Tiger Shark

Tiger Shark


Tiger Shark


Tiger Shark


Tiger Shark

Tiger sharks (Galeocerdo cuvier) belong to the large-sized species of sharks. Their size is comparable to that of the white sharks (Carcharodon carcharias), but they look somewhat less massive. Although they have been proven to be involved in some accidents involving humans, they are still less well-known among the average population. This is due partly to the fact that science has not concerned itself so much with this species. Outside of Hawaii there are few projects specifically involving tiger sharks. This is somewhat surprising since the tiger shark is one of the largest and perhaps most common predator in the Bahamas, and the second most caught shark species in the Western Atlantic, the Gulf of Mexico and Cuba (see the special report "U.S. Government Shark Research Project" in this edition.) So a lot of mythologies revolve around tiger sharks, especially with members of the local island populations. They are one of the few shark species who not only change their body forms but also their skin pattern during growth. Another unique feature of tiger sharks are their astonishingly formed teeth, allowing them to rip apart almost any prey.

Biology of the tiger shark (Galeocerdo cuvier)

Appearance

The exterior appearance of a tiger shark is relatively atypical for the family of gray sharks (Carcharhinidae) for its body is longer and the snout is not pointed but noticeably flat and edgy. Tiger sharks are the only species of gray sharks with suction holes (Spiraculi). They were given the name because of the tiger-like pattern on their body during their youth, a pattern which slowly disappears with age and which eventually may only be faintly or no longer visible. This coloring probably serves as camouflage because the young usually stay close to the coast, directly below the water surface, and with their stripes they very much resemble the shadows of waves in the water.

Size and age

Tiger sharks usually grow to a length of 5.5 meters, however, assumably they may sometimes even grow to a size of more than 7 meters (Fourmanoir 1961). Their maximum age can only be estimated, but they can definitely reach a minimum age of 12 years. Adult tiger sharks do not really have any more enemies for their size prevents them from being chased by other shark species. Only the young are exposed to this kind of pressure.

Food

Tiger sharks are noted for having the widest food spectrum of all sharks. They can eat almost anything, from turtles to birds, as well as other sharks and fish. Besides normal prey they even eat garbage like tires, nails or car license plates, as sometimes documented by examinations of their stomach contents. For this reason they acquired the reputation of being "garbage eaters" and were considered primitive. In reality, it is exactly their diverse food palette and unique chewing mechanism which today puts them into a different light, for their apparent lack of specialization indicates a much higher development. Tiger sharks are special because they feed on a broad spectrum of prey rather than being specialized on specific prey. A shark species which can grow to a length of 5 meters thus has a selective advantage when its prey is not restricted (in the sense of their evolution). Sharks of this size need a lot of energy and any decrease in the numbers of one prey could well pose a threat to such highly specialized forms of life.

The chewing mechanism

The tiger shark's teeth and jaws is what differentiates this species from other gray sharks and generally from most other shark species. While the teeth of other sharks which hunt swimming prey as a rule are designed to cut in the upper jaw region and to grab and hold onto possible prey in the lower jaw, tiger sharks have rows of almost 24 identical teeth both in the upper and lower jaws. These teeth have both a cutting as well as sawing region. The flatter, rear tooth component protects the large saw from the tooth pressure which amounts to three tons per square centimeters (see photo).

Also in contrast to other sharks, their jaws have a square rather than round form. The jaw cartilage meets almost at a right angle in the middle of the snout, which gives this shark its typical appearance.

Reproduction

Pregnancy with tiger sharks lasts between 15 and 16 months. Normally, the young are born with a length of 50 to 70 centimeters, but depending on where they are born, the young may also be much larger. For example, in the region around Hawaii their size at birth reaches 80 to 90 centimeters. The average number of pups per litter is 41 (Crow, 1995), whereby the spectrum fluctuates between 10 all the way up to 80 (Compagno, 1984). Females appear to bear young only every three years. It is not known if males have a similar cycle, but generally they are presumed to have more of an annual cycle. Tiger sharks are the only species of gray shark who do not bear their young live with a placenta (placentally viviparous), but reproduce aplencentally viviparous. It is unclear if this should be considered a more primitive method of reproduction.

Distribution

Tiger sharks are found almost worldwide in tropical and moderate coastal regions, preferring murky waters and estuaries. In addition to these areas they are found near island groups such as the Marshall Islands, Hawaii, Tahiti or the Galapagos.

