SHARKS OF ALIWAL SHAOL

Durban’s Aliwal Shoal is world renowned as a mecca for species of sharks. On this reef both large and small sharks make their home. Many are resident, some are seasonal visitors, and some, like the giant whale shark, are just passing through. Here we would like to share with you five species that we love encountering during our shark snorkeling and SCUBA diving expeditions to Aliwal Shoal

Blacktip Shark (Carcharhinus limbatus)

The blacktip shark is found in warm coastal waters around the world—where it often encounters people. The animal frequents bays, estuaries, coral reefs, and the shallow waters off beaches and river mouths. During summer some blacktip sharks migrate to typically cooler waters, including those off Cape Cod, Massachusetts, while others stay put in warmer equatorial waters year-round. Blacktips have been observed to live in sexually segregated schools except during the mating season.

Blacktip sharks are sometimes spotted above the water. They leap above the surface, rotate several times, and splash down on their backs. This dramatic display is sometimes part of a stealthy feeding method they use to strike at schools of bony fishes near the water surface. The sharks hurl themselves at the fish from below. Blacktip sharks supplement their diets with skates, stingrays, squids, and some crustaceans. These sharks have also been known to follow fishing boats and feed on discarded bycatch.

IUCN conservation status

This IUCN assessment is based on the information published in the 2005 shark status survey (Fowler et al. 2005). The Blacktip Shark (Carcharhinus limbatus) is a modest-sized species that is frequently captured in commercial and recreational fisheries. Its meat is well-regarded and its fins are highly marketable. The Blacktip Shark is widespread in warm-temperate, subtropical and tropical waters throughout the world. It frequents inshore waters as adults and has inshore nursery areas, making it highly vulnerable to fishing pressure and human-induced habitat alteration.

Worldwide distribution

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Identification

The blacktip shark has a robust, streamlined body with a long, pointed snout and relatively small eyes. The five pairs of gill slits are longer than those of similar requiem shark species. The jaws contain 15 tooth rows on either side, with two symphysial teeth (at the jaw midline) in the upper jaw and one symphysial tooth in the lower jaw. The teeth are broad-based with a high, narrow cusp and serrated edges. The first dorsal fin is tall and falcate (sickle-shaped) with a short free rear tip; no ridge runs between the first and second dorsal fins. The large pectoral fins are falcate and pointed.

Behaviour

The blacktip shark is an extremely fast, energetic predator that is usually found in groups of varying size. Segregation by sex and age does not occur; adult males and non-pregnant females are found apart from pregnant females, and both are separated from juveniles.

Feeding

Fish make up some 90% of the blacktip shark’s diet. A wide variety of fish have been recorded as prey for this species: sardines, herring, anchovies, ladyfish, sea catfish, cornetfish, flatfish, threadfins, mullet, mackerel, jacks, groupers, snook, porgies, mojarras, emperors, grunts, butterfish, tilapia, triggerfish, boxfish, and porcupinefish. They also feed on rays and skates, as well as smaller sharks such as smoothhounds and sharpnose sharks. Crustaceans and cephalopods are occasionally taken.

Distribution

The blacktip shark has a worldwide distribution in tropical and subtropical waters. In the Atlantic, it is found from Massachusetts to Brazil, including the Gulf of Mexico and the Caribbean Sea, and from the Mediterranean Sea, Madeira, and the Canary Islands to the Democratic Republic of the Congo. It occurs all around the periphery of the Indian Ocean, from South Africa and Madagascar to the Arabian Peninsula and the Indian subcontinent, to Southeast Asia. In the western Pacific, it is found from southern China to northern Australia, including the Philippines and Indonesia. In the eastern Pacific, it occurs from Baja California to Peru. It has also been reported at a number of Pacific islands, including New Caledonia, Tahiti, the Marquesas, Hawaii, Revillagigedo, and the Galápagos.

