Large Marine Predators Research Articles

Empirical evidence that large marine predator foraging behavior is consistent with area-restricted search theory.

When prey is patchily distributed, predators are expected to spend more time searching for food in proximity of recent prey captures before searching in other areas. This behavior, known as area-restricted search, results in predators remaining localized in areas where prey had been detected previously because of the higher probability of encountering additional prey. However, few studies have tested these predictions on marine species because of the difficulties of observing feeding behavior. In this study, we utilized passive acoustic detections of echolocating dolphins to identify foraging behavior. C-PODs (click train detectors) were deployed for two years with an acoustic recorder attached to the same mooring during the second year. The time series of feeding buzzes, indicative of foraging behavior, revealed that both bottlenose (Tursiops truncatus) and common dolphins (Delphinus delphis) were more likely to stay in the area longer when foraging activity was high at the beginning of the encounter. The probability of foraging was also higher following previous foraging activity. This suggests that dolphins were feeding on spatially patchy prey and previous foraging experience influenced their movement behavior. This is consistent with the predictions of area-restricted search behavior, a nonrandom foraging strategy.

Biologging Tags Reveal Links Between Fine-Scale Horizontal and Vertical Movement Behaviors in Tiger Sharks (Galeocerdo cuvier)

An understanding of the role that large marine predators play in structuring trophic flow and nutrient cycling in marine ecosystems requires knowledge of their fine-scale (m-km) movement behaviours. In this study, biologging tags were used to reveal new insights into the three-dimensional fine-scale movement ecology of tiger sharks (Galeocerdo cuvier) at Ningaloo Reef, Western Australia. Tags deployed on 21 sharks in April-May 2017 for durations of 5-48 hours recorded both physical parameters such as depth and temperature, and, through the use of accelerometers, gyroscopes and compasses, in-situ measurements of animal trajectory and locomotion. Animal-borne-video enabled the validation of behavioural signatures, mapping of habitat, and recording of interactions with prey. Collectively, these data were used to examine the link between vertical (oscillations) and horizontal (tortuosity) movements, and link sensor data to prey interactions recorded by the video. This biologging approach revealed complex movements that would otherwise be invisible within the time-depth records provided by traditional tagging techniques. The rate of horizontal turning was not related to vertical oscillations, suggesting that vertical movements occur independently of searching behaviours in tiger sharks. These animals displayed tortuous movements possibly associated with prey searching for 27% of their tracks, and interactions with prey elicited varied responses including highly tortuous paths and burst movements. Accurate speed measurements and GPS anchor points will considerably enhance the value of magnetometer data in future studies by facilitating more accurate dead-reckoning and geo-referencing of area-restricted search behaviours.

Trophic Relationships and Niche Partitioning of Red Drum Sciaenops ocellatus and Common Snook Centropomus undecimalis in Coastal Estuaries of South Florida

Large marine predators occupying the same spatial arena exhibit a variety of temporal and behavioral differences to minimize competition for habitat and prey. Here, we examined two such species in the Florida Everglades, red drum Sciaenops ocellatus and snook Centropomus undecimalis, to evaluate niche separation based on diet and multiple stable isotope (white muscle, δ13C, δ15N, δ34S) analyses. For these two estuarine predators, our results indicated that although dietary niche overlap was broad, different feeding modes (spatial and behavioral) allow niche partitioning. The diet of red drum was dominated by pink shrimp Farfantepenaeus duorarum and other demersal species. For snook, although their diet included significant numbers of pink shrimp, it was largely dominated by pelagic and epibenthic fishes. Mean red drum δ13C signatures (− 10.5 to − 20.8‰) differed significantly between areas and were strongly correlated with both area-specific seagrass concentration and amount of incidental seagrass ingestion. Mean snook δ13C signatures were generally depleted (− 20.9 to − 22.4‰) with the exception of one area (− 14.1‰). Red drum and snook diet and mean δ15N signatures (10.1‰, 10.8‰, respectively) indicated they were both mid-trophic-level consumers. Mean red drum δ34S signatures were significantly depleted (− 0.31‰) in the seagrass-dominated area, but enriched (2.03 to 3.78‰) in the other areas and indicated benthic but no pelagic sources of primary production. Mean snook δ34S signatures varied widely (0 to 20‰) among areas suggesting dependence on benthic (benthic algae and seagrass) and pelagic (phytoplankton) sources of primary production.

