Christopher Hart, 2011Cape Fur Seals
Seal islands in False Bay and Mossel Bay, South Africa support huge populations of fur seals, with seal island in False Bay harboring around 75,000 seals at peak season. Fur seals spend most of the year at sea, but nesting grounds are never fully abandoned and are often revisited by females and young. Research has shown that photoperiod, or environmental cues of variation in daylength, is an obligatory proximate cause (a required direct environmental cue) for the onset of mating cycles in cape fur seals (Stewardson, Bester and Oosthuizen). After the seal fur trade vastly diminished in the late 20th century, seal populations in southern Africa steadily rose to their current levels and provided a more stable dietary option for the top marine predator in the region, white sharks.
Seals in these ecosystems play a very prominent ecological role. Cape fur seals feed primarily on fish and squid with a very small portion of their diet being crustaceans, such as crabs. With population estimates of wild fur seals in South African waters being around one million, marine ecology is very dependent on the seals. As seals were hunted heavily in the 1800's and most of the 1900's, pressures on fish and other dietary items of seals were far less. Trophic roles of seals were proven as their populations steadily increased after hunting bans and increased predation on fish and squid effected the ecosystem. Fluctuations in seal populations carry very important implications for trophic dynamics in marine ecosystems. White sharks acting as population control for seal colonies is therefore even more important in maintaining stable ecosystems in southern Africa.
Sharks residing permanently near southern Africa have different diets depending on maturity. When sharks reach a certain length (usually around 3 meters) their diet shifts from fish to primarily marine mammals (Martin, et. al). White sharks have also aligned migrational cues to correlate with cape fur breeding season. Young seals are easier to hunt and as seals return in vast numbers to their colony sites, the sharks take advantage and maximize encounters with their selected prey. Despite the tenacity and appetite of white sharks, balance is maintained in this ecosystem. As an apex predator, white sharks play a critical role in maintaining diversity in southern African marine systems and their disappearance in response to raised sea temperatures would have dramatic biological effects.
Figure 6. Predatory behaviours in white sharks at Seal Island, South Africa: (A) Polaris breach; (B) surface broach; (C) lateral broach; (D) inverted broach; (E) surface lunge; (F) surface intercept above water; (G) lateral roll; (H) surface arc; (I) lateral snap; (J) direct surface approach; (K) killing bite; (L) surface grasp horizontal approach; (M) subsurface carry; (N) surface feed; and (O) lateral head shake.