Tigers of the Sea
Garbage can with fins, waste-basket of the sea, the bane of surfers... Cheryl-Samantha Owen goes on assignment to find out more about the infamous tiger shark.
Armed with an arsenal of gear, including acoustic and pop-up archival satellite tags, underwater radio listening stations, cameras, a modified tagging speargun and shark slings, the neoprene-clad researchers that brave the waves off South Africa’s east coast are not your average white-lab-coat data crunchers. Vic Peddemors of the University of KwaZulu-Natal and Macquarie University, Australia, Malcolm Smale and Matt Dicken of Bayworld Centre for Research and Education in Port Elizabeth, and sharkdiving expert Mark Addison, have spent countless hours at sea. Initiated and funded by the Save Our Seas Foundation (SOSF), their mission is to uncover the secrets of the infamous tiger shark Galeocerdo cuvier.
First described in 1822, the genus name Galeocerdo is derived from the Greek galeos or ‘shark’ and the Latin cerdus, while the species name cuvier honours 18th-century French naturalist Georges Cuvier. The tiger shark is one of the world’s largest predatory fish, growing up to five metres (and possibly even as long as 7.2 metres) in length and weighing as much as 1 000 kilograms. Its broad, blunt snout and large black eyes are unmistakable, and its unique saw-edged and cockscomb-shaped teeth cater perfectly for the appetite they have to sate.
The exquisite beauty of the tiger shark’s skin may seem to pay homage to its furry, jungle-inhabiting namesake, but as sharks evolved millions of years before their mammalian counterparts, nature’s patent on tiger stripes was actually registered at sea. In clear water, light dances on the sharks’ backs, jumping across the vivid bars and spots (which fade from black to a gentler grey with age) and highlighting the golden sheen that coats their skin like molten honey. In motion they are hypnotic, their purposeful swimming a culmination of elegance and grace developed over millennia.
Colloquially, they are known variously as ‘a garbage can with fins’, ‘wastebasket of the sea’ or more sinisterly as ‘the man-eater shark’ and ‘bane of surfers’. Their bad rap stems partly from some of the items found in the stomachs of captured animals and partly from occasional attacks on surfers and divers in the tropics. The discovery of licence plates, pieces of rubber tyres, seabirds, sea snakes, marine iguanas and even the tattooed arm of a murdered mobster explains why these sharks are thought to have a diet more indiscriminate than that of a goat.
Although indigestible garbage (and even human remains) sometimes does end up in the guts of scavenging sharks, their natural and preferred prey, depending on their geographical region, includes sea turtles, bony fishes, sharks, rays, seabirds, dolphins and squid. Studies of the diets of tiger sharks found off South Africa’s east coast show that rays and other sharks form the most significant part of their diet – tiger sharks in other locations consume far less of these species. These studies also concluded that tigers probably play a critical role as apex predators in regulating dolphin populations along the KwaZulu- Natal (KZN) coast, and that they have a surprising taste for toxic puffer- and porcupine fish.
“Sea turtles seem to appear regularly only on tiger shark menus; no other shark is known to prey on them consistentlyTiger sharks use the specialised structure of their teeth – heavy, oblique cusps with coarse serrations – together with a rolling jaw motion to saw prey apart and to bite through hard, bony turtle shells. Sea turtles seem to appear regularly only on tiger shark menus; no other shark is known to prey on them consistently. Indeed, fossil records show that tiger sharks evolved during the same period as the tough-armoured sea turtles – about 100 million years ago during the Upper Cretaceous.
Solitary feeders that hunt primarily at night, tiger sharks either head into deeper oceanic water or move closer inshore as darkness falls, depending on their location. They often hunt in turbid water and where fresh water flows into the sea. As they grow, so the structure of their teeth – and their diet – changes. Younger sharks tend to target inshore reef fish, moving up the food chain to turtles, birds and sharks when they are larger.
This wide spectrum of culinary choice may be one of the secrets to their survival. Tiger sharks are very large animals and require megadoses of kilojoules. The fact that they are able to take such advantage of the marine smorgasbord means that they are less likely to run out of food than more selective feeders.
Amzani, jarjur knaza and ma’o tore tore – the Swahili, Arabic and Tahitian names for the tiger shark point to just a few of the locations it frequents. A relatively common coastal and open ocean species, it can be found from the waters around oceanic islands to estuaries, and it inhabits tropical and warm-temperate seas throughout the world. In Africa, the species ranges from Cape St Francis on South Africa’s southern coast up the east coast to the Red Sea, and from Morocco in the west possibly as far south as Angola.