Hawaiian monk seals exhibit behavioral flexibility when targeting prey of different size and shape.

Animals use diverse feeding strategies to capture and consume prey, with many species switching between strategies to accommodate different prey. Many marine animals exhibit behavioral flexibility when feeding to deal with spatial and temporal heterogeneity in prey resources. However, little is known about flexibility in the feeding behavior of many large marine predators. Here, we documented the feeding behavior and kinematics of the endangered Hawaiian monk seal (Neomonachus schauinslandi, n=7) through controlled feeding trials. Seals were fed multiple prey types (e.g. night smelt, capelin, squid and herring) that varied in size and shape to examine behavioral flexibility in feeding. Hawaiian monk seals primarily used suction feeding (91% of all feeding trials) across all prey types, but biting, specifically pierce feeding, was also observed (9% of all feeding trials). Suction feeding was characterized by shorter temporal events, a smaller maximum gape and gape angle, and a fewer number of jaw motions than pierce feeding; suction feeding kinematic performance was also more variable compared with pierce feeding. Seals showed behavioral flexibility in their use of the two strategies. Suction feeding was used most frequently when targeting small to medium sized prey and biting was used with increasing frequency on larger prey. The feeding kinematics differed between feeding strategies and prey types, showing that Hawaiian monk seals adjusted their behaviors to particular feeding contexts. Hawaiian monk seals are opportunistic marine predators and their ability to adapt their feeding strategy and behavior to specific foraging scenarios allows them to target diverse prey resources.

Future Research Directions on the “Elusive” White Shark

White sharks, Carcharodon carcharias, are often described as elusive, with little information available due to the logistical difficulties of studying large marine predators that make long-distance migrations across ocean basins. Increased understanding of aggregation patterns, combined with recent advances in technology have, however, facilitated a new breadth of studies revealing fresh insights into the biology and ecology of white sharks. Although we may no longer be able to refer to the white shark as a little-known, elusive species, there remain numerous key questions that warrant investigation and research focus. Although white sharks have separate populations, they seemingly share similar biological and ecological traits across their global distribution. Yet, white shark’s behavior and migratory patterns can widely differ, which makes formalizing similarities across its distribution challenging. Prioritization of research questions is important to maximize limited resources because white sharks are naturally low in abundance and play important regulatory roles in the ecosystem. Here, we consulted 43 white shark experts to identify these issues. The questions listed and developed here provide a global road map for future research on white sharks to advance progress toward key goals that are informed by the needs of the research community and resource managers.

Complex biogeographical patterns support an ecological connectivity network of a large marine predator in the north‐east Atlantic

AbstractAimThe knowledge of a species biogeographical patterns greatly enhances our understanding of geographical ecology, which can improve identifying key conservation needs. Yet, this knowledge is still scarce for many marine top predators. Here, we aim to analyse movement patterns and spatial structuring of a large predator, the short‐finned pilot whale Globicephala macrorhynchus, over a wide geographical area.LocationNorth‐east Atlantic, in Macaronesian archipelagos (Azores, Madeira and Canaries) and Iberian Peninsula (Sagres).MethodsWe used likelihood techniques to estimate residency times and transition probabilities and carried out social analysis from individual photographic identification data, and analysed year‐round distribution from effort‐related sightings, collated between 1999 and 2015.ResultsThe best‐fitting models included emigration and reimmigration and showed different residency times within each archipelago. A total of 26 individual movements from 21 individuals (from a sample of >2,300 individuals) were recorded between Madeira and the neighbouring archipelagos, and heterogeneous transition probabilities were estimated within and between areas. A social network diagram showed associations from animals with distinct residency patterns. Higher significant sighting rates were recorded during autumn in the Azores and Madeira.Main conclusionsThe variation in site fidelity and year‐round occupancy among areas of the Macaronesia is consistent with some degree of population structuring, which combined with a connectivity network and seasonal inflows from animals inhabiting offshore waters, support the development of a complex social and geographical ecology in short‐finned pilot whales. The combination of techniques applied in this study was an effective way to estimate parameters of movement, which could be a good practice to be used for other scenarios and species.

