Accelerometers Identify New Behaviors and Show Little Difference in the Activity Budgets of Lactating Northern Fur Seals (Callorhinus ursinus) between Breeding Islands and Foraging Habitats in the Eastern Bering Sea

We tagged 82 lactating northern fur seals (Callorhinus ursinus) with tri-axial accelerometers and magnetometers on two eastern Bering Sea islands (Bogoslof and St. Paul) with contrasting population trajectories. Using depth data, accelerometer data and spectral analysis we classified time spent diving (30%), resting (~7%), shaking and grooming their pelage (9%), swimming in the prone position (~10%) and two types of previously undocumented rolling behavior (29%), with the remaining time (~15%) unspecified. The reason for the extensive rolling behavior is not known. We ground-truthed the accelerometry signals for shaking and grooming and rolling behaviors—and identified the acceleration signal for porpoising—by filming tagged northern fur seals in captivity. Speeds from GPS interpolated data indicated that animals traveled fastest while in the prone position, suggesting that this behavior is indicative of destination-based swimming. Very little difference was found in the percentages of time spent in the categorical behaviors with respect to breeding islands (Bogoslof or St. Paul Island), forager type (cathemeral or nocturnal), and the region where the animals foraged (primarily on-shelf <200m, or off-shelf > 200m). The lack of significant differences between islands, regions and forager type may indicate that behaviors summarized over a trip are somewhat hardwired even though foraging trip length and when and where animals dive are known to vary with island, forager type and region.     

Inter-island variation in the diet of female northern fur seals (Callorhinus ursinus) in the Bering Sea

The diet of adult female northern fur seals ( Callorhinus ursinus ) is examined through the analysis of faecal material collected during the summer breeding season at three breeding locations in the Bering Sea: St. Paul Island (1988, 1990) and St. George Island (1988, 1990) of the Pribilof Islands Group (USA), and Medny Island (1990) of the Commander Islands Group (Russia). Prey consumption varies annually and accordingly with the physical and biological environment surrounding each island. Juvenile walleye pollock ( Theragra chalcogramma ) is the most common prey of northern fur seals from St. Paul Island; the island is surrounded by a broad neritic environment with widely separated frontal zones and is the greatest distance from the continental shelf-edge. Gonatid squid ( Gonatopsis borealis/Berryteuthis magister and Gonatus madokail Gonatus middendorffi ) were the most common prey of northern fur seals from Medny Island; the island is surrounded by a compressed neritic environment and is adjacent to the continental shelf-edge and the oceanic marine environment. A combination of walleye pollock and gonatid squid is consumed by northern fur seals from St. George Island; the island has a surrounding oceanographic environment intermediate between the other two islands.
Variability in predation on walleye pollock is consistent with fishery information concerning the relative abundance and availability of walleye pollock around St. George and St. Paul Islands. The abundance and availability of these prey resources during the summer breeding season are key factors which influence the health and growth of the northern fur seal populations in the Bering Sea.     

Seasonal diving behaviour in lactating subantarctic fur seals on Amsterdam Island

Diving behaviour was investigated in female subantarctic fur seals (Arctocephalus tropicalis) breeding on Amsterdam Island, Indian Ocean. Data were collected using electronic Time Depth Recorders on 19 seals during their first foraging trip after parturition in December, foraging trips later in summer, and during winter. Subantarctic fur seals at Amsterdam Island are nocturnal, shallow divers. Ninety-nine percent of recorded dives occurred at night. The diel dive pattern and changes in dive parameters throughout the night suggest that fur seals follow the nycthemeral migrations of their main prey. Seasonal changes in diving behaviour amounted to the fur seals performing progressively deeper and longer dives from their first foraging trip through winter. Dive depth and dive duration increased from the first trip after parturition (16.6 ± 0.5 m and 62.1 ± 1.6 s respectively, n=1000) to summer (19.0 ± 0.4 m and 65 ± 1 s, respectively, n=2000) through winter (29.0 ± 1.0 m and 91.2 ± 2.2 s, respectively, n=800). In summer, subantarctic fur seals increased the proportion of time spent at the bottom during dives of between 10 and 20 m, apparently searching for prey when descending to these depths, which corresponded to the oceanic mixed layer. In winter, fur seals behaved similarly when diving between 20 and 50 m, suggesting that the most profitable depths for feeding moved down during the study period. Most of the dives did not exceed the physiological limits of individuals. Although dive frequency did not vary (10 dives/h of night), the vertical travel distance and the time spent diving increased throughout the study period, while the post-dive interval decreased, indicating that subantarctic fur seals showed a greater diving effort in winter, compared to earlier seasons. Accepted: 1 August 1999     

