Wide-ranging Predators Research Articles

Spatially heterogeneous habitat use across distinct biogeographic regions in a wide-ranging predator, the Persian leopard

Spatially heterogeneous habitat use across distinct biogeographic regions in a wide-ranging predator, the Persian leopard

Environmental conditions and marine heatwaves influence blue whale foraging and reproductive effort.

Animal behavior is motivated by the fundamental need to feed and reproduce, and these behaviors can be inferred from spatiotemporal variations in biological signals such as vocalizations. Yet, linking foraging and reproductive effort to environmental drivers can be challenging for wide-ranging predator species. Blue whales are acoustically active marine predators that produce two distinct vocalizations: song and D calls. We examined environmental correlates of these vocalizations using continuous recordings from five hydrophones in the South Taranaki Bight region of Aotearoa New Zealand to investigate call behavior relative to ocean conditions and infer life history patterns. D calls were strongly correlated with oceanographic drivers of upwelling in spring and summer, indicating associations with foraging effort. In contrast, song displayed a highly seasonal pattern with peak intensity in fall, which aligned with the timing of conception inferred from whaling records. Finally, during a marine heatwave, reduced foraging (inferred from D calls) was followed by lower reproductive effort (inferred from song intensity).

Recent jaguar records confirm the conservation value of the Baritú–Tariquía corridor between Argentina and Bolivia

AbstractEffective transboundary corridors play a crucial role in jaguar Panthera onca conservation. Local residents reported jaguar sightings along the Baritú–Tariquía Biological Corridor, which prompted us to carry out this camera-trap survey. We surveyed an area of 1,243 km2 across the corridor to confirm jaguar presence. We used 50 single camera stations, with cameras placed c. 5 km apart. We placed the cameras along trails, streams and mountain ridges. We recorded jaguars at seven sites across the Corridor; at least three different individuals were identified. These records confirm the presence of the jaguar in the Baritú–Tariquía Biological Corridor between Argentina and Bolivia, a trans-frontier area of the Austral Yungas facing multiple threats but hosting one of the southernmost jaguar populations. Conservation efforts in border regions can promote collaboration and synergies between agencies and other conservation stakeholders, with important implications for wide-ranging predators such as jaguars and their habitats.

Environmental correlates of activity and energetics in a wide-ranging social carnivore

BackgroundEnvironmental conditions can influence animal movements, determining when and how much animals move. Yet few studies have quantified how abiotic environmental factors (e.g., ambient temperature, snow depth, precipitation) may affect the activity patterns and metabolic demands of wide-ranging large predators. We demonstrate the utility of accelerometers in combination with more traditional GPS telemetry to measure energy expenditure, ranging patterns, and movement ecology of 5 gray wolves (Canis lupus), a wide-ranging social carnivore, from spring through autumn 2015 in interior Alaska, USA.ResultsWolves exhibited substantial variability in home range size (range 500–8300 km2) that was not correlated with daily energy expenditure. Mean daily energy expenditure and travel distance were 22 MJ and 18 km day−1, respectively. Wolves spent 20% and 17% more energy during the summer pup rearing and autumn recruitment seasons than the spring breeding season, respectively, regardless of pack reproductive status. Wolves were predominantly crepuscular but during the night spent 2.4 × more time engaged in high energy activities (such as running) during the pup rearing season than the breeding season.ConclusionIntegrating accelerometry with GPS telemetry can reveal detailed insights into the activity and energetics of wide-ranging predators. Heavy precipitation, deep snow, and high ambient temperatures each reduced wolf mobility, suggesting that abiotic conditions can impact wolf movement decisions. Identifying such patterns is an important step toward evaluating the influence of environmental factors on the space use and energy allocation in carnivores with ecosystem-wide cascading effects, particularly under changing climatic conditions.