Decline of coastal apex shark populations over the past half century

Overexploitation of large apex marine predators is widespread in the world’s oceans, yet the timing and extent of declines are poorly understood. Here we reconstruct a unique fisheries-independent dataset from a shark control programme spanning 1760 km of the Australian coastline over the past 55 years. We report substantial declines (74–92%) of catch per unit effort of hammerhead (Sphyrnidae), whaler (Carcharhinidae), tiger shark (Galeocerdo cuvier) and white sharks (Carcharodon carcharias). Following onset of the program in the 1960s, catch rates in new installations in subsequent decades occurred at a substantially lower rate, indicating regional depletion of shark populations over the past half a century. Concurrent declines in body size and the probability of encountering mature individuals suggests that apex shark populations are more vulnerable to exploitation than previously thought. Ongoing declines and lack of recovery of vulnerable and protected shark species are a cause for concern.

Dusky sharks (Carcharhinus obscurus) undertake large-scale migrations between tropical and temperate ecosystems

Understanding the large-scale migrations of marine predators can allow better representation of their population dynamics. The migration biology of dusky sharks (Carcharhinus obscurus), a cosmopolitan large marine predator with very low resilience to fishing, was quantified using a large-scale network of acoustic receivers deployed across Western Australia. Time-series plotting of individual shark detections and modified logistic modelling were used to determine the timing of acoustically tagged sharks’ seasonal migration, the proportion of the population migrating, and the size at which sharks start to migrate. Large (>200 cm fork length) dusky sharks migrated between areas closed (north) and open (south) to commercial shark fishing. There was limited evidence that smaller sharks occurred in the northern study area, whereas several larger individuals of both sexes undertook repeated north–south displacements, moving between disparate ecosystems within the Indian Ocean (21.7°S–35.4°S) and covering round-trip distances of 2000–3000 km per migratory event. For migrating individuals, the probability of occurring in the north was high in the austral winter–spring and low (males) to moderate (females) during the austral summer–autumn.

Ontogenetic partial migration is associated with environmental drivers and influences fisheries interactions in a marine predator

Abstract The ability to predict animal movement based on environmental change is essential for understanding the dynamic nature of their spatial ecology, and in turn the effectiveness of conservation strategies. We used a large marine predator that displays partial migration (the tiger shark Galeocerdo cuvier) as a model to test the role of oceanic conditions in predicting the space-use of different size classes. By using generalized additive mixed models (GAMMs), we revealed that environmental variables (sea surface temperature, primary productivity, thermal fronts, and bathymetry) had much greater predictive power for the movements of large, migratory tiger sharks than for small, resident individuals. We also found that coverage of tiger shark movements within “shark sanctuaries” (protected areas specifically for sharks) in the northwest Atlantic could be increased from 12 to 52% through inclusion of Bermuda’s waters. However, as large tiger sharks are migratory, over 80% of potential longline fisheries interactions would still occur outside the boundaries of even the expanded protected areas. This emphasises that management of highly migratory species needs to be dynamic and account for changing interactions with fisheries over time, which in a changing climate may rely on predicting movements based on oceanic conditions to be effective.

