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Dispersal of male and female Antarctic fur seals (Arctocephalus gazella) I.L. Boyd, D.J. McCafferty, K. Reid, R. Taylor, and T.R. Walker

Abstract: This study examined the foraging locations of adult male and female Antarctic fur seals (Arctocephalus gazella) in the Scotia Sea during the postbreeding period. Satellite transmitters were used to track adult males and females and to obtain information about dive depths. Male fur seals migrated away from the breeding area during the postbreeding period whereas females remained close to the breeding grounds and foraged in the same area during two consecutive years. The most intensive foraging by females was associated with the edge of the continental shelf of South Georgia. Males dived deeper than females. Counts of males at South Georgia and at the South Orkney Islands support the result from satellite tracking data showing that males move from South Georgia to the South Orkney Islands at the end of the breeding season. Unlike males, females were limited in their foraging range by the necessity to return to feed dependent young, so breeding sites are likely to be located close to foraging areas that are optimal for females. Locations used for feeding by females were avoided by males, either because they were suboptimal for males or because foraging by females at South Georgia causes local depletion of food, and males, which have the option to forage further afield, can forage more successfully in regions where there are no females. Comparison with fisheries data also suggests that these fur seals are targeting the most abundant exploitable prey. Résumé : Cette étude porte sur les lieux d’alimentation des otaries des Kerguelen (Arctocephalus gazella) adultes, mâles et femelles, dans la mer de Scotia après la période de reproduction. On a utilisé des émetteurs satellitaires pour pister des mâles et des femelles adultes et pour obtenir de l’information sur leur profondeur de plongée. Les otaries mâles se sont éloignées de l’aire de reproduction après l’accouplement tandis que les femelles sont demeurées à proximité et se sont alimentées dans la même région durant deux années consécutives. C’est à la bordure de la plate-forme continentale de l’île South Georgia que les femelles se sont alimentées le plus intensivement. Les mâles plongeaient plus profondément que les femelles. Les dénombrements des mâles à l’île South Georgia et aux îles South Orkney confirment les résultats du pistage satellitaire qui montre que les mâles passent de l’île South Georgia aux îles South Orkney à la fin de la période de reproduction. À la différence des mâles, les femelles sont limitées quant à leur aire d’alimentation du fait qu’elles doivent retrouver leurs petits pour les nourrir, de sorte que les sites de reproduction doivent probablement se trouver près des aires d’alimentation qui sont optimales pour les femelles. Les lieux d’alimentation des femelles étaient évités par les mâles, soit qu’ils étaient sous-optimaux pour ces derniers, soit parce qu’en s’alimentant à l’île South Georgia, les femelles réduisent localement les ressources alimentaires, et les mâles, qui peuvent aller s’alimenter à distance des aires de reproduction, peuvent mieux se nourrir dans des régions exemptes de femelles. La comparaison avec les données sur les pêches laisse aussi penser que ces otaries recherchent la proie exploitable la plus abondante. [Traduit par la Rédaction]

Introduction The impact of predator populations on their prey has potential significance for the management of marine resources (Larkin 1996). Until recently, it has been difficult to estimate the spatial impact of predators on resources, even though reasonable estimates of the total impact of a predator population have been made (e.g., Lavigne et al. 1985; Butterworth et al. 1995; Shelton et al. 1995; Guinet et al. 1996). The introduction of methods of tracking seabirds (Jouventin and Weimerskirch 1990; Prince et al. 1997) and marine mammals (e.g., Martin et al. 1993; McConnell and Fedak 1996; Stewart et al. 1996) using

Received February 3, 1997. Accepted October 30, 1997. J13857 I.L. Boyd,1 D.J. McCafferty, K. Reid, R. Taylor, and T.R. Walker. British Antarctic Survey, Natural Environment Research Council, Madingley Road, Cambridge CB3 OET, U.K. 1

Author to whom all correspondence should be addressed. e-mail: [email protected]

