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Allee effects play an important role in the dynamics of many populations and can increase the risk of local extinction. However, some authors have questioned the weight of evidence for Allee effects in wild populations. We therefore exploited a natural experiment provided by two adjacent breeding colonies of contrasting density to investigate the potential for Allee effects in an Antarctic fur seal (Arctocephalus gazella) population that is declining in response to climate change-induced reductions in food availability. Biometric time-series data were collected from 25 pups per colony during two consecutive breeding seasons, the first of which was among the worst on record in terms of breeding female numbers, pup birth weights and foraging trip durations. In previous decades when population densities were higher, pup mortality was consistently negatively density dependent, with rates of trauma and starvation scaling positively with density. However, we found the opposite, with higher pup mortality at low density and the majority of deaths attributable to predation. In parallel, body condition was depressed at low density, particularly in the poor-quality season. Our findings shed light on Allee effects in wild populations and highlight a potential emerging role of predators in the ongoing decline of a pinniped species.

The efficiency with which individuals extract energy from their environment defines their survival and reproductive success, and thus their selective contribution to the population. Individuals that forage more efficiently (i.e., when energy gained exceeds energy expended) are likely to be more successful at raising viable offspring than individuals that forage less efficiently. Our goal was to test this prediction in large long-lived mammals under free-ranging conditions. To do so, we equipped 20 lactating Antarctic fur seals (Arctocephalus gazella) breeding on Kerguelen Island in the Southern Ocean with tags that recorded GPS locations, depth and tri-axial acceleration to determine at-sea behaviours and detailed time-activity budgets during their foraging trips. We also simultaneously measured energy spent at sea using the doubly-labeled water (DLW) method, and estimated the energy acquired while foraging from 1) type and energy content of prey species present in scat remains, and 2) numbers of prey capture attempts determined from head acceleration. Finally, we followed the growth of 36 pups from birth until weaning (of which 20 were the offspring of our 20 tracked mothers), and used the relative differences in body mass of pups at weaning as an index of first year survival and thus the reproductive success of their mothers. Our results show that females with greater foraging efficiencies produced relatively bigger pups at weaning. These mothers achieved greater foraging efficiency by extracting more energy per minute of diving rather than by reducing energy expenditure. This strategy also resulted in the females spending less time diving and less time overall at sea, which allowed them to deliver higher quality milk to their pups, or allowed their pups to suckle more frequently, or both. The linkage we demonstrate between reproductive success and the quality of individuals as foragers provides an individual-based quantitative framework to investigate how changes in the availability and accessibility of prey can affect fitness of animals.

In animal ecology, a question of key interest for aquatic species is how changes in movement behavior are related in the horizontal and vertical dimensions when individuals forage. Alternative theoretical models and inconsistent empirical findings mean that this question remains unresolved. Here we tested expectations by incorporating the vertical dimension (dive information) when predicting switching between movement states (\"resident\" or \"directed\") within a state-space model. We integrated telemetry-based tracking and diving data available for four seal species (southern elephant, Weddell, antarctic fur, and crabeater) in East Antarctica. Where possible, we included dive variables derived from the relationships between (1) dive duration and depth (as a measure of effort), and (2) dive duration and the postdive surface interval (as a physiological measure of cost). Our results varied within and across species, but there was a general tendency for the probability of switching into \"resident\" state to be positively associated with shorter dive durations (for a given depth) and longer postdive surface intervals (for a given dive duration). Our results add to a growing body of literature suggesting that simplistic interpretations of optimal foraging theory based only on horizontal movements do not directly translate into the vertical dimension in dynamic marine environments. Analyses that incorporate at least two dimensions can test more sophisticated models of foraging behavior.