Short-finned pilot whales exhibit behavioral plasticity in foraging strategies mediated by their environment

Predators adapt their foraging behavior to exploit a variety of prey in a range of environments. Short-finned pilot whales are wide-ranging predators in tropical and sub-tropical oceans, but most previous studies of their foraging ecology have been conducted near oceanic islands. We deployed sound- and movement-recording tags on 43 short-finned pilot whales off Cape Hatteras, North Carolina, USA, to measure their foraging behavior in a continental shelf-break ecosystem and investigate how variation in the environment shapes their behavior. Overall, the foraging behavior of pilot whales off Cape Hatteras was similar to that of their counterparts from island-associated habitats. Off Cape Hatteras, pilot whales made foraging dives as deep as 1077 m (mean: 445 m), lasting up to 23 min (mean: 12.8 min), with sprints (pursuit at speeds over 3 m s-1and up to 6.9 m s-1) in more than half of foraging dives. However, tagged whales off Cape Hatteras produced higher buzz rates (11.3 buzzes dive-1), foraged more extensively in daytime hours, and engaged in more frequent benthic foraging than island-associated ecotypes. By parsing the echoic scene generated by the animal’s own echolocation clicks, we show that pilot whales off Cape Hatteras frequently exploit bathymetric features for foraging, with benthic dives resulting in higher prey capture attempts than pelagic dives. The ability of these predators to strategically adapt foraging strategies to local habitat features likely contributes to their ecological success and may allow them to adjust to shifts in prey distributions in a rapidly changing Anthropocene ocean.

Feeding Ecology of Elusive Caribbean Killer Whales Inferred From Bayesian Stable Isotope Mixing Models and Whalers’ Ecological Knowledge

Investigating the feeding ecology of marine predators is critical for understanding their roles and functional importance in ecosystems. However, assessing the diet of large and wide-ranging predators can be challenging, particularly in the case of rare and elusive species. Killer whales (Orcinus orca) are large apex predators in global oceans, but very little is known about their feeding ecology in tropical ecosystems, particularly in the Caribbean Sea. Killer whales are distributed throughout the Caribbean, and are a regular target of artisanal whalers in St. Vincent and the Grenadines (West Indies). Here we investigate the feeding ecology of Caribbean killer whales using a combination of stable isotope analysis (δ15N, δ13C) and traditional ecological knowledge (TEK) data from whalers operating from St. Vincent. Potential prey data from the Caribbean region included large pelagic teleosts, sea turtles, oceanic sharks, cetaceans, and oceanic cephalopods. Mixing models suggest that the contribution of odontocete cetaceans to the diet of killer whales is the most important (overall mean contribution: 60.4%), particularly mesopelagic delphinids (Lagenodelphis hosei; 26.4%, SD = 0.14), large teuthophageous odontocetes (Physeter macrocephalus,Globicephala macrorhynchus; 20.0%, SD = 0.14), and epipelagic delphinids (Stenella attenuata; 14%, SD = 0.14). Oceanic sharks (Carcharhinus longimanus) had a contribution of 17.0% of the diet of killer whales (SD = 0.13), and other potential prey had a relatively marginal contribution. TEK data suggest that whalers believe killer whales feed almost exclusively on cetaceans, particularly odontocete species that they hunt (e.g.,G. macrorhynchus). Stable isotope and TEK data were consistent to some extent, although TEK data are qualitative and based on a limited number of observations. Despite some limitations (including sample size for both methods), this study highlights the value of combining independent data sources and methodologies to investigate the ecological roles of marine predators in data-poor regions.

Spatio-temporal patterns of foraging behaviour in a wide-ranging seabird reveal the role of primary productivity in locating prey

Predicting the distribution and behaviour of animals is a fundamental objective in ecology and a cornerstone of conservation biology. Modelling the distribution of ocean-faring species like seabirds remains a significant challenge due to ocean dynamics, colony-specific effects and the vast ranges seabirds can cover. We used a spatial and behavioural approach to model the distribution of the Manx shearwaterPuffinus puffinus, a pelagic, central-place forager that can cover great distances while foraging. GPS data from birds tagged in 2 colonies over 3 yr were modelled with a range of environmental predictors of marine productivity. For both colonies, transitions to foraging behaviour correlated with chlorophylla, and the distribution of foraging behaviour was also associated with areas of high chlorophyllaconcentration in coastal but not offshore areas for one colony. Furthermore, there was evidence for colony differences in habitat use, prevalence of nocturnal foraging, and for some competitive exclusion on foraging grounds, even though the colonies were 170 km apart. Despite the extensive dataset, our models had modest predictive power, which we suggest can probably only be improved by including biotic interactions, including more direct measures of food resource distribution. Our results highlight the importance of including spatial complexity and data from multiple sites when predicting the distribution of wide-ranging predators, because patterns of distribution and habitat use likely differ across the range of a population.

