The World’s Largest Fish

The whale shark (Rhincodon typus) is the world