1. Energy expenditure is an important component of foraging ecology, but is extremely difficultto estimate in free-ranging animals and depends on how animals partition their timebetween different activities during foraging. Acceleration data have emerged as a new way todetermine energy expenditure at a fine scale but this needs to be tested and validated in wildanimals.2. This study investigated whether vectorial dynamic body acceleration (VeDBA) could accuratelypredict the energy expended by marine predators during a full foraging trip. We alsoaimed to determine whether the accuracy of predictions of energy expenditure derived fromacceleration increased when partitioned by different types of at-sea activities (i.e. diving, transiting,resting and surface activities).3. To do so, we equipped 20 lactating northern (Callorhinus ursinus) and 20 lactating Antarcticfur seals (Arctocephalus gazella) with GPS, time-depth recorders and tri-axial accelerometersand obtained estimates of field metabolic rates using the doubly labelled water (DLW)method. VeDBA was derived from tri-axial acceleration, and at-sea activities (diving, transiting,resting and surface activities) were determined using dive depth, tri-axial acceleration andtravelling speed.4. We found that VeDBA did not accurately predict the total energy expended by fur sealsduring their full foraging trips (R2 = 036). However, the accuracy of VeDBA as a predictorof total energy expenditure increased significantly when foraging trips were partitioned byactivity and when activity-specific VeDBA was paired with time-activity budgets (R2 = 070).Activity-specific VeDBA also accurately predicted the energy expenditures of each activityindependent of each other (R2 > 085).5. Our study confirms that acceleration is a promising way to estimate energy expenditures offree-ranging marine mammals at a fine scale never attained before. However, it shows that itneeds to be based on the time-activity budgets that make up foraging trips rather than beingderived as a single measure of VeDBA applied to entire foraging trips. Our activity-basedmethod provides a cost-effective means to accurately calculate energy expenditures of fur sealsusing acceleration and time-activity budgets, that can be transfered to studies on other species.

The waters off Sanriku (located on the northeastern coast of Honshu Island, Japan) provide the southernmost habitats of northern fur seals ( Callorhinus ursinus ) during winter and spring in the western North Pacific. The southward flowing cold Oyashio current and northward-flowing warm Kuroshio extension mix there, making the area highly productive. Northern fur seals migrate into these waters from the breeding rookeries for feeding, and the locations of the southern margins of their habitats vary yearly. The key questions for understanding the seasonal migration patterns are “why” and “how” the species utilize these waters as the southernmost habitat. We estimated the density and abundance of northern fur seals using standard line-transect theory combined with habitat modeling. The spatial patterns of animal density were analyzed using generalized additive models with seven static and dynamic environmental covariates, and those covariates were selected based on Akaike’s information criterion (AIC). The lowest AIC model included depth, sea surface temperature, slope, and gradient in sea surface temperature. This model estimated well the spatial patterns of the density of the species, in which fur seals were widely distributed in the study areas, but less frequently encountered between the isobaths 100 m and 200 m. These spatially separated habitats suggest that the shelf break and offshore front play an important role in creating the feeding grounds of fur seals. On the other hand, sea surface temperature positively correlated with fur seals’ density up to 14°C. This may indicate that further warm waters work as a temperature barrier, and fur seals concentrate on the edge of suitable temperature ranges.

Intensive human exploitation of the Antarctic fur seal (Arctocephalus gazella) in its primary population centre on sub-Antarctic South Georgia, as well as on other sub-Antarctic islands and parts of the South Shetland Islands, in the eighteenth and nineteenth centuries rapidly brought populations to the brink of extinction. The species has now recovered throughout its original distribution. Non-breeding and yearling seals, almost entirely males, from the South Georgia population now disperse in the summer months far more widely and in higher numbers than there is evidence for taking place in the pre-exploitation era. Large numbers now haul out in coastal terrestrial habitats in the South Orkney Islands and also along the north-east and west coast of the Antarctic Peninsula to at least Marguerite Bay. In these previously less- or non-visited areas, the seals cause levels of damage likely never to have been experienced previously to fragile terrestrial habitats through trampling and over-fertilisation, as well as eutrophication of sensitive freshwater ecosystems. This increased area of summer impact is likely to have further synergies with aspects of regional climate change, including reduction in extent and duration of sea ice permitting seals access farther south, and changes in krill abundance and distribution. The extent and conservation value of terrestrial habitats and biodiversity now threatened by fur seal distribution expansion, and the multiple anthropogenic factors acting in synergy both historically and to the present day, present a new and as yet unaddressed challenge to the agencies charged with ensuring the protection and conservation of Antarctica’s unique ecosystems.

