Predator - Prey Interactions between Great White Sharks and Cape Fur Seals in False Bay, South Africa.

Predation is one of the most fundamental interactions in nature and one of the most inherently fascinating. But predation is rarely observed in the wild. Seal Island, in False Bay, South Africa, provides unique opportunities to observe natural predation by White Sharks on Cape Fur Seals and to observe social interactions among both species.

 
Methods used in the study of the Great White Sharks and Cape Fur Seals

As part of a larger study instigated by Rob Lawrence of African Shark Eco-Charters and Chris Fallows Apex Predators, predator-prey interactions between Great White Sharks and Cape Fur Seals at Seal Island, False Bay, South Africa, were investigated by direct observations of ad libidem encounters between these two species at the surface during August and September 2000. Observations were made from the platform of an 8-metre Butt-Cat boat equipped with paired Johnson 150 HP outboard engines, Garmin GPSMAP 235 with chronometer and sounder; a [brand an model?] ‘fish finder’, and a [brand and model?] VHS marine radio. Water temperature was measured with a [brand and model] electronic thermometer and water clarity was measured using a standard Secci disc.

Predatory events were usually first detected at the surface by one or more of the following indicators;

1. Incoming seals abruptly changing course,
2. Seals suddenly switching from porpoising to rapid zig-zag leaping,
3. A group of travelling seals suddenly exploding from the water in multiple directions,
4. A Great White Shark breaching, with or without a seal in its mouth,
5. A bloody splash, often accompanied by a spreading oily slick, or
6. Kelp Gulls or other seabirds wheeling over or plunging repeatedly toward a discrete region of sea surface.

Upon detection, the research vessel was piloted toward the one or more indicator(s), approaching as closely as possible without manifestly altering the behaviour of either shark or seals. During a predatory event, every effort was made to avoid blocking a seal’s escape path to the Island.

To facilitate analysis, the waters surrounding Seal Island were divided into Inshore (within 100 metres of the island) and Offshore (beyond 100 metres of the island) regions. The Inshore waters of Seal Island were further subdivided into six sectors. For purposes of codification, Cape Fur Seals were divided into four broad categories:

1. pup (neonates with black fur),
2. first year (young of the year up to 1 metre in length),
3. cow and immature bulls (brown-furred individuals 1 to 1.5 metres in length), and
4. adult bulls (mature males with prominent sagital crest and greater than 1.5 metres in length).

Shark length was estimated by reference to the width of the research vessel, which measures 2.5 metres from gunnel to gunnel. Outcome of each attack was categorized as either an unsuccessful Attempted Predation (AP), in which the seal escaped, or a Kill (K), in which the seal did not.

To the extent that direct observation permitted, date, time of attack, seal category, region, island sector, water depth, estimated shark length, and attack outcome were recorded for each predator-prey interaction.

As noted previously, two basic types of porpoising have been noted at Seal Island.

1. High porpoising is most often near (within 100 metres) the shore and is often followed by minor course changes. Therefore, this behaviour may help seals get their bearings on beaching or rafting sites.
2. In contrast, low porpoising is typically observed relatively far (more than 100 metres) from shore and often aborted in favour of anti-predator movements.

Thus, this behaviour may be a way for seals to maximise sub-surface vigilance and thereby reduce their vulnerability to Great White Sharks.

Once a kill had been made, proximity of the vessel did not seem to significantly affect the shark’s behaviour.

Seal Island in False Bay provides a unique natural laboratory where Great White Shark predation on Cape Fur Seals and intra-specific interactions among both species can be readily observed in intimate detail from relatively close distances. Due to the large number of White Sharks and Seals in the region, ample opportunities exist here for in situ experiments on such matters as sensory discrimination, cognitive psychology, space utilisation, population size and structure of either (or both) species.