Anti-Predatory Strategies of Cape Fur Seals at Seal Island
From direct observation and the data collected, Cape Fur Seals apparently
reduce their vulnerability to White Sharks by:
Taking advantage of the expanded vigilance of entire groups;
sub-surface vigilance while rafting is accomplished via
assuming a head-down posture, with only the tail and the
tip of the rear flippers showing above the surface
Leaving Seal Island as coordinated groups of 8 to 12 animals;
multiple groups - ranging from 2 to as many as 5 - leave
the island at intervals of approximately 45 seconds
Single or small groups (2-5) of individuals executing
a finely controlled zig-zagging evasive maneuver when a
White Shark is spotted stalking below them; this tactic
is referred to as "working
When an individual is actively pursued by a White Shark,
riding its slipstream - usually mid-body, at the level of
its dorsal fin - to remain out of reach of the shark's jaws;
this tactic is referred to as "on
When a group is "hit" (attacked) by a White Shark,
the individual seals 'explode' from the water in all directions,
presumably serving to confuse the predator - perhaps sufficiently
to allow some or all of them to escape; this tactic is relatively
infrequent but highly spectacular
When an individual or group returns to the island, swimming
the last 50 metes or so underwater, presumably because this
tactic reduces vulnerability to attack by White Sharks
When any or all of the aforementioned tactics
fail and a predatory attack is successful, the surviving seals
become extremely vigilant - often to the point of seeming
momentarily stunned - but are, in fact, hyper-alert.
In short, the Cape Fur Seals' main anti-predatory strategies rely
on vigilance and agility.
Porpoising in Cape Fur Seals
As noted previously, two basic types of porpoising have been noted
at Seal Island.
High porpoising is most often near (within 100 metres)
the shore and is often followed by minor course changes. Therefore,
this behavior may help seals get their bearings on beaching or
Low porpoising is typically observed relatively
far (more than 100 metres) from shore and often aborted in favor
of anti-predator movements. Thus, this behavior may be a way for
seals to maximize sub-surface vigilance and thereby reduce their
vulnerability to White Sharks.
Curiosity in Cape Fur Seals
Cape Fur Seals are intensely curious animals, using their large,
well developed eyes and sensitive mouths to investigate virtually
everything in their environment. The individual at right is
clearly craning its neck to get a better look at the photographer
(my lovely wife, Anne). But when White Sharks are about, such
curiosity is abandoned and all eyes search anxiously beneath the surface. Curiosity
may have killed the proverbial cat, but it probably won't snuff one
of these bear-descended explorers.
[ Note from R.
Aidan Martin ] I would
like to acknowledge that much of the information on Seal Island presented
here was provided by two very talented South African naturalists,
Chris Fallows and Rob Lawrence, of African Shark Eco-Charters.
I would also like to acknowledge my debt to South African diver
Alison Kock, who provided valuable information on the Island's
underwater topography. Were it not for Chris's, Rob's, and
Ali's generosity in sharing what they have learned, I could not
have gotten a conceptual handle on what's going on at Seal Island
as quickly as I did.