A long-term study by Australian Institute of Marine Science fish biologist Dr. Mark Meekan found that while male whale sharks grow more quickly, the females of the species tend to grow the biggest.
The study found that males plateau at an average adult length of about eight or nine meters while females can continue to grow to an average adult length of about 14 meters. Females can actually reach a reported 18 meters long.
“That’s absolutely huge—about the size of a bendy bus on a city street,” he said. “But even though they’re big, they’re growing very, very slowly. It’s only about 20cm or 30cm a year.”
In conducting the research, scientists visited Western Australia’s Ningaloo Reef for 11 seasons between 2009 and 2019. They tracked 54 whale sharks as they grew—a feat made possible by a unique ‘fingerprint’ of spots on each whale shark that can be used to identify individual fish.
For the females, there are huge advantages to being big, Meekan said.
“Only one pregnant whale shark had ever been found, and she had 300 young inside her,” he said. “That’s a remarkable number, most sharks would only have somewhere between two and a dozen. So these giant females are probably getting big because of the need to carry a whole lot of pups.”
“If you’re a very slow-growing animal and it takes you 30 years or more to get to maturity, the chances of disaster striking before you get a chance to breed is probably quite high,” he said.
“And that’s a real worry for whale sharks.”