected to be widespread potentially leaving behind individuals with smaller oral gapes and more compact bodies. This might have repercussions for food webs by altering foraging and predation.     

Density‐dependent,central‐place foraging in a grazing herbivore: competition and tradeoffs in time allocation near water

Optimal foraging theory addresses one of the core challenges of ecology: predicting the distribution and abundance of species. Tests of hypotheses of optimal foraging, however, often focus on a single conceptual model rather than drawing upon the collective body of theory, precluding generalization. Here we demonstrate links between two established theoretical frameworks predicting animal movements and resource use: central‐place foraging and density‐dependent habitat selection. Our goal is to better understand how the nature of critical, centrally placed resources like water (or minerals, breathing holes, breeding sites, etc.) might govern selection for food (energy) resources obtained elsewhere – a common situation for animals living in natural conditions. We empirically test our predictions using movement data from a large herbivore distributed along a gradient of water availability (feral horses, Sable Island, Canada, 2008–2013). Horses occupying western Sable Island obtain freshwater at ponds while in the east horses must drink at self‐excavated wells (holes). We studied the implications of differential access to water (time needed for a horse to obtain water) on selection for vegetation associations. Consistent with predictions of density‐dependent habitat selection, horses were reduced to using poorer‐quality habitat (heathland) more than expected close to water (where densities were relatively high), but were free to select for higher‐quality grasslands farther from water. Importantly, central‐place foraging was clearly influenced by the type of water‐source used (ponds vs. holes, the latter with greater time constraints on access). Horses with more freedom to travel (those using ponds) selected for grasslands at greater distances and continued to select grasslands at higher densities, whereas horses using water holes showed very strong density‐dependence in how habitat could be selected. Knowledge of more than one theoretical framework may be required to explain observed variation in foraging behavior of animals where multiple constraints simultaneously influence resource selection.     

A sense of scale: Foraging cetaceans' use of scale‐dependent multimodal sensory systems

Research on cetacean foraging ecology is central to our understanding of their spatial and behavioral ecology. Yet, functional mechanisms by which cetaceans detect prey across different scales remain unclear. Here, I postulate that cetaceans utilize a scale‐dependent, multimodal sensory system to assess and increase prey encounters. I review the literature on cetacean sensory systems related to foraging ecology, and hypothesize the effective scales of each sensory modality to inform foraging opportunities. Next, I build two “scale‐of‐senses” schematics for the general groups of dolphins and baleen whales. These schematics illustrate the hypothetical interchange of sensory modalities used to locate and discriminate prey at spatial scales ranging from 0 m to 1,000 km: (1) vision, (2) audition (sound production and sound reception), (3) chemoreception, (4) magnetoreception, and somatosensory perception of (5) prey, or (6) oceanographic stimuli. The schematics illustrate how a cetacean may integrate sensory modalities to form an adaptive foraging landscape as a function of distance to prey. The scale‐of‐senses schematic is flexible, allowing for case‐specific application and enhancement with improved cetacean sensory data. The framework serves to improve our understanding of functional cetacean foraging ecology, and to develop new hypotheses, methods, and results regarding how cetaceans forage at multiple scales.     

Short-term cues used by foraging trout in a California stream

Stream salmonids choose foraging locations to maximize the energy benefit of foraging within the constraints of size-mediated dominance hierarchies and predation risk. But, because stream habitats are temporally variable, fish must use a search process to monitor changing habitat conditions as a means of locating potentially-better foraging locations. I explored the cues used by the cutthroat trout, Oncorhynchus clarki clarki, when searching for food at the pool scale by artificially increasing prey availability at different locations by using special feeders and by manipulating pool velocities. Behavior of individually marked fish was monitored from stream bank platforms under unmanipulated control conditions and under seven experimental sets of conditions involving different combinations of feeder location and velocity manipulation. Under natural conditions fish elected to forage in the deepest (>50 cm), fastest (0.10–0.25 m s⁻¹) locations and within 1 m of structure cover, but would readily move to shallower (<30 cm) water away from cover if velocities were manipulated to be highest there. Although fish did not locate feeders unless they were placed in high-velocity areas, when high velocity was provided fish would move into very shallow water (<20 cm) if prey were delivered there. Responses of individual trout to manipulations indicated that water velocity was the main physical cue used by fish to decide where to forage, and that fish could also learn about new food sources by observing conspecifics. Overall, results indicated fish were not “perfect searchers” that could quickly locate new food resources over short time scales, even when the new resources were within a few meters of the fish’s normal foraging location. When given the correct cues, however, fish could detect new food sources and defend them against subordinate fish. Movement of new fish into and out of the study pools during the ten-day observation period was common, consistent with the idea that trout used movement as a means of exploring and learning about habitat conditions at the reach scale.     