The utility of bioenergetics modelling in quantifying predation rates of marine apex predators: Ecological and fisheries implications

Predators play a crucial role in the structure and function of ecosystems. However, the magnitude of this role is often unclear, particularly for large marine predators, as predation rates are difficult to measure directly. If relevant biotic and abiotic parameters can be obtained, then bioenergetics modelling offers an alternative approach to estimating predation rates, and can provide new insights into ecological processes. We integrate demographic and ecological data for a marine apex predator, the broadnose sevengill shark Notorynchus cepedianus, with energetics data from the literature, to construct a bioenergetics model to quantify predation rates on key fisheries species in Norfolk Bay, Australia. We account for the uncertainty in model parameters by incorporating parameter confidence through Monte Carlo simulations and running alternative variants of the model. Model and parameter variants provide alternative estimates of predation rates. Our simplest model estimates that ca. 1130 ± 137 N. cepedianus individuals consume 11,379 (95% CI: 11,111–11,648) gummy sharks Mustelus antarcticus (~21 tonnes) over a 36-week period in Norfolk Bay, which represents a considerable contribution to total predation mortality on this key fishery species. This study demonstrates how the integration of ecology and fisheries science can provide information for ecosystem and fisheries management.

The trophic role of a large marine predator, the tiger shark Galeocerdo cuvier

Tiger sharks were sampled off the western (Ningaloo Reef, Shark Bay) and eastern (the Great Barrier Reef; GBR, Queensland and New South Wales; NSW) coastlines of Australia. Multiple tissues were collected from each shark to investigate the effects of location, size and sex of sharks on δ13C and δ15N stable isotopes among these locations. Isotopic composition of sharks sampled in reef and seagrass habitats (Shark Bay, GBR) reflected seagrass-based food-webs, whereas at Ningaloo Reef analysis revealed a dietary transition between pelagic and seagrass food-webs. In temperate habitats off southern Queensland and NSW coasts, shark diets relied on pelagic food-webs. Tiger sharks occupied roles at the top of food-webs at Shark Bay and on the GBR, but not at Ningaloo Reef or off the coast of NSW. Composition of δ13C in tissues was influenced by body size and sex of sharks, in addition to residency and diet stability. This variability in stable isotopic composition of tissues is likely to be a result of adaptive foraging strategies that allow these sharks to exploit multiple shelf and offshore habitats. The trophic role of tiger sharks is therefore both context- and habitat-dependent, consistent with a generalist, opportunistic diet at the population level.

Thermal stratification drives movement of a coastal apex predator

A characterization of the thermal ecology of fishes is needed to better understand changes in ecosystems and species distributions arising from global warming. The movement of wild animals during changing environmental conditions provides essential information to help predict the future thermal response of large marine predators. We used acoustic telemetry to monitor the vertical movement activity of the common dentex (Dentex dentex), a Mediterranean coastal predator, in relation to the oscillations of the seasonal thermocline during two summer periods in the Medes Islands marine reserve (NW Mediterranean Sea). During the summer stratification period, the common dentex presented a clear preference for the warm suprathermoclinal layer, and adjusted their vertical movements following the depth changes of the thermocline. The same preference was also observed during the night, when fish were less active. Due to this behaviour, we hypothesize that inter-annual thermal oscillations and the predicted lengthening of summer conditions will have a significant positive impact on the metabolic efficiency, activity levels, and population dynamics of this species, particularly in its northern limit of distribution. These changes in the dynamics of an ecosystem’s keystone predator might cascade down to lower trophic levels, potentially re-defining the coastal fish communities of the future.

The effects of familiarity on the social interactions of juvenile lemon sharks, Negaprion brevirostris

Investigating individual preferences using Social Network Analysis (SNA) has contributed to understanding group formations, which hold significant evolutionary value, and the dynamics occurring within them across the animal kingdom. Groups of sharks have been observed for centuries, and understanding the mechanisms that facilitate their formation furthers our comprehension of the adaptive value of sociality. However, there is a dearth of quantitative analyses on these mechanisms. The preference for familiars can influence group formation and facilitate an increased level of fitness. This preference has been documented across many vertebrate classes, but has not been shown to have an overall effect on partner preference in chondrichthyan fishes. This deficiency is significant due to the taxa's ecological diversification and importance. In this study, we used behavioral experiments to assess the potential role of familiarity in group formation and on social behavior in a large marine vertebrate. Juvenile lemon sharks, Negaprion brevirostris, (N=23) in Bimini, Bahamas, were separated into holding pens according to their size and given 14days to familiarize with their cohort. Two pairs of sharks were selected from separate holding pens and introduced in a social network pen for a behavioral trial. SNA indicated that N. brevirostris showed a significant preference for familiar individuals, performing more social interactions, such as closely following, with individuals from their holding pen. We suggest that this preference for familiars is developed through repeated interactions and facilitates the advantages of maintaining stable behavioral interactions, which results in spatial isolation from unfamiliars. These findings advance our understanding of the mechanisms driving social structure in N. brevirostris and other large marine predators, which contribute to an animal's overall fitness and evolutionary adaptability.

