On the Tiger Shark (Coursera Course Writing Assignment #2)

Submitted as part of work in the Cousera course on Marine Megafauna last week:

The tiger shark sits atop the ocean food chain as an apex predator.  As a striped shark and a potential man-eater, it easily earns the name of ”sea tiger.” Cool facts: Tiger sharks hatch from eggs, yet are live born! Tiger sharks are voracious! They are known to eat anything that swims and many things, like crustaceans, that live on the sea bottom and don’t swim. Like their furry namesake, tiger sharks have a reflective layer in their eyes behind the retina to help them see in the dark.



As a shark, the tiger shark (Galeocerdo cuvier, Hawaiian name mano nihui) falls under the class Chondrichthyes, which are the cartilagenous fishes. Tiger sharks are in the taxonomic family of the requiem sharks (family Carcharhinidae), which are migratory, live-bearing warm water sharks which generally live in shallower waters at least part of the time. As so-called ground sharks, requiem sharks possess nictitating membranes, which are special, thin, moveable eyelid-like surfaces which allow the tiger shark to blink to remove the debris often present in brackish or shallow waters. The tiger shark’s order, the ground sharks or Carcharhiniformes, differs from the order Lamniformes, which includes the great white sharks and thresher sharks, because those related species, in the different order Lamniformes, lack nictitating membranes. Tiger sharks have their own subfamily, Galeocerdinae, and genus, Galeocerdo, as the only ovoviviparous (that is, its eggs hatch internally and the young are born live when fully developed) species in the family of the requiem sharks.  The requiem shark family also includes related species such as the bull shark and blacktip reef shark.
Tiger sharks are large sharks, reaching an average of about 400 cm, or 13 feet in length. Mature female sharks are usually about 10% larger than males of similar age. Body mass at maturity is around 600 kg, with exceptionally large females over 550 cm length and 1000 kg in mass. The tiger shark has a torpedo-shaped body thicker in mid portion than at its front and back, an elongated and broad nose, one dorsal and two pectoral fins, a barrel-shaped chest, and a narrower than average for sharks caudal keel area at the base of its tail fin. Tiger shark pups are born spotted, and these spots change to stripes as they mature. Juvenile tiger sharks have dark stripes on their sides as well as back, but these side stripes eventually tend to fade in older mature sharks, which then usually have visible stripes on the back but a more even dark coloration on the sides. Like most sharks, tiger sharks show a typical fish body countershading coloration, with darker bluish or greenish gray coloration on their top and sides and a paler white or yellow underbelly. Tiger sharks range throughout tropical and subtropical ocean waters. They are found mostly in coastal habitats rather than mid ocean regions of the Atlantic and Indian oceans, but are more widely distributed in the tropical and subtropical Pacific within both coastal and mid ocean regions, in association with the more widely distributed land of the islands in the tropical Pacific ocean. As generalist predators, they are likely more populous than shark species which are more restricted in their prey types, but as with most fish species, accurate population numbers are not available because we lack accurate census methods.  Estimates based on fishery data (tiger sharks caught per year) suggest to me a population of very roughly 5 to 10 million.
Tiger sharks use a wide variety of coastal and oceanic habitats during their 30 to 40 year lifespan.   They breed about every 3 years, with a period of gestation of 14-16 months.  Litter size ranges from 10 and 80 offspring. Mated females tend to move from more oceanic areas to larger island and coastal areas to give birth.  As stated above, this is ovoviviparous live birth, with eggs hatching internally. Pups may then possibly spend more time in warmer coastal regions, and then as juveniles the sharks will spend more and more of their time in less sheltered, more oceanic and colder environments. Sexual maturity is reached around age 6 to 8 years.
Adult tiger sharks may swim long distances for foraging daily, moving to patrol shallow coastal waters and then deeper areas up to 1200 meters depth, alternating sometimes many times daily in “yo-yo” style. When cruising in a foraging area, they may move slowly and stealthily, but they are capable of a sudden rush of high speed to seize prey in their sharp teeth and strong jaws. They are solitary hunters, but are able to cooperate without conflict when groups are seen scavenging large carcasses. A recent study in the Coral Sea concluded that tiger sharks which forage in shallow waters along the coast may later dive to depths of over 1000 m, covering a volume of over 2300 cubic km of ocean in a year. Tiger sharks eat a wide variety of fish, shellfish, seabirds, snakes, and turtles. They also eat mammals including dugongs, seals and dolphins. They are even known to eat other sharks, including other tiger sharks. Their “yo-yo” alternate movement, foraging from deep to shallow waters and back, may be a strategy to help them keep prey species from becoming wary of the shark’s presence, since the shark will not remain in any given spot for long.
Tiger sharks are the second most common (just behind the great white sharks) cause of shark attacks on humans. A recently published study (Papastamatiou 2014)  suggests that 25% of mature female tiger sharks in the Hawaiian islands region migrate per year from the region around the remote French Frigate Shoals atoll to the main Hawaiian islands, such as Maui, during late summer/early fall, perhaps to give birth. This migration coincides historically with an increase in tiger shark attacks in Hawaii during that season of year.
Conservation concerns:  Tiger sharks adapt readily to the presence of human structures and ships and can eat a varied diet in response to changes in types of prey, such as those caused by human over-fishing of other fish species. They are not currently considered endangered. Like most large sharks, they are threatened by commercial fishing pressures, such as those caused by commercial fishing for shark fins to supply culinary preferences in eastern Asia. Overall fishing catch rates for this species have been in continual decline over the past 25 years, so it seems that human fishing has adversely affected the size of the population.  There are few measures in place to protect the tiger shark population, and indeed culling to decrease local populations near shores used by human swimmers has been recently practiced in some locales. The World Conservation Union (IUCN) presently lists the tiger shark as "Near Threatened" throughout its range. One weak movement toward increasing the chances of the species' survival has been that quotas for shark fishing have been created to limit commercial fishing in the Atlantic, where tiger sharks are included in the large coastal group quota, which has an annual quota of 1,285 tons of several types of large sharks.

