G.J. Greg Hofmeyr

Postsealing genetic variation and population structure of two species of fur seal (Arctocephalus gazella and A. tropicalis)

Commercial sealing in the 18th and 19th centuries had a major impact on the Antarctic and subantarctic fur seal populations (Arctocephalus gazella and A. tropicalis) in the Southern Ocean. The intensive and unrestricted nature of the industry ensured substantial reductions in population sizes and resulted in both species becoming locally extinct at some sites. However, both species are continuing to recover, through the recolonization of islands across their former range and increasing population size. This study investigated the extent and pattern of genetic variation in each species to examine the hypothesis that higher levels of historic sealing in A. gazella have resulted in a greater loss of genetic variability and population structure compared with A. tropicalis. A 316‐bp section of the mitochondrial control region was sequenced and revealed nucleotide diversities of 3.2% and 4.8% for A. gazella and A. tropicalis, respectively. There was no geographical distribution of lineages observed within either species, although the respective ΦST values of 0.074 and 0.19 were significantly greater than zero. These data indicate low levels of population structure in A. gazella and relatively high levels in A. tropicalis. Additional samples screened with restriction endonucleases were incorporated, and the distribution of restriction fragment length polymorphism (RFLP) and sequence haplotypes were examined to identify the main source populations of newly recolonized islands. For A. tropicalis, the data suggest that Macquarie Island and Iles Crozet were probably recolonized by females from Marion Island, and to a lesser extent Ile Amsterdam. Although there was less population structure within A. gazella, there were two geographical regions identified: a western region containing the populations of South Georgia and Bouvetøya, which were the probable sources for populations at Marion, the South Shetland and Heard Islands; and an eastern region containing the panmictic populations of Iles Kerguelen and Macquarie Island. The latter region may be a result of a pronounced founder effect, or represent a remnant population that survived sealing at Iles Kerguelen.

Changes in population sizes and distribution of fur seals at Marion Island

Abstract Population censuses of the Antarctic fur seal (Arctocephalus gazella) and the sub-Antarctic fur seal (A. tropicalis) were conducted during the 1994/1995 breeding season at Marion Island. Pup numbers, determined from direct counts and a mark-recapture experiment, were used to estimate population sizes. Pup numbers of A. tropicalis showed a mean annual change of 2.0% over the previous 6 years, culminating in an estimated total population of 49, 523 for 1994/1995. The population appears to be entering the maturity phase of population growth and may therefore have recovered from the effects of uncontrolled sealing that ended in the early twentieth century. Numbers at the major colonies on Marion Island showed little change since 1989 and these sites may have reached carrying capacity. The extension of breeding to other parts of the island continues. Over the same period, A. gazella pup numbers showed a mean annual change of 17% and the total population numbered 1,205 in 1994/1995. This species has possibly entered the rapid recolonisation phase of population growth. A few hybrid seals were found.

Pup growth and maternal attendance patterns in Subantarctic fur seals

Samples of pups from the expanding populations of Subantarctic fur seals (Arctocephalus tropicalis) atMarion and Gough islands were weighed at specific ages between birth and weaning in the period 1993-2000. Growth was estimated and compared between years, sexes, sites of different density, and populations. Pups of comparable age were consistently heavier at Marion Island than at Gough Island, probably reflecting relative prey resource abundance. No changes in pupgrowth rates were apparent over the study period, nor was there any evidence that growth was affected by ENSO events. Measures of pup growth at Marion Island were complemented by measures of maternal attendance behaviour during three summer andwinter seasons. More years of data are required to determine patterns between attendance, growth, resource conditions and population abundance trends.

Participation in the winter haulout by southern elephant seals (Mirounga leonina)

Southern elephant seals haulout on land to moult, breed and for a third, unknown reason, which we refer to as the winter haulout. We used long-term mark-recapture data to estimate participation levels in the winter haulout by southern elephant seals at Marion Island. There was no evidence that participation levels varied between cohorts or between years. Participation differed between sexes, with males being more likely to haulout in winter, except in the first year of life where participation was equal. Within each sex, both age and reproductive status influenced participation, but age seemed to be the most influential determinant. Generally, immature male individuals hauled out year after year in winter. The results did not allow speculation as to the purpose(s) of the winter haulout.

Median pupping date, pup mortality and sex ratio of fur seals at Marion Island

Modelling fur seal populations requires the accurate assessment of demographic parameters such as age-specific mortality. Owing to the highly variable mortality rates that pups are subject to, mortality of this age class is perhaps the most important factor determining the number of surviving individuals within each cohort. Early pup mortality,sex ratio and median pupping date were determined for sympatric populations of Subantarctic fur seals (Arctocephalus tropicalis) and Antarctic fur seals (A. gazella) at Marion Island, Southern Ocean. Mortality for this species was density dependent, varying from 0.8% at low density sites to 10.9% at high density sites. More accurate methods employed at low-density sites showed a substantially higher pup mortality at three weeks of 1.3–3.4% for Subantarctic fur seals. The same method yielded a mortality estimate at four weeks of age of 1.1–5.1% for Antarctic fur seals. Despite the underestimate inherent in island-wide counts for Subantarctic fur seals, these estimates are still useful for observing temporal and spatial patterns. Sex ratios were at parity for newborns of both species. The sex ratio of eight-week-old Subantarctic fur seal pups was also at parity, which is unusual for fur seal populations. The median pupping dates determined for Subantarctic fur seals and Antarctic fur seals did not differ substantially from previous estimates.

