In the span of 6 years during a study performed in False Bay, researchers recorded 2088 natural predatory interactions with a success rate of 47.3%. That turns out to be 988 seals eaten by sharks, only counting predations above water and only those observed by researchers (Martin, et. al). This is also just predation activity in one bay along the southern African coast and excludes all events out at sea. These statistics illustrate the amazing predatory prowess of white sharks and the impact they have on controlling fur seal populations.
White sharks are known for being veracious predators, and rightly so. These fish can reach large sizes and weights, upwards of 20 feet and 4,200 pounds, respectively. With ample food, white sharks can eat around two percent of their bodyweight per day and around ten percent in one week. As previously stated, once sharks reach maturity and attain large enough body size, they feed almost exclusively on marine mammals, especially fur seals in South Africa. White shark energetic demands and population numbers lead to their role in contributing to population control of fur seals. There is not primary data on what effects would be expected to limit seal populations if sharks were removed from the ecological equation. However, I feel two main factors could limit seal populations in the absence of sharks.
Given the vast number of seals in the area (around one million), lack of pressure from higher trophic levels as well as continued expansions of hunting bans would likely lead to an initial increase in seal populations. Seals, marine mammals, have high metabolic rates and consume large quantities of food (fish and squid) relative to their body size. Without their main predator present, seal population booms would likely deplete local preferred fish populations and lead to collapses of food chains and overall weakening of the marine ecosystem.
Availability of breeding space
Seal Island in False Bay, South Africa is the largest breeding ground for fur seals in the country. This island is only around 800 meters by 50 meters and space upon which to inhabit the island is understandably limited. Many of the islands and beaches currently used as breeding ground for seals in South Africa are saturated and cannot not support increased numbers of seals. If population numbers increase for seals in the future, either new breeding grounds will need to be established or lack of breeding space will act as a population control.
Seal population control also carries indirect impacts for humans. Seals feed on fish, squid and crabs, some of which are also fished by humans. By controlling seal impacts on fish populations, white sharks are indirectly benefiting fishing industries. Shark predations in False Bay, South Africa are also a substantial source of income and tourism and alterations of the ecosystem could carry very serious financial ramifications. Through either indirect impacts to humans, or direct roles in the ecology of southern African marine systems, white sharks serve a critical role and their reactions to climate changed need to be taken seriously.
Christopher Hart, 2011