A tiger shark expert’s biography:
Carl Meyer, PhD, FIBiol
Dr. Carl Meyer is an assistant researcher at the Hawai'i Institute of Marine Biology.  His  current research focuses on the ecology and management of sharks and reef fishes. He has interests in the movement patterns, habitat use and trophic ecology of sharks and fishes, and the navigational abilities of sharks. His research addresses a variety of issues of management concern including impacts of shark ecotourism, shark predation on critically endangered species, effectiveness of Marine Protected Areas (MPAs) and impacts of human recreational activities in MPAs. 

References:
Yannis P. Papastamatiou, Carl G. Meyer, Felipe Carvalho, Jonathon J. Dale, Melanie R. Hutchinson, and Kim N. Holland (2013) Telemetry and random-walk models reveal complex patterns of partial migration in a large marine predator. Ecology 94:2595–2606.http://www.esajournals.org/doi/pdf/10.1890/12-2014.1
Meyer CG, O'Malley JM, Papastamatiou YP, Dale JJ, Hutchinson MR, Anderson JM, Royer MA, Holland KN. (2014) Growth and maximum size of tiger sharks (Galeocerdo cuvier) in Hawaii. PLoS One. Jan 8;9(1):e84799.http://www.plosone.org/article/info:doi/10.1371/journal.pone.0084799.
Werry J, Planes S, Berumen M, Lee K, Braun C, Clua E (2014). "Reef-Fidelity and Migration of Tiger Sharks, Galeocerdo cuvier, across the Coral Sea". PLOS ONE.  2014 Jan 8;9(1): e83249. http://www.plosone.org/article/info:doi/10.1371/journal.pone.0083249
Fitzpatrick R, Thums M, Bell I, Meekan MG, Stevens JD, et al. (2013) A comparison of the seasonal movements of tiger sharks and green turtles provides insight into their predator-prey relationship. PLoS ONE 7(12): e51927. doi: 10.1371/journal.pone.0051927. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0051927

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