Winter’s Bounty: As ‘breach season’ rolls in…

Winter’s Bounty: As ‘breach season’ rolls in…

Posted by Rabi'a Ryklief on May 5, 2013

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.

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.

seasonal patterns

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.
spatial patterns

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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