Shark Read online

  The blue shark is brilliantly coloured, being shiny indigo blue on its upper surface, paling to an even white underneath the slender body. The snout and head are streamlined, the eyes large and white-rimmed, the dorsal fin modest, the upper lobe of the tail fin elongated and slender, and the pectoral fins long and winglike. This graceful and elegant shark is described as an ‘open-ocean glider’.34

  The blue shark eats a broad range of prey, mainly relatively small teleosts and cephalopods, although as an open-ocean opportunist it will take shipwreck victims. One adaptation to existence in an open-ocean environment where food can be scarce is the presence of gill rakers that prevent even tiny food items escaping. It is not uncommon in coastal waters, making it unusual as a species that is both a fringe littoral and oceanic–epipelagic inhabitant.

  The blue shark’s inshore feeding strategies follow prey movements. The Hudson River, flowing through New York City, continues on as the submerged Hudson Canyon, a kilometre-deep rift in the continental shelf off the coast. Between 1979 and 1996, a seminal project tagged feeding blue sharks active in this inshore water. During the day the sharks swam slowly near the surface, but periodically dived to depths approaching 400 metres, before surfacing again. Sharks were observed to make up to six dives during daylight hours, with a diving technique formally described as ‘remarkably regular vertical oscillations . . . with some [sharks] dropping very rapidly’.35 The shark’s return to the surface may have something to do with thermoregulation, given the temperature differences between the water layers.

  Blue shark migrations are associated with their reproductive cycle. Tagging studies revealed populations mating in waters off the Caribbean Sea and east coast of the United States. The pregnant female stores the male sperm in sacs in her reproductive tract and sets off towards Europe, planing through the ocean in the currents of the Gulf Stream and the North Atlantic Gyre—the aquatic equivalent of albatrosses circumnavigating Antarctica in the winds of the Roaring Forties. At some point in the journey, possibly as long as twenty months after the act of copulation, the female’s eggs are fertilised. The timing is perhaps linked with the female’s own maturation process. Litters that can exceed 100 live young are born in pupping waters off the west coast of Europe. The female then continues on to the north-west coast of Africa, in order to link up with currents that will return her to the Caribbean Sea. Such an incredible journey could total up to 15 000 kilometres, and may take the shark about two years to complete.

  Blue shark pupping grounds are also known in the Southwest Atlantic, the Northern Pacific and the South-west Pacific, and there may well be others. Why such a diverse range? There are multiple reasons, according to longtime blue shark researcher John Stevens: ‘The oceanic environment is huge and food resources are often scarce in nutrient-poor open ocean waters. Pupping areas are often in more productive zones so that there is sufficient food to grow the pups rapidly out of the predation window, and separating different components of the population by area also reduces intraspecific competition’.36

  Bull shark (Carcharhinus leucas) (Plate 5)

  This is the last of the requiem sharks to be described here. The bull shark is the only large saltwater predator also capable of living in freshwater rivers and lakes. It grows to about four metres and weighs in excess of 200 kilograms—significantly heavier than the longer, leaner oceanic whitetip. The bull shark is variously described as stout, robust, unpredictable and pugnacious: a mixture of physiological and behavioural characteristics that underscore its reputation as a vigorous warm-water predator and scavenger.

  The bull shark has a noticeably short, blunt snout (its French name is requin bouledogue) and large, triangular serrated teeth. The eyes are quite small, well-adapted to shallow, often murky or silty water in which eyesight is less important than other senses in detecting food. The heavy, fusiform body is propelled by a large, powerful tail, the first dorsal fin has a large base and a distinct triangular form, and the pelvic fins and second dorsal fin are also well-developed. The fact that the bull shark inhabits marine, estuarine and freshwater systems means that it takes a range of prey, with a concentration on teleosts, rays and smaller sharks (including its own kind).

  The bull shark has a notorious reputation as a maneater, partly because, as an inshore and freshwater species, it comes into contact with people more often than other large predators, and also because it is a somewhat indiscriminate forager. Many shark researchers believe that attacks automatically attributed to great whites are just as likely to be the work of the bull shark. The dentition of the two species is similar and analysis of the bite marks can be inconclusive.

