Sharks: The Big Five – Ocean’s Ultimate Predators

The ocean, like Africa, has its own big five, sharks. Shark, there are few words that have such attention-grabbing power. The most feared is the great white. There's also the biggest fish in the ocean, the whale shark. Most sharks swim alone or in loose groups, but hammerheads can congregate in their hundreds. To be surrounded 360 by sharks is, for some, a must-do experience. The tiger of the ocean is one of the few sharks that is known on rare occasions to kill people. But the bull shark claims the title of the most dangerous to humankind. At up to 6 meters long and the biggest predatory fish in the ocean, this is the shark many crave to see, yet hope never to meet unexpectedly. Close encounters from a boat provide a powerful natural high. It's a definitive adrenaline bus that's only topped by cage diving with these awe-inspiring fish. Free diving, the ultimate experience, is only for professionals who know the lie of the ocean floor and which sharks are relatively safe to swim with. Remarkably, scientists have found each one has a different character. Some are shy and cautious, others bold and confident. The fact that free diving is possible at all shows that the propaganda surrounding great whites, generated originally from the infamous film Jaws, is largely unfounded. This monster movie terrified audiences. It exploited the iconic image of the dorsal fin as a symbol of death at sea. Initially, the impact of the film had tragic consequences for great whites and many other shark species. Jaws did irreparable damage to the great white shark's reputation. Every time there's another shark attack, panic reactions and hyped press coverage reinforce the old stereotype. Killing sharks was regarded as ridding the sea of a pestilent menace. But sharks have been in the ocean for 400 million years. They deserve our respect and to be better understood. To many people's surprise, Jaws also had a positive knock-on effect. It inspired fascination and changed the world by raising ocean awareness. In the Bahamas, a dead bull shark is brought in for a scientist to discover the cause of death. It's a perfect opportunity to teach the next generation just how amazing sharks really are. I came up with a key identifying feature of a bull shark. Where this dorsal fin comes in, it's directly in line of posterior to the back of the pectoral fin. 50 years ago, it was extremely hard to get funds to study sharks. Now, the world is hungry for information about them. Marine biologists and educators have urged people to shed the perception of the great white shark as a vengeful, man-eating machine in favor of a complex, misunderstood, ancient sea creature. Great white sharks are not wanton killers. And in fact, they don't want to eat humans at all. They do occasionally bite people. But it's now thought that this is due to curiosity or mistaken identity. In South Africa, surfers and swimmers are warned not to go into the sea when the water is rough or murky. When they can't see clearly, great white sharks Great whites are thought to mistake the outline of a swimmer or surfer for seals, their natural prey. Stealthy, skilled predators, the sharks launch their attack from depth, biting as they close in on their target. When they grasp a human, they more often than not let go. The triangular, serrated teeth of the great white are efficient cutting tools for biting large chunks from mammalian prey. Tragically, for their mistaken human victims, their first bite can cause fatal damage. The reason they let go is because it's thought that people are not nutritious enough for great whites to be worth hunting. Seals and sea lions are the mainstay of an adult's diet. These marine mammals have a good layer of blubber to keep them warm in a cold sea. Fat produces twice the energy of muscle, so it's an ideal food for these sharks. Surprisingly, great whites are not, like most fish, cold-blooded. They can regulate their body temperature, raising it up to 14 degrees in cold seas. Being relatively warm-blooded is one of the factors that have allowed great whites to inhabit temperate waters that are too cold for many other shark species. A warm body allows the sharks' muscles to work more efficiently, and it is this that enables them to produce bursts of energy needed to catch their prey. The great whites living off the coast of California hunt elephant seals, as well as sea lions and harbor seals. Off the coast of South Africa, here near Cape Town, they target Cape fur seals. The common name, white shark, is a misnomer. Although the shark does present a striking white belly when hauled out of the water, it's more slate gray than white. An adult swimming close to the seabed is well-camouflaged and hard for a seal to spot from above. Unable to see them, but knowing sharks may be below, adult seals travel fast. They present the great whites with a speeding objective that is hard to target. Pups, still learning to perfect their swimming skills, are easier to catch. Seal milk is rich. The pups grow fast, and at weaning, 50% of their weight can be fat. The seals give birth on offshore islands. Researchers have found that the sharks congregate near the islands when the weaned pups have to venture into the open ocean to hunt for fish. Great whites are thought to grow about 25 to 30 centimeters a year, so need rich food to enable them to achieve this. The young seals are not yet fast or proficient swimmers. They can't hold their breath underwater for long, and their high