Can. J. Fish. Aquat. Sci. 55: 845–852 (1998)

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satellite transmitters has begun to provide the level of information necessary to determine spatial patterns of impact. This study was designed as a first step towards estimating the dispersal of a marine mammal population that could eventually lead to estimates being made of the spatial impact it has on its prey. In addition, dispersal patterns of marine top predators may be used to infer important variation in the ecology and vulnerability of metapopulations or of the different sex and age classes. For example, among mammals, sexual dimorphism of body size is most evident in marine species, and this may lead to differences in the ecology of the two sexes. Adult male sperm whales (Physeter macrocephalus) are three times the mass of adult females and there is a strong effect of latitude on the sex ratio of the population, with the largest males being found at the highest latitudes (Best 1979). Male elephant seals (Mirounga sp.), which are three to six times larger than females, also forage in different areas than females (Hindell et al. 1991; LeBoeuf et al. 1993; Stewart and DeLong 1993). In this study, we examine the spatial distribution of male and female Antarctic fur seals (Arctocephalus gazella) during the postbreeding period (January–March) when both sexes © 1998 NRC Canada


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Fig. 1. Study region and geographic distribution of locations of Antarctic fur seals obtained from PTTs carried by each animal. Open circles are the locations of females in 1994–1995, the grey area is the location of females in 1995–1996, and solid circles are the locations of males in 1992–1993. The location of Bird Island, where the transmitters were deployed, is shown by the white star. The tracks of each male are shown individually and each position is linked by a line whereas the tracks of females are not shown individually. Arrows indicate the direction of movement of males 1 and 2. The 500-m isobath is shown.

depend mainly on a diet of krill (Euphausia superba) with a smaller proportion of fish (Daneri and Coria 1992; Reid and Arnould 1996). At this time, adult males, which are up to six times the mass of adult females, require food to recover from the mass lost during the breeding season (Boyd and Duck 1991), while most adult females are foraging to sustain themselves and a dependent pup (Boyd et al. 1991; Arnould et al. 1996b). Therefore, there is likely to be a high predator demand through this period. Moreover, the foraging range of lactating females is presumed to be restricted by the requirement to return to the pup, which remains ashore throughout the 4 months of lactation. There is little information about the dispersion patterns of marine mammals while foraging at sea. In cases where marine mammals interact with commercial fisheries, there is a need to understand the spatial impact and distribution of these predators. Therefore, this study examined the hypothesis that (i) geographic distribution and dive depths should differ between the sexes in a marine mammal that has sexual dimorphism of body size and (ii) using current instrumentation, it is possible to measure the relative impact of a marine mammal

predator at a spatial and temporal scale that will be useful to fisheries management.

Materials and methods Study area The study was centred on two specific locations in the region of the Scotia Sea: Bird Island (South Georgia) and Signy Island (South Orkney Islands) (Fig. 1). The southern Scotia Sea is covered seasonally by pack ice that occasionally extends as far north as South Georgia in late winter. Daily counts of the total number of adult male Antarctic fur seals were made at specific locations at Bird Island and Signy Island to document seasonal abundance patterns (Boyd 1989). These locations were consistent across years (1992–1995 at Bird Island and 1992–1994 at Signy Island). At Bird Island, counts were made between 1 November and 10 January and these covered the breeding season. Relatively small numbers of males are found at Bird Island, and South Georgia in general, at other times of year (Boyd 1989). At Signy Island, counts began on 1 January and continued until the sea froze, usually during June. No counts were made before 1 January because fur seals are not normally present before this date. © 1998 NRC Canada

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Boyd et al. Table 1. Deployments of PTTs on adult male and female Antarctic fur seals from Bird Island during the austral summers of 1992–1993, 1994–1995, and 1995–1996. Tag No. w3815 (male 1) w3817 (male 2) w3811 (male 3) w2403 w2943 w4687 w5201 w5319 w1639 w1753 w2385 w2935 w3805 w3847 w4605 w4633 w5149 w5344 w5346 w5347 w5350 w5351 w5354 w5355 a