Generalist seabirds as biomonitors of ocean mercury: The importance of accurate trophic position assignment

Monitoring mercury concentration in the marine environment is pivotal due to the risks that mercury intake poses to the ecosystem and human health. It is therefore of interest to make reliable, comparative measurements over large geographic areas. Here, the utility of wide-ranging generalist seabirds as mercury biomonitors at an ocean basin scale was assessed, using the Cory's Shearwater as a model species. The mercury concentration in flight feathers moulted at distant non-breeding areas of geolocator-tracked birds was quantified, reflecting contamination in various geographic areas. Compound-specific isotope analysis of amino acids was used to obtain comparable trophic positionestimates controlled for baseline isoscape. Birds that remained resident in the Canary Current integrated less mercury into their feathers than those that migrated to either the Benguela or Agulhas currents. Residents also occupied a significantly lower trophic position during the non-breeding season than migrants, largely explaining the difference in mercury exposure. Both mercury concentration and trophic position were similar in individuals spending the non-breeding period in the Benguela and Agulhas currents. This paper highlights the importance of accurate trophic position calculation in order to understand mercury exposure in wide-ranging predators and for meaningful spatial comparisons.

SealNet: A fully-automated pack-ice seal detection pipeline for sub-meter satellite imagery

Antarctic pack-ice seals, a group of four species of true seals (Phocidae), play a pivotal role in the Southern Ocean foodweb as wide-ranging predators of Antarctic krill (Euphausia superba). Due to their circumpolar distribution and the remoteness and vastness of their habitat, little is known about their population sizes. Estimating pack-ice seal population sizes and trends is key to understanding how the Southern Ocean ecosystem will react to threats such as climate change driven sea ice loss and krill fishing. We present a functional pack-ice seal detection pipeline using Worldview-3 imagery and a Convolutional Neural Network that counts and locates seal centroids. We propose a new CNN architecture that detects objects by combining semantic segmentation heatmaps with binary classification and counting by regression. Our pipeline locates over 30% of seals, when compared to consensus counts from human experts, and reduces the time required for seal detection by 95% (assuming just a single GPU). While larger training sets and continued algorithm development will no doubt improve classification accuracy, our pipeline, which can be easily adapted for other large-bodied animals visible in sub-meter satellite imagery, demonstrates the potential for machine learning to vastly expand our capacity for regular pack-ice seal surveys and, in doing so, will contribute to ongoing international efforts to monitor pack-ice seals.

Seasonal occurrence and site fidelity of juvenile bronze whalers (Carcharhinus brachyurus) in a temperate inverse estuary

Assessing the movements and habitat use of wide-ranging predators is critical for understanding their role within an ecosystem and vulnerability to anthropogenic threats. The bronze whaler (Carcharhinus brachyurus) is a large-bodied, slow-growing shark species that is widely distributed throughout temperate Australian waters. Currently, habitat use, spatio-temporal presence and fine-scale movement information is unknown for Southern Australian waters. This study investigated the spatio-temporal patterns of occurrence, habitat use, and residency of bronze whalers in Gulf St. Vincent, South Australia. Forty-seven predominately small juvenile bronze whalers (83% of tagged sharks < 150 cm TL) were tagged with acoustic transmitters from 2009 to 2014 and monitored for periods of up to 55 months (range 516–1634 days; mean ± standard error 825 ± 47). The seasonal presence of bronze whalers peaked in spring–early autumn (September–April). Site fidelity was high, with 77% of tagged sharks detected over multiple years and 36% detected over 3 years. Low estimates of mean residency index (Ri = 0.05 ± 0.01) indicated that time spent within the array was limited. Shark occurrence was driven by a combination of factors, which included water temperature, season, moon illumination, and wind speed. Manly–Chesson’s Electivity Index showed bronze whalers exhibited the highest affinity to seagrass (Posidonia spp.), suggesting that this habitat plays an important role in the early life history stages. The slow growth and low productivity of bronze whalers combined with its predictable nearshore occurrence result in this species having an increased susceptibility to anthropogenic threats, which highlights the need for continued monitoring and vulnerability assessments.