Simple models for studying complex spatiotemporal patterns of animal behavior

Minimal mathematical models able to explain complex patterns of animal behavior are essential parts of simulation systems describing large-scale spatiotemporal dynamics of trophic communities, particularly those with wide-ranging species, such as occur in pelagic environments. We present results obtained with three different modelling approaches: (i) an individual-based model of animal spatial behavior; (ii) a continuous taxis-diffusion-reaction system of partial-difference equations; (iii) a ‘hybrid’ approach combining the individual-based algorithm of organism movements with explicit description of decay and diffusion of the movement stimuli. Though the models are based on extremely simple rules, they all allow description of spatial movements of animals in a predator–prey system within a closed habitat, reproducing some typical patterns of the pursuit-evasion behavior observed in natural populations. In all three models, at each spatial position the animal movements are determined by local conditions only, so the pattern of collective behavior emerges due to self-organization. The movement velocities of animals are proportional to the density gradients of specific cues emitted by individuals of the antagonistic species (pheromones, exometabolites or mechanical waves of the media, e.g., sound). These cues play a role of taxis stimuli: prey attract predators, while predators repel prey. Depending on the nature and the properties of the movement stimulus we propose using either a simplified individual-based model, a continuous taxis pursuit-evasion system, or a little more detailed ‘hybrid’ approach that combines simulation of the individual movements with the continuous model describing diffusion and decay of the stimuli in an explicit way. These can be used to improve movement models for many species, including large marine predators.

Quantifying ontogenetic stable isotope variation between dermis and muscle tissue of two pelagic sharks

Comparative analysis of isotope values from different tissues can capture temporal variation in the trophic and foraging behavior of difficult to study large marine predators, reveal- ing either uniform or variable ecological roles over time. The isotopic values (δ 13 C and δ 15 N) of dermis, and muscle tissue of silky Carcharhinus falciformis and blue sharks Prionace glauca sam- pled in the northeast central Pacific were analyzed to quantify ontogenetic inter- and intra-tissue isotopic variation. Consistent differences in δ 15 N values occurred between dermis and muscle tis- sue for both species (2.5 ± 0.4 ‰ and 2.1 ± 0.3 ‰, respectively), while tissue differences in δ 13 C values were more variable between species (2.3 ± 0.6 ‰ and 0.7 ± 0.6 ‰, respectively), likely a result of tissue composition. The overall δ 15 N and δ 13 C values of dermis and muscle were highly correlated for blue sharks and for silky sharks with the exception of silky shark δ 13 C values. This pattern indicates that dermis isotope values are able to provide a proxy for muscle tissue, similar to that previously reported for fin, accepting dermis-specific diettissue discrimination factors. Tissue-specific ontogenetic isotopic variation for the silky shark, and the low regression slope value between dermis and muscle δ 13 C values, however, may suggest that dermis and muscle tissue have different isotopic turnover rates. These data demonstrate that dermis yields valuable isotope data to examine the trophic ecology and feeding/movement behavior of sharks, but further work is required to address dermis-specific turnover rates and diettissue discrimination factors.

Does optimal foraging theory predict the foraging performance of a large air-breathing marine predator?

