In the battle for the biggest fish in the ocean, it seems slow and steady wins the race as it has been announced that female whale sharks tip the scale compared to males in later life. Due to their comparatively explosive growth rate, it was previously thought that male whale sharks were the biggest fish in the ocean, but new research published in the journal Frontiers in Marine Science has revealed that this growth spurt plateaus. Meanwhile, female whale sharks that grow slowly and steadily continue to increase in size long after males have stopped, meaning they reach a great size even if it takes them a little longer to get there.
The decade-long study was led by Australian Institute of Marine Science (AIMS) fish biologist Dr Mark Meekan, who worked with a team of scientists at Western Australia's Ningaloo Reef for 11 seasons from 2009 to 2019. Their findings revealed that while the males grew faster, their rate of growth eventually plateaued, giving an average adult length of around 8 to 9 meters (26 to 30 feet). By comparison, females grew slower but continued to grow for longer, giving them an average adult length of about 14 meters (46 feet). The study even recorded individuals at a whopping 18 meters long (60 feet).
"That's absolutely huge, about the size of a bendy bus on a city street," said Meekan in a statement. "But even though they're big, they're growing very, very slowly. It's only about 20cm or 30cm (8-12 inches) a year."
The team monitored the growth of 54 whale sharks year-on-year, made possible thanks to the unique spots on whale sharks, which are effectively fingerprints that can be used to identify individuals. They recorded over 1,000 whale shark lengths using stereo-video cameras mounted on an underwater frame operated by 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 said. "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."
According to Meekan, the study is the first to prove that male and female whale sharks grow differently and offers an explanation as to why growing to be such a chonk is a boon for the majestic giants.
"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."
While an impressive feat, the discovery is a cause of concern for conservationists as these threatened fish are at a greater risk of coming to harm before being able to reproduce given they take around 30 years to reach maturity. Aesop’s tortoise might’ve hurried up a little if they’d been on the “endangered” IUCN Red List.