Behavior

Very little is known about the behavior of tiger sharks. They tend to be active at dusk or at night, demonstrating different behavioral patterns in various regions, depending on the time of day. Usually they stay in very flat regions in the evening and at night, preferring to retreat to lower depths during the day. Youngsters appear to be more active during the day than in the evening and demonstrate less hesitation to appear directly underneath the water surface. Although several animals may appear at the scene of food simultaneously, most larger animals tend to go their own way.

Encounters with humans

People who are not used to these animals should avoid them whenever possible. They are very curious and may be rather persistent when encountering skin divers who chase and harpoon fish. Even though the accident rate should not be overemphasized, it cannot be denied that most accidents in the tropics are ascribed to tiger sharks. Nevertheless, the danger of being bitten by a tiger shark is still relatively small - as is the case for all other shark species. In Hawaiian waters, a region frequented by numerous tiger sharks, the accident rate does not exceed one per year.

Mythology

It is somewhat astonishing that the old Hawaiians gave the same name to both the tiger shark and white shark: "Niuhi". Many shark species found in Hawaiian waters were honored as being sacred and were even considered reincarnations of dead family members. The "Niuhi" were, however, more feared than adored. Still, both species played a role in local mythology. Legends suggest that many kings living in the historical Hawaiian environment acquired their premonition of future events by consuming the eyes of the "Niuhi". It is said that even the mother of the most famous king of Hawaii, King Kamehameha (born around 1753 and having died on May 8, 1819) asked for "Niuhi" eyes during her pregnancy because they supposedly would enhance the leadership qualities of the future king she was carrying. Tiger sharks were always considered a very special shark species not only in the Pacific but also in the Maldives, where they were called "Femunu".

Great White Shark

Great White Shark


Great White Shark


Great White Shark



Great White Shark Attacking


Distribution and Habitat Of Great White Sharks
Great White Sharks live in almost all the cold or temperate waters of the

planet, with greater concentrations in the southern coasts of Australia , in South Africa , California , and to a degree in the Central Mediterranean and the Adriatic Sea . The densest known population is found around Dyer Island , South Africa where up to 31 different white sharks have been documented by Michael Scholl of White Shark Trust in a single day[citation needed]. It can be also found in tropical waters like those of the Caribbean and has been recorded off Mauritius . It is also a pelagic fish, but recorded or observed mostly in coastal waters in the presence of rich game like otariids, cetaceans, other sharks and large bony fish species. It is considered an open-ocean dweller and is found from the surface to as deep as 1280 meters, but is most often seen near the surface.

Anatomy and appearance
An illustration of the Great White sharkThe Great White Shark has a robust large conical-shaped snout. It has almost the same size upper and lower lobes on the tail fin (like most mackerel shark, but unlike most other sharks). It is pale to dark gray and has a white belly.

Great Whites, like many other sharks, have rows of teeth behind the main ones, allowing any that break off to be rapidly replaced. Their teeth are unattached to the jaw and are retractable, like a cat's claws, moving into place when the jaw is opened. Their teeth also rotate on their own axis (outward when the jaw is opened, inward when closed). The teeth are linked to pressure and tensor-sensing nerve cells. This arrangement seems to give their teeth high tactile sensitivity.

Size Of Great White Sharks
While the average length of a Great White is 4 to 5 m (13 to 16 ft), females generally being larger than males, the question of the maximum size of Great White sharks has been subject to much debate, conjecture, and misinformation. Richard Ellis and John E. McCosker, both academic shark experts, devote a full chapter in their book The Great White Shark (1991) to analysis of various accounts of extreme size.

Today, most experts contend that the Great White's "normal" maximum size is about 6 m (20 ft), with a maximum weight of about 1900 kg (4200 lb). Any claims much beyond these limits are generally regarded as doubtful, and are closely scrutinized.

For some decades, many standard ichthyology reference books listed an 11 m (36 feet) Great White captured in south Australian waters near Port Fairy in the 1870s as the largest individual. While this was the commonly accepted maximum size, reports of 7.5 to 10 m (25 to 30 ft) Great Whites were common and often deemed credible.

Some researchers questioned the reliability of this measurement of the Port Fairy shark, noting it was much larger than any other accurately reported Great White. The question was settled in the 1970s, when J.E. Reynolds examined the Port Fairy shark's jaws and "found that the Port Fairy shark was of the order of 5 m (17 feet) in length".[1]

Ellis and McCosker write that "the largest White Sharks accurately measured range between 19 and 21 feet [about 5.8 to 6.4 meters], and there are some questionable 23-footers [about 7 meters] in the popular — but not the scientific — literature". Furthermore, they add that "these giants seem to disappear when a responsible observer approaches with a tape measure." (For more about legendary measurements, see The Submarine (shark)).