Year

Sex

Number of locationsa

Start date of deployment

Duration of deployment (days)

Seal mass (kg)

1993–1994 1993–1994 1993–1994 1994–1995 1994–1995 1994–1995 1994–1995 1994–1995 1995–1996 1995–1996 1995–1996 1995–1996 1995–1996 1995–1996 1995–1996 1995–1996 1995–1996 1995–1996 1995–1996 1995–1996 1995–1996 1995–1996 1995–1996 1995–1996

Male Male Male Female Female Female Female Female Female Female Female Female Female Female Female Female Female Female Female Female Female Female Female Female

43 61 58 2 9 7 5 21 15 26 33 4 5 2 24 39 3 10 1 3 42 7 2 1

19 Dec. 1992 21 Dec. 1992 19 Dec. 1992 27 Dec. 1994 5 Feb. 1995 13 Jan. 1995 4 Dec. 1995 23 Jan. 1995 29 Jan. 1996 21 Jan. 1996 26 Jan. 1996 7 Jan. 1996 19 Feb. 1996 1 Feb. 1996 10 Feb. 1996 19 Feb. 1996 7 Dec. 1995 8 Dec. 1995 15 Dec. 1995 17 Dec. 1995 5 Jan. 1996 5 Jan. 1996 11 Jan. 1996 12 Jan. 1996

50 22 29 7 9 9 5 7 17 15 17 6 13 17 17 18 7 8 14 9 19 14 17 18

159.0 145.0 122.5 33.0 40.0 31.5 38.5 41.5 40.5 38.5 46.0 39.0 37.5 43.5 38.5 47.0 46.0 34.5 41.5 42.0 41.5 46.0 40.0 37.0

Only Argos location classes 0–3 included.

Table 2. Mean error (±SD), measured in terms of distance from a fixed point at 54.002°S, 38.0567°W, of position fixes obtained from Argos in relation to location class provided by Argos. Location class 3 2 1 0 A B

Table 3. Results of comparisons made between the dive depth distributions of pairs of Antarctic fur seal individuals and grouped by year and sex.

Mean distance (m)

Minimum distance (m)

Maximum distance (m)

n

Comparison

1 228 ± 957 1 115 ± 362 1 566 ± 1 082 3 779 ± 2 587 18 843 ± 42 604 22 841 ± 43 926

383 467 534 1140 1077 2532

5 368 1 763 6 153 8 595 133 017 131 028

25 21 29 7 14 8

Satellite tracking Both male and female fur seals were given flipper tags (Dalton Supplies, Henley-on-Thames, U.K.) and tracked at sea using satellitelinked platform transmitter terminals (PTTs) with the Argos satellite system (Argos 1989). PTTs were deployed on three adult males that were holding breeding territories during December 1992 (Table 1). Males were immobilised chemically using Zoletil (Boyd et al. 1990) and the PTTs (1.5 kg; Sea Mammal Research Unit, Natural Environment Research Council) were attached to the fur on the dorsal side of the neck with epoxy glue (Fedak et al. 1983). These PTTs measured and relayed the maximum depth of the dive made immediately before a transmission. Each transmission received by the satellite contained this information as a digital string in addition to the unique identification code of each transmitter. Depth was measured to an accuracy of 1 m from 10 to 35 m and to 5 m for depths >35 m. The maximum depth resolution was 250 m. Information about dives to 10 m depth that were transmitted to the satellite in relation to latitude.

