SHARKS

DISCOVER

The waters around Dyer Island has one of the densest populations of Great White Sharks in the world. Attracted by the colony of roughly 60 000 seals on Geyser Rock separated from Dyer Island by the world-famous Shark Alley, the apex predators are regularly sighted by thousands of visitors who visit the area to cage dive each year. This makes the area ideal for studying this apex predator.

Much of the life of the Great White Shark remains unknown. The Dyer Island Conservation Trust believes that Science Saves Sharks. Our team targets research actions that will enable policy makers and conservation planners to make sound conservation policy decisions that will prevent further population decline.

The Great White Shark is currently listed as vulnerable by the IUCN Red List, but plummeting numbers has put them on the brink of moving into the endangered classification. The world-wide population figure is uncertain, but estimates have suggested that only between 3 000 to 5 000 remain.

We use our dedicated research vessel, aptly named Lwazi, meaning “seeking knowledge” in isiXhosa, to carry out our research work.

Estimating white shark numbers is challenging and requires a combination of data, such as tagging, historical fisheries as well as photo identification.The Dyer Island Conservation Trust produced the first population estimate for Gansbaai in 2013 of 800-1000 individuals using photo ID and automated software. Currently, a national estimate is underway using a varied method approach

ACOUSTIC TAGGING:

Our marine biologists explore the movement ecology of sharks within the greater Dyer Island area through the use of acoustic tags. Sharks are fitted (tagged) with acoustic transmitters.

A permanent network of 13 receivers, anchored to listening stations across the greater area record shark movements via these receivers. The data gathered from the “shark tracks” allows our marine biologists to analyse shark movements. This work is done in collaboration with the African Tracking Platform, which incorporates data from all tagged species along the coastline.

SATELLITE TRACKING:

Our collaborative studies conducted together with Ocearch in 2012 revealed the larger migratory movement patterns of sharks within the bay and along the Southern African coast. To protect the Great white shark we need to understand which areas they frequent.  This knowledge could indicate possible breeding regions and areas where they face greater risks. 

Information discovered during the satellite tracking studies reveal that:

  •     The Gansbaai Great White Sharks are moving into areas such as Mozambique where they face the threat of being killed by local fishermen. 
  •     The shark nets and drumlines in Kwazulu Natal waters off South Africa’s coastline are another hazard they must face. 

Because the great white shark is a slow breeding species that only reach sexual maturity late in life (approx. 20 years), it is important to understand their population dynamics.

IDENTIFYING INDIVIDUAL GREAT WHITE SHARKS:

Two methods of photo identification can be used to identify individual sharks from one another.

  • Dorsal fin identification: Great White Sharks can be identified by their dorsal fins. Using the trailing edge of the dorsal fin, specific notch marking sequences can be distinguished. These markings are individual to each shark and can now be recognised accurately by using the latest computer programme technology.
  • Sub-surface identification: Under-water film footage and photography enable us to identify Great White Sharks even more accurately by matching specific pigmentation markings on three marked areas of each shark. These areas are the gills, pelvic fins and tails.

The data analysing  team used a software programme called Darwin (usually used for dolphin fin identification) to assist with the identification of 532 individual sharks. Data gathered after the long-term identification study revealed that the numbers of sharks found in the Dyer Island/Gansbaai area was only half of what was previously assumed to be present in the area. Based on an open population size estimate the team could suggest that an estimate of between 808 to 1008 sharks visit this area. This study, a first for the area and for South Africa, was crucial in the establishment of a national database. As a result of this co-operative effort and, the intensive research programme conducted by the Dyer Island Conservation Trust (DICT), the area is now an internationally recognised centre for exploring the world of the Great White Shark.

SHARK NECROPSIES:

The Dyer Island Conservation Trust team are equipped to collect carcasses of washed up sharks and conduct necropsies to not only determine the cause of death but also to collect vital information. The necropsy team will collect samples as required by members of the White Shark Research Group in South Africa and Department of Environmental Affairs for a multitude of research projects.

The following information and samples are collected during each necropsy:

  • extensive morphometric measurements to obtain accurate size information,
  • parasites such as copepods and nematodes are extracted and preserved,
  • muscle tissue is preserved for genetics,
  • diet and physiology studies and all the wounds measured and examined closely,
  • vertebrae are collected for age growth studies and,
  • all organs are inspected weighed and measured.