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

Fur seal trends at the South Shetland Islands and South Georgia

The overall fur seal population of the South Shetland Islands has been increasing since counts began in the mid-1960s (Oliva et al. 1987, Aguayo and Torres 1993). In the Elephant Island area, comprising Elephant Island and the Seal Islands archipelago [ILLUSTRATION FOR FIGURE 1 OMITTED], fur seals increased from four breeding colonies with 13 pups observed in February of 1966 (Aguayo and Torres 1967, Aguayo 1978) to a population producing at least 1096 pups in 1993/1994 (Table 3). The 3.8% annual rate of increase in pup numbers at the four locations counted in 1986/1987, 1991/1992, and 1993/1994 (Table 3) was much lower than the 11% annual increase in pup counts from 1986/1987 and 1991/1992 at Cape Shirreft (Livingston Island) and nearby San Telmo Island, the largest fur seal colonies in the South Shetlands (5313 pups [Aguayo and Torres 19931). Clearly, the portion of the population in the Elephant Island vicinity has been growing much more slowly than that of Livingston Island.

Antarctic fur seal colonies at South Georgia re-established themselves more quickly following exploitation, which ended in [approximately]1907 (Bonner 1968). There, the species' recovery probably began from a remnant group of seals at Bird and Main Islands off the northwest end of South Georgia (Bonner 1968). Pups were first seen there in the 1930s (Mackintosh 1967) and Bonner and Laws (1964) estimated that by 1956 there were 13 000 fur seals there. The South Georgia fur seal population was estimated to be [greater than] 1.5 x [10.sup.6] individuals in 1990/1991 (Boyd 1993). The annual rate of increase in pup production was as high as 13.5% (Wickens and York 1997) or 16.8% (Payne 1977) in the 1960s and early 1970s, dropping to 9.8% between 1976/1977 and 1990/1991 as densities reached saturation at the sites that were colonized first (Boyd 1993). In comparison, the South Shetland Island population has grown only to [approximately]30 000 seals, even though it has probably been free from harvesting for 20 yr longer than the South Georgia population (Bonner 1968). Growth rates of the main colonies in the South Shetlands have not exceeded 11%/yr even though densities of the breeding colonies are much lower than most at South Georgia (J. L. Bengtson and P. L. Boveng, personal observation).

Top-down control of fur seal populations

Although we have shown that leopard seals can reverse the recovery of a local population of fur seals such as the NC colony at Seal Island, it remains uncertain whether any broader population of fur seals is limited or regulated by predators. In the remainder of this paper, we discuss circumstantial evidence that is consistent with a significant role of leopard seals in the dynamics of the fur seal population in the South Shetland Islands. We then discuss the species interactions between leopard and fur seals, and the similarity of certain aspects of those interactions to other vertebrate predator-prey systems, especially with regard to whether density-dependent regulation of fur seals by leopard seals is plausible. Finally, we speculate briefly about why predation by leopard seals has apparently increased in importance, at least at NC on Seal Island in recent years.