We propose a continuous-time version of the correlated random walk model for animal telemetry data. The continuous-time formulation allows data that have been nonuniformly collected over time to be modeled without subsampling, interpolation, or aggregation to obtain a set of locations uniformly spaced in time. The model is derived from a continuous-time Ornstein-Uhlenbeck velocity process that is integrated to form a location process. The continuous-time model was placed into a state—space framework to allow parameter estimation and location predictions from observed animal locations. Two previously unpublished marine mammal telemetry data sets were analyzed to illustrate use of the model, by-products available from the analysis, and different modifications which are possible. A harbor seal data set was analyzed with a model that incorporates the proportion of each hour spent on land. Also, a northern fur seal pup data set was analyzed with a random drift component to account for directed travel and ocean currents.

While the majority of studies have concluded that sleep deprivation causes detrimental effects on various cognitive processes, some studies reported conflicting results. We examined the effects of a 108-h total sleep deprivation (TSD) on working memory in the northern fur seal, an animal with unusual sleep phenomenology and long-range annual migrations. The performance of fur seals was evaluated in a two-choice visual delayed matching to sample (DMTS) task, which is commonly used to evaluate working memory. In baseline conditions, the performance of fur seals in a DMTS task based on the percentage of errors was somewhat comparable with that in nonhuman primates at similar delays. We have determined that a 108-h TSD did not affect fur seals’ performance in a visual DMTS task as measured by overall percentage of errors and response latencies. On the contrary, all fur seals improved task performance over the study, including the baseline, TSD and recovery conditions. In addition, TSD did not change the direction and strength of the pattern of behavioral lateralization in fur seals. We conclude that a 108-h TSD did not interfere with working memory in a DMTS test in northern fur seals.

Understanding the effects of human exploitation on the genetic composition of wild populations is important for predicting species persistence and adaptive potential. We therefore investigated the genetic legacy of large-scale commercial harvesting by reconstructing, on a global scale, the recent demographic history of the Antarctic fur seal (Arctocephalus gazella), a species that was hunted to the brink of extinction by 18th and 19th century sealers. Molecular genetic data from over 2,000 individuals sampled from all eight major breeding locations across the species’ circumpolar geographic distribution, show that at least four relict populations around Antarctica survived commercial hunting. Coalescent simulations suggest that all of these populations experienced severe bottlenecks down to effective population sizes of around 150–200. Nevertheless, comparably high levels of neutral genetic variability were retained as these declines are unlikely to have been strong enough to deplete allelic richness by more than around 15%. These findings suggest that even dramatic short-term declines need not necessarily result in major losses of diversity, and explain the apparent contradiction between the high genetic diversity of this species and its extreme exploitation history.

The marginal male hypothesis is a mechanism proposed to explain gregariousness in pinnipeds. Here, we explore whether this mechanism, combined with density-dependent pup survival, can also account for heterogeneity in density across colonies, as observed in Antarctic fur seals ( Arctocephalus gazella ). We built a discrete-time matrix model inspired by the Antarctic fur seal to investigate how differences in density between two colonies can emerge through mate choice and density-dependent pup mortality. Our model assumes a heritable male colony preference that is coupled to competitive ability, i.e. a local siring advantage. Female colony preference is modelled as an independent trait that is allowed to evolve separately. Pup survival decreases with local density. We observe slight differences in density between the colonies at equilibrium. When siring advantage is high and the survival penalty for males with siring advantage is low, the colony that is preferred by the high-quality males becomes the less dense colony. Our model serves as a proof of concept that such density differences can emerge, although the predicted density differences are much smaller than those observed in the field. We expect that by including additional environmental factors, future models will explain more of the observed variation in density.