Modeling habituation of introduced predators to unrewarding bird odors for conservation of ground-nesting shorebirds.

Foraging mammalian predators face a myriad of odors from potential prey. To be efficient, they must focus on rewarding odors while ignoring consistently unrewarding ones. This may be exploited as a nonlethal conservation tool if predators can be deceived into ignoring odors of vulnerable secondary prey. To explore critical design components and assess the potential gains to prey survival of this technique, we created an individual-based model that simulated the hunting behavior of three introduced mammalian predators on one of their secondary prey (a migratory shorebird) in the South Island of New Zealand. Within this model, we heuristically assessed the outcome of habituating the predators to human-deployed unrewarding bird odors before the bird's arrival at their breeding grounds, i.e., the predators were "primed." Using known home range sizes and probabilities of predators interacting with food lures, our model suggests that wide-ranging predators should encounter a relatively large number of odor points (between 10 and 115) during 27d of priming when odor is deployed within high-resolution grids (100-150m). Using this information, we then modeled the effect of different habituation curves (exponential and sigmoidal) on the probability of predators depredating shorebird nests. Our results show that important gains in nest survival can be achieved regardless of the shape of the habituation curve, but particularly if predators are fast olfactory learners (exponential curve), and even if some level of dishabituation occurs after prey become available. Predictions from our model can inform the amount and pattern in which olfactory stimuli need to be deployed in the field to optimize encounters by predators, and the relative gains that can be expected from reduced predation pressure on secondary prey under different scenarios of predator learning. Habituating predators to odors of threatened secondary prey may have particular efficacy as a conservation tool in areas where lethal predator control is not possible or ethical, or where even low predator densities can be detrimental to prey survival. Our approach is also relevant for determining interaction probabilities for devices other than odor points, such as bait stations and camera traps.

Ecosystem Oceanography of Seabird Hotspots: Environmental Determinants and Relationship with Antarctic Krill Within an Important Fishing Ground

The discipline of ecosystem oceanography provides a framework for assessing the role of mesoscale physical processes on the formation and occurrence of biological hotspots. We used shipboard surveys over nine years to investigate environmental determinants of seabird hotspots near the Antarctic Peninsula, a region experiencing rapid climate change and an expanding krill fishery. We hypothesize that seabird hotspots are structured by mesoscale ocean conditions that reflect differences in prey distribution within oceanic and coastal waters. We used generalized additive models to quantify functional relationships of seabird hotspots with krill biomass, and a suite of remotely sensed environmental variables, such as eddy kinetic energy. The spatial organization, changes in intensity, and distribution shifts of seabird hotspots indicate different environmental drivers within coastal and oceanic domains and reflect the seasonal variability of the ecosystem. Our results indicate at least eight mesoscale hotspot zones that represent ecologically important areas where significant krill and predator biomass may be concentrated. Our ecosystem assessment of seabird hotspots identified critical foraging habitat and provided reference points to benefit research on estimating their trophic impacts on Antarctic ecosystems and potential effects from the krill fishery. Our approach is generally applicable to other pelagic ecosystems that are structured by hydrographic fronts and eddies, and containing schooling forage species shared by multiple wide-ranging predators. Furthermore, identification of biological hotspots is useful for the designation of marine protected areas most critical to potentially endangered wildlife and fisheries resources.

Dynamic habitat corridors for marine predators; intensive use of a coastal channel by harbour seals is modulated by tidal currents.