Optimal foraging theory (OFT) suggests that air-breathing diving animals should minimize costs associated with feeding under water (e.g. travel time, oxygen loss) while simultaneously maximizing benefits gained from doing so (e.g. foraging time, energy gain). Humpback whales, Megaptera novaeangliae , foraging along the Western Antarctic Peninsula appear to forage according to OFT, but the direct costs and benefits in terms of their behaviours (e.g. allocation of time) have not been examined. We compared the foraging behaviour of humpback whales in this region inferred from multisensor high-resolution recording tags to their behaviour predicted by OFT time allocation models assuming the following currencies were being maximized: (1) the proportion of time spent foraging, (2) the net rate of energetic gain and/or (3) the ratio of energy gained to energy expended (i.e. efficiency). Model predictions for all three currencies were similar, suggesting any of these OFT models were suitable for comparison with the observed data. However, agreement between observed and optimal behaviours varied widely depending on the physiological and behavioural values used to derive optimal predictions, highlighting the need for an improved understanding of cetacean physiology. Despite this, many of the theoretical OFT predictions were supported: shallow dives (i.e.

Evaluation of an operational ocean model configuration at 1/12° spatial resolution for the Indonesian seas (NEMO2.3/INDO12) – Part 2: Biogeochemistry

Abstract. In the framework of the INDESO (Infrastructure Development of Space Oceanography) project, an operational ocean forecasting system was developed to monitor the state of the Indonesian seas in terms of circulation, biogeochemistry and fisheries. This forecasting system combines a suite of numerical models connecting physical and biogeochemical variables to population dynamics of large marine predators (tunas). The physical–biogeochemical coupled component (the INDO12BIO configuration) covers a large region extending from the western Pacific Ocean to the eastern Indian Ocean at 1/12° horizontal resolution. The NEMO-OPA (Nucleus for European Model of the Ocean) physical ocean model and the PISCES (Pelagic Interactions Scheme for Carbon and Ecosystem Studies) biogeochemical model are running simultaneously ("online" coupling), at the same resolution. The operational global ocean forecasting system (1/4°) operated by Mercator Océan provides the physical forcing, while climatological open boundary conditions are prescribed for the biogeochemistry. This paper describes the skill assessment of the INDO12BIO configuration. Model skill is assessed by evaluating a reference hindcast simulation covering the last 8 years (2007–2014). Model results are compared to satellite, climatological and in situ observations. Diagnostics are performed on nutrients, oxygen, chlorophyll a, net primary production and mesozooplankton. The model reproduces large-scale distributions of nutrients, oxygen, chlorophyll a, net primary production and mesozooplankton biomasses. Modelled vertical distributions of nutrients and oxygen are comparable to in situ data sets although gradients are slightly smoothed. The model simulates realistic biogeochemical characteristics of North Pacific tropical waters entering in the archipelago. Hydrodynamic transformation of water masses across the Indonesian archipelago allows for conserving nitrate and oxygen vertical distribution close to observations, in the Banda Sea and at the exit of the archipelago. While the model overestimates the mean surface chlorophyll a, the seasonal cycle is in phase with satellite estimations, with higher chlorophyll a concentrations in the southern part of the archipelago during the SE monsoon and in the northern part during the NW monsoon. The time series of chlorophyll a anomalies suggests that meteorological and ocean physical processes that drive the interannual variability of biogeochemical properties in the Indonesian region are reproduced by the model.

Food limitation of sea lion pups and the decline of forage off central and southern California.

California sea lions increased from approximately 50 000 to 340 000 animals in the last 40 years, and their pups are starving and stranding on beaches in southern California, raising questions about the adequacy of their food supply. We investigated whether the declining sea lion pup weight at San Miguel rookery was associated with changes in abundance and quality of sardine, anchovy, rockfish and market squid forage. In the last decade off central California, where breeding female sea lions from San Miguel rookery feed, sardine and anchovy greatly decreased in biomass, whereas market squid and rockfish abundance increased. Pup weights fell as forage food quality declined associated with changes in the relative abundances of forage species. A model explained 67% of the variance in pup weights using forage from central and southern California and 81% of the variance in pup weights using forage from the female sea lion foraging range. A shift from high to poor quality forage for breeding females results in food limitation of the pups, ultimately flooding animal rescue centres with starving sea lion pups. Our study is unusual in using a long-term, fishery-independent dataset to directly address an important consequence of forage decline on the productivity of a large marine predator. Whether forage declines are environmentally driven, are due to a combination of environmental drivers and fishing removals, or are due to density-dependent interactions between forage and sea lions is uncertain. However, declining forage abundance and quality was coherent over a large area (32.5–38° N) for a decade, suggesting that trends in forage are environmentally driven.