  Like most shark species, there is much still to learn about the bull shark. Richard Pillans of the University of Queensland, in his study of the physiological ecology of bull sharks in the Brisbane River, determined that the animal’s liver, kidneys, gills and rectal glands modify the body’s salt levels according to the degree of salinity in the water around it. The kidneys maintains equilibrium by retaining or discharging salt-concentrated urea and the gills undergo a cellular alteration to accommodate the changing salinity of the water flow.37 Pillans also identified bull shark nursery grounds in Queensland estuarine systems; the first known in the South Pacific.

  Other studies have concentrated on why these sharks move between different habitats. Bull sharks resident at the Bahamas for nine months of each year mysteriously vanish for the remaining three months. A pilot conservation program, Project Aware, tracked a number of individuals. One adult female headed directly to Florida and entered a freshwater system that bull sharks use as a nursery ground. Researchers are also studying a Pacific Ocean population of bull sharks, using data about their movements to understand more about their behaviour. The bull sharks are not always as fearsome as their reputation:

  The bull sharks we study off Viti Levu in the country of Fiji stay deep at 30 to 40 meters depth in the water column. We also wanted to target specific individual sharks which could yield the most useful data. Since we did not want to traumatize the sharks through conventional scientific catching techniques to deploy our tags we chose to intimately enter the bull sharks’ world and interact with them face to face. Swimming among two dozen or more large bull sharks at 30 meters is a life changing experience and we were immediately awestruck by their beauty, majesty and grace. They would allow us to approach them and attach the tag into their dorsal musculature just below the first dorsal fin with a custom designed tagging stick. Over the course of the year 2004 we equipped a total of 11 personally selected adult bull sharks with satellite tags . . .38

  The broad and fast-flowing rivers of the south-east coast of Africa carry great amounts of silt; their rivermouths are often muddy-brown, turbulent and home to Zambezi sharks—as the bull shark is known locally. The Zambezi shark is considered to be a major killer off the popular swimming beaches of the South African province of Natal. This may be so but, according to a local dive operator:

  Diving with Bull Sharks in South Africa and Mozambique is quite safe—if you respect the sharks and stick to the rules. No scuba diver has ever been attacked by a Bull Shark while diving, and we intend to keep it that way. We expect to start seeing Bull Sharks from September, with the most sightings from November to May. Sightings in other months are rare. The animals are not fed, baited or conditioned at all—the Bull Sharks are completely wild. The Bull Sharks are curious of nature—therefore we often see them as soon as we hit the water because they have been waiting to find out if it is a fishing boat or not (clever buggers). The splash of divers doing backward rolls into the water causes the Bull Sharks to scatter at first. They often come very close to inspect the divers on both the descent and ascent, but easily lose interest.39

  Bull sharks are found in many freshwater systems. They travel enormous distances inland in rivers such as the Mississippi and Amazon. A Caribbean population that mates and breeds near the mouth of Nicaragua’s San Juan River regularly migrates 200 kilometres upriver,
negotiating three sets of large rapids, into Lake Nicaragua. An individual takes a week or more to make the journey, the purpose of which is thought to be associated with the breeding cycle, because the sharks do not form a permanent population in the lake (unlike the entirely freshwater Nicaraguan sawfish). Lake Nicaragua averages a shallow thirteen metres, which suits the bull shark.

  The bull shark is generally thought of as ‘slow’, perhaps because of its bulky body, but it has an impressive turn of speed. The cartilage of its jaws is unusually rigid (due to a concentrated calcification process) and in attack, its mouth is almost squarely open. This makes it extremely efficient at tackling and extracting great chunks from large prey. It’s a feeding strategy supported by an unusually muscular neck/nape, a musculature which increases with age. These physical characteristics have led the bull shark to be compared to terrestrial predator–scavengers such as the Tasmanian devil, famous for its ferocious-looking gape and large, bullish neck and head on a proportionately small body.