Previous studies have found that predators utilise habitat corridors to ambush prey moving through them. In the marine environment, coastal channels effectively act as habitat corridors for prey movements, and sightings of predators in such areas suggest that they may target these for foraging. Unlike terrestrial systems where the underlying habitat structure is generally static, corridors in marine systems are in episodic flux due to water movements created by tidal processes. Although these hydrographic features can be highly complex, there is generally a predictable underlying cyclic tidal pattern to their structure. For marine predators that must find prey that is often patchy and widely distributed, the underlying temporal predictability in potential foraging opportunities in marine corridors may be important drivers in their use. Here, we used data from land-based sightings and 19 harbour seals (Phoca vitulina) tagged with high-resolution GPS telemetry to investigate the spatial and temporal distribution patterns of seals in a narrow tidal channel. These seals showed a striking pattern in their distribution; all seals spent a high proportion of their time around the narrowest point of the channel. There was also a distinctive tidal pattern in the use of the channel; sightings of seals in the water peaked during the flood tide and were at a minimum during the ebb tide. This pattern is likely to be related to prey availability and/or foraging efficiency driven by the underlying tidal pattern in the water movements through the channel.Significance StatementTo maximise foraging efficiency, predators often make use of narrow constrictions in habitat to intercept prey using these corridors for movement. In the marine environment, narrow channels may act as corridors, and sightings of predators suggest that they may target these for foraging. Despite this, there is little information on how individual predators use such areas. Here, we investigate how individual harbour seals use a narrow coastal channel subject to strong tidal currents; results showed that seals spent the majority of their time at the narrowest point of the channel foraging during peak tidal currents. This highlights the importance of narrow channels for marine predators and suggests that this usually wide-ranging predator may restrict its geographic range to forage in the channel as a result of increased prey availability and/or foraging efficiency driven by water movements through the narrow corridor.Electronic supplementary materialThe online version of this article (doi:10.1007/s00265-016-2219-7) contains supplementary material, which is available to authorized users.

Contrasting movements and connectivity of reef-associated sharks using acoustic telemetry: implications for management.

Understanding the efficacy of marine protected areas (MPAs) for wide-ranging predators is essential to designing effective management and conservation approaches. The use of acoustic monitoring and network analysis can improve our understanding of the spatial ecology and functional connectivity of reef-associated species, providing a useful approach for reef-based conservation planning. This study compared and contrasted the movement and connectivity of sharks with different degrees of reef association. We examined the residency, dispersal, degree of reef connectivity, and MPA use of grey reef (Carcharhinus amblyrhynchos), silvertip (C. albimarginatus), and bull (C. leucas) sharks monitored in the central Great Barrier Reef (GBR). An array of 56 acoustic receivers was used to monitor shark movements on 17 semi-isolated reefs. Carcharhinus amblyrhynchos and C. albimarginatus were detected most days at or near their tagging reef. However, while C. amblyrhynchos spent 80% of monitoring days in the array, C. albimarginatus was only detected 50% of the time. Despite both species moving similar distances (< 50 km), a large portion of the population of C. albimarginatus (71%) was detected on multiple reefs and moved more frequently between reefs and management zones than C. amblyrhynchos. Carcharhinus leucas was detected less than 20% of the time within the tagging array, and 42% of the population undertook long-range migrations to other arrays in the GBR. Networks derived for C. leucas were larger and more complex than those for C. amblyrhynchos and C. albimarginatus. Our findings suggest that protecting specific reefs based on prior knowledge (e.g., healthier reefs with high fish biomass) and increasing the level of protection to include nearby, closely spaced reef habitats (< 20 km) may perform better for species like C. albimarginatus than having either a single or a network of isolated MPAs. This design would also provide protection for larger male C. amblyrhynchos, which tend to disperse more and use larger areas than females. For wide-ranging sharks like C. leucas, a combination of spatial planning and other alternative measures is critical. Our findings demonstrate that acoustic monitoring can serve as a useful platform for designing more effective MPA networks for reef predators displaying a range of movement patterns.

Diet composition and foraging success in generalist predators: Are specialist individuals better foragers?

