January 9, 2014
Great Whites May Live Much Longer Than Previously Thought
Brett Smithfor redOrbit.com - Your Universe Online
Great white sharks have always been considered one of the longer living fish, but a new study in the journal PLOS ONE has found that they actually live much longer than previously believed.
Using a radiocarbon analysis, study researchers from the Woods Hole Oceanographic Institution (WHOI) and National Oceanic and Atmospheric Administration (NOAA) discovered that male great whites can live up to 73 years and females can live up to 40.
"Our results dramatically extend the maximum age and longevity of white sharks compared to earlier studies," said study author Li Ling Hamady, a Joint Program student at WHOI. "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."
The conventional method to determine the age of a fish relies on analyzing growth increments in mineralized tissue like ear bones, vertebrae, and fin rays. As these tissues grow during a fish's life, they form annual rings in the process, comparable to the growth rings in trees.
The problem with using this method in great whites is the oscillating light and dark banding patterns in shark vertebrate can be thin and less distinct than in other species. The bands also don't necessarily signify annual growth.
"Ageing sharks has traditionally relied on counting growth band pairs, like tree rings, in vertebrae with the assumption that band pairs are deposited annually and are related to age," said study author Lisa Natanson, a fisheries biologist at NOAA's Northeast Fisheries Science Center (NEFSC).
"In many cases, this is true for part or all of a species' life, but at some point growth rates and age are not necessarily in sync. Growth rates slow as sharks' age. Deposition rates in vertebrae can change once the sharks reach sexual maturity, resulting in band pairs that are so thin they are unreadable. Age is therefore frequently underestimated. "
Based on these growth-band methods, previous studies identified the oldest white shark individuals from the southwestern Pacific Ocean as 22 years old and the western Indian Ocean as 23 years old.
In the new study, the researcher team decided to take advantage of a unique human activity: thermonuclear device testing that took place during the 1950s and 1960s. Radioactive carbon from the tests was eventually mixed into the ocean and was integrated into the tissues of marine organisms living at the time. The distinct radiocarbon signature gave the study team a specific point in time that could be identified in the vertebra layers – essentially a 'time stamp' to help resolve the age of an organism.
In the study, researchers used the National Ocean Sciences Accelerator Mass Spectrometry Facility at WHOI to analyze collagen in the white shark vertebrae. Samples of shark tissue were culled from white sharks caught in the Atlantic Ocean from 1967 to 2010 and archived at the NEFSC. The samples were also photographed through a stereomicroscope to count growth bands.
"This research demonstrates the power of applying cutting-edge techniques in isotope geochemistry to answer fundamental questions in ocean ecology," said study author Simon Thorrold, a biologist at WHOI. "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."
"These findings change the way we model white shark populations and must be taken into consideration when formulating future conservation strategies," added study author Greg Skomal, a WHOI adjunct scientist and Marine Fisheries biologist.