 

Repoduction

As with other requiem sharks, the blacktip shark exhibits vivipary (live young). Females typically give birth to four to seven (range one to 10) pups every other year, making use of shallow coastal nurseries that offer plentiful food and fewer predators. Although adult blacktip sharks are highly mobile and disperse over long distances, they are philopatric and return to their original nursery areas to give birth. This results in a series of genetically distinct breeding stocks that overlap in geographic range.

 

At Aliwal Shoal

Blacktip at Aliwal Shoal are encountered throughout the water column, including the surface. They are spotted year around, and are the primary shark that we interact with on our shark expeditions.

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Tiger Shark (Galeocerdo cuvier)

Tiger sharks are named for the dark, vertical stripes found mainly on juveniles. As these sharks mature, the lines begin to fade and almost disappear. Tiger sharks are consummate scavengers, with excellent senses of sight and smell and a nearly limitless menu of diet items. They have sharp, highly serrated teeth and powerful jaws that allow them to crack the shells of sea turtles and clams. The stomach contents of captured tiger sharks have included stingrays, sea snakes, seals, birds, squids, and even license plates and old tires.

They are heavily harvested for their fins, skin, and flesh, and their livers contain high levels of vitamin A, which is processed into vitamin oil. They have extremely low repopulation rates, and therefore may be highly susceptible to fishing pressure. They are listed as near threatened throughout their range.

IUCN conservation status

This IUCN assessment is based on the information published in the 2005 shark status survey (Fowler et al. 2005). They are a large (>550 cm), omnivorous shark, that is common world wide in tropical and warm-temperate coastal waters. It is a relatively fast growing and fecund species. The tiger shark is caught regularly in target and non-target fisheries. There is evidence of declines for several populations where they have been heavily fished, but in general they do not face a high risk of extinction. However, continued demand, especially for fins, may result in further declines in the future.

Worldwide distribution

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Identification

The tiger shark commonly attains a length of 3.25–4.25 m (10 ft 8 in–13 ft 11 in) and weighs around 385–635 kg (849–1,400 lb). Sometimes, an exceptionally large male tiger shark can grow up to 4 m (13 ft 1 in). Females are larger, and exceptionally big ones can reportedly measure over 5 m (16 ft 5 in). One female specimen caught off Australia reportedly measured 5.5 m (18 ft 1 in) long and weighed an exceptional 1,524 kg (3,360 lb), although her weight is thought to have been bolstered by her pregnant state at the time. Even larger specimens have been reported, but are unconfirmed.

Feeding

A tiger shark will eat any type of fish, shark, animal, and small entity it can find in the water. They are very curious by nature and will taste anything that comes their way. The stomachs of tiger sharks have been found with some very unique items inside of them. This includes pieces of boats and ships, jewelry, clothing, tires, books, and more. If it finds its way into the water there is a good chance a tiger shark is going to dine on it. Tiger sharks tend to have a very big appetite but they will also eat when they aren’t hungry just because they see something around them that they enjoy. The variation in weight has to do with how much food is readily available in the area where they live.

Distribution

The Tiger Shark has a worldwide distribution in tropical and warm temperate seas. Randall (1992) described its distribution as follows: ‘In the western Atlantic it ranges from Cape Cod to Uruguay, including the Gulf of Mexico, Bermuda and islands of the Caribbean; in the eastern Atlantic it is found on the West African coast from Morocco to Angola; it remains unknown from the Mediterranean Sea, but there are reports from Iceland and the United Kingdom (these were probably based on vagrants transported there during a warm year by the Gulf Stream) (Compagno 1984). It occurs throughout the Indo-Pacific region from the northern Red Sea to South Africa and east through the islands of Oceania and northern New Zealand (though not yet reported from Easter Island); in the eastern Pacific it ranges from southern California to Peru, including the Galapagos and Revillagigedo Islands.’

Evolution

The shark was first described by Peron and Lesueur in 1822, and was given the name Squalus cuvier. Müller and Henle in 1837 renamed it Galeocerdo tigrinus.[7] The genus, Galeocerdo, is derived from the Greek galeos, which means shark, and the Latin cerdus, the word for the hard hairs of pigs. It is often colloquially called the man-eater shark.