The largest specimen Ellis and McCosker endorse as reliably measured was 6.4 m (21 ft) long, caught in Cuban waters in 1945 (they note, however, that other experts have argued this individual might have been a few feet shorter). There have since been claims of larger Great Whites, but, as Ellis and McCosker note, verification is often lacking and these extraordinarily large Great Whites have, upon examination, all proved of average size. For example, a female said to be 7.13 meters (over 23 feet) was fished in Malta in 1987 by Alfredo Cutajar. In their book, Ellis and McCosker agree this shark seemed to be larger than average, but they did not endorse the measurement. In the years since, experts eventually found reason to doubt the claim, due in no small part to conflicting accounts offered by Cutajar and others. A BBC photo analyst concluded that even "allowing for error ... the shark is concluded to be in the 18 ft [5.5m] range and NO WAY approaches the 23 ft [7 m] reported by Abela." (as in original) [2]

According to the Canadian Shark Research Centre, the largest accurately measured Great White shark was a female caught in August 1983 at Prince Edward Island off the Canadian coast ( North Atlantic ) and measured 6.1 m (20 ft). The shark was caught by David McKendrick a local resident from Alberton, West Prince[citation needed].

The question of maximum weight is complicated by an unresolved question: when weighing a Great White, does one account for the weight of the shark's recent meals? With a single bite, a Great White can take in up to 14 kg (30 lb) of flesh, and can gorge on several hundred pounds or kilograms of food.

Ellis and McCosker write that "it is likely that [Great White] sharks can weigh as much as 2 tons", but also note that the largest verified examples weigh in at about 1.75 short tons (1.6 metric tons).

The largest Great White recognized by the International Game Fish Association is one landed by Alf Dean in south Australian waters in 1959, weighing 1208 kg (2664 lb). Several larger Great Whites caught by anglers have since been verified, but were later disallowed from formal recognition by IGFA monitors for rules violations.

Diet
Great White sharks primarily eat fish, smaller sharks, turtles, dolphins, and pinnipeds such as seals and sea lions. They are apex predators; the only animals known to attack them are other Great Whites, sperm whales and orcas.

Great Whites are partially warm-blooded, keeping most of their body up to 14°C above the surrounding water, which would suggest a high metabolism. Despite that, the few estimates that have been made, suggest they're economical with their calories and can go weeks between meals. Due to problems keeping Great Whites in captivity, no concrete figures for this exist.

Behavior
White sharks' reputation as ferocious predators is well-earned, yet they are not (as was once believed) indiscriminate "eating machines". They typically hunt using an "ambush" technique, taking their prey by surprise from the bottom. Sometime they swim so fast that they actually jump out of the water while chasing/attacking seals. This is one of only a few sharks that can jump fully out of the water, the others are Thresher shark, Shortfin mako, Longfin mako, Spinner shark, Blacktip reef shark, Salmon shark, Porbeagle shark and the Copper shark. This is the only shark known to regularly lift its head above the sea surface to gaze at other objects such as prey; this is known as "spy-hopping", this behaviour have also been seen in at least one group of Blacktip reef sharks but this might be a behaviour learned from interaction with humans. It is theorized that the shark may also be able to smell better this way, since smells travel through air faster than through water.

More than any documented attack, Steven Spielberg's 1975 film Jaws provided the Great White with the image of a "man eater" in the public mind. While Great Whites have been responsible for occasional fatalities in humans, they typically do not target humans as prey: for example, in the Mediterranean Sea there were 31 confirmed attacks against humans in the last two centuries, only a small number of them deadly. Many incidents seem to be caused by the animals "test-biting" out of curiosity. Great White Sharks are known to perform test-biting with buoys, flotsam, and other unfamiliar objects as well, and might grab a human or a surfboard with their mouth (their only tactile organ) in order to determine what kind of object it might be.

Other incidents seem to be cases of mistaken identity, in which a shark ambushes a bather or surfer, usually from below, believing the silhouette it sees on the surface is a seal. Many attacks occur in waters with low visibility, or other situations in which the shark's senses are impaired. It has been speculated that the species typically does not like the taste of humans, or at least that the taste is unfamiliar.

Humans, in any case, are not healthy for Great White sharks to eat because the sharks' digestion is too slow to cope with the human body's high ratio of bone to muscle and fat. Accordingly, in most recorded attacks, Great Whites have broken off contact after the first bite. Fatalities are usually caused by loss of blood from the initial limb injury rather than from critical organ loss or from whole consumption.