Elucidating fishing effects in a large-predator dominated system: The case of Darwin and Wolf Islands (Galápagos)

Fifty years of artisanal fishing history in Galápagos characterized by boom and bust of resources has changed the natural marine community structure close to fishing ports in the Galápagos Marine Reserve. Nonetheless, the remote Darwin and Wolf Islands are still in a “near natural state” harboring high species richness unique for the archipelago and high biomass. Marine life that aggregates to the pinnacles linked to the wider oceanic seascape makes them interesting and unique systems in terms of community structure and study of on-shore/offshore trophic interactions. Although >200km from local fishing ports the two islands also support an artisanal fishery targeting large demersal predators of high trophic levels. They are areas prioritized for conservation actions by Galápagos National Park, and local non-governmental organizations given their natural heritage importance, value to local fishers and revenue from dive tourism attracted by the promise of large marine predator aggregations such as sharks. Given that catch data upon which to base conservation recommendations is sparse it is difficult to accurately estimate present day fishing trends in Darwin and Wolf. We used a trophic steady-state model that connects benthic and pelagic communities to assess the potential effects of a change in fishing intensity in the two islands system. A predator–prey matrix was built using the Ecopath with Ecosim (EwE) software with total biomass estimated at 937tkm−2. Large fish aggregations accounted for 19%, primary producers for 9.0% and non-fish (seabirds, marine mammals and sea turtles) for less than 0.1% of the total living biomass. The total system size in terms of overall flows was 16,652tkm−2year−1, four times larger than a generic seamount model but smaller than that modeled for a Galápagos upwelling system based upon the western Bolivar Channel region. Consumption and respiration dominated the flows in the system (53.4 and 31.8% respectively). The Darwin and Wolf system shows a mature state, dominated by respiration, and top–down control via sharks and benthic predatory fish as the most important system compartments in terms of biomass. Model simulations suggest that heavy fishing pressure on top predators would greatly change the system's trophic structure. The nature of impacts depended on the types of gears simulated. Reduced natural predator densities through fishing exert strong direct and indirect influences on the system towards sea urchin-dominated benthos.

Diving deeper into individual foraging specializations of a large marine predator, the southern sea lion.

Despite global declines in the abundance of marine predators, knowledge of foraging ecology, necessary to predict the ecological consequences of large changes in marine predator abundance, remains enigmatic for many species. Given that populations suffering severe declines are of conservation concern, we examined the foraging ecology of southern sea lions (SSL) (Otaria flavescens)-one of the least studied otariids (fur seal and sea lions)-which have declined by over 90% at the Falkland Islands since the 1930s. Using a combination of biologging devices and stable isotope analysis of vibrissae, we redress major gaps in the knowledge of SSL ecology and quantify patterns of individual specialization. Specifically, we revealed two discrete foraging strategies, these being inshore (coastal) and offshore (outer Patagonian Shelf). The majority of adult female SSL (72% or n = 21 of 29 SSL) foraged offshore. Adult female SSL that foraged offshore travelled further (92 ± 20 vs. 10 ± 4 km) and dived deeper (75 ± 23 vs. 21 ± 8 m) when compared to those that foraged inshore. Stable isotope analysis revealed long-term fidelity (years) to these discrete foraging habitats. In addition, we found further specialization within the offshore group, with adult female SSL separated into two clusters on the basis of benthic or mixed (benthic and pelagic) dive behavior (benthic dive proportion was 76 ± 9 vs. 51 ± 8%, respectively). We suggest that foraging specialization in depleted populations such as SSL breeding at the Falkland Islands, are influenced by foraging site fidelity, and could be independent of intraspecific competition. Finally, the behavioral differences we describe are crucial to understanding population-level dynamics, impediments to population recovery, and threats to population persistence.