fat content makes them buoyant. They have no experience of sharks and haven't learned how to evade an attack. A good seal meal may provide enough energy to sustain a shark for a week or more. The sharks themselves bear the unmistakable tooth marks of their own kind. The subordinates get put in their place with bites from dominant individuals. Great whites are mavericks among sharks. They have colonized cold temperate waters where seals are abundant. By gorging on seal fat, the shark derives thermal energy from the same insulating tissue that keeps its prey warm. And they can travel great distances. Research has shown that sharks from California can swim to Hawaii and back. Another tagged in South Africa showed up in Western Australia. There's new information about their movements and habits off the coast of South Africa. Young great whites swim along the coast hunting bottom dwelling fish. Once they grow a little bigger, they eat bigger fish like mackerel, tuna, and bonito. Only when they reach about three meters long do they start feeding on seals and sea lions. Tagging research has shown that great whites come inshore much more frequently and much closer to people than anyone had previously suspected. The very small number of attacks by great whites again suggests that people are not on their menu. Adults are now known to rest inshore where there is a slack in the current or tide. An open water species, great whites generally avoid kelp forests. Smaller species, like the pajama and other cat sharks, seek shelter in the forest. Seven gill sharks also frequent the kelp. Although they hunt the smaller species at night, there seems to be an uneasy truce as they rest in the forest during the day. While some great whites stay near the coast all year round, others move offshore and remain in deep water for four to six months. The more we find out about great whites, the more we turn previous beliefs about them upside down. Fascinating and intriguing they are. Wanton man-eaters they are not. Even so, people should be wary of all big sharks, except for plankton feeders like the whale shark. Tourists worldwide make great efforts to swim with the whale shark. Despite being the world's largest fish, it is no threat to us. Here in Mozambique, snorkeling or diving with one of these incredible fish can make for an experience of a lifetime. It's one that is on a par with seeing Africa's better known, terrestrial, and iconic big five. Lions, rhinos, leopard, buffalo, and elephant. Whale shark ecotourism is worth millions of dollars annually worldwide, with many small communities benefiting financially. Practically nothing was known about the biology of this shark up until about 15 years ago. Since that time, there has been a veritable explosion of interest in the world's largest fish. A fish, not a mammal. The whale shark originated about 60 million years ago. It can reach up to 14 meters and probably live up to 100 years old. It is migratory and seasonally turns up at regular food sources. Between November and January every year, off the coast of Djibouti at the Horn of Africa, hundreds of mainly male juvenile whale sharks gather to feed. Offshore upwellings and rich surface waters. This phenomenon triggers plankton blooms that in turn can initiate a spectacular gathering of giants. It's obvious that these sharks are all young, as the tails of juveniles have larger upper than lower lobes. The spots and patterns are unique to each individual. Photographic records enable scientists to count populations and assess which sharks return each year. Very similar spot patterns support the idea that some sharks may be littermates. The Djibouti aggregations are exceptional in that they contain the smallest whale sharks found in the world. Some only two meters long. It's thought that at this length, the sharks are just reaching their first birthday. Where they are born and spend their first year of life is unknown. It's possible that this region is a sort of entry point for young whale sharks. When they grow bigger and can compete with larger whale sharks, they move on to other aggregation sites. Where they come from, where they go, and why there are more males than females at Djibouti remain unanswered questions. Whale sharks feed on plankton, fish eggs, and small fish. They actively gulp in water and filter food from it using their pink gill rakers. The sharks feed day and night, maximizing their intake when food is available. In the Maldives, plankton blooms generally occur between May and November. The surfeit of food attracts whale sharks, but they are not alone in exploiting this resource bonanza. Spectacular feeding trains of hundreds of manta rays glide by in a steady stream. Barrel rolling and intent on feeding, the mantas often bump into each other, as well as the whale sharks. When the plankton sinks to the seabed, the mantas bottom feed, turning their cephalic lobes outwards rather than inwards to maximize the flow through their gill rakers. The whale shark has a circum-global distribution in tropical and warm temperate seas. Moving between seasonal food sources means they can travel vast distances. The longest recorded whale shark journey spanned 13,000 kilometers and took over 36 months. Researchers have discovered that whale sharks make very deep and long dives, down to 1,280 meters, where temperatures drop to around 3 degrees. This is the deepest dive of any shark. Tags that record depth and acceleration have revealed that deep dives may make traveling long distances more