The tiger shark is a member of the order Carcharhiniformes, the most species-rich order of sharks, with more than 270 species also including the small catsharks and hammerhead sharks. Members of this order are characterized by the presence of a nictitating membrane over the eyes, two dorsal fins, an anal fin, and five gill slits. It is the largest member of the Carcharhinidae family, commonly referred to as requiem sharks. This family consists of mostly slender but powerful mid- to large-sized sharks and includes some other well-known sharks, such as the blue shark (Prionace glauca), lemon shark (Negaprion brevirostris), and bull shark (Carcharhinus leucas).

Repoduction

Female tiger sharks give birth to live young. The pups remain inside of their mother for up to 16 months with 14 months being the average. They can give birth to anywhere from 10 to 80 pups at a time. They will be on their own from the second they are born. Females are ready to reproduce when they are about 8 years of age. Males are ready when they are 6-7 years of age.

At Aliwal Shoal

Tiger Sharks at Aliwal Shoal are encountered primarily during the summer months, and particularly when water temperature rises above 23’C. They are a cautious shark, and frequently chumming must occur for a number of hours to attract them to a boat and divers.

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Raggedtooth Shark (Carcharias taurus)

The Ragged Tooth Shark is also known as the Sand Tiger Shark or the Grey Nurse Shark. Although rather frightening in appearance, this species is relatively placid and docile, only likely to make an attack if cornered or provoked. Because of its somewhat scary look and its slow, passive movements, this is the ideal shark for a manmade aquarium, and can often be seen on display in such facilities all over the world.

The Raggie has a stocky body and reaches a length of just over two metres, with the female being slightly larger than the male. They weigh between 90 and 160 kilograms when mature. The top of the body (the dorsal side) is a grey-bronze colour, while the underneath is a much lighter variation thereof. They also have a rather hunched appearance with a sharp, pointy nose or snout.

IUCN conservation status

This IUCN assessment is based on the the information published in the 2005 shark status survey (Fowler et al. 2005). The Raggedtooth Shark (Carcharias taurus) is a large, coastal shark with a disjunct distribution, occurring in most subtropical and warm temperate oceans, except for the Eastern Pacific. It has a strongly K-selected life history and produces only two large pups per litter. As a result, annual rates of population increase are very low, greatly reducing its ability to sustain fishing pressure. Populations in several locations have been severely depleted by commercial fishing, spearfishing and protective beach meshing, requiring the introduction of specific management measures.

Worldwide distribution

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Behaviour

The Ragged tooth shark is a nocturnal animal and spends the day times hidden in caves and gutters, paying little attention to divers. They make little or no effort or exertion during the day, conserving their energy for their night-time hunting.

Identification

Exposed ragged appearing teeth in jaw. Nares and upturned snout create a snarling appearance. Bulky mid body. First dorsal fin set far back over pelvic fins. Anal fin large – same size as pelvic fins and second dorsal. Dorsal colouration brown to greyish gold usually with scattered darker spots or blotches. Ventral colouration pale.

Distribution

Ragged tooth sharks roam the epipelagic and mesopelagic regions of the ocean, sandy coastal waters, estuaries, shallow bays, and rocky or tropical reefs, at depths of up to 190 metres (623 ft). The sand tiger shark can be found in the Atlantic, Pacific and Indian Oceans, and in the Mediterranean and Adriatic Seas.