Home‐range size and habitat use of European Nightjars Caprimulgus europaeus nesting in a complex plantation‐forest landscape

In Europe, the consequences of commercial plantation management for birds of conservation concern are poorly understood. The European Nightjar Caprimulgus europaeus is a species of conservation concern across Europe due to population depletion through habitat loss. Pine plantation‐forest is now a key Nightjar nesting habitat, particularly in northwestern Europe, and increased understanding of foraging habitat selection is required. We radiotracked 31 Nightjars in an extensive (185‐km²) complex conifer plantation landscape in 2009 and 2010. Home‐range 95% kernels for females, paired males and unpaired males were an order of magnitude larger than song territories of paired males, emphasizing the importance of habitats beyond the song territory. Nightjars travelled a mean maximum distance of 747 m from the territory centre each night. Home‐range placement relative to landscape composition was examined by compositional analysis. Pre‐closure canopy forest (aged 5–10 years) was selected at all scales (MCP, 95% and 50% kernels), with newly planted forest (aged 0–4 years) also selected within 50% kernels. For telemetry fixes relative to habitat composition within 2 km of their territory centre, individuals again selected pre‐closure and newly planted forest, and also grazed grass heath. Open ungrazed habitat was not selected, with implications for open habitat planning for biodiversity conservation within public‐owned forests. Despite the Nightjars’ selection for younger growth, moth biomass was greater in older forest stands, suggesting that foraging site selection reflects ease of prey capture rather than prey abundance. Within large plantation‐forest landscapes, a variety of growth stages is important for this species and our results suggest that grazing of open habitats within and adjacent to forest will additionally benefit the European Nightjar.     

Foraging modes of stream benthic fishes in relation to their predation effects on local prey density

Habitat use and foraging behavior of two benthic insectivorous gobies, Rhinogobius sp. CO (cobalt type) and Rhinogobius sp. DA (dark type), were examined in relation to their predation effects on local prey density in a small coastal stream in southwestern Shikoku, Japan. Correlations among the foraging range, frequency of foraging attempts and current velocity indicated that individuals using fast-current habitats had small foraging ranges and infrequently made foraging attempts while those in slow currents frequently foraged over large areas. The former and the latter were recognized as ambush and wandering foragers, respectively. Interspecific comparisons of habitat use, foraging behavior and prey preference suggested that Rhinogobius sp. CO selectively forage mobile prey by ambushing in fast currents, whereas Rhinogobius sp. DA randomly forage available prey by wandering in slow-current habitats. A cage experiment was conducted to assess prey immigration rate and the degree of predation effects on local prey density in relation to current velocity. The results of the experiment support, at least in part, our initial predictions: (1) prey immigration rates increase with current velocity and (2) the effects of fish predation on local prey density are reduced as current velocity increases. Overall results illustrated a link between the foraging modes of the stream gobies and their predation effects on local prey density: fish adopt ambush foraging in fast currents, where the decrease in prey density tends to be less, whereas fish actively forage over large areas in slow currents, where the decrease in prey is relatively large.     

Within-colony feeding selectivity by a corallivorous reef fish: foraging to maximize reward?

Foraging theory predicts that individuals should choose a prey that maximizes energy rewards relative to the energy expended to access, capture, and consume the prey. However, the relative roles of differences in the nutritive value of foods and costs associated with differences in prey accessibility are not always clear. Coral‐feeding fishes are known to be highly selective feeders on particular coral genera or species and even different parts of individual coral colonies. The absence of strong correlations between the nutritional value of corals and prey preferences suggests other factors such as polyp accessibility may be important. Here, we investigated within‐colony feeding selectivity by the corallivorous filefish, Oxymonacanthus longirostris, and if prey accessibility determines foraging patterns. After confirming that this fish primarily feeds on coral polyps, we examined whether fish show a preference for different parts of a common branching coral, Acropora nobilis, both in the field and in the laboratory experiments with simulated corals. We then experimentally tested whether nonuniform patterns of feeding on preferred coral species reflect structural differences between polyps. We found that O. longirostris exhibits nonuniform patterns of foraging in the field, selectively feeding midway along branches. On simulated corals, fish replicated this pattern when food accessibility was equal along the branch. However, when food access varied, fish consistently modified their foraging behavior, preferring to feed where food was most accessible. When foraging patterns were compared with coral morphology, fish preferred larger polyps and less skeletal protection. Our results highlight that patterns of interspecific and intraspecific selectivity can reflect coral morphology, with fish preferring corals or parts of coral colonies with structural characteristics that increase prey accessibility.     

