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

Ecology, Dec, 1998 by Peter L. Boveng, Lisa M. Hiruki, Michael K. Schwartz, John L. Bengtson

INTRODUCTION

Antarctic fur seals (Arctocephalus gazella) occupy an upper trophic position in the Antarctic marine food web, feeding mainly on krill (Euphausia superba) and various fish species (Croxall and Pilcher 1984, Green et al. 1989, Green et al. 1991, Reid 1995, Reid and Arnould 1996). Variability in Antarctic fur seal populations, and in populations of many other marine endotherms, has usually been associated with prey-related ("bottom-up" [e.g., Hunter and Price 1992]) forces often mediated by environmental variability (e.g., Antarctic fur seals [Boyd et al. 1994, Croxall ctal. 1988], other seals [Trillmich and Oho 1991], and seabirds [Croxall et al. 1988, Ainley et al. 1995, Monaghan 1996]). Although seals and seabirds are often considered top predators, many species are themselves preyed upon by other species of seals, killer whales, or sharks. Thus, despite the usual focus on bottom-up forces controlling seals and seabirds, there is considerable scope for top-down forces to play important roles. In this paper we examine evidence for control of an Antarctic fur seal population by leopard seals (Hydrurga leptonyx).

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The Antarctic fur seal population in the South Shetland Islands [ILLUSTRATION FOR FIGURE 1 OMITTED] is presently recovering from early 19th-century commercial exploitation and near extinction (O'Gorman 1961, Aguayo and Torres 1967, Bengtson et al. 1990). The number of fur seals increased from a breeding colony of as few as 32 seals at Livingston Island in 1959 (O'Gorman 1961) to a population estimated to contain 19000 seals in 1987, when most of the breeding areas were found to be on or near Elephant, King George, and Livingston Islands (Bengtson et al. 1990). However, only the breeding colonies at Elephant and Livingston Islands have expanded since the 1960s (Aguayo 1978, Oliva et al. 1987, Bengtson et al. 1990, Aguayo and Torres 1993), indicating that the recovery of the fur seal population in the South Shetlands has been mostly limited to the vicinities of these two islands [ILLUSTRATION FOR FIGURE 1 OMITTED]. This recovery provides an opportunity to investigate the forces responsible for changes in vital rates (reproduction, mortality, immigration, and emigration) that ultimately determine the relative success of breeding colonies and the fur seal population as a whole.

To document demographic changes associated with the recovery of the subpopulation of Antarctic fur seals in the vicinity of Elephant Island, we monitored abundance and reproduction annually during austral summers from 1986/1987 to 1994/1995. This research included daily observations at two colonies in the Seal Islands (where the greatest concentration of fur seals is found), annual censuses of other colonies in the Seal Islands, and less frequent censuses of colonies on or near Elephant Island that were less easily accessible from our Seal Island field camp (Bengtson et al. 1990; [ILLUSTRATION FOR FIGURE 1 OMITTED]). Because leopard seals were frequently observed at Seal Island and occasionally seen to take fur seal pups, we also documented leopard seal presence and behavior. Our objectives in this study were to: (1) describe patterns in fur seal pup production throughout the breeding seasons of 1986/1987 to 1994/1995 at Seal Island, including breeding chronology, total numbers of births, mortality rates of pups, and the effects of predation by leopard seals; (2) document abundance and trends of the Antarctic fur seal population breeding on and near Elephant Island; (3) interpret trends in fur seal abundance in the vicinity of Elephant Island within the larger context of the fur seal populations of the South Shetland Islands and South Georgia (1300 km east-northeast [ENE] of Elephant Island); and (4) assess the importance of predation by leopard seals in limitation or regulation of these populations.

METHODS

Fur seal pup production and mortality at Seal Island

We monitored pup production of Antarctic fur seals annually from 1986/1987 through 1994/1995 at Seal Island, Antarctica (60 [degrees] 59[minutes] S, 55 [degrees] 23[minutes] W, [ILLUSTRATION FOR FIGURE 1 OMITTED]). Live and dead fur seal pups were counted daily or every second day from early December to early March during the 1989/1990 through 1994/1995 breeding seasons. Pups were counted less frequently or for shorter periods during the 1986/1987 through 1988/1989 seasons. Pups were usually counted between the hours of 1000 and 1600 and during low tide, if possible. Pups were counted visually by a single observer (on nine occasions when counts were replicated by independent observers, the coefficient of variation among observers was 3%). Observers counted from several vantage points affording views of each of two colonies, North Annex (NA) and North Cove (NC), which are situated in adjacent protected coves, each with a shallow lagoon connected to the open sea by two channels. The channels leading to the lagoon at NA are awash only during high tides and occasional periods of large ocean swells; the channels leading to the NC lagoon are flooded at all but the lowest tides. The breeding areas of NA and NC are [approximately]25 m apart at their nearest points, are separated by a rock ridge that prevents overland movement of fur seals, and cover [approximately]490 [m.sup.2] and 1300 [m.sup.2], respectively.

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

Ecology, Dec, 1998 by Peter L. Boveng, Lisa M. Hiruki, Michael K. Schwartz, John L. Bengtson

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