Sex‐related differences in the postmolt distribution of Weddell seals (Leptonychotes weddellii) in the southern Weddell Sea

The population of Weddell seals (Leptonychotes weddellii) in the southern Weddell Sea is in a unique position on the continental shelf edge, with vast shelf waters to the south, and deep Southern Ocean to the north. We describe sex‐related differences in the winter distribution of this population, from data collected by 20 conductivity‐temperature‐depth satellite relay data loggers deployed in February 2011 at the end of the annual molt. The regional daily speed was calculated, and a state‐space model was used to estimate behavioral states to positions along individuals’ tracks. GLMMs estimated that males and smaller individuals, diving in shallower water, traveled less far per day of deployment (males 14.6 ± 2.26 km/d, females 18.9 ± 2.42 km/d), and males were estimated to dive in shallower water (males 604 ± 382 m, females 1,875 ± 1,458 m). Males and smaller individuals were also estimated to be more resident; males spent an average 83.4% ± 7.7% of their time in a resident behavioral state, compared to females at 74.1% ± 7.1%. This evidence that male and female Weddell seals in the southern Weddell Sea are adopting different strategies has not been shown elsewhere along their circumpolar distribution.     

Individual foraging site fidelity in lactating New Zealand fur seals: Continental shelf vs. oceanic habitats

Wide‐ranging marine central place foragers often exhibit foraging site fidelity to oceanographic features over differing spatial scales (i.e., localized coastal upwellings and oceanic fronts). Few studies have tested how the degree of site fidelity to foraging areas varies in relation to the type of ocean features used. In order to determine how foraging site fidelity varied between continental shelf and oceanic foraging habitats, 31 lactating New Zealand fur seals (Arctocephalus australis forsteri 1 ) were satellite tracked over consecutive foraging trips (14–108 d). Thirty‐seven foraging trips were recorded from 11 females that foraged on the continental shelf, in a region associated with a coastal upwelling, while 65 foraging trips were recorded from 20 females that foraged in oceanic waters. There were no significant differences in the mean bearings (to maximum distance) of individual's consecutive foraging trips, suggesting individual fidelity to foraging areas. However, overlap in area and time spent in area varied considerably between continental shelf and oceanic foragers. Females that foraged on the continental shelf had significantly greater overlap in consecutive foraging trips when compared to females that foraged in oceanic waters (overlap in 5 × 5 km grid cells visited on consecutive trips 55.9%± 20.4% and 13.4%± 7.6%, respectively). Females that foraged on the continental shelf also spent significantly more time within the same grid cell than females that foraged in oceanic waters (maximum time spent in 5 × 5 km grid cells: 14%± 5% and 4%± 2%, respectively). This comparatively high foraging site fidelity may reflect the concentration of productivity associated with a coastal upwelling system, the Bonney Upwelling. Lower foraging site fidelity recorded by seals that foraged in oceanic waters implies a lower density/larger scale habitat, where prey are more dispersed or less predictable at fine scales, when compared to the continental shelf region.     

