Population Growth of Antarctic Fur Seals: Limitation by a Top Predator, the Leopard Seal?

Abstract

Abstract. 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 consistent 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.

A small unmanned aerial system for estimating abundance and size of Antarctic predators

“…In addition to capturing images for estimating abundance of animals, aerial photographs allow post-processing of morphometrics in target species which can be used to track changes in animal size, body shape, and nutritive condition (Perryman and Lynn 2002;Miller et al 2012). Thus, we wanted to test the feasibility of using a VTOL to estimate size and condition of leopard seals, an apex predator that affects Antarctic populations of penguins and seals (Siniff and Stone 1985;Boveng et al 1998;Ainley et al 2005;Schwarz et al 2013). Similarly, aerial methods to collect data on size and identification of individual marine mammals, including pinnipeds and cetaceans, may help reduce disturbance associated with capture and traditional measurement.…”

Detecting Density Dependence in Recovering Seal Populations

“…Most seal populations therefore comprise large proportions of adults, and the intrinsic population growth rate is usually low (Fowler 1981;Boveng et al 1998;Harkonen et al 2002). This makes seal populations comparably resistant to short-term changes in the environment.…”

“…This makes seal populations comparably resistant to short-term changes in the environment. Population dynamics of seals are primarily affected by intra-and inter-specific competition and predation (Boveng et al 1998;Mathews and Pendleton 2006), disease (Heide-Jørgensen et al 1992;Harding et al 2002), and reduced diversity or abundance of prey species (Innes et al 1981;Merrick et al 1997;Krafft et al 2006;Bowen et al 2007).…”

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

“…Demographic parameters of these species have been described in detail in a number of studies (Hes & Roux 1983;Doidge et al 1984a;Bester 1987;Kerley 1987;Shaughnessy & Goldsworthy 1990;Bester 1995;Hofmeyr et al 2005;Hofmeyr et al 2006). Differences between various populations (Croxall & Gentry 1987;Wickens & York 1997), between sites within populations (Doidge et al 1984a;Boveng et al 1998) and between years (Doidge et al 1984a;Lunn & Boyd 1993;Boyd & Murray 2001;Hofmeyr et al 2005;Reid & Forcada 2005), have also been noted, requiring that monitoring of these factors be ongoing and widespread.…”