Where Do Hawaii's Great White Sharks Come From?
Sept. 5, 2012
-- Five rays and sharks are receiving priority attention at this week's IUCN World Conservation Congress held in Jeju, Republic of Korea. The congress is the world's largest conservation event. One of the five sharks is the porbeagle, shown here. Porbeagles are vulnerable globally, and are classified as critically endangered in the Northeast Atlantic. Their low reproductive capacity combined with over-fishing has led to severe population declines over several parts of its range. "Sharks and rays have traveled the Earth for more than 400 million years," said Dr. Cristián Samper, president and CEO of the Wildlife Conservation Society and keynote speaker at the Jeju congress. "Yet, in only recent decades, many of these species have become threatened from overfishing and, in some instances, have disappeared entirely from major portions of their range." He added, "The potential loss of one of only two groups of the world's living fishes is a crisis the world community must take decisive action to address. We are calling for governments around the world to vigorously support CITES international trade regulations and strengthen fisheries management and protection measures for shark and ray species. We cannot continue to allow the destruction of these wonders of evolution."
Manta Rays Fate Worse Than Sharks
The oceanic whitetip shark is critically endangered in the Northwest and Central Atlantic Ocean. It's frequently sought after for its fins, used to make shark fin soup. Usually the rest of the dead or dying shark is then tossed back into the sea. Andrew Brierley, a University of St. Andrews marine ecologist, told Discovery News that when sharks like this die off, the deaths can cause a domino effect of other ecosystem losses. Shellfish, for example, may go into decline because they are consumed more by predators that the sharks would normally prey upon. "The trophic cascade brought about by the increasing demand for shark fin soup has not only left once economically valuable bivalve fisheries in crisis, but has precipitated an ecological and culinary bankruptcy," explained Brierley.
Scalloped hammerhead sharks are endangered all around the globe. This tropical species forms large migratory schools at certain times of the year. The aggregating behavior, distinct from that of other sharks, makes the shark particularly vulnerable to fishing. "We estimate that many millions of sharks are killed annually through both legal and illegal, unregulated and unreported fishing for the trade in fins, the prime ingredient in shark fin soup," said Dr. Rachel Graham, director of WCS's Gulf and Caribbean Sharks and Rays Program. "The high price for fins has caused the global shark fishery to expand far beyond what is sustainable. The need for international regulation and enforcement has never been greater."
Shark Paradise Found
Many rays, such as these, are also in peril. The Wildlife Conservation Society and over 35 government agencies and NGO partners at the congress additionally highlighted population loss problems associated with so-called "devil rays," which feed on planktonic crustaceans. Small schooling fishes become trapped on their specialized gill rakers. Rays are frequently captured in target fisheries and as bycatch across much of their range. One reason is that the gill rakers are dried and exported for the Asian medicinal market.
Dr. Dwayne Meadows, NOAA/NMFS/OPR
The reef manta ray, shown here, is among the world's largest fishes. Both it and the giant manta ray can grow several feet across. They are slow growing, however, typically giving birth to only one pup every two to three years. They are migratory and occur in small, highly fragmented populations that are sparsely distributed across the world’s tropics. Manta rays are captured in targeted fisheries and incidentally as bycatch. In addition, manta rays are used for human consumption, shark bait, and -- as for devil rays -- are increasingly sought for their gill rakers. "The international trade in shark and ray products, including fins, meat, and other body parts, is driving shark and ray fisheries around the world, and most of these are unmanaged or only minimally managed," said Dr. John Robinson, WCS's executive vice president for Conservation and Science. "Lack of controls on fisheries and international trade puts species at risk, but also jeopardizes sustainable fisheries, ecosystems, and food security." The Convention on International Trade in Endangered Species of Wild Fauna and Flora will meet in Thailand in March 2013.
PHOTOS:Shark Teeth Weapons Reveal Surprises
Although it's a relatively rare occurrence, great white sharks have occasionally been sighted near the Hawaiian Islands throughout history (and even prehistory -- teeth from white sharks have been found in artifacts from ancient islanders).
The small number of sightings and lack of young sharks seen, however, suggests that there isn't one group of sharks that lives in the region, and that the animals that have been spotted are drifters who have wandered far from their native waters. But whence do they come?
A recent study compiled all records of great white shark sightings around Hawaii in modern history -- a total of 13 -- and all published reports of satellite-tracked sharks that reached the region, a total of 22. By comparing the records with data from tracking studies, it appears the sharks most likely come from the population centered in the eastern Pacific, along the west coast of Mexico and the United States.
Scientists came to that conclusion after noting that all of the sharks seen in the fall around the Hawaiian Islands are female. That matches the life history of the great white sharks in the eastern Pacific. Female great white sharks from this group complete a two-year migration from Guadalupe Island, Mexico, where they mate, after which they roam far and wide into the Pacific, before returning to Baja California to give birth to pups (after which they go back to Guadalupe Island). Males, on the other hand, complete a one-year migration from the island, and thus wouldn't be around in the fall.
There is also a population of great white sharks centered near Japan that could reach the Hawaiian Islands, but the scientists found no evidence for that in the study, published recently in the Journal of Marine Biology.
Modern records of great white sharks "date back to May 1926, when the remains of a man who apparently drowned in waters of Haleiwa, Oahu, were recovered 16 days later in the stomach of a large shark landed" nearby, the authors wrote in the study. The man, a soldier, was identified primarily by his swim trunks, which were issued by the U.S. Army.
"We learned that (great) white sharks occur in Hawaii across a broader part of the annual cycle than previously thought -- we recorded observations from every month except November," said study author Kevin Weng, a researcher at the University of Hawaii, Manoa, in a statement from the school.
More From OurAmazingPlanet:
On the Brink: A Gallery of Wild Sharks
In Images: The Fantastic Creatures of Shark Island
Images: Sharks & Whales from Above
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