John L. Bengtson

Population growth of Antarctic fur seals : Limitation by a top predator, the leopard seal?

Antarctic fur seals (Arctocephalus gazella) in the South Shetland Islands are recovering from 19th-century exploitation more slowly than the main population at South Georgia. To document demographic changes associated with the recovery in the South Shetlands, we monitored fur seal abundance and reproduction in the vicinity of Elephant Island during austral summers from 1986/1987 through 1994/1995. Total births, mean and variance of birth dates, and average daily mortality rates were estimated from daily live pup counts at North Cove (NC) and North Annex (NA) colonies on Seal Island. Sightings of leopard seals (Hydrurga leptonyx) and incidents of leopard seal predation on fur seal pups were recorded opportunistically during daily fur seal research at both sites. High mortality of fur seal pups, attributed to predation by leopard seals frequently observed at NC, caused pup numbers to decline rapidly between January and March (i.e., prior to weaning) each year and probably caused a long-term decline in the size of that colony. The NA colony, where leopard seals were never observed, increased in size during the study. Pup mortality from causes other than leopard seal predation appeared to be similar at the two sites. The number of pups counted at four locations in the Elephant Island vicinity increased slowly, at an annual rate of 3.8%, compared to rates as high as 11% at other locations in the South Shetland Islands. Several lines of circumstantial evidence are con- sistent with the hypothesis that leopard seal predators limit the growth of the fur seal population in the Elephant Island area and perhaps in the broader population in the South Shetland Islands. The sustained growth of this fur seal population over many decades rules out certain predator-prey models, allowing inference about the interaction between leopard seals and fur seals even though it is less thoroughly studied than predator-prey systems of terrestrial vertebrates of the northern hemisphere. Top-down forces should be included in hypotheses for future research on the factors shaping the recovery of the fur seal population in the South Shetland Islands.

Arctic fox home range characteristics in an oil-development area

Spring and summer home ranges and local movements of arctic foxes (Alopex lagopus) were studied from 1975 to 1977 at the Prudhoe Bay oil-development area in northern Alaska. Twenty-seven adult and 62 juvenile foxes were captured, marked, and released. Nine adults and 5 juveniles were equipped with radio collars and monitored during 1976 and 1977. Home range size was 20.8 +/- 12.5 (SD) km/sup 2/ for 4 adult foxes and 3.7 +/- 1.7 km/sup 2/ for 5 juveniles. Home range configuration was similar for all marked members of individual families. Adult foxes were nocturnal and territorial. Foxes used oil-development sites for feeding, resting, and denning. Use of these became more comon late in the rearing season, as juveniles became more mobile. A major fluctuation in the availability of natural foods did not appear to alter ues of developed areas by foxes. The number of juvenile foxes observed at Prudhoe Bay decreased from 1976 to 1977, but the decrease was less pronounced than in a nearby undisturbed area.

Estimating Food Consumption of Free-Ranging Manatees in Florida

GRINNELL, J., J. S. DIXON, AND J. M. LINSDALE. 1937. Fur-bearing mammals of California. Univ. California Press, Berkeley. 777pp. HARGIS, C. D. 1981. Winter habitat utilization and food habits of the pine marten (Martes americana) in Yosemite National Park. M.S. Thesis, Univ. California, Berkeley. 57pp. KLECKA, W. R. 1975. Discriminant function analysis. Pages 434-467 in N. H. Nie, C. H. Hall, J. G. Jenkins, K. Steinbrenner, and D. H. Bent, eds. Statistical package for the social sciences. 2nd ed. McGraw-Hill Book Co., New York, N.Y. 675pp. KOEHLER, G. M., AND M. G. HORNOCKER. 1977. Fire effects on marten habitat in the SelwayBitterroot Wilderness. J. Wildl. Manage. 41:500505.

Diving patterns of a Ross seal (Ommatophoca rossii ) near the eastern coast of the Antarctic Peninsula

Abstract In January 1987 we documented the diving patterns of a female Ross seal (Ommatophoca rossii) in the marginal pack-ice zone near the eastern coast of the Antarctic Peninsula for 2 days using a microprocessor-based time-depth recorder. The seal hauled out during the day and dived continually when in the water at night. Dives averaged 110 m deep and 6.4 min long; the deepest dive was 212 m and the longest 9.8 min. Dives were deepest near twilight and shallowest at night; this pattern suggests that the seals prey, presumably mid-water squid and fish, may have been making vertical migrations or changing predator-avoidance behavior in response to diel light patterns. The dives of this Ross seal were substantially deeper, on average, than those of crabeater seals (Lobodon carcinophagus), which forage in the same areas on Antarctic krill (Euphausia superba).

Relationships between brood size and parental provisioning performance in chinstrap penguins during the chick guard phase

Abstract Chinstrap penguins (Pygoscelis antarctica) normally lay two eggs, but brood size is often reduced by mortality during incubation or after hatching. We hypothesized that this variation in brood size would affect the parents’ foraging behavior and their chick provisioning performance. We studied patterns of adult foraging trip duration and frequency, food load delivery, and chick growth rates in relation to brood size during the guard phase in four breeding seasons (1991–1994) on Seal Island, Antarctica. Within a given year, parents with two chicks made more frequent foraging trips to sea and may have transported larger food loads to the nest; however, the duration of foraging trips was unrelated to brood size. Overall, parents with two chicks spent ∼15% more time at sea than parents with only one chick. Both the frequency and duration of foraging trips varied between years. Foraging trip duration may partly reflect the birds’ foraging radius, which probably varies with time in response to shifts in krill distribution. Chick growth rate varied betwen years, but was related to brood size only in 1992, when chicks from two-chick broods grew significantly more slowly than chicks from one-chick broods. Food loads transported to chicks, as well as chick growth rates, were highest in 1994, when concurrent hydroacoustic studies indicated that regional krill biomass was severely depressed. This apparent anomaly suggests that the spatial scale of the krill survey may have been too coarse to detect some high-density krill aggregations within the penguins’ foraging range.