  Hammerhead sharks (Plate 6)

  There are nine species of this shark, some of which are quite small, but the largest, the great hammerhead (Sphyrna mokarran), grows to about six metres and weighs 450 kilograms or more. Hammerheads are widespread in tropical and temperate waters, from coastal shallows to semi-oceanic depths of about 300 metres. During the summer they migrate, moving polewards into cooler waters. They are not oceanic.

  It is both apposite and opposite to link hammerheads and threshers, the exceptional tails of the latter matched by the heads of the former, which supply both their scientific name (sphyrna means ‘hammer’) and common names: the winghead shark, the bonnethead shark, the mallethead shark, the scoophead shark and the scalloped hammerhead.

  S. mokarran has a powerful fusiform body, a tall, pointed dorsal fin, big pelvic fins and a large upper caudal lobe. The dermal denticles are closely spaced, and the mouth a semicircle set well back underneath the flattened head. The teeth are relatively small, triangular and serrated. It is an active predator, with a wide prey base and a preference for stingrays. Litters of live young may number more than 40, but great hammerheads are solitary animals, unlike other hammerhead species, such as the scalloped hammerhead, which school in large numbers: groups numbering in the low hundreds are not uncommon.

  In a species family known for its unusual heads, S. mokarran is so unusual that its head has acquired its own generic name, a ‘cephalofoil’, defined as the lateral extensions of the head of a hammerhead. Descriptions of S. mokarran refer to ‘its nearly rectangular head’;40 its ‘T-shaped head’;41 or the fact that the ‘front margin of the head is nearly straight . . . [with] eyes located at the tips of laterally expanded blades’.42

  These sharks’ widespread distribution and preference for shallow waters mean that researchers have been able to work with them over many years, but there remains a degree of uncertainty over the precise evolutionary solution achieved by the head shape. University of Hawaii researchers, using electrically charged dipoles to simulate prey, hypothesised that the ‘unique head morphology of sphyrnid sharks might have evolved to enhance electrosensory capabilities’,43 but concluded that ‘although the sphyrnid head morphology does not appear to confer a greater sensitivity to prey-simulating dipole electric fields, it does provide (1) a greater lateral search area, which may increase the probability of prey encounter, and (2) enhanced maneuverability, which may aid in prey capture’.44

  One or more of four possibilities—binocular vision, stereo smell, electroreception, manoeuvrability—remain the accepted explanations for the shape. The latter is intriguing, posing to the University of Hawaii researchers the question, ‘does the hammerhead help this shark to turn on a pinhead?’45 Such precision could do for the hammerhead what the tail may do for the thresher shark. Furthermore, the enormous head of the winghead shark (Eusphyra blochii) is about half the length of this shark—the same proportion as the thresher’s tail to its body. The common name of this species scarcely requires explanation: ‘In practice, the head is transformed into a wing able to support and aid changes of direction.’46 This shark grows to about 1.8 metres.

  Schooling behaviour also remains something of a mystery. Hammerhead schools can number in hundreds and form part of distinct behaviour patterns. They gather during the day, generally at seamounts. Their interactions are highly social, with numerous displays, some of which include use of the hammer as a butting instrument. There is a dominance hierarchy, by which the largest females occupy the middle of the school and are often approached by males who otherwise seem not to be part of the group. It would appear, therefore, that mate selection is one function of the school. Towards evening, the school disperses and each animal spends a solitary night feeding in deeper waters. They then return to the same seamount the next day. The schools move from one seamount to another, although they are not unique fish in this. According to the longtime hammerhead shark researcher Peter Klimley, whose work has focused on the scalloped hammerhead, ‘There is a whole assemblage of species that move north and south via seamount stepping stones’.47 As a highly visible marine species whose numbers are under threat, the migratory habits of schooling hammerheads are being closely monitored by Klimley and his team at the University of California Davis. The team is using electromagnetic fields associated with seamounts to map their journeys—that is, creating geomagnetic shark roads—an approach Klimley hopes will prove the critical importance of seamounts and encourage governments to turn the areas around them into marine reserves.

  Tiger shark (Galeocerdo cuvier)

  This requiem shark is the only member of its genus, in contrast for example to the closely related Carcharhinus genus which contains about 30 species. There is some doubt about its maximum length, but specimens measuring 7.5 metres and weighing over 800 kilograms have been recorded.

