After a decade of study, scientists have reached the conclusion that female whale sharks are the largest fish in the oceans. Male whale sharks grow rapidly, and reach an average length of about eight or nine meters in adulthood. Females grow more slowly than males, but overtake the males and reach an average adult length of around fourteen meters. Some even grow to eighteen meters long, noted Australian Institute of Marine Science fish biologist Dr Mark Meekan, who led the research, which has been reported in Frontiers in Marine Science.
"That's absolutely huge: about the size of a bendy bus on a city street," said Meekan. "But even though they're big, they're growing very, very slowly. It's only about 20 centimeters or 30 centimeters a year."
For this study, the researchers conducted fieldwork at Western Australia's Ningaloo Reef over eleven seasons from 2009 to 2019. Whale sharks each have a unique set of spots, so the scientists were able to identify and track 54 individual whale sharks while they grew.
The team took over 1000 measurements of what sharks with stereo-video cameras, noted AIMS marine scientist Dr. Brett Taylor. "It's basically two cameras set up on a frame that you push along when you're underwater," he explained. "It works the same way our eyes do so you can calibrate the two video recordings and get a very accurate measurement of the shark."
Along with the research on wild whale sharks, the researchers also used data obtained from whale sharks in captivity. This research is the first to show that male and female whale sharks grow in different ways, said Meekan.
"Only one pregnant whale shark had ever been found, and she had 300 young inside her," Meekan 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."
In 2016, whale sharks were listed as endangered. Collisions with ships and fishing pose serious threats to these fish.
"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."
"This paper has really re-written what we know about whale shark growth," added Taylor.