Biologist Douglas Long writes that the Great White's "role as a menace is exaggerated; more people are killed in the U.S. each year by dogs than have been killed by White sharks in the last 100 years." [3]

Many "shark repellents" have been tested, some using scent, others using protective clothing, but to date the most effective is an electronic beacon worn by the diver/surfer that creates an electric field which disturbs the shark's sensitive electro-receptive sense, the Ampullae of Lorenzini.

Capabilities
Great Whites like all other sharks have an extra sense given by the Ampullae of Lorenzini, which enables them to detect the electromagnetic field emitted by the movement of living animals. A Great White's sense of sight is useful, but the shark does not depend on it. A shark primarily uses its extra senses (i.e, Electrosense and Mechanosense) to locate prey from far off. Then, the shark uses smell and hearing to further verify that its target is food. At close range, the shark utilizes sight for the attack. The shark will often in ambush deliver a massive disabling bite and then back off to allow the prey to expire. This tactic allows the animal to avoid combat with dangerous prey, such as sea lions. It also has allowed occasional rescue of humans bitten by the animal, though it appears to attack humans mostly in error.

Reproduction
There is still a great deal that is unknown about Great White behavior, such as mating habits. Birth has never been observed, but several pregnant females have been examined. Great Whites are ovoviviparous, the eggs developing in the female's uterus, hatching there and continuing to develop until they are born, at which point they are perfectly capable predators. The embryos can feed off unfecundated eggs. The delivery takes place in the period transitioning spring and summer.

The young, which number 8–9 (with a maximum of perhaps 14) for a single delivery, are about 1.5 m (5 ft) long when born. Their teeth are provided with small side cusps. They grow rapidly, reaching 2 meters of length in the first year of life. Almost nothing, however, is known about how and where the Great White mates. It should be noted that there is some evidence that points to the near-soporific effect as the result of a large kill (such as a large whale) possibly inducing mating.

A White Shark can reproduce when a male's length is around 3.8 meters and a female's length is around 4.5 to 5 meters. Their lifespan has not been definitively established, though many sources estimate 30–40 years. It would not be unreasonable to expect such a large marine animal to live longer however.

Great White Sharks in captivity
All attempts to keep a Great White Shark in captivity prior to August 1981 lasted 11 days or less. However, that month a Great White broke previous records by lasting 16 days in captivity at SeaWorld San Diego before being released into the wild.

In 1984, the Monterey Bay Aquarium in Monterey , California housed its first Great White, which died after 10 days. In July 2003, Monterey researchers captured a small female and kept it in a large, netted pen off Malibu for five days, where they had the rare success of getting the shark to feed in captivity before it was released. [5] It was not until September 2004 that the aquarium made history by becoming the first aquarium in the world to place a Great White on long-term exhibit. The young female, who was caught off the coast of Orange County, was kept in the aquarium's massive 1 million-gallon Outer Bay exhibit for 198 days before her successful release back to the wild in March 2005. She was tracked for 30 days after her release.

Probably the most famous Great White to be kept in captivity was a female named " Sandy ," which in August 1980 became the first and only Great White to be housed at the Steinhart Aquarium in San Francisco , California . She was returned to the wild because she would not eat anything given to her and constantly bumped against the walls

Bull Shark

Bull Shark


Bull Shark


Bull Shark


Bull Shark

Distribution

Bull sharks (Carcharhinus leucas) are found in tropical and subtropical waters almost everywhere in the world, whereby they prefer to live close to, or even in rivers. They belong to a minority species, capable of living in sweet water as sexually mature individuals. They are thus found in various rivers such as the Mississippi, the Amazon or the Zambezi. In the Mississipi river they are found all the way up to Illinois, and in the Amazon as far inland as 3,500 km. But whole populations, not just single individuals, also live in such sweet water lakes as Lake Nicaragua or Lake Isabel in Guatemala. Their capability of tolerating varying saltwater levels means they can also be found in brackish water systems, like the Indian River System near Cape Canaveral (Florida). Bull sharks often live so long in sweet water that their metabolism adjusts to the missing salt.
Identifying marks and appearance

Bull sharks may grow to a length of 350 cm and weigh about 230 kg. Their massive appearance is rather striking. Their short and round snout is a conspicuous feature, and in contrast to other gray shark species from the Carcharhinidae family, bull sharks have very small eyes. Their most noticeable characteristic is, however, the dorsal fin which is shaped like a triangle. The bodyÔs colors are subdued, with a dark-gray back and a white belly. With pups, the end of the fins are black, but this coloring disappears as they grow older.
Feeding