In South Africa, a near-shore reef called the Aliwal Shoal, which lies five kilometres off Scottburgh on the KZN coast, is known as Tiger Shark Central. Constantly frequented by a suite of other cartilaginous fish, from ragged-tooth and blacktips to dusky sharks, the shoal – a marine protected area or MPA – is a shark lover’s hotspot. Tigers are also often found off Protea Banks and Ponta d’Ouro in Mozambique, but at Aliwal Shoal they are almost always encountered during the warmer season from December to June. Happily, this coincides with the province’s peak tourist season and in recent years a thriving shark-diving industry has sprung up around the reef (see ‘Dancing with Tigers’ on page 58).
Tag-and-release data in the US have shown that while some tiger sharks remain in one place, others travel great distances. In tropical areas they are often present year-round, whereas in warmtemperate areas such as the Aliwal Shoal there is a seasonal abundance of tigers during the warmer months. It seems that tourists are not the only species to vacate the KZN coast in winter; most of these cryptic predators disappear each year when the water temperature drops.
“The exquisite beauty of the tiger shark’s skin may seem to pay homage to its furry jungle–inhabiting namesakeAddison, the owner of Blue Wilderness, an underwater filming logistics and expeditions company, has been observing tigers at Aliwal for more than a decade. For him, ‘diving with tiger sharks over the past 14 years has been one of the undoubted highlights of my diving life as well as being a great myth exploder’. He has noted that 97 per cent of the individuals he’s seen here are females, with few sightings of immature or male sharks and none of pups. Some of the smaller females frequent the shoal throughout the year, but the three-metre-plus mature females visit only between February and May, when the visibility is good and the seas are calm and warm. Temperature, breeding or food availability may play a part in their movements, but scientists are still trying to piece together the clues that will tell us why they come and why they go.
‘Understanding the tiger shark’s local and regional movements and how it uses its habitat is essential for making informed conservation decisions about protecting the species,’ says SOSF Director Chris Clarke. ‘Answering research questions to a high degree of accuracy is often less complicated on land than in the sea, where large marine animals such as sharks range across ocean basins, making observation difficult.’
TIGERS AT A GLANCE:
Maximum size: 7.2 metres and 1 000 kilograms.
Life cycle: Female tiger sharks reach maturity at about three metres and/or seven years of age. 14 to 16 months after they’ve mated, between 10 and 80 pups are born. The young are 60 centimetres at birth and can live for up to 25 years.
Mythbuster: The embryos of tiger sharks fight each other while in their mother’s womb, the survivors being the baby sharks that are born. FALSE (this is how ragged-tooth sharks do it).
Threats: Fishing, both recreational and commercial. In addition to their meat, sharks provide fins for soup, skin for leather and their livers are used to produce vitamin A oil, which is used in skincare products and supplements.
“Scientists are still trying to piece together the clues that will tell us why they come and why they goThe SOSF tiger shark research project was started in 2006 to improve current understanding of this important apex predator. ‘We hope the results of our research will also be used to develop sound advice for divers and swimmers along this coast,’ says Clarke. Information gained from tiger shark research is proving invaluable for global conservation efforts aimed at pulling the species back from the precipice. The study has focused on four aspects of tiger shark research. Behaviour and movements off the KZN coast are tracked using acoustic tagging techniques; longrange movements are plotted by pop-up archival satellite tags; manual tracking is done via pinging V16 tags; and photographic ‘body-printing’ of individual tiger sharks is ongoing.
One would imagine that tagging a mature female tiger shark is a bit like dancing with wolves (chaotic and potentially lethal), but in fact the sharks are remarkably cooperative and the team runs a slick operation. First, Addison free-dives down to a shark and places a hook in the corner of its mouth. Then the boat crew brings the animal to the surface and quickly calms it by placing a plastic cover over its eyes.
“One would imagine that tagging a mature female tiger shark is a bit like dancing with wolves – chaotic and potentially lethalWith the shark pinned to the boat in a tailor-made sling, holes are rapidly drilled through its dorsal fin with a specialised corer that also provides genetic samples. Once the tag is bolted to the fin, the shark is released. Rather than turning on its captors, it swims away. ‘The fact that the sharks often return to baited dives shortly after the experience suggests that they are not in pain or traumatised,’ says Peddemors.
Three of the four pop-up archival satellite tags deployed revealed a startling pattern in the sharks’ journeys (the fourth malfunctioned, leaving its subject’s whereabouts a mystery). When Smale downloaded the dive profiles recorded by the satellite, a vertical line marking each dive zigzagged across the screen. The sawtooth pattern it formed was characteristic of a shark ‘bounce-diving’ through the water column. The tiger sharks were conducting frequent dives to more than 300 metres, constantly moving from the surface to the seafloor and up again. One shark even descended below 800 metres, breaking its tagged Hawaiian cousin’s record for maximum swimming depth by a factor of almost three.
Aside from revealing an incredible diving aptitude, these results show that the animals travel through temperatures that vary by as much as 10 °C, suggesting that the tiger shark is not confined to being a ‘warm-water and tropical’ species as was previously thought. Utilising the entire water column, especially in the open ocean where food is scarce, is like hunting in 3D rather than 2D. It allows the shark to pick up the scent trails and other signals of potential prey across the whole ocean larder, instead of being confined to a single shelf.