Foraging habitats based on the diet of female northern fur seals (Callorhinus ursinus) on the Pribilof Islands, Alaska

Scats (fecal samples) collected between 1987 and 2000 on northern fur seal Callorhinus ursinus rookeries of St Paul ( n =2968) and St George Islands ( n =1203), Alaska, were used to examine the relationship between breeding sites and food habits of adult female seals. On the basis of the frequency of occurrence (FO) and per cent minimum number of individual prey (%MNI) in scats, juvenile walleye pollock Theragra chalcogramma and gonatid squid Gonatopsis borealis/Berryteuthis magister and Gonatus madokai/Gonatus middendorffi were the dominant prey species consumed overall. Other primary prey (FO>5%) included Pacific sand lance Ammodytes hexapteus , Pacific herring Clupea pallasi , northern smoothtongue Leuroglossus schmidti , Atka mackerel Pleurogrammus monopterygius , Pacific salmon ( Oncorhynchus spp.) and other squid of the Gonatus genus. We identified five rookery complexes from a cluster analysis of the FO of primary prey in scats. Rookery complexes were separated geographically and each was further defined by characteristic patterns in the representation of prey types typically associated with specific hydrographic domains. Diet differences were observed among rookeries on the north and south side of St George Island and on the east, south and south-west side of St Paul Island. The rookery clusters observed in this study provide evidence of resource partitioning among adult female northern fur seals and have important implications for fur seal conservation and management.     

Activity‐specific metabolic rates for diving,transiting, and resting at sea can be estimated from time–activity budgets in free‐ranging marine mammals

Time and energy are the two most important currencies in animal bioenergetics. How much time animals spend engaged in different activities with specific energetic costs ultimately defines their likelihood of surviving and successfully reproducing. However, it is extremely difficult to determine the energetic costs of independent activities for free‐ranging animals. In this study, we developed a new method to calculate activity‐specific metabolic rates, and applied it to female fur seals. We attached biologgers (that recorded GPS locations, depth profiles, and triaxial acceleration) to 12 northern (Callorhinus ursinus) and 13 Antarctic fur seals (Arctocephalus gazella), and used a hierarchical decision tree algorithm to determine time allocation between diving, transiting, resting, and performing slow movements at the surface (grooming, etc.). We concomitantly measured the total energy expenditure using the doubly‐labelled water method. We used a general least‐square model to establish the relationship between time–activity budgets and the total energy spent by each individual during their foraging trip to predict activity‐specific metabolic rates. Results show that both species allocated similar time to diving (~29%), transiting to and from their foraging grounds (~26–30%), and resting (~8–11%). However, Antarctic fur seals spent significantly more time grooming and moving slowly at the surface than northern fur seals (36% vs. 29%). Diving was the most expensive activity (~30 MJ/day if done non‐stop for 24 hr), followed by transiting at the surface (~21 MJ/day). Interestingly, metabolic rates were similar between species while on land or while slowly moving at the surface (~13 MJ/day). Overall, the average field metabolic rate was ~20 MJ/day (for all activities combined). The method we developed to calculate activity‐specific metabolic rates can be applied to terrestrial and marine species to determine the energetic costs of daily activities, as well as to predict the energetic consequences for animals forced to change their time allocations in response to environmental shifts.     

Vagrant Antarctic fur seals at Gough Island in 2009

The Antarctic fur seal Arctocephalus gazella at Gough Island (40°20′S, 09°54′W) in the South Atlantic Ocean, first seen in October/November 2005, was recorded again in September–October 2009. Up to three different individual Antarctic fur seals were sighted on a single day, on a particular beach. A total of seven different individuals were recorded over a 3-week period, well before the onset of the breeding (pupping) season of the resident population of Subantarctic fur seals A. tropicalis. Positively identified individuals were all male, mostly subadult and lean. Only a fraction (~20%) of the available beaches was searched, and it is unknown if the Antarctic fur seals were still present at Gough Island during the austral summer breeding season of southern fur seals.     

