The world's biggest fish is the mysterious whale shark, which can grow as heavy as 18,000kg and as large as 18m, though most top out a bit less than that. It also appears these behemoths can live to be 130 years old, new observations and modelling have shown.
It's a challenge to study how these whale sharks grow over time, or even how many of them there are. Past research typically relies on dead shark bones and leaves room for inaccuracy. Now, a team of scientists working in the Maldives is filling the gaps in scientific knowledge on these huge creatures.
"Estimating the ages of sharks in general is difficult," Mahmood Shivji, a professor from Nova Southeastern University in Florida, told Gizmodo.
Scientists estimate the age of whale sharks based on dead specimens' vertebrae, which appear to have rings much like trees do. But it's unclear how these rings form over time and whether they're actually a marker for the number of years the shark has been around.
These scientists visited places where the whale sharks congregated over multiple years to take measurements. Grad student Cameron Perry, first author of the study from Nova Southeastern University, free-dived with the sharks in the Maldives repeatedly.
The team took tape measurements, as well as measurements with lasers and cameras, to deduce the size. They tracked new and returning individuals, taking data on 186 encounters with 44 sharks, mostly juveniles, over 10 years. They could recognise repeat visitors based on the spot patterns on their backs.
The scientists plugged this data into a mathematical model - equations based on previous observations and data of how the whale sharks grow over time that can estimate how old the shark would be at certain sizes. The model calculated that the male sharks matured at 25 years old and lived to be 130.
It's not the first estimate of whale shark longevity, but it falls within the range of other estimates that span anywhere from 79 to 174 years. It's also similar to estimates based on counting vertebrae.
There were too few females to make an estimate of female whale shark longevity, however. The paper is published in Marine and Freshwater Research.
Knowing how big these sharks can get and how long they can live is important for conservation reasons. "A more thorough grasp of age and growth parameters will lead to better estimates of the ability of whale shark populations to recover from overexploitation and is vital for effective management plans", the scientists write. It's easier to tell how many sharks there are if you know how long the sharks live.
This is a model, though, meaning it's an informed estimate - scientists haven't directly confirmed a 130-year-old whale shark. Still, these measurements are the best that scientists have at this stage, said Shivji and the same techniques could perhaps work with other species of sharks.
"This is a new method of measuring live sharks that can be applied to other places," said Shivji. "You don't have to rely on killing the animals or on animals that are captured in fisheries".