Number of great whites is stable

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No, there hasn’t been a population explosion of great white sharks since this species was protected in South African waters.

Before 1991, when it was formally protected, the population was decreasing at 1.3% a year, but in a best case scenario, it has since been increasing at perhaps 1.6 % – still “way, way lower” than the biologically possible maximum population growth rate of between 4% and 11.9%.

And because it is an apex predator that grows very slowly, matures sexually very late and produces only a small number of young of between two and 10, it is classified as “highly vulnerable”.

These were some of the data presented during the Shark Working Group briefing on Thursday by scientist Alison Kock of the Save Our Seas Foundation, who is researching the movements of great whites in False Bay as part of her PhD thesis through UCT.

Responding to some suggestions that increased shark attacks in False Bay in recent years were because of a “population explosion” since 1991, Kock said this was “physiologically not possible”. The small potential increase in the number of sharks could not explain the increased number of attacks.

She said data on this species, which grows to a length of about 6,4 metres, was scarce.

The best available “guestimate” for the great White population for the coastline between Struisbaai and Richards Bay was put at 1 279 by the Natal Sharks Board and the Oceanographic Research Institute (ORI) in 1993.

There was no historical data for numbers in False Bay, and anecdotal evidence had “major limitations”. During 2004/5, she had identified a minimum of 128 in False Bay, but this was “very preliminary” data.

The presence of great whites in False Bay correlated “very strongly” with the number of neonates and newly weaned seals learning to swim and leaving Seal Island for the first time in the winter, May to September, when they were about five- to six months old.

False Bay had proportionately the largest number of great Whites over 3,25m of any South African study area, Kock noted.

“So it could be a critical area housing the adult and possibly reproductive stock, which is probably the most important sector of the population.”

“But there are very few sexually mature females over 5m observed in False Bay and in South Africa.

“So my personal feeling is that we may not have a sexually mature population here.”

Source: iol.co.za