The white shark is a marine apex predator that is able to maintain biodiversity through direct and indirect predation effects and thus represents a keystone species essential to the functioning of coastal marine ecosystems.
The white shark is a marine apex predator that is able to maintain biodiversity through direct and indirect predation effects and thus represents a keystone species essential to the functioning of coastal marine ecosystems. As is the case with most sharks, the white shark is characterised by a K-selected life history i.e. it is a slow growing, late maturing, and long-lived species with low fecundity. Despite its circum-global distribution, many aspects of the basic biology of white sharks are still poorly understood.
South Africa is internationally recognised as one of the global centres of abundance for white sharks and hosts a genetically distinct population from those found in the North Atlantic and Australasia. White sharks actively migrate along the entire South African coast and between identified aggregation sites i.e. False Bay, Gansbaai and Mossel Bay. South Africa thus hosts a metapopulation with movement between the different local populations.
Threats, both direct and indirect are anthropogenic in nature. Direct threats to this species includes incidental bycatch by fisheries, and targeting for sports fishing, the curio trade (jaws, teeth, meat and fins), and bather protection meshing installations (Natal Sharks Board). Indirect threats encompass the degradation of inshore habitats as a result of coastal development and the decline in prey species, particularly teleosts prey (bony fish) due to over-fishing. However, the largest indirect threat to white sharks in South Africa arises from public pressure to remove its protection status due to occasional shark-human accidents.
Photographic identification techniques are used as a non-invasive tool to collect data on white sharks aggregating at Mossel Bay. Long-term information on abundance is required to determine population trends to yield further insight into the population health of this vulnerable, protected species.
The main research aims of this project are:
1. To produce robust annual population estimates that will form baseline data for the ongoing monitoring of population health
2. To define the population structure to ascertain what aspect of the population is utilising Mossel Bay i.e. size classes and sex ratios
3. To investigate the seasonal and spatial patterns in abundance to infer habitat use within Mossel Bay, providing insight into the ecology of this species
The white shark was one of the first elasmobranch species on which photographic identification techniques were implemented. Although white sharks are considered to be a wide-ranging species, studies show that they return to specific localities at regular intervals thus making photographic identification techniques feasible for estimating population sizes.
Long-term information on the population status and composition of the white shark, a vulnerable species (IUCN) is currently derived from the captures of sharks in the Natal Sharks Board bather protection nets. Thus, a cost-effective and non-consumptive method for collecting long-term data on the relative and absolute trends in population metrics and composition can be gained through photographic identification of individuals. This non-invasive method is ideal for sampling protected species such as the white shark, in comparison to data derived from the captures of individuals from bather protection nets. By applying mark-recapture models to data from uniquely identified individuals, researchers can obtain robust estimates of a population’s size. Such estimates, especially when obtained over an extended time period, are important in order to assess the effectiveness of conservation measures in South Africa.
From a global perspective, the population metrics of the white shark observed at aggregation sites off central California (US) and Guadalupe Island (Mexico) have recently been established, thus placing pressure on South Africa to start producing baseline data of its own.
It is imperative to obtain robust estimates of population numbers over time to facilitate the detection of population trends. Global fisheries-dependent studies have documented declining trends for white shark populations, however, there are no global records documenting an increase in abundance for this species. It is important to note that no inferences may currently be made regarding the trends in the white shark’s population as long-term data sets are required for this. However, at present, valuable insight can be derived on the inter-annual abundance of this species.
From these preliminary results, white sharks appear to be rarer than popularly perceived. This is in contradiction to the general public’s perception that the population has increased since its protection in 1991. This misconception can be attributed to an increased number of water-users, increased cognisance of the presence of sharks, and over publicized reports on shark-attack incidents in which the white shark is assumed the prime suspect.
Essentially, sound population estimates are required to effectively manage and monitor populations. It has been suggested that management should actively protect juveniles and adults for effective recovery of declining populations. As a population trend cannot yet be determined, it is safer to err on the side of caution by supporting the current protection status afforded to the white shark.
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