Leucistic Antarctic fur seals at Bouvetøya

One leucistic and one partially leucistic Antarctic fur seal Arctocephalus gazella were seen at Bouvetøya during the 1996/97 austral summer. Both likely came from South Georgia, where this colour morph is common. No individuals of this colour morph were sighted during three subsequent expeditions to Bouvetøya. The prevalence of this colour morph in the abundant populations of the Scotia Arc may be due to founder effect, as at least one leucistic animal was present at South Georgia when the Antarctic fur seal was close to extinction.

SEASONAL SURVIVAL AND THE RELATIVE COST OF FIRST REPRODUCTION IN ADULT FEMALE SOUTHERN ELEPHANT SEALS

Abstract The annual cycle of adult female southern elephant seals (Mirounga leonina) can be divided into 2 pelagic phases, separated by relatively short terrestrial phases: breeding and molting. We used resighting data collected from tagged female southern elephant seals at Marion Island during 1986–1999 to investigate seasonal survival during the 2 pelagic phases in relation to reproductive experience. Mean postbreeding (pelagic phase between breeding and molting, about 62 days) survival of primiparous females was 0.830 compared to 0.912 for more-experienced females. Postmolting (pelagic phase between molting and breeding, about 255 days) survival was 0.847 (0.960 when rescaled to 62 days for comparative purposes) and was not dependent on reproductive experience. Postbreeding survival of experienced females was higher than postmolting survival, but per unit time the opposite applied. A 2-stage survival model, in which survival was constrained to be constant before 1994 (when the population was declining) and from 1994 onward (during the stable phase), had overwhelming support from the data. Postbreeding survival of primiparous females increased from 0.799 before 1994 to 0.880 from 1994 onward. Postmolting survival of all females also increased from 0.817 to 0.872 over the same period. Postbreeding and postmolting mortality risk varied independently over time, demonstrating the importance of an intra-annual approach in population studies of southern elephant seals.

Post-breeding at-sea movements of three central-place foragers in relation to submesoscale fronts in the Southern Ocean around Bouvetøya

Abstract At-sea behaviour of central-place foraging fur seals and penguins in the Southern Ocean is understudied during the latter stages of parental care and the subsequent pre-moulting period. This biologically important period is costly to investigate due to the risk (or certainty) of losing tracking instruments when the animals moult. Early in this period, parents must meet the increasing demands of larger, more mobile offspring that are still nutritionally dependent and then the parents must recover lost body condition prior to the onset of their annual moult. This study reports late-season, at-sea movement patterns of macaroni penguins, chinstrap penguins and adult female Antarctic fur seals from the subantarctic island Bouvetøya, in relation to remotely-sensed oceanographic features. Foraging trips differing significantly in direction and distance travelled compared to those performed earlier in the breeding season, coincide with the time when offspring would be expected to become independent. On these trips, macaroni penguins moved towards the Polar Front while chinstrap penguins and Antarctic fur seals moved southward. Individuals from all three species appeared to target submesoscale ocean features once they were presumed to have been released from the constraints of feeding their young and were able to travel greater distances from the colony.

Evidence for density dependent population regulation in southern elephant seals in the southern Indian Ocean

ABSTRACT The means by which populations are regulated form a central theme in conservation biology, and much debate has revolved around density dependence as a mechanism driving population change. Marion Island (46°54′S, 37°45′E) is host to a relatively small breeding population of southern elephant seals, which like its counterparts in the southern Indian and southern Pacific Oceans, have declined precipitously over the past few decades. An intensive mark–recapture study, which commenced in 1983, has yielded a long time-series of resight data on this population. We used the program MARK to estimate adult female survival in this population from resight data collected over the period 1986–1999. Including concurrent population counts as covariates significantly improved our mark–recapture models and suggests density dependent population regulation to be operational in the population. Although predation may have been involved, it is far more likely that density dependent regulation has been based on a limited food supply. A significant increase in adult female survival was evident which is likely to have given rise to recent changes in population growth.

Counts of southern elephant seals, Mirounga leonina, at Bouvet Island

Abstract Southern elephant seals were counted and classified into subjective sex-age classes on a weekly basis during expeditions to Bouvet Island in the austral summers of 1996/1997 and 1998/1999. The expeditions coincided with the moulting period of elephant seals aged one year and older. The presence of weaned pups at the principal haulout site, Nyrøysa/Westwindstranda, during the latter expedition, indicates that breeding took place here during 1998. Elephant seal counts from previous expeditions are summarised.