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Great white sharks

Great white sharks found to have extreme longevity

Wednesday, January 22, 2014 by: David Gutierrez, staff writer
Tags: great white sharks, extreme longevity, marine life

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(NaturalNews) Great white sharks, formerly thought to have a maximum lifespan of about 20 years, can actually live as long as human beings, according to a new study that may have serious implications for ocean conservation.

The study was led by the Woods Hole Oceanographic Institution and published in the journal PLOS ONE.

"White sharks in the northwest Atlantic are considerably older than previous age estimates," researcher Li Ling Hamady said.

Great white sharks are among the ocean's top predators. Although they are most common in temperate and subtropical waters, they migrate over vast distances and can be found in almost any part of the ocean.

Nuclear tests provide key

The most common method for estimating age in fish involves counting layers of mineralized tissue in certain body parts that grow consistently throughout the animal's life. Much like tree rings, the number of bands present can indicate an animal's age.

Previous studies, assuming that shark growth bands are deposited annually, have pegged the oldest great white sharks at 22 and 23 years of age, identified in the southwestern Pacific and western Indian oceans, respectively.

But the assumption that growth bands are deposited annually - or even that their rate of deposition is consistent throughout life - has remained untested. In addition, the banding patterns in great white sharks tend to be narrower and less distinct than in other species, making them more difficult to count.

"Traditionally, ageing sharks has relied on the assumption that band pairs are annual. In many cases this has been proven correct for part or all of a species life, however in more and more cases this is being disproven," said researcher Lisa Natanson of the National Oceanic and Atmospheric Administration's Northeast Fisheries Science Center lab in Narragansett, R.I.

For the new study, researchers took advantage of the fact that nuclear testing during the 1950s and 1960s flooded the oceans with radioactive carbon, or carbon-14. Because carbon-14 degenerates into non-radioactive carbon at a consistent rate, the researchers were able to use carbon-14 concentrations to date the different tissue layers in the vertebrae of four male and four female great white sharks caught in the northwestern Atlantic Ocean between 1967 and 2010.

The study showed that the oldest of the four females was 40 years old, while the oldest male was 73. If a lifespan of 70 years or more is typical, that would make great white sharks among longest lived of all cartilaginous fishes.

The study also confirmed that, while growth bands are indeed laid down annually in small to medium-large great white sharks, the rate of band deposition may change later in life. In addition, many bands become so thin that they are effectively uncountable.

"This research demonstrates the power of applying cutting-edge techniques in isotope geochemistry to answer fundamental questions in ocean ecology," researcher Simon Thorrold said. "The radiocarbon time stamp in white shark vertebrae provides irrefutable evidence of white shark longevity that had proved to be impossible to verify using traditional age estimation methods."

Conservation implications

The study also shows that great white sharks grow and mature much more slowly than had previously been thought. This implies that the species is even more vulnerable to overfishing, habitat degradation and other extinction risks than had been previously thought.

"These findings change the way we model white shark populations and must be taken into consideration when formulating future conservation strategies," researcher Greg Skomal said.

"Understanding longevity of the species, growth rate, age at sexual maturity, and differences in growth between males and females are especially important for sustainable management and conservation efforts," Hamady said.

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