Cape Town - The unmistakable shape of a large fin slicing through the water towards a hapless swimmer has been one of Hollywood’s most enduring symbols of horror, conjuring up visions of gore, death and destruction.
The reality, of course, is usually very different, and particularly so at the undisputed shark-watching capital of the world – Gansbaai – where fascinated visitors pay hundreds, sometimes thousands, of rand to watch these dorsal fins and the awesome creatures they’re attached to – great white sharks, or white sharks – in their natural habitat.
One group of avid shark-watchers at Gansbaai observed these fins with very close attention for a full five years. The results of their observations have just been published in PLOS ONE, an inclusive, peer-reviewed, open-access resource from the Public Library of Science – and make for fascinating, but worrying, reading.
The group are five marine biologists who worked for the Dyer Island Conservation Trust, named for the island a few kilometres from Gansbaai and where a shallow channel is known as “Shark Alley”, because of the many great whites patrolling here to snap up unsuspecting Cape fur seal pups from the breeding colony on adjoining Geyser Rock.
The scientists worked aboard a shark cage-diving boat from local company Marine Dynamics, whose owner, Wilfred Chivell, is the founder-chairman of the trust, and collectively they made 1 647 trips between January 2007 and December 2011.
They collected more than 20 000 digital photos of white shark dorsal fins to identify and count these apex predators.
“Photo identification has been developed as a non-invasive method of mark and recapture in which distinctive features of an individual can be used to recognise it against the rest of the population during different samples, over extensive time periods,” they write in their article, Gauging the threat: The first population estimate for white sharks using photo identification and automated software.
“The first dorsal fin of white sharks is often characterised by distinctive shapes, notches, scarring and pigmentation patterns, which can be used to recognise individuals over many years. From such photo ID data, mark-recapture methods can be applied to estimate population sizes…”
The researchers eventually ended up with 1 683 high-quality images, which were imported into a computer program named Darwin.
This software traces the fin and assigns a fixed spacing of points along its leading and trailing edges, and then compares it to other images in the catalogue. It was the first study to successfully use this software for white shark identification, and the researchers found that they could identify 532 unique individual white sharks.
Some of their sharks were given obvious names – Slashfin, Notch – while others ranged from the evocative (Big Wolfgang, Vindication, Zebra and Darwin) to the prosaic (Jimmy, Bob).
Darwin was photographed numerous times, while Zebra was captured on nine occasions.
“The scientists then analysed their data on the individual sharks with another program, Mark, which uses “maximum likelihood” models to estimate population parameters. This estimated the size of the Gansbaai “superpopulation” of White Sharks, some of which move between the Cape, KwaZulu-Natal and Mozambique – one even swam to Western Australia and back – at somewhere between 808 and 1008 individuals, with a mean of just 908.”
Although this figure was comparable to a previous South African population estimate conducted 17 years ago by Geremy Cliff and colleagues from the Natal Sharks Board, it was only half of the current widely accepted estimate of 2 000 sharks.
“Despite this species being protected since 1991, such a low (population) estimate and lack of recovery rate suggests the southern African white shark is not receiving adequate protection for population growth,” the scientists concluded. “These results highlight the need for effective protection measures within the entire home range of the southern African white shark.”
Shark researcher Alison Kock, who heads the City of Cape Town’s Shark Spotters programme, said it was “extremely exciting” to see research of this calibre being produced for white sharks.
“The results support the notion that South Africa is home to the largest concentration of white sharks in the world and reminds us of the important role South Africa plays in contributing to the global conservation of this threatened species,” she said.
“Whether this population estimate represents the entire South African white shark population, or is more specific to the aggregation site in Gansbaai, will be addressed in future studies…
“Regardless, this population estimate provides a valuable baseline from which one can monitor changes in the population over time at a major centre of abundance.
“Increasingly, threats are being identified for white sharks across their range – for example, bycatch in fisheries – and information like this is vital to monitor changes… and advocate for improved conservation strategies.”
l The white shark (Carcharodon carcharias) is protected in South Africa, Namibia, the US, Australia, Malta, the Maldives, Mexico, New Zealand and Israel. It is listed as “vulnerable” on the IUCN’s Red List of Threatened Species.
l On the web: www.sharkwatchsa.com/projects/great-white-shark-population - Sunday Argus