As we move from
the end of autumn towards the beginning of winter in South Africa, we are also drawing
ever nearer to “breach season”.
Mossel Bay is a
well-known white shark aggregation site on the Southern Cape coast and we are
lucky to have these beauties present year-round. Although we see a spectrum of sizes,
it is interesting to note that 80% of our white sharks are between 2 – 3m. This
means that most of our white sharks are feeding predominantly on fish and
smaller elasmobranchs. It is only around the 3m-mark that white sharks begin to
target marine mammals, such as Cape fur seals.One of the sites that we sample
in Mossel Bay is at a Cape fur seal island colony that is roughly 800m from the
shore. Despite the small size of this seal colony (est. 5000 individuals) it
still represents a valuable food source.
A white shark attacking a Cape fur seal in
Mossel Bay, South Africa
Below is a graph
representing the average number of white sharks sighted per unit effort (SPUE),
in this case per hour, for each season.This graphical representation of the
data highlights a distinct peak in abundance during the winter months.
Insight into the
life-history of this seal species shows that the winter months coincide with
the time seal pups leave the island to forage offshore for the first time. The
lack of experience in predator-prey games make these seal pups an easy target
for white sharks. Thus it’s easy to see why, during winter, the mean number of
sharks seen per hour is 1.57 in comparison to the average of 1.3 sharks per
hour across the four seasons.
Over the past
few weeks white shark activity has increased drastically at the seal
colony.Although we are still seeing the 2-3m sized white sharks, there appears
to be a greater proportion of individuals that are over the 3m-mark utilising
this area.
As we move into
winter and the environmental conditions favor the ambush-strategies of white
sharks stalking Cape fur seals, it is certainly a very exciting time to be out
at sea!!
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