The influence of large wood on brown trout (Salmo trutta) behaviour and surface foraging

1. Changes in riparian vegetation owing to forest harvesting may affect the input of large wood, a major structural element, to streams. Studies of large wood impacts on stream fish have focused on population‐level responses, whereas little attention has been given to how wood affects fish behaviour. 2. In a laboratory stream experiment, we tested how two size classes of brown trout, Salmo trutta, (mean size of 85 and 125 mm), alone and together, responded to a gradient of large wood in terms of activity, foraging on terrestrial drift and interactions between conspecifics. 3. The results showed that the presence of large wood significantly reduced the overall activity of the fish, the number of agonistic interactions between individuals and the proportion of captured prey. However, activity decreased relatively more than the proportion of captured prey, resulting in a significant positive net effect of wood on the number of prey captures per time spent active (PTA). This indicates that trout living in habitats with high wood density may have a higher net energy gain than trout living in habitats with less wood. 4. There were no observable size‐class differences in the benefits of large wood or in the utilisation of surface‐drifting terrestrial prey. 5. These results suggest that the presence of large wood may be an important factor shaping stream communities and that a lack of structural complexity may decrease energy gain, increase agonistic interactions and, consequently, lower the production of brown trout.     

Space‐use strategies of wintering Ovenbirds in Belize: causes and consequences

For long‐distance migrants, territoriality and prey biomass during the non‐breeding season have been linked to body condition that can carry over to affect spring migration and breeding events. For Ovenbirds (Seiurus aurocapilla), studies in Jamaica showed that body condition in mid‐season depended on leaf litter prey biomass and declined seasonally as conditions became drier, and that individuals with sedentary (territorial) and wandering space‐use strategies did not differ in age or body condition. During October and November 2010–2011, we radio‐tracked Ovenbirds in Belize to determine if space‐use patterns differed with age and sex, if space‐use strategy influenced foraging behavior or body condition, and if areas used by wanderers and territory owners differed in food abundance or habitat characteristics. Most Ovenbirds (41 of 51, 80.4%) possessed small (1‐ha) territories with largely non‐overlapping cores, but 22.5% (10 of 51) of birds were wanderers (～7‐ha home range). Early season space use was predicted by age class, with most wanderers (90%, 9 of 10) being first‐year birds and most territory owners (63%) being older birds. Sex ratios of wanderers and territory owners did not differ. We found that wanderers may have been at a disadvantage because they had significantly lower foraging rates and lower relative body masses than territorial birds, although baseline corticosterone levels did not differ. Habitat characteristics of areas used by wanderers and territory owners did not differ nor did biomass of ground‐surface arthropods, likely because ranges of most wanderers overlapped those of territory owners. Using stable hydrogen isotopes in feathers, we found that first‐year Ovenbirds that were wanderers tended to have a more northern natal origins than sedentary birds, though the difference was not significant. Longer migration distances could delay arrival and reduce competitive ability. Our results suggest that wandering may not be an alternative and equally successful strategy, at least early in the season, and instead young birds may be competitively excluded from territory ownership.     

DIVERSITY‐DEPENDENT CLADOGENESIS AND TRAIT EVOLUTION IN THE ADAPTIVE RADIATION OF THE AUKS (AVES: ALCIDAE)

Through the course of an adaptive radiation, the evolutionary speed of cladogenesis and ecologically relevant trait evolution are expected to slow as species diversity increases, niches become occupied, and ecological opportunity declines. We develop new likelihood‐based models to test diversity‐dependent evolution in the auks, one of only a few families of seabirds adapted to underwater “flight,” and which exhibit a large variety of bill sizes and shapes. Consistent with the expectations of adaptive radiation, we find both a decline in rates of cladogenesis (a sixfold decline) and bill shape (a 64‐fold decline) evolution as diversity increased. Bill shape diverged into two clades at the basal cladogenesis event with one clade possessing mostly long, narrow bills used to forage primarily on fish, and the other with short thick bills used to forage primarily on plankton. Following this initial divergence in bill shape, size, a known correlate of both prey size and maximum diving depth, diverged rapidly within each of these clades. These results suggest that adaptive radiation in foraging traits underwent initial divergence in bill shape to occupy different food resources, followed by size differentiation to subdivide each niche along the depth axis of the water column.     