MOVEMENTS OF SOUTHERN ELEPHANT SEALS AND SUBANTARCTIC FUR SEALS IN RELATION TO MARION ISLAND

Based primarily on an intensive marking/resighting program conducted at Marion Island (46°54'S, 37°45'E) in the Southern Ocean, the inter-island movements of southern elephant seals, Mirounga leonina , and fur seals Arctocephalus spp., were investigated to elucidate their little known pelagic phase. Southern elephant seals, in particular immature animals, readily move between the proximate Marion and Prince Edward islands. Some range as far afield as Iles Crozet, approximately 1,000 km distant where they haul out for the summer molt or during an autumn resting phase. The exchange of individuals between Marion Island and Iles Crozet during the return of immatures for the molt after a winter at sea, suggests overlapping of the foraging ranges of the two populations. Despite their wanderings, the majority of M. leonina from Marion Island probably feed in the proximity of the island, and relocate onto the island for breeding, molting and resting. Of the fur seals, only a few A. tropicalis were seen away from their natal island, in some cases covering distances in excess of 2,000 km. displaying a remarkable dispersal capacity.     

Prey selection of subadult male northern fur seals (Callorhinus ursinus) and evidence of dietary niche overlap with adult females during the breeding season

During the breeding season northern fur seals (Callorhinus ursinus) congregate on the Pribilof Islands in large numbers creating the potential for intraspecific competition. Due to the declining trend in the Pribilof Islands population of fur seals, it is important to understand how prey resources are partitioned among the population. Fur seals exhibit a high degree of sexual dimorphism resulting in energetic differences among age and sex classes. Therefore, we hypothesized that subadult male and adult female fur seals would differ in the type and size of prey consumed. We examined the diets of subadult male (age 2–8; mean mass 28–176 kg) and adult female (age ≥ 3 yr; mean mass 13–50 kg) seals on St. Paul Island from 1992 to 2000. Prey remains found in fecal samples were compared using niche overlap indices. There was nearly complete dietary niche overlap between subadult male and adult female fur seals. Walleye pollock (Theragra chalcogramma), Pacific salmon (Oncorhynchus spp.), Pacific herring (Clupeia pallasi), and cephalopods were common prey items found in the diets of both groups. We found differences in the size of pollock consumed and that geographic location of sample collection may be important in determining diet differences. Our results indicate high levels of dietary overlap among subadult male and adult female fur seals.     

Individual Foraging Strategies Reveal Niche Overlap between Endangered Galapagos Pinnipeds

Most competition studies between species are conducted from a population-level approach. Few studies have examined inter-specific competition in conjunction with intra-specific competition, with an individual-based approach. To our knowledge, none has been conducted on marine top predators. Sympatric Galapagos fur seals (Arctocephalus galapagoensis) and sea lions (Zalophus wollebaeki) share similar geographic habitats and potentially compete. We studied their foraging niche overlap at Cabo Douglas, Fernandina Island from simultaneously collected dive and movement data to examine spatial and temporal inter- and intra-specific competition. Sea lions exhibited 3 foraging strategies (shallow, intermediate and deep) indicating intra-specific competition. Fur seals exhibited one foraging strategy, diving predominantly at night, between 0–80 m depth and mostly at 19–22 h. Most sea lion dives also occurred at night (63%), between 0–40 m, within fur seals'' diving depth range. 34% of sea lions night dives occurred at 19–22 h, when fur seals dived the most, but most of them occurred at dawn and dusk, when fur seals exhibited the least amount of dives. Fur seals and sea lions foraging behavior overlapped at 19 and 21 h between 0–30 m depths. Sea lions from the deep diving strategy exhibited the greatest foraging overlap with fur seals, in time (19 h), depth during overlapping time (21–24 m), and foraging range (37.7%). Fur seals foraging range was larger. Cabo Douglas northwest coastal area, region of highest diving density, is a foraging “hot spot” for both species. Fur seals and sea lions foraging niche overlap occurred, but segregation also occurred; fur seals primarily dived at night, while sea lions exhibited night and day diving. Both species exploited depths and areas exclusive to their species. Niche breadth generally increases with environmental uncertainty and decreased productivity. Potential competition between these species could be greater during warmer periods when prey availability is reduced.     