Factors affecting individual diet specialization in generalist populations and the relationship between diet and foraging success remain poorly studied, particularly in terrestrial wide-ranging predators. We studied whether individual variations in diet in Montagu's harrier males (determined through a combination of direct foraging observations and pellet analysis) were associated with patterns of foraging habitat selection and foraging success of 12 radiotracked males during the breeding period. We found important differences in diet composition and breadth between individuals. Diet diversity was negatively related to hunting success: the most efficient individuals in terms of hunting success had the most specialized diet. This study also suggests an important role of individual foraging habitat selection in explaining individual diet, as the proportion of different prey types in the diet was associated with habitat composition within the home range, with higher proportion of those habitats that held higher abundances of their more frequent prey. This study thus provides evidence of individual diet specialization having a knock-on effect on foraging efficiency in a wide-ranging raptor and highlights the role of individual behaviour as a driving force of intra-population niche variation.

Providing insights on habitat connectivity for male brown bears: A combination of habitat suitability and landscape graph-based models

The combined study of movement patterns and habitat selection, behaviors that are affected by landscape structure, is of critical importance for effective conservation of wildlife. In the same context, maintaining connectivity is a key component for facilitating movement and ensuring species persistence. The brown bear is a wide-ranging predator that moves across large areas, crossing various habitats of different quality. Here, we provide a methodological framework that allowed us to explore the relative importance of habitat quality upon movement patterns and connectivity. First, we developed habitat-suitability models by using telemetry data of eight male brown bears (12,893 radio-locations) collected in Greece. Next, we combined habitat-suitability maps with real movement data to develop graph models that represent movement networks as inter-connected patches of different quality. Network analyses revealed that movement networks demonstrate some well-studied properties regarding their structure and robustness (i.e. scale-free and small-world), providing insights on the contribution of patches of different quality to connectivity. Less suitable patches were found to play a critical role for facilitating brown bear's movement and landscape connectivity. Our findings raise the importance of considering the entire habitat of brown bear in conservation planning rather than isolated patches of high quality, as it might have been revealed by simple habitat-suitability models. Our results highlight the need of assessing the importance of the intervening matrix for facilitating connectivity and movement specifically in the case of large and highly mobile species that do not perceive landscape as strictly dichotomous.

Millennial-scale isotope records from a wide-ranging predator show evidence of recent human impact to oceanic food webs

Human exploitation of marine ecosystems is more recent in oceanic than near shore regions, yet our understanding of human impacts on oceanic food webs is comparatively poor. Few records of species that live beyond the continental shelves date back more than 60 y, and the sheer size of oceanic regions makes their food webs difficult to study, even in modern times. Here, we use stable carbon and nitrogen isotopes to study the foraging history of a generalist, oceanic predator, the Hawaiian petrel (Pterodroma sandwichensis), which ranges broadly in the Pacific from the equator to near the Aleutian Islands. Our isotope records from modern and ancient, radiocarbon-dated bones provide evidence of over 3,000 y of dietary stasis followed by a decline of ca. 1.8‰ in δ(15)N over the past 100 y. Fishery-induced trophic decline is the most likely explanation for this sudden shift, which occurs in genetically distinct populations with disparate foraging locations. Our isotope records also show that coincident with the apparent decline in trophic level, foraging segregation among petrel populations decreased markedly. Because variation in the diet of generalist predators can reflect changing availability of their prey, a foraging shift in wide-ranging Hawaiian petrel populations suggests a relatively rapid change in the composition of oceanic food webs in the Northeast Pacific. Understanding and mitigating widespread shifts in prey availability may be a critical step in the conservation of endangered marine predators such as the Hawaiian petrel.

Factors Influencing Home-Range Sizes of Eastern Indigo Snakes in Central Florida

Wide-ranging snake species are particularly sensitive to landscape fragmentation, and understanding area requirements is important for their conservation. We used radiotelemetry to quantify how Eastern Indigo Snake home-range sizes were influenced by sex, land cover, and the length of time (weeks) individuals were radio tracked. We found that Eastern Indigo Snakes had the largest home ranges among other snake species studied. Female home ranges averaged 44 and 76 ha, respectively, for kernel and minimum convex polygon estimators. Male home ranges averaged 156 and 202 ha, respectively, for kernel and minimum convex polygon estimators. Many animal species respond to habitat fragmentation by using larger areas than in unfragmented landscapes, but we found that Indigo Snakes in fragmented landscapes used much smaller areas. The length of time that snakes were tracked had almost no influence on home-range size compared to sex and land cover type. Our results suggest that maintaining populations of this large wide-ranging predator will require large conservation areas with minimum fragmentation.