Sex‐specific prey partitioning in breeding piscivorous birds examined via a novel,noninvasive approach

Piscivorous birds frequently display sex‐specific differences in their hunting and feeding behavior, which lead to diverging impacts on prey populations. Cormorants (Phalacrocoracidae), for example, were previously studied to examine dietary differences between the sexes and males were found to consume larger fish in coastal areas during autumn and winter. However, information on prey partitioning during breeding and generally on sex‐specific foraging in inland waters is missing. Here, we assess sex‐specific prey choice of Great Cormorants (Phalacrocorax carbo) during two subsequent breeding seasons in the Central European Alpine foreland, an area characterized by numerous stagnant and flowing waters in close proximity to each other. We developed a unique, noninvasive approach and applied it to regurgitated pellets: molecular cormorant sexing combined with molecular fish identification and fish‐length regression analysis performed on prey hard parts. Altogether, 364 pellets delivered information on both, bird sex, and consumed prey. The sexes differed significantly in their overall prey composition, even though Perca fluviatilis, Rutilus rutilus, and Coregonus spp. represented the main food source for both. Albeit prey composition did not indicate the use of different water bodies by the sexes, male diet was characterized by higher prey diversity within a pellet and the consumption of larger fish. The current findings show that female and male cormorants to some extent target the available prey spectrum at different levels. Finally, the comprehensive and noninvasive approach has great potential for application in studies of other piscivorous bird species.     

Reduction of predation risk under the cover of darkness: Avoidance responses of mayfly larvae to a benthic fish

Summary Mayfly larvae of Paraleptophlebia heteronea (McDunnough) had two antipredator responses to a nocturnal fish predator (Rhinichthys cataractae (Valenciennes)): flight into the drift and retreat into interstitial crevices. Drift rates of Paraleptophlebia abruptly increased by 30 fold when fish were actively foraging in the laboratory streams but, even before fish were removed, drift began returning to control levels because larvae settled to the substrate and moved to areas of low risk beneath stones. This drifting response was used as an immediate escape behavior which likely decreases risk of capture from predators which forage actively at night. Surprisingly, drift most often occurred before contact between predator and prey, and we suggest that in darkness this mayfly may use hydrodynamic pressure waves for predator detection, rather than chemical cues, since fish forage in an upstream direction. Although drifting may represent a cost to mayfly larvae in terms of relocation to a new foraging area with unknown food resources, the immediate mortality risk probably out-weighs the importance of staying within a profitable food patch because larvae can survive starvation for at least 2 d. In addition to drifting, mayflies retreated from upper, exposed substrate surfaces to concealed interstitial crevices immediately after a predator encounter, or subsequent to resettlement on the substrate after predator-induced drift. A latency period was associated with this response and mayflies remained in these concealed locations for at least 3 h after dace foraging ceased. Because this mayfly feeds at night and food levels are significantly lower in field refugia under stones, relative to exposed stone surfaces, predator avoidance activity may limit foraging time and, ultimately, reduce the food intake of this stream mayfly.     

Interspecific territoriality in males of the tube blenny <Emphasis Type="Italic">Neoclinus bryope</Emphasis> (Actinopterygii: Chaenopsidae)

Territoriality was investigated in the tube blenny Neoclinus bryope (Actinopterygii: Chaenopsidae) at rocky intertidal areas of Banda Beach, Tateyama Bay, central Japan. Males used small holes as spawning nests, usually staying at the nest and maintaining the area while showing exclusive behaviors. Their home ranges were limited to areas within 30 cm distance from the nest for over 2 months. Four heterospecific fishes were threatened when they approached to within 6–14 cm of the nest holes, and two species of carnivorous snails were removed at points 0–30 cm from the nest entrance. There were no significant differences between the distances from the nest entrances to the points defended against fish and those used for foraging. As the four heterospecific species have similar feeding habits to those of N. bryope, the area defended against fishes may function as a foraging territory. At 24 h following the removal of nest owners, carnivorous snails had gathered to actively prey on eggs, indicating that the area defended against snails may function as a territory for protection against egg predators. Although the positions of females where males started courtship displays were significantly farther than the foraging points and the positions of threatened fishes, males displayed no territoriality against conspecific males. The fact that males did not leave the nest hole during the courtship suggests that it may be costly to maintain a courtship territory. These results show that males of tube blenny maintain territories for egg guarding and for protecting food resources around their nest holes in the spawning season.     