Feeding

The majority of prey items of sand tigers are demersal (i.e. from the sea bottom), suggesting that they hunt extensively on the sea bottom as far out as the continental shelf. Bony fish (Teleosts) form about 60% of sand tigers food, the remaining prey comprising sharks and skates. In Argentina, the prey includes mostly demersal fishes, e.g. the striped weakfish (Cynoscion guatucupa). The most important elasmobranch prey is the bottom-living smooth-hound shark (Mustelus sp.). Benthic (i.e. free-swimming) rays and skates are also taken. Stomach content analysis indicates that smaller sand tigers mainly focus on the sea bottom and as they grow larger they start to take more benthic prey. This perspective of the diet of sand tigers is consistent with similar observations in the north west Atlantic[14] and in South Africa where large sand tigers capture a wider range of shark and skate species as prey, from the surf zone to the continental shelf, indicating the opportunistic nature of sand tiger feeding. Off South Africa, sand tigers less than 2 m (6 ft 7 in) in length prey on fish about a quarter of their own length; however, large sand tigers capture prey up to about half of their own length. The prey items are usually swallowed as three or four chunks.

Repoduction

The Ragged Tooth Shark is ovoviviparous, meaning that eggs are fertilised and hatched inside the mother and that the pups continue to live internally until they are big and strong enough to be born alive. The mother continues to produce unfertilised eggs inside her uterus so that the pups have sustenance while they are yet to be born, essentially making them cannibalistic (this is a process known as oophagy). Only two pups are born (one from each uterus) and they each measure approximately 100 cm at birth.

At Aliwal Shoal

Raggedtooth at Aliwal Shoal are encountered during the winter months (June – October) at Aliwal Shoal. Their pressence is part of an annual reproductive migration from the eastern cape. Raggedtooth sharks are encountered only by SCUBA divers as they prefer to stay close to the reef and residing in caves and under overhangs.

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Dusky Shark (Carcharhinus obscurus)

The dusky shark swims in tropical and temperate oceans worldwide, cruising from depths of 1,300 feet (400 meters) near the continental shelf all the way in to the surf zone and other shallow inshore waters.Dusky sharks are long-distance swimmers known for seasonal, temperature-driven migrations that males and females undertake in separate groups. Local patterns vary but the sharks often head toward the Poles in summer and return to the Equator in winter on sea voyages that have been known to top 2,000 nautical miles.

Despite their wanderlust, adult female dusky sharks are homebodies when it comes to reproduction. These animals give birth in the same continental regions where they were born. This natal site fidelity means that dusky sharks around the globe live in distinct populations that are not replenished by wandering or migrating animals. Such local pride has a downside, however, because the isolated communities are more susceptible to localized overfishing pressures.

 

IUCN conservation status

The Dusky Shark is a large wide-ranging coastal and pelagic warm water species, which is among the slowest-growing, latest-maturing of known sharks, bearing small litters after a long gestation period. Its very low intrinsic rate of increase renders this species among the most vulnerable of vertebrates (including the great whales and sea turtles) to depletion by fisheries. Unfortunately the dusky shark is difficult to manage or protect because it is taken with other more productive sharks in mixed species fisheries, and has a high mortality rate when taken as bycatch. This species’ fins are highly valued. Time series data are available from the Northwest and Western Central Atlantic, where catch rates have declined. A recent stock assessment of the fishery off southwestern Australia estimated that CPUE of this species declined by >75% from the early 1970s-2004. Given the very high intrinsic vulnerability of this species to depletion, significant estimated declines in several areas of its range and inferred declines in highly fished areas from which data are not available, C. obscurus is assessed as Vulnerable globally.

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Identification

One of the largest members of its genus, the dusky shark commonly reaches a length of 3.2 m (10 ft) and a weight of 160–180 kg (350–400 lb); the maximum recorded length and weight are 4.2 m (14 ft) and 347 kg (765 lb) respectively. This shark has a slender, streamlined body with a broadly rounded snout no longer than the width of the mouth. The medium-sized, circular eyes are equipped with nictitating membranes (protective third eyelids). The mouth has very short, subtle furrows at the corners and contains 13-15 (typically 14) tooth rows on either side of both jaws. The upper teeth are distinctively broad, triangular, and slightly oblique with strong, coarse serrations, while the lower teeth are narrower and upright, with finer serrations.