Similar bounce-diving behaviour was observed by Dicken via a boat-based receiver. He conducted multiple daytime tracks of tigers using continuous pinger tags that transmitted depth and temperature information about the sharks being tracked to the boat. Subsequently, the team stalked a tiger shark for 24 hours. This added a new dimension to the study and gave immediate insight into how different individuals utilise the reef. It also turned up surprises of a different sort. At one stage, the shark swam beneath all 10 recreational dive boats without being seen. Despite its species’ reputation, this particular individual proved to be docile and wary, and if anything, felt threatened by the presence of the divers.
Eighteen VR2 listening stations were deployed in a grid pattern within the Aliwal Shoal MPA. Acting as the ears of the reef, they recorded the movements of tagged sharks whenever they roamed the area. In March 2007, however, a gargantuan storm struck the coast, and its 14-metre waves and six metres of sediment erosion destroyed most of the stations and the precious data they harboured. The few units that survived revealed that sharks return to the area many times during the course of the day and come back in multiple years. In addition, the photographic identification catalogue (based on the unique patterns on the sharks’ dorsal and pectoral fins) is supporting Addison’s visual sightings of the tigers that return to the shoal each year. Much of the data still awaits analysis, holding promise of new discoveries.
DANCING WITH TIGERS
Yes, tiger sharks are big predators, but knowledge of their behaviour has progressed to the point where, with the right operator, free-diving with them is a perfectly sane (and safe) thing to do. Cheryl-Samantha Owen explains.
The sharks are attracted to a sardinefilled perforated steel drum suspended approximately five metres below the surface of the water. The drum releases a pungent brew of fish oils that moves with and is distributed by the current. Sharks pick up the scent and follow the ‘chum’ slick back to its source, the boat. When a shark is sighted, the buoyed drum is released from its anchor to be taken up by the current. The divers and snorkellers then drift in the water column perpendicular to and staying out of the chum slick. A safety diver keeps watch from the surface, making sure that divers do not drift into the slick.
When? The highest concentration of tiger sharks at Aliwal Shoal is in the summer months, from December to June.
Experience? Says Mark Addison from Blue Wilderness, ‘We have taken our understanding of these magnificent animals to a level where the experience needed is very low – you can watch the sharks from the boat, or surface snorkel, free-dive or scuba dive with them.’
How safe is it? It’s all about choosing your operator carefully. Blue Wilderness takes some 1 500 people a year on shark-diving expeditions, without incident.
For more information or to book a tour, contact http://www.bluewilderness.co.za
‘One of the most important and exciting things about the SOSF project is that it assisted in breaking down the barrier of belief across the world that tiger sharks are ferocious man-eaters,’ says Peddemors. No formal protection for the species exists and with most of their time spent outside MPAs, the sharks face a perilous ocean filled with longliners, unrestricted fishing and shark nets. In addition, they may be affected by concentrations of toxic pollutants contained in the prey they eat. Over-exploitation from targeted and bycatch fisheries, which kill the sharks for their fins and flesh, has garnered the mighty tiger shark, alongside many other shark species, a place on the IUCN Red List of threatened species.
“Of all the sharks I have worked with, the tiger is by far the most beautiful and gentle. Its… patient and unexcitable demeanour gives it a stately presence, inspiring feelings of wonder rather than fearKZN’s infamous shark nets (essentially gill nets that ‘work’ by reducing populations) alone kill a reported average of 48 tiger sharks a year. In an area where ecotourism dollars are earned because of the presence of tigers, these deaths are bad news. Dicken’s 2009 report on the value of tiger shark dive tourism states that ‘… the direct value of tiger shark diving to the Aliwal Shoal region was just under R12.5-million [around US$1.6-million]’. As with many other species worldwide, the tiger sharks that live off South Africa’s east coast are more valuable alive for ecotourism and education than they are as fins, jaws, teeth and meat.
The web of life within the ocean ecosystem is so complex that we have not yet uncovered all its secrets. We are still trying to understand the role that many species play in relation to their environment, but it is safe to say that every species of shark has adapted over millennia to a particular habitat and the loss of any apex predator has an impact. Sharks have survived for some 400 million years – appearing at least 200 million years earlier than the dinosaurs and 396 million years before the first hominids evolved.
Scientists and dive operators alike are battling to save the tigers of the sea. In the words of Mark Addison: ‘Of all the sharks I have worked with, the tiger is by far the most beautiful and gentle. Its slow and elegant movement coupled with a patient and unexcitable demeanour give it a stately presence, inspiring feelings of wonder rather than fear.’ So go on, take a plunge into its world, bust a few myths and you could help one of the ocean’s most important apex predators to survive its greatest challenge yet. Us.
This article originally appeared in the May 2010 edition of Africa Geographic
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