20

Signy Island

0 54 55 56 57 58 59 69 61 62

Latitude (degrees south) was also used to analyse this record for individual dives. Dives were defined as excursions below the surface to depths >2 m. However, to provide a comparison with the data obtained from the PTTs on males, only dives to >10 m are presented. In December–March 1994–1995 and 1995–1996, PTTs (1 W, 250–500 g; Wildlife Computers) were deployed on five and 16 lactating adult females, respectively (Table 1). PTTs were deployed for two to four consecutive foraging trips (5–19 days) while females were foraging to support themselves and a dependent pup ashore at Bird Island. Females were restrained by the method described by Gentry and Holt (1982) and PTTs were attached with epoxy glue in the same way as for adult males except that the instruments were positioned in the middle of the back along the dorsal midline. PTTs on females also incorporated a data logger (256 kilobytes of memory) that recorded depth measured (accuracy 1 m) at 10-s intervals throughout the period of deployment or until the memory was exhausted (10–15 days). These data were recovered from the logger when the instrument was removed at the end of the deployment period. Depth information was used to reconstruct a continuous record of diving during the deployments using purpose-built software (Boyd 1996). As for the PTTs placed on males, those on females had a minimum transmission interval of 45 s (no duty cycling) and they also only operated when the antenna was above the water because of a submergence sensor close to the base of the antenna. The instrument stopped operating after 1 h without submergence. Accuracy of geographical locations from PTTs and data analysis The geographical location of animals was calculated from the Doppler shift of each transmission received by the satellite and from the reception of a minimum of three transmissions during each satellite pass. The location of the transmitter was estimated to have a precision of 0.001° of latitude and longitude ( 111 m). The Argos system divided the precision of calculated locations into six classes representing a gradation from high (class 3) to low confidence (class B) (Table 2). We quantified the degree of error associated with each of

these classes by examining the locations received from two PTTs placed at the site of deployment which was fixed using the Global Positioning System. The average location provided by Argos for the transmitters placed at a known location was within 1–2 km for location classes 1, 2, and 3 (Table 2). The maximum distance from the site for these three location classes was 6 km. On average, location class 0 gave a position 3.8 km from the actual location, with a maximum of 8.6 km (Table 2). Location classes A and B were clearly less precise (Table 2), with maximum potential errors of 130 km. Since we expected animals to be moving over ranges of 100–1000 km, we rejected the location classes A and B as having insufficient reliability. Therefore, the remainder of the analysis used location classes 0–3 only. To examine the distribution of females when at sea, the position of each dive made by females during 1996 was estimated by interpolation along a direct line between successive locations provided by the satellite. This assumed that animals moved at approximately constant speeds between successive points and that the position of each dive could be obtained from the time of the dive as an indication of the distance travelled between successive position fixes. The position determined in this way was rounded to the nearest 0.1° of latitude or longitude ( 11 km). Assumptions and statistics When comparing the distributions of dive depths among different sexes and years, it is necessary to account for differences among individuals. The lack of normality in the distributions of dive depths meant that the nonparametric Kolmogorov–Smirnov test was used to examine differences between the distributions of dive depths among individuals and groups. Since the Kolmogorov–Smirnov D-statistic is a measure of the absolute difference between two distributions, comparisons among years and between the sexes were made using the mean D-statistic for paired comparisons between individuals in each group. Therefore, by comparing the results of comparisons between the pooled samples for each age and sex class with the comparisons at the level of individuals, it was possible to examine the potential effects among sex and year classes. We have assumed that fur seals only feed when they dive and that diving is primarily associated with feeding or search for food.

Results Postbreeding movements of males and females During December–March, adult female Antarctic fur seals were distributed over the continental shelf of South Georgia and beyond the shelf edge to the northwest of the island whereas males travelled south towards the region of the Antarctic Peninsula (Fig. 1). Females ranged up to 350 km from Bird Island and, of the 261 locations, all were clustered in the same region in both years (Fig. 1). Of the three male fur seals that were tracked, one (male 1, Fig. 1) reached the South Orkney Islands. This animal was also sighted by personnel from the British Antarctic Survey Base at Signy Island. The same individual also made a foraging trip to the south of the South Orkney Islands where it remained over the continental shelf. It then returned to the South Orkney Islands before moving southwest when the transmissions stopped after 50 days of operation (Table 1). Between 11 and 24 January, male 1 transited from Bird Island to Signy Island, a distance of 886 km, at an average speed of 1.0 m⋅s–1. Evidence from the number of dives recorded that were >10 m depth suggests that male 1 did not feed intensively until he reached the shelf region of the South Orkney Islands (Fig. 2). Male 2 also travelled towards Signy Island, after departing © 1998 NRC Canada