Distribution

The range of the dusky shark extends worldwide, albeit discontinuously, in tropical and warm-temperate waters. In the western Atlantic Ocean, it is found from Massachusetts and the Georges Bank to southern Brazil, including the Bahamas and Cuba. In the eastern Atlantic Ocean, it has been reported from the western and central Mediterranean Sea, the Canary Islands, Cape Verde, Senegal, Sierra Leone. In the Indian Ocean, it is found off South Africa, Mozambique, and Madagascar, with sporadic records in the Arabian Sea. In the Pacific Ocean, it occurs off Japan, mainland China and Taiwan, Vietnam, Australia, and New Caledonia in the west, and from southern California to the Gulf of California.

Behaviour

This shark is coastal and pelagic in its distribution, where it occurs from the surf zone to well offshore, and from the surface to depths of 400 m. Because it is poorly adapted to osmoregulate at lower salinities, it is not commonly found in estuaries. Tagging studies in the southwestern Indian Ocean. the Northwest Atlantic, Gulf of Mexico, and the southeastern Indian Ocean have all shown that the Dusky shark is a highly migratory species. The longest distance between tagging and recapture is 2,052 nautical miles, and the longest period at liberty 15.8 years. Movements normally show seasonal patterns, with adults moving into more temperate areas as temperatures rise in summer. Movements of adults are longer than those of neonates and juveniles, although juveniles of approximately a year old have been recorded moving as much as 742 nautical miles off South Africa. Major nursery areas for Dusky Sharks have been identified off the KwaZulu-Natal coast of South Africa, the New Jersey to South Carolina coast of the United States, and the southwest coast of Australia.

Feeding

The dusky shark is a generalist that takes a wide variety of prey from all levels of the water column, though it favors hunting near the bottom. A large individual can consume over a tenth of its body weight at a single sitting. The bite force exerted by a 2 m (6.6 ft) long dusky shark has been measured at 60 kg (130 lb) over the 2 mm2 (0.0031 in2) area at the tip of a tooth. This is the highest figure thus far measured from any shark, though it also reflects the concentration of force at the tooth tip. Dense aggregations of young sharks, forming in response to feeding opportunities, have been documented in the Indian Ocean.

The known diet of the dusky shark encompasses pelagic fishes, including herring and anchovies, tuna and mackerel, billfish, jacks, needlefish and flyingfish, threadfins, hairtails, lancetfish, and lanternfish; demersal fishes, including mullets, porgies, grunts, and flatheads, eels, lizardfish, cusk eels, gurnards, and flatfish; reef fishes, including barracudas, goatfish, spadefish, groupers, scorpionfish, and porcupinefish; cartilaginous fishes, including dogfish, sawsharks, angel sharks, catsharks, thresher sharks, smoothhounds, smaller requiem sharks, sawfish, guitarfish, skates, stingrays, and butterfly rays; and invertebrates, including cephalopods, decapod crustaceans, barnacles, and sea stars.

Repoduction

The dusky shark is viviparous: the developing embryos are initially nourished by a yolk sac, which is converted into a placental connection to the mother once the yolk supply is exhausted. Mating occurs during spring in the northwestern Atlantic, while there appears to be no reproductive seasonality in other regions such as off South Africa. Females are capable of storing masses of sperm, possibly from multiple males, for months to years within their nidamental glands (an organ that secretes egg cases). This would be advantageous given the sharks’ itinerant natures and low natural abundance, which would make encounters with suitable mates infrequent and unpredictable.]

With a gestation period estimated at up to 22–24 months and a one-year resting period between pregnancies, female dusky sharks bear at most one litter of young every three years. The litter size ranges from 3 to 16, with 6 to 12 being typical, and does not correlate with female size.

At Aliwal Shoal

Dusky sharks at Aliwal Shoal are seasonal visitors arriving over the winter months. They frequently intermix with the resident blacktip sharks during chumming operations. Most guests cannot tell the difference between the two species, although alert observers will see the smaller dorsal fin and more muted colouration.

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Whale Shark (Rhincodon typus)

The whale shark is the biggest fish and shark in the world. These gentle marine giants roam the oceans around the globe, generally alone. However, large numbers of whale sharks often gather in areas with abundant plankton food—making them prime tourist attractions. Its enormous mouth (nearly five feet wide) engulfs large quantities of tiny plankton that it filters through its gills as it swims.