Bull sharks feed on a wide variety of water animals, including bony fishes, mollusks, crabs and at times even other sharks and rays, for they belong to the few species of sharks with cannabalistic traits.
Reproduction

Like all other members of the Carcharhinidae family, bull sharks are born alive (viviparous) and the litter may include 1-3 pups. Pregnancy lasts 10 to 11 months. Males reach sexual maturity between 14 and 15 years, females only at about age 18. Although bull sharks do not rear their young like most other shark species, they bear their young in protected coastal areas. It follows that most young bull sharks are found in the flat, coastal regions around the world. These "nurseries" increase their chance of survival. Bull shark youngsters grow slowly and are thus exposed to predators for fairly long periods of time, whereby larger sharks represent the main danger. However, larger predators avoid the flat regions.
Exploitation and fishing

This shark species lives primarily in shore areas and coastal regions, and thus often becomes victim to coastal fishing activities. They are primarily caught with long-lines but also frequently turn up as incidental catch. Although bull sharks rank "low" on the list of endangered species (IUCN status: lower risk), (also see article in this issue on shark protection titled "Weak shark protection") it must be assumed that the species will acquire the IUCN status of an endangered species in the foreseeable future, not least because of the increasing destruction of their nurseries by man.
Accidents

Since bull sharks often live in rivers or their estuaries, accidents with humans may occur, which accounts for their relatively bad reputation in various regions of the world. In South Africa they are feared almost as much as the white sharks (Carcharodon carcharias). And frequently, it is nearly impossible to determine which species was really involved in an accident. The victims rarely remember the exact sequence of events and their descriptions of the sharks are often inaccurate. As a result the species involved can only be identifed by bite marks, but bull sharks and white sharks have very similar triangular teeth.

Spiny Dogfish

Spiny Dogfish


Spiny Dogfish


Spiny Dogfish

Spiny Dogfish

Facts

Also known as:	piked dogfish, common spinyfish and white-spotted spurdog

Kingdom	Animalia
Phylum	Chordata
Class	Chondrichthyes
Order	Squaliformes
Family	Squalidae
Genus	Squalus (1)
Size	Male length: up to 100 cm (2) Female length: up to 124 cm (2)

Status

Classified as Vulnerable (VU) on the IUCN Red List.

Description

The common name ‘dogfish' was given by fisherman to small sharks due to their habit of hunting shoals of fish in ‘packs' (3). The spiny dogfish is a small, slim fish with a narrow, pointed snout and distinctive white spots (4). The back is slate grey to brown and the belly is pale grey to white. It has two dorsal fins, the first of which is smaller (3). They both have a spine which can inject venom causing strong pain lasting for several hours, and very occasionally death in humans (4). The pectoral fins are curved and have rounded tips (3).

Range

Distributed along coastlines, the spiny dogfish is found in the western Atlantic, eastern Atlantic, western Pacific and eastern Pacific, as well as the Mediterranean Sea and the Black Sea (4).

Habitat

Found from the surface to a depth of 900 metres, the spiny dogfish is thought to tolerate in temperatures of between 7 and 15 degrees Celsius.

Biology

Said to be the most abundant living shark, the spiny dogfish is a slow, inactive swimmer and forms massive feeding aggregations of thousands of individuals. Tending to be same-sex and same-size shoals, they prey on shoals of bony fish, as well as octopuses, smaller sharks, squid, crabs and shark egg cases (3). They are highly migratory, moving towards the equatorial side of their range during winter (2).

With estimates of between 20 years and 75 years, the spiny dogfish is thought to be a very long-lived fish that matures late and reproduces slowly, with gestation lasting two years – the longest of any vertebrate (1) (2) (3). An ovoviviparous species, spiny dogfish develop in eggs within the female, and gain nourishment from their yolk sacs, After four to six months, these eggs are shed, but the embryos continue to develop inside the female, still living off the yolk sac attached to their abdomens. Finally, after another 18 to 20 months of development, six to seven live young are born, measuring 20 to 33 centimetres.

Despite possessing venom-delivering spines on each of its two dorsal fins, the spiny dogfish is eaten by cod, red hake, goosefish, other spiny dogfish, larger sharks, seals, and orcas (2).

Threats

The spiny dogfish is considered to be the most abundant living shark, yet two particular subpopulations in the northwest and northeast Atlantic Ocean are considered to be at risk due to massive fishing pressure. This shark is caught for food, liver oil, and used to make sand paper, vitamins, leather, fertiliser, pet food and fish meal (1) (4). At a time of peak abundance between 1900 and 1910, it is estimated that up to 27 million spiny dogfish were caught off the Massachusetts coast every year (5).