Biggest fish in the sea are females, major 10-year study finds

The largest fish on Earth are female, research suggests.

A ten-year study of the endangered whale shark has revealed that they grow at different rates depending on their sex.

While males of the species Rhincodon typus - the biggest known living fish on the planet - grow faster, they stop at an average length of eight or nine metres (29 feet) when they reach sexual maturity at around the age of 30.

Female whale sharks grow more slowly but eventually overtake the males, reaching an average of 14 metres at about 50-years-old.

The research backs up previous observations of whales sharks in nature. So far only one male, spotted in the equatorial Atlantic - has been measured at over 13 metres. The rest are all females, with the largest measuring over 18 metres.

“That’s absolutely huge—about the size of a bendy bus on a city street,” said fish biologist Dr Mark Meekan, who led the research at the Australian Institute of Marine Science.

“But even though they’re big, they’re growing very, very slowly. It’s only about 20cm or 30cm a year.”

Dr Meekan said the research, carried out at western Australia's Ningaloo Reef between 2009 and 2019, had "rewritten what we know about whale sharks".

But why do female whale sharks grow bigger than males?

Dr Meakan believes it gives them an advantage when it comes to ensuring the survival of the species.

“Only one pregnant whale shark has ever been found, and she had 300 young inside her,” Dr 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.”

The difference in growth rates may also explain why large groups of whale sharks in tropical regions are almost entirely male.

“They gather to exploit an abundance of food so they can maintain their fast growth rates,” said Dr Meekan.

The bad news for whale sharks is that their immense size and long life - perhaps between 100 and 150 years - makes them more vulnerable.

A decline in numbers due to overfishing led to the species being classified as endangered on the IUCN Red List in 2016.

Whale sharks are also threatened by ship strikes, microplastics littering the ocean, and warming seas due to the climate crisis.

“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,” Dr Meekan said.

“And that’s a real worry for whale sharks.”

Each individual whale shark was identified by their unique ‘fingerprint’ of spots.

The team at the Australian Insitute of Marine Science used stereo-video cameras to film 54 whale sharks over 11 seasons at Ningaloo reef between 2009 and 2019.

In total the scientists recorded more than 1000 whale shark measurements.

Dr Brett Taylor explained: “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.”