Diving behaviour and foraging location of female southern elephant seals from Patagonia

Our aim was to describe the free-ranging diving pattern and to determine the location of foraging of pregnant female southern elephant seals, Mirounga leonina , from Peninsula Valdes, Argentina. This colony is unusual in two respects: it is removed from deep water by a broad shallow shelf (345–630 km wide), and colony numbers have been increasing in recent years in contrast to numbers from other southern hemisphere colonies that are stable or in decline. Microprocessor controlled, geolocation-time-depth recorders were deployed on four females, recording a total of 15,836 dives (270 dive days) during the period February to April, 1992. Departing seals crossed the continental shelf quickly (54–5–62–1 h) and did not show signs of foraging until reaching deep water, due east of the colony in the South Atlantic Ocean. Diving was virtually continuous (93% of the time underwater) with overall mean (±S.D.) rates of 2.5±0.2 dives/h, mean dive durations of 22.8 ± 7.1 min (maximum dive duration = 79 min) with 1.6±0.6min surface intervals between dives, and dive depths of 431±193m (maximum dive depth = 1,072 m). The diving pattern of females from Patagonia is similar to that of seals from colonies where numbers are decreasing (Macquarie stock) or are stable (South Georgia Island). Our subjects did not, however, feed in or south of the Antarctic Polar Front, or in cold waters along the Antarctic coast, where seals from declining or stable colonies forage.     

Summer diving and haul‐out behavior of leopard seals (Hydrurga leptonyx) near mesopredator breeding colonies at Livingston Island,Antarctic Peninsula

Leopard seals are conspicuous apex predators in Antarctic coastal ecosystems, yet their foraging ecology is poorly understood. Historically, the ecology of diving vertebrates has been studied using high‐resolution time‐depth records; however, to date such data have not been available for leopard seals. Twenty‐one time‐depth recorders were deployed on seasonally resident adult females in January and February between 2008 and 2014. The average deployment length was 13.65 ± 11.45 d and 40,308 postfilter dives were recorded on 229 foraging trips. Dive durations averaged 2.20 ± 1.23 min. Dives were shallow with 90.1% measuring 30 m or less, and a mean maximum dive depth of 16.60 ± 10.99 m. Four dive types were classified using a k‐means cluster analysis and compared with corresponding animal‐borne video data. Dive activity (number of dives/hour) was concentrated at night, including crepuscular periods. Haul‐out probabilities were highest near midday and were positively correlated with available daylight. Visual observations and comparisons of diving activity between and within years suggest individual‐based differences of foraging effort by time of day. Finally, dive and video data indicate that in addition to at‐surface hunting, benthic searching and facultative scavenging are important foraging strategies for leopard seals near coastal mesopredator breeding colonies.     

At-sea behavior of South American fur seals: Influence of coastal hydrographic conditions and physiological implication

At-sea behavior and effects of hydrographic conditions on the pelagic habitat use of South American fur seals (Arctocephalus australis) seasonally inhabiting the surrounding waters of shallow rocky reefs off Punta Mogotes (Mar del Plata, Argentina) were analyzed integrating geographic locations of fur seal groups (FSGs) with coastal hydrographic conditions and behavioral data in a Geographic Information System. Punta Mogotes rocky reefs represent a potentially high quality patch foraging area, crucial to a central place foraging species during their pelagic dispersion at sea. Fur seal behavior at-sea was strongly influenced by hydrographic conditions such us bathymetry, Beafourt sea state and sea surface current direction. Fur seals General Use Area (GUA) was associated with the 10 m isobaths, whereas Critical Use Area (CUA) was almost completely enclosed within the 5 m isobaths. A concentration-dispersion dynamic trend according to sea state was evident (GUA Beafourt ≤3 = 3.3 km² vs. GUA Beafourt >3 = 1.7 km²), with a “use area displacement” according to sea surface current direction. A general prevalence of long, at-sea resting periods (passive floating was the most frequently performed behavior, and usually for long periods) and a differential occurrence of each behavior associated with Beafourt sea states were detected. During calm seas (Beafourt ≤3), fur seals exhibit passive floating, occupying extended areas, and drifting according to sea surface current direction. With increasing sea states (Beafourt sea state >3), fur seals tended to perform shallow prolonged immersion and directional movements, and concentrated in restricted areas weakly affected by currents. The importance of floating periods at-sea, probably associated with resting and digestion, was interpreted as an energy conserving strategy that would allow an increase overall foraging efficiency. Results suggested that the ability of fur seals to perform certain behaviors that will allow completing physiological process and ultimately determine reproduction and survival success, would be conditioned by the hydrographic regime at foraging areas. This last could be extrapolated to other fur seal species spending long times at-sea, both as part of migration movements or during typical long foraging round trips.     