efficient. The sharks glide deep down and then gently beat their tails and ascend at a very shallow angle. In this way, the whale sharks travel a maximum range with minimum energy expenditure. Once back at the surface, they rest and warm up. They cannot regulate their body temperature to the same degree as great whites. The secret life of this colossal yet elusive giant is very slowly beginning to be unraveled. But there is still so much to discover. In Scottish waters, another huge ocean traveler, the basking shark, is the second largest living fish. While not in the top five, it is a close contender, so deserves recognition. Recent research has revealed cutting-edge discoveries about this shark, too. In contrast to the whale shark, the basking shark is a passive feeder that is unable to pump water across its gills. The plankton that it feeds on is filtered out by pinkish brush-like gill breakers. Every so often, it closes its jaws and gulps three to five times to swallow. Basking sharks are seen feeding at the surface. But as deep-sea shrimps have been found in their stomachs, it's apparent that they feed at depth as well. A feeding basker can filter up to 2,000 tons of water per hour. Many adult basking sharks bear white scars. Those on the pectoral fin may be due to the male biting the female in a mating embrace. The sharks rubbing against one another during courtship can result in abrasions on their noses. Below the caudal fin on the back of this shark are some long, white scars caused by lampreys. Lampreys are eel-like, jawless, primitive vertebrates. These are parasitic, feeding on the flesh and fluids of their hosts. While lampreys may be an irritation, they are not the reason for the basker's dramatic decline. Basking shark numbers are now at an all-time low. There are possibly only 8,000 left worldwide. They're protected in British waters, but recent research has shown that this is not enough. Satellite tags have revealed that some of these sharks make remarkable trans-oceanic migrations. A female tagged off the west coast of England traveled over 9,500 kilometers, sometimes at a depth of around 1,200 meters, all the way to Newfoundland off the east coast of Canada. While trans-oceanic migrations take the sharks out of protected areas, recent evidence points to a slow recovery of the species in British waters. No such positive story for the scalloped hammerhead. Their numbers have recently plummeted, a tragedy not only for the sharks, but for the people whose dream it is to swim with them. Spectacles like this off the coast of Costa Rica are high on many divers' must-see list. Hammerheads have long been regarded as dangerous, but it's been found that these sharks generally do not attack unless provoked, and there have been no fatalities. Hammerheads have a fused upper jaw and small mouth. They're not adapted for prey the size of humans, but to grab fish, squid, octopus, and crustaceans. Their hammer-shaped head is thought to have evolved to maximize the area of electro-sensory pores in the skin. These special sensors provide the shark with a sort of sixth sense. They're used to detect chemical, physical, and thermal changes, as well as the electrical fields of prey species, including those buried in the seabed. The hammer shape allows these sharks to scan significantly larger areas than other shark species. The positioning of the eyes, mounted well out on the sides of the head, also give them good 360-degree vision so they can see above and below at all times. Just like humans, scalloped hammerheads can get a suntan. When swimming in shallow water or close to the surface, shark skin produces more of the pigment melanin and darkens considerably to protect sharks from the ultraviolet light of the sun. Recent research revealed that these hammerheads congregate in schools near seamounts or underwater mountains by day. The mount is a vital cleaning station where wounds can be tended and parasites and dead skin removed by cleaner fish. The sharks invite the cleaners to them by floating or swimming slowly just above the mound. Hammerheads are not the only sharks around the seamount. Threatened white-tip reef sharks rest in sheltered crevices. These small sharks can suck water into their mouths and pump it out over their gills to breathe. Unlike the hammerheads, they can remain stationary to sleep during the day. While hammerheads migrate, visiting the seamount seasonally when the water is warm, the white-tips stay within a particular area for years, time and again returning to the same shelter. Why hammerheads should swim in schools was a mystery. Prey fish congregate to gain safety in numbers, but hammerheads are at the top of the food chain, so have little need to gather together for protection. Painstaking research using video cameras and acoustic tags has revealed the schools may be mating aggregations. Most of the sharks are females that compete with each other for a central position. Males select mates from those around the core, and then the pair leaves to mate in seclusion. At night, hammerheads swim out, each one alone to feed up to 20 kilometers away. The white-tip reef sharks also become active after dusk, but they hunt in groups. Their elongated bodies allow them to wriggle into crevices and between rocks to flush out hidden prey. A turtle is unfazed by the pack of sharks and carries on picking algae off the rocky seabed. The sharks are after smaller prey, like crustaceans and sleeping fish. Flushed from a hiding hole, fish face a host of hungry