The whale shark, like the world’s second largest fish, the basking shark, is a filter feeder. In order to eat, the beast juts out its formidably sized jaws and passively filters everything in its path.  The mechanism is theorized to be a technique called “cross-flow filtration,” similar to some bony fish and baleen whales.

The whale shark’s flattened head sports a blunt snout above its mouth with short barbels protruding from its nostrils. Its back and sides are gray to brown with white spots among pale vertical and horizontal stripes, and its belly is white. Its two dorsal fins are set rearward on its body, which ends in a large dual-lobbed caudal fin (or tail).

 

IUCN conservation status

Based on count data, modelled population estimates and habitat availability, 75% of the global Whale Shark population is inferred to occur in the Indo-Pacific, and 25% in the Atlantic. A variety of datasets present declines of 40-92%, inferring an overall decline of 63% in the Indo-Pacific over the last 75 years (three generations), resulting in a subpopulation assessment of Endangered A2bd+4bd. In the Atlantic, the overall population decline is considered to be lower at  ≥30%, resulting in a subpopulation assessment of Vulnerable A2b+4b. Given the bulk of the global population occurs in the Indo-Pacific, the overall global decline is inferred to be ≥50%. Globally, the Whale Shark is therefore assessed as Endangered A2bd+4bd.

Worldwide distribution

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Identification

The whale shark (Rhincodon typus Smith, 1828) is the world’s largest fish, reaching 15 meters (m) and 18 metric tons (Colman 1997). The head is broad and flattened with a large terminal mouth, miniscule teeth, and large gill slits. The eyes are small and located just behind the mouth on each side of the head. There are three prominent ridges along the back. The first dorsal fin is larger than the second dorsal fin and is set midway back on the shark. Whale sharks have distinctive markings of pale white spots and stripes on their dark dorsal surface, which resembles a checkerboard pattern. It has been suggested that this checkerboard pattern functions as camouflage in the pelagic environment (Wilson and Martin, 2004). Since the arrangement of spots is specific to the individual, photographic identification libraries are being complied for whale shark populations around the world (ECOCEAN).

Behaviour

Whale sharks are solitary creatures. They don’t shy away from sharing feeding grounds with other whale sharks, though. The Red Sea is a popular area for juvenile whale sharks to hang out together and eat, for example. There isn’t much else known about these types of sharks and their social habits. They haven’t been studied as well as other sea creatures, according to the IUCN.

Feeding

These sharks don’t attack and tear apart their prey like most many of their relatives. While they are meat-loving carnivores, whale sharks are filter feeders. They open their mouths, let water come in and their bodies filter out food, and release the water and any debris back into the ocean.

Plankton are their main food source, but they also eat shrimp, algae and other marine plant material, sardines, anchovies, mackerels, squid, tuna and albacore. They also eat fish eggs. Whale sharks will wait as long as 14 hours for fish to spawn on reefs. Then, they will swoop in and eat the eggs.

Distribution

Whale sharks tend to like warmer areas and are found in tropical waters all over the world. Some have been spotted in cooler waters, such as those off the coast of New York. Most whale sharks — 75 percent — are found in the Indian and Pacific oceans; 25 percent in the Atlantic, according to the International Union for Conservation of Nature (IUCN).

Repoduction

Whale shark females produce eggs, but the young hatch inside of the mother instead of in the water like most fish. Then, the female gives birth to about 300 live young. Many never make it to maturity, though.

Whale sharks have a long childhood. At the age of 25, the offspring are ready to have their own young. They may live 100 to 150 years.

At Aliwal Shoal

Whale sharks are infrequent visitors to Aliwal Shoal, and the coastal waters of the Aliwal Shoal Marine Protected area. During summer months we occasionally across juvenile whale sharks traveling behind the surf zone. Research suggests that these sharks are juvenile males.

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