Movements and home ranges of monk seals in the main Hawaiian Islands

Hawaiian monk seals (Neomonachus schauinslandi) began recolonizing the main Hawaiian Islands (MHI) roughly 20 yr ago. The species’ abundance is still declining, but the subpopulation in the MHI is increasing by 6.5% per year. This difference may be due to differences in prey availability or habitat quality between the northwest (NWHI) and main Hawaiian Islands, which could be reflected in the movements and behavior of the seals. For example, foraging trip durations may be shorter in areas with higher forage quality. From 2007 to 2014 we deployed GPS phone tags on the islands of Molokai (n = 7), Kauai (n = 6), and Oahu (n = 6) to study movements and dive behavior. Foraging trips typically lasted 0.57 d (IQR: 0.34–0.83) and seals traveled 18 km (IQR: 10.2–30.5) per trip. Seals began benthic dives shortly after entering the water, with most dives to depths of 12–32 m. The median 95% and 50% kernel density isopleths for seals in the MHI were 149.2 km² and 23.2 km², respectively. The duration and distance of foraging trips in the MHI were shorter than that observed in other studies from the NWHI, suggesting that foraging habitat is currently better in the MHI.     

Inter‐colony foraging area segregation quantified in small colonies of Adélie Penguins

Theoretical models on the movement of colonial animals predict that neighbouring colonies may segregate their foraging areas, and many seabird studies have reported the presence of such segregations. However, these studies have often lacked the appropriate null model to test the effect of neighbouring colonies on foraging areas, especially in small colonies or in short‐ranging species. Here, we examined the foraging areas of Adélie Penguins Pygoscelis adeliae from two neighbouring (2 km apart) colonies by using bird‐borne GPS loggers. The field study was conducted at Hukuro Cove colony (104 pairs) and Mizukuguri Cove colony (338 pairs) in Lützow‐Holm Bay, East Antarctica. We obtained GPS tracks for 504 foraging trips from 48 chick‐rearing Adélie Penguins and quantified the degree of overlap in the foraging areas between two colonies. We also produced simulated movement tracks by using correlated random‐walks assuming no inter‐colony competition and quantified the degree of overlap in the simulated foraging areas. Finally, we compared the results from real GPS tracks with those from simulated tracks to examine the effect of neighbouring colonies on Adélie Penguin movement. The results indicate that the degree of overlap was significantly smaller in real tracks than in simulated tracks. In real tracks, the foraging area of the smaller Hukuro Cove colony extended to the other side of the larger Mizukuguri Cove colony, unlike in simulated tracks. Consequently, we suggest that Adélie Penguins from two neighbouring colonies segregated their foraging areas and that the larger colony appeared to affect the foraging area of the smaller colony.     