Forest succession and prey availability influence the strength and scale of terrestrial‐aquatic linkages in a headwater salamander system

1. Trophic linkages between terrestrial and aquatic ecosystems are common and sensitive to disruption. However, there is little information on what causes variation in the strength and spatial scale of these linkages. 2. In the highly aquatic adults of the headwater salamander Gyrinophilus porphyriticus (family Plethodontidae), use of terrestrial prey decreases along a gradient from early‐ to late‐successional riparian forests. To understand the cause of this relationship, we tested the predictions that (i) terrestrial prey abundance is lower in late‐successional forests, and (ii) G. porphyriticus adults cannot move as far from the stream to forage in late‐successional forests, thus limiting access to terrestrial prey. 3. We established 100‐m long study reaches on six headwater streams in the Hubbard Brook Experimental Forest, New Hampshire. Three reaches were in early‐successional forests and three were in late‐successional forests. We conducted pitfall trapping for invertebrate prey in June and July of 2005, with three traps at 0, 2, 5 and 10 m from the stream at each reach. In June, July and August of 2004 and 2005, nighttime salamander surveys were conducted at each reach along ten, 10‐m long by 2.5‐m wide transects perpendicular to the stream. 4. Abundance of terrestrial prey was consistently lower in late‐successional forests, suggesting that consumption of terrestrial prey by G. porphyriticus is affected by prey abundance. Contrary to our prediction, G. porphyriticus adults moved farther from the stream in late‐successional forests, suggesting that habitat conditions in late‐successional forests do not limit movement away from the stream, and that lower abundances of terrestrial prey in these forests may cause salamanders to move farther from streams. 5. Our results provide novel insight on the extent of terrestrial habitat use by G. porphyriticus. More broadly, these results indicate that major habitat gradients, such as forest succession, can affect the strength and scale of terrestrial‐aquatic linkages. Application of this insight to the design of vegetation buffers along headwater streams would have widespread benefits to freshwater ecosystems.     

Evidence of hypoxic foraging forays by yellow perch (Perca flavescens) and potential consequences for prey consumption

1. Previous studies in a variety of ecosystems have shown that ecologically and economically important benthic and bentho‐pelagic fishes avoid hypoxic (<2 mg O₂ L^?1) habitats by moving vertically or horizontally to more oxygenated areas. While avoidance of hypoxic conditions generally leads to a complete shift away from preferred benthic prey, some individual fish continue to consume benthic prey items in spite of bottom hypoxia, suggesting complex habitat utilisation and foraging patterns. For example, Lake Erie yellow perch (Perca flavescens) continue to consume benthic prey, despite being displaced vertically and horizontally by hypolimnetic hypoxia. 2. We hypothesised that hypolimnetic hypoxia can negatively affect yellow perch by altering their distribution and inducing energetically expensive foraging behaviour. To test this hypothesis, we used drifting hydroacoustics and trawl sampling to quantify water column distribution, sub‐daily vertical movement and foraging behaviour of yellow perch within hypoxic and normoxic habitats of Lake Erie’s central basin during August‐September 2007. We also investigated the effects of rapid changes in ambient oxygen conditions on yellow perch consumption potential by exposing yellow perch to various static and fluctuating oxygen conditions in a controlled laboratory experiment. 3. Our results indicate that, while yellow perch in general avoid hypoxic conditions, some individuals undertake foraging forays into hypoxic habitats where they experience greater fluctuations in abiotic conditions (pressure, temperature and oxygen concentration) than at normoxic sites. However, laboratory results suggest short‐term exposure to low oxygen conditions did not negatively impact consumption potential of yellow perch. 4. Detailed understanding of sub‐daily individual behaviours may be crucial for determining interactive individual‐ and ecosystem‐level effects of stressors such as hypoxia.