Food web dynamics in the Scotia Sea in summer: A stable isotope study

The pelagic food web of the Scotia Sea was studied by analysing natural abundances of nitrogen and carbon stable isotopes of primary producers and pelagic consumers, sampled from the seasonal ice edge in the south to the Antarctic Polar Front in the north. The analysis covered, within a single mid-summer period, particulate organic matter (POM) and 38 taxa, ranging from suspension feeding copepods and salps to omnivorous euphausiids, pelagic fish and higher, land-based predators including fur seals, penguins and flying birds. Spatial variation in δ 15N of POM correlated well with nutrient availability and primary productivity. Latitudinal differences in δ 13C of POM were closely linked to variations in temperature, nutrients and productivity depending on the frontal region sampled. This translated to equivalent (although smaller) regional δ 13C differences among higher trophic levels. The trophic positions of species based on isotope values broadly agreed with previously published dietary data with three important exceptions. First, the carnivorous amphipod Themisto gaudichaudii had anomalously low δ 15N values. Second, Euphausia superba had δ 15N values that were also surprisingly low, considering the abundant literature suggesting its omnivory. Third, the copepod Rhincalanus gigas, considered a suspension feeder, had unexpectedly high δ 15N values rather more in keeping with omnivorous feeding. The consumer δ 15N values ranged from 1.2‰ (min.) measured in Salpa thompsoni (designated here as trophic level (TL) 2 across all regions) to 15.2‰ (max.) measured in white-chinned petrels ( Procellaria aequinoctialis, calculated as TL5 relative to the TL2 of salps). Excluding seabirds, the resulting food chain length of 3.7 TL (above POM at TL1) was lower than in most other Southern Ocean and temperate marine pelagic ecosystems. The majority (60%) of vertebrate predators occupied only 1–1.5 trophic levels above the herbivorous suspension feeders such as krill. This indicates the existence of the classic short food chain of POM–suspension feeder–vertebrate predator. However the presence of trophic levels 4 and above indicates the existence of alternative trophic pathways, for example involving myctophid fish or carrion, and that some wide-ranging predators which breed at South Georgia also feed outside the region. This conclusion is supported first by the continuum of δ 15N values between krill, suspension feeding copepods and myctophid fish, and secondly by higher trophic levels in several of the myctophid species in the low-krill region of the northern Scotia Sea, suggesting latitudinal differences in food web structure and food chain length.

Stable isotopes reveal the trophic position and mesopelagic fish diet of female southern elephant seals breeding on the Kerguelen Islands

Trophic interactions between organisms are the main drivers of ecosystem dynamics, but scant dietary information is available for wide-ranging predators during migration. We investi- gated feeding habits of a key consumer of the Southern Ocean, the southern elephant seal Miroun- gia leonina, by comparing its blood δ 13 C and δ 15 N values with those of various marine organisms, including crustaceans, squid, fishes, seabirds and fur seals. At the end of winter, δ 13 C values (-23.1 to -20.1‰) indicate that female elephant seals forage mainly in the vicinity of the Polar Front and in the Polar Frontal Zone. Trophic levels derived from δ 15 N values (trophic level = 4.6) show that the southern elephant seal is a top consumer in the pelagic ecosystem that is dominated by colossal squid. The mean δ 15 N value of seals (10.1 ± 0.3‰) indicates that they are not crustacean eaters, but instead feed on crustacean-eating prey. Surprisingly, most of the previously identified prey species have isotope δ 13 C and δ 15 N values that do not fit with those of potential food items. The most singular pattern to emerge from δ 15 N values of predators and prey is that female seals are likely to feed on myctophid fishes, not squid or Patagonian toothfish. We therefore suggest that they occupy a unique trophic niche amongst the guild of air-breathing, diving vertebrates by feeding on mesopelagic fish at great depths. In turn, this finding emphasizes the role of myctophids in oceanic waters and will help to quantify and model fluxes of matter and energy within the pelagic ecosystem of the Southern Ocean.