  The tiger shark is widespread in tropical waters and is the primary apex predator in the planet’s warm coastal waters. Newborn and juvenile tiger sharks have distinctive black vertical bars and spots, which become less pronounced in adulthood, with the skin a uniform grey. The underside is typically white. The adult shark’s snout is usually described as blunt, wedge-shaped and robust; a shape that allows the animal to turn its head rapidly and shake it vigorously from side to side when feeding. The eyes are large and the mouth huge, almost as wide as the head. The teeth in both jaws are broad-based, flat, heavily serrated and asymmetrical, or ‘cockscomb’. The cartilage of the jaws is strengthened with multiple layers of calcium salts called tesserae, normally found in a single layer in shark jaws, but the force of the bite of the tiger shark—and the great white—requires this strengthening.

  As the shark matures, its body forward of the dorsal fin continues to bulk up. The first dorsal fin is large and upright, and acts as a fulcrum, helping the shark to pivot and turn quickly; the long, downswept pectoral fins act as wings in providing lift; and the elongated tail generates powerful forward thrust.

  The tiger shark is found in a variety of shallow water habitats such as harbours, estuaries, lagoons, river mouths and inshore reefs, where it feeds at night on surface, midwater and bottom prey. It has also been recorded at depths of at least 150 metres, which means that it has a wide prey base. It is a migratory species, moving into cooler waters in summer. The tiger shark is the only ovoviviparous requiem shark and the females give birth in shallow pupping grounds.

  The tiger shark has been condemned variously as the ‘dustbin’ of the sea because it eats ‘everything’, and as a ferocious maneater lurking in shallow murky waters. Neither charge is true. Like any other shark, it has evolved slowly over a great period of time to take full advantage of its particular niche in the marine ecosystem. Certainly, its diet is cosmopolitan:

  The stomach contents of 811 specimens of G. cuvier caught in the Townsville area [in shark nets] were examined. Of these, 31.2% were empty . . . The main prey groups were teleosts and sea snakes, but crabs, turtles and birds were also important. Other prey g
roups included dolphins, dugongs, sharks, rays and squids. A number of items of garbage were also recorded, including plastic bags, aluminium foil, clothing, a sugar bag, prawn netting and kitchen scraps . . . The proportion of teleosts in the diet decreased with increasing shark size. Other food groups that decreased with increasing shark size were squids and birds. Concomitant with the decrease in these groups was an increase in the occurrence of turtles, crabs and sharks in larger individuals.48

  This is not the diet of a true scavenger; rather, it is euryphagic (eating a wide range of foods). The oceans are cluttered with human refuse, any amount of which may have been ingested by the prey rather than the predator. This does not mean that the tiger shark doesn’t eat garbage: behaviourally it is described as curious and assertive; and feeding is a major component of behaviour. The significant fact in the quote is that as tiger sharks grow they eat a greater proportion of turtles. This relates to their complex dentition. It is increasingly believed that tiger sharks’ teeth have evolved to take advantage of large turtles (adult green turtles reach 100 centimetres and weigh about 150 kilograms). The tiger shark’s heavily serrated teeth ‘are reminiscent of a can opener’.49 When the tiger shark clamps the turtle between its teeth and shakes its head from side to side, the teeth saw through the carapace top and bottom.

  Concurrent but unrelated Australian studies are adding to the meagre understanding of this unique shark. A tagging program at Raine Island off Cape York Peninsula, northeastern Australia, has revealed a new understanding about the movement patterns of the species. The tagging site was chosen because tiger sharks congregate in large numbers around the island in summer, in order to feed off its turtles and seabirds. Researchers attract the shark alongside their boat with bait, then clamp its tail with a ‘shark claw’ attached to a rope and a float, the drag of which, in a modern echo of the shark caller’s technique, quickly immobilises the animal without in any way harming it. The tag (a satellite transmitter, long-life batteries, aerial and saltwater switch all encased in a waterproof resin pod) is then attached to its dorsal fin, and the signals it transmits enable the shark’s movements to be tracked.