mouths. Communal life is not for all sharks. The great hammerhead swims alone. The largest species of its kind, it is reported to reach around six meters in length. Endangered and wary, it's not often lured within range for scientists to study. Although potentially dangerous, it rarely attacks humans. It sometimes behaves inquisitively towards divers and, like all large sharks, should be treated with respect. Great hammerheads are heavily fished for their large fins for the Asian shark fin soup trade. In some areas, numbers are down by 80%. All large shark species are suffering, including the stunning tiger of the sea. Striped like its namesake, this shark not only shares a title with the world's biggest cat, but also a reputation as a man-eater. The tiger shark is a stealthy, powerful predator that is found worldwide in tropical and subtropical waters. They are opportunistic feeders that will scavenge and are sometimes known as the trash cans of the seas. They do occasionally kill people, but are not manhunters. They don't go out to feed on swimmers, but with their undiscerning palate, they don't swim away after biting a person, as great whites frequently do. Recent research in South Africa has shown that the larger tigers tend to stay offshore. Only the younger, immature sharks come inshore, and when they do, they avoid people and divers. This better understanding reveals that tiger sharks are not as dangerous to humans as previously thought. Many tigers spend time in deep, open ocean far offshore, then move inland to shallow seas. They have the ability to drastically switch their habitats comfortably. Few other shark species show this flexibility. One that does is the bull shark. This large predator is just as at home in deep seas as it is in shallow coastal waters. People enter its domain on a daily basis. This close proximity to humans and the shark's bold, inquisitive, and fearless nature make it the most dangerous of all sharks. It's thought that it was probably bull sharks, not great whites, that caused the deaths of the five people bitten and killed in 1916 that sparked the story of Jaws. Bull sharks get their name from their short, blunt snout, as well as their pugnacious disposition and a tendency to headbutt their prey before attacking. Like most shark scientists, Swiss-born biologist Eric Ritter believes that people are not part of any shark diet. Even with bulls, it's a case of victims being in the wrong place at the wrong time. To prove his point, Eric waded into bull shark-infested shallows. Keeping relaxed, with his heart and breathing rate low, he walked among them. OK, hands up, hands up, hands up. Sharks use their jaws to mouth and investigate unfamiliar items in their domain. A bull tests Eric to find out what he is. Although most sharks have good eyesight, the water can be murky. In low visibility, sharks investigate anything unusual with their mouths. Tragically, even just a test with a shark's razor-sharp teeth can be fatal for a thin-skinned human. In 2002, a bull bit Eric while he was filming a shark special. Though he was seriously injured, Eric believed the attack was a mistake. He said that the shark took an exploratory bite and then got stressed. Bulls make no such mistakes when taking chunks out of manta rays. At first, it was thought that great whites were biting the ray's wings, when in doubt, blame great whites. Research into bite size and jaw shape has revealed that the mantas are most likely being bitten by bull sharks. Like scalloped hammerheads, the mantas regularly visit seamount cleaning stations. Keeping the wound edges clean probably stops the spread of infection. No one knows where or when bull sharks target these rays. In some areas, bull sharks don't bite rays at all. Perhaps it's learned behavior, with sharks copying each other in specific locations to perpetuate the hunting technique. For an ultimate shark encounter high, shark reef off Pacific Harbor in Fiji is a top destination. Here, people can dive with bulls and seven other shark species. To attract the sharks, local divers collect fish heads and offal from a fish factory. The bait is lowered into place in wheelie bins. Tawny nurse sharks have learned how to get at the prize within. Thousands of fish congregate to pick up scraps and grab what they can. Sharks are vital to the health of the ocean. Take them out, and the pyramid that is the food chain collapses, and the ocean becomes out of balance. We all need the ocean to absorb carbon dioxide and recycle fresh water. The sea needs its top predators. We lose sharks at our peril. It may seem foolhardy to hand feed bulls, but the Fiji shark divers know how to work and swim with them safely. The sharks are conditioned through food rewards to approach only from one side and to take food from one particular diver, identified by their different colored hoods. Researchers and the local divers have found that the sharks are trainable and have individual personalities. The feeders know which ones are very gentle and which ones need reminding of their manners occasionally. In Fiji, thousands of people have had thrilling dives with these incredible predators. No one here has been attacked or bitten. The odds of being a victim of a shark attack worldwide are extremely low. About one in six million in the United States. In fact, people are at a greater risk of being bitten and killed by a dog than a shark. The opportunity for such close encounters like this with bull sharks is a truly amazing experience. An ultimate shark high.