Great Whites Could Be Longest-Lived Sharks
Previously it was thought that great whites died of old age starting in their 20s or so. The new study, published in PLoS ONE, finds that their lifespan is more like that of humans, with great whites reaching 70 or more.
“Our results dramatically extend the maximum age and longevity of white sharks compared to earlier studies,” said lead author Li Ling Hamady.
“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,” said Hamady, a graduate student at the Woods Hole Oceanographic Institution.
In all fish, age is determined by analyzing increments in mineralized tissue, such as otoliths (ear bones), vertebrae and fin rays. These grow throughout a fish’s life, adding annual rings, similar to growth rings in trees.
In great whites, determining age is even more challenging. Their vertebrae are constructed of layers of tissue created sequentially over an individual’s lifetime, but the alternating light/dark banding patterns can be narrow and less distinct than in other species. And the bands don’t necessarily signify annual growth.
“Traditionally, ageing sharks has relied on the assumption that band pairs are annual,” said co-author Lisa Natanson of NOAA’s Northeast Fisheries Science Center. “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.”
To get around that problem, the researchers cleverly took advantage of radiocarbon produced by thermonuclear device testing done during the 1950s and 1960s.
Radiocarbon mixed from the atmosphere into the ocean, and was incorporated into the tissues of marine organisms living during that time period. The rise in radiocarbon gave the researchers a specific spot in time pinpointed in the vertebra layers, which were used to help determine age.
“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,” said co-author Simon Thorrold, a biologist at Woods Hole Oceanographic Institution and Hamady’s Ph.D. advisor.
Now that we know great whites can reach such advanced ages, that knowledge can be applied to conservation efforts. Since great whites grow slowly and mature late, their populations could be even more sensitive to fishing, environmental and other pressures.
Image: Greg Skomal, MA Marine Fisheries