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Fig. 3. Interpolated foraging locations of 16 female Antarctic fur seals from Bird Island (54°S, 38°W) during the postbreeding period (December–March) in 1995–1996. The greyscales show the percentage frequency of all dives occurring in each 0.1° × 0.1° square. The 500-, 1000-, and 2000-m isobaths are shown.

South Georgia on 2 January, but transmissions stopped on 22 January when this animal had completed about two thirds of the transit to Signy Island. The mean speed of travel during this period was 0.4 m⋅s–1. Male 3 was tracked between 28 December and 17 January and remained at South Georgia after having initially moved south on a parallel track with the other two seals. He then changed course to the southeast and eventually spent 5 days ashore at a location on the south coast of South Georgia before once again moving southwest approximately parallel with his initial track and with those of males 1 and 2. Soon after leaving South Georgia for a second time, the transmitter on this male stopped. Both males 2 and 3 appeared to feed intensively in the region of South Georgia after leaving Bird Island, as judged by diving activity (Fig. 2). Male 1 returned to his breeding site at South Georgia during the following winter and male 3 held his territory again during the following two breeding seasons. Male 2 was not seen again. The transmitters from both males 1 and 3 had evidently been lost because the hairs holding the transmitters had broken. There was a tendency for females to dive more frequently over the shallower zone of the continental shelf to the northwest of South Georgia and over the area of the contental slope, although females also extended their foraging range over deeper water (Fig. 3). However, the dive depths of both males and females were considerably shallower than the ocean depths in the regions in which foraging took place (Fig. 4 compared with Figs. 1 and 3).

Patterns of male abundance The number of male fur seals ashore at the study site on Bird Island reached a peak in early December and declined through late December and into January (Fig. 5). At Signy Island, there were no male fur seals at the study site through most of November and December, but abundance began to increase through January, with a peak in February. Fur seal abundance declined again to zero by late June (Fig. 5). Dive depths of fur seals Although there was overlap in the maximum dive depths of male and female fur seals (Fig. 4), when all dives were combined, the depth distribution of males in 1992–1993 was greater than for females in the same year (D = 0.468, P < 0.002). This distribution was also different from females in the other two years (P < 0.001 in both cases). To examine the effect that variation between individuals may have on these differences, the Kolmogorov–Smirnov D-statistic was also calculated for all possible paired comparisons between individuals and the means were compared among sex and year (Table 3). This showed that 97% (1031/1058) of the comparisons of dive depth distributions were different (P < 0.05). The mean D-statistic for paired comparisons between individuals among groups (Table 3) showed that there was an overall greater difference between the dive depth distributions of males in 1992–1993 and females in 1992–1993 and 1995– 1996. The dive depth distribution for males in 1992–1993 was © 1998 NRC Canada

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Fig. 4. Frequency distributions of dive depths of the three male Antarctic fur seals tracked during 1992–1993, lactating females during 1992–1993, and the females that were tracked during 1994–1995 and 1995–1996.

12 9 6 3 0

Males (1992/93) n = 200

% frequency

20

40

60

10 8 6 4 2 0 10 8 6 4 2 0 10 8 6 4 2 0

80

100

120

Females (1992/93) n = 27 006

20

40

60

80

100

120

Females (1994/ 95) n = 11 495

20

40

60

80

100

120

Females (1995/96) n = 55 953

20

40

60

80

100

120

Depth (m) more similar to females in 1994–1995. Dive depth distributions for the same sexes within specific years were generally more similar to each other than distributions compared across years and sexes.