The behavioural response of Australian fur seals to motor boat noise

Australian fur seals breed on thirteen islands located in the Bass Strait, Australia. Land access to these islands is restricted, minimising human presence but boat access is still permissible with limitations on approach distances. Thirty-two controlled noise exposure experiments were conducted on breeding Australian fur seals to determine their behavioural response to controlled in-air motor boat noise on Kanowna Island (39°10'S, 146°18'E). Our results show there were significant differences in the seals' behaviour at low (64-70 dB) versus high (75-85 dB) sound levels, with seals orientating themselves towards or physically moving away from the louder boat noise at three different sound levels. Furthermore, seals responded more aggressively with one another and were more alert when they heard louder boat noise. Australian fur seals demonstrated plasticity in their vocal responses to boat noise with calls being significantly different between the various sound intensities and barks tending to get faster as the boat noise got louder. These results suggest that Australian fur seals on Kanowna Island show behavioural disturbance to high level boat noise. Consequently, it is recommended that an appropriate level of received boat sound emissions at breeding fur seal colonies be below 74 dB and that these findings be taken into account when evaluating appropriate approach distances and speed limits for boats.     

Telemetry tags increase the costs of swimming in northern fur seals,Callorhinus ursinus

Animal‐borne instruments have become a standard tool for collecting important data from marine mammals. However, few studies have examined whether placement of these data loggers affects the behavior and energetics of individual animals, potentially leading to biasing data. We measured the effect of two types of relatively small data loggers (<1% of animals’ mass and front profile) on the swimming speeds and energy expenditure of four female northern fur seals (Callorhinus ursinus) while swimming at depth. Swim speeds and rates of oxygen consumption were measured as the trained fur seals repeatedly swam an underwater circuit, with or without the tags. We found the placement of either tested tag significantly affected both the behavior and energetics of the fur seals in our study. Diving metabolic rate increased an average of 8.1%–12.3% (depending on tag type) and swim speed decreased an average of 3.0%–6.0% when wearing the tags. The combined changes in velocities and metabolic rates resulted in a 12.0%–19.0% increase in the total energy required by the fur seals to swim a set distance. The demonstrated effects of tags on behavior and energy expenditure may bias data sets from wild animals and potentially incur longer‐term impacts on the studied animals.     

DENSITY DEPENDENCE IN NORTHERN FUR SEALS (CALLORHINUS URSINUS)

This paper summarizes, updates, and interprets information on density-dependent dynamics of populations of the northern fur seal ( Callorhinus ursinus ). Density-dependent changes observed in these populations have involved various aspects of growth (body length, body weight, tooth weight, and size of other skeletal parts, especially the skull), survival, age at maturation, incidence of disease, and time spent foraging. For the population of northern fur seals on St. Paul Island of the Pribilof Islands, which was observed during a major increase and during two significant declines, density-dependent changes that occurred during the growth of the population were reversed during the declines. The Robben Island population in the western Pacific declined after 1965, and the decline was accompanied by changes similar to those observed during the declines on the Pribilof Islands. Although data are not available for all age and sex classes, it appears that most or all components of the populations exhibit similar changes. The overall implication of these changes is that current populations on Robben Island and the Pribilof Islands are reduced to levels below what could be supported by the resources available in their environments. Density dependence for this species is consistent with that of other large mammals, specifically in that vital rates for fur seals ate related to density in a nonlinear fashion.     

Fur seals at Macquarie Island: post-sealing colonisation,trends in abundance and hybridisation of three species

Commercial sealers exterminated the original fur seal population at Macquarie Island in the early 1800s. The first breeding record since the sealing era was not reported until March 1955. Three species of fur seal now occur at Macquarie Island, the Antarctic (Arctocephalus gazella), subantarctic (A. tropicalis) and New Zealand (A. forsteri) fur seal. Census data from 54 breeding seasons in the period 1954–2007 were used to estimate population status and growth for each species. Between the 1950s and 1970s, annual increases in pup production for the species aggregate were low. Between 1986 and 2007, pup production of Antarctic fur seals increased by about 8.8% per year and subantarctic fur seals by 6.8% per year. The New Zealand fur seal, although the most numerous fur seal species on Macquarie Island, has yet to establish a breeding population, due to the absence of reproductively mature females. Hybridisation among species is significant, but appears to be declining. The slow establishment and growth of fur seal populations on Macquarie Island appears to have been affected by its distance from major population centres and hence low immigration rates, asynchronous colonisation times of males and females of each species, and extensive hybridisation.