Data Collection on Slashfin: Shark Diversity & Conservation

Since starting my scientific internship at the Marine Dynamics Academy, I have been able to acquire various practical skills in research and data collection, specifically on the shark cage diving boat Slashfin. Data collection occurs on every single trip operated, meaning data is acquired almost daily.

South Africa is home to over 200 species of shark, of which 40 species of shark and rays are IUCN Red Listed, with conservation statuses ranging from Least Concern to Critically Endangered. Sharks and rays share the trait of a cartilaginous skeleton, a physical feature of elasmobranch species, both of which can be sighted in Gansbaai. 

Sharks are fascinating animals with physical abilities and features that make them one of the most effective marine predators. 

Firstly, these animals have a unique feature called the Ampullae of Lorenzini, which are small jelly-filled pores on the shark’s skin that appear as tiny dots. These pores detect the electric fields produced by prey, assisting sharks in locating and catching their food. Secondly, dermal denticles are tiny tooth-like structures that cover the skin of sharks. They reduce drag, improve swimming efficiency and protect against abrasions and parasites. Sharks, like other fish, absorb oxygen through their gills. They extract dissolved oxygen from the water as it passes over their gills, enabling them to breathe efficiently underwater. Most larger pelagic sharks must continuously swim in order to breathe efficiently, with a consistent flow of oxygenated water over the gills keeping them alive. Benthic shark species, such as the smaller catsharks, are able to rest on the seafloor and breathe using a method called buccal pumping. An opening and closing motion of the mouth forces water over the gills, aided by openings behind the eyes called spiracles, allowing the animal to avoid consistent swimming. An additional sense, and one of the most effective of sharks, is the lateral line system. This is a fluid-filled canal running along the length of the body, extending over the head. As it is so close to the surface of the shark’s skin, it is sensitive to changes in water pressure and movement, helping sharks to navigate, locate prey and perceive their environment.

On the Marine Dynamics shark boat, Slashfin, it is possible to observe multiple different species of shark, most notably the Great White shark, which I haven’t had the chance to see yet, but hope to, before I return to France.
 
The Great White shark (Carcharodon carcharias) can measure up to 6 metres in length at total size and live to 70 years, weighing up to 2000 kg at full size. They predominantly feed on marine mammals, pelagic fish and rays. These sharks are widespread but found mainly in temperate seas such as the hotspots found in South Africa, Australia, New Zealand, the north-eastern United States, California and the Pacific coast of Mexico. The white shark is a highly migratory and vulnerable species, with South Africa being the first to offer protection to the species in 1991. The development of human activities, generating pollution and overfishing, has led to the disappearance of natural marine habitats that are essential to the survival of shark populations. In Gansbaai, and the wider coast of South Africa, the white shark became targeted by killer whale activity, reducing sightings in the region as a result of area avoidance in response to the predation. The liver of white sharks takes up roughly two-thirds of the body cavity and is very nutritionally dense, hence the targeted feeding by killer whales. Shark species rely on their liver to control buoyancy in the water as they lack a swim bladder, a feature of bony fish. Loss of biodiversity and unregulated practices concerning cartilaginous fish, such as by-catch and direct fishing, contribute towards the threats shark species face globally, with over one-third of all species being threatened with extinction. 

Next, we can observe the Bronze whaler shark (Carcharhinus brachyurus). These sharks are found in temperate to subtropical waters with a distinctive bronze or copper colouration on their dorsal side. They can reach lengths of 3.3 to 3.5 metres and feed on bony fish, cephalopods (squid and octopus) and crustaceans. The most notable trait of the Bronze Whaler shark is their schooling behaviour in a feeding environment, where multiple individuals work together to improve feeding success. On the IUCN Redlist, these sharks are currently listed as Vulnerable, as a result of a decreasing population trend. Unlike the white shark, which is ovoviviparous, the Bronze whaler shark is viviparous. 
Ovoviviparous animals do not feed their embryos directly via a placenta with the embryos depending entirely on the nutritional reserves present in the egg yolk. In viviparous animals, development takes place entirely internally with the embryonic development of the young inside the maternal body, often receiving nutrients directly via a placenta. Bronze whaler sharks can give birth to 7-24 individuals, with a long gestation period ranging from 12-21 months. 

The third shark species that can be sighted is the Broadnose Sevengill shark (Notorynchus cepedianus), named due to the fact them having seven gill slits as opposed to the five that the majority of other shark species have. Also nicknamed cow sharks, this species is found all over the world in temperate coastal waters. They can live from 30 to 50 years and measure up to 3 metres in length. They play an important role in marine ecosystems as apex predators, feeding on a wide variety of prey such as fish, seals, rays and other sharks. As with other shark species, these sevengills are threatened by habitat degradation, overfishing and targeted capture. The Broadnose sevengill shark has a distinctive grey colouration with white and black spots. This can be used as a unique fingerprint to identify specific individuals with photo identification, assisting with population estimates and insight into habitat usage. 

The last magnificent species we can observe is the Short-tailed ray (Dasyatis brevicaudata). Whilst not a shark, these animals are very closely related due to their skeleton made of cartilage. As a bottom-dwelling species, they feed on molluscs, small squid, crustaceans, worms and small fish. They are found exclusively in the southern hemisphere and are one of the largest species of stingray that can be found globally, reaching lengths of up to 4 metres.

To study these species, data sheets are utilised on the boat to record the names and numbers of each species, as well as the time when the first individuals arrived at the dive site. Through this, we can record all the data and study the distribution of each species as a function of time and temperature and how they evolve within the ecosystems. Photography is also used to identify certain physical characteristics (diseases, wounds, bites, etc.) and applied to photo identification, focusing on the dorsal fins for Great white sharks, scars and unique markings for Bronze whalers and finally spot patterns for both the Broadnose sevengill and Short-tail stingray. 

In addition to the recording of marine species, water property measurements are taken utilising a multiparameter meter called a YSI. The physical properties of the water, such as temperature and dissolved oxygen content, provide key information about the habitat and environment utilised by these animals. This data provides the opportunity to highlight correlations between certain conditions and animal sightings or behavioural changes, better understanding the relationship between the sharks and the effects of changing environmental factors. 

The time I spent on board the Slashfin boat observing sharks and carrying out data collection was one of my favourite activities and gave me a better understanding of their distribution and abundance, allowing me to participate in their conservation within the ecosystem.
 
Biblio sources : 
https://apexpredators.com/fr/great-white-sharks-leaving-south-african-coasts/
https://dict.org.za/dict-projects/sharks/
https://sharkwatchsa.com/en/the-trip/the-boat-and-cage/
 
Written by Lou Barruel-Brussin