Discussion Geographical distribution This study has documented the spatial distribution of Antarctic fur seals so that, at least for females, it may be possible to provide a fine-scale ( 10 km) estimation of their spatial impact on prey resources. It demonstrates that, using intensive deployments of satellite transmitters and appropriate filtering and verification of positional information, the foraging locations of individuals can be measured across consecutive years. This has the potential to add a fine-scale spatial dimension to estimates of predator impact and of potential competition with fisheries. Although only three males were tracked in one year and, of these, only one animal was observed to haul out at Signy Island, this provides a link between the temporal distribution of numbers of males at South Georgia compared with the South Orkney Islands (Fig. 5). It also seems probable that male 2 eventually moved to the South Orkney Islands and all three males tracked by satellite were located in separate

geographical areas from adult females during the postbreeding period of the annual cycle. Both anecdotal observations and counts of animals at South Georgia (Boyd 1989; Reid 1995; this study) suggest that males are either more widely distributed during the postbreeding period or they tend to leave the region of South Georgia at this time. Several lines of evidence suggest that many of these males migrate towards the South Orkney and South Shetland Islands at the end of the breeding season. These include the annual increase in the numbers of males at the South Orkney Islands following the end of the breeding season at South Georgia (Fig. 5), the satellite tracks of the three individuals examined in this study (Fig. 3), and regular observations of male fur seals at the South Orkney Islands with flipper tags that had been applied at South Georgia (British Antarctic Survey archive data). Jablonski et al. (1987) also showed peak numbers of male Antarctic fur seals in the South Shetland Islands during February and March. Moreover, the increasing number of males observed at the South Orkneys since the early 1970s (Smith 1988) has changed in parallel with the increasing numbers at South Georgia where ~96% of the world population breeds (Boyd 1993). This supports the view that the same individual males use both sites at different times of year. Previous detailed analysis of the foraging behaviour of female Antarctic fur seals (Croxall et al. 1985; Boyd and Croxall 1992; Boyd et al. 1994; Arnould et al. 1996a; Boyd 1996) has shown intensive foraging activity during the postbreeding period when most female fur seals are supporting dependent young. Females are restricted in their foraging range by the potential starvation duration of their pups, which remain ashore and have no source of food other than maternal milk until weaning. Therefore, unlike males, females must exploit local resources. This study has shown that males move to forage in different geographical areas during the postbreeding period. Although two of the males tracked in this study appeared to feed close to South Georgia, as judged by their diving activity (Fig. 2), they fed in a different region than females. These males were also apparently feeding in a region where there are no breeding females occupying the adjacent coastline (Boyd 1993). Differences in dive depths Programming methods may have biased the estimated dive depth distribution of males. For example, mainly deeper dives will be included if the male fur seals spent more time at the surface between deep dives than between shallow dives because more time will have been available to send information to the satellite after deeper dives. However, since the transmitter was designed to transmit immediately on reaching the surface and dives to 380 000 at South Georgia; Boyd 1993), even though large breeding colonies of fur seals have developed recently at the nearby South Shetland Islands (Bengtson et al. 1990). This may indicate that the local foraging conditions at the South Orkney Islands are not appropriate for females during nursing or that other local environmental peculiarities of the South Orkney Islands, such as prolonged sea ice cover in some years, may make this site unsuitable for breeding. Finally, krill fishery data from this region (Everson 1992) show that male and female fur seals from South Georgia are

Acknowledgements We thank colleagues at the British Antarctic field stations at Bird Island and Signy Island who assisted with data collection and animal handling. In particular, we thank Luke Bullough and Simon Brockington for carrying out counts of male fur seals at Signy Island, Bernie McConnell for providing advice on the use of the staellite transmitters purchased from the Sea Mammal Research Unit, and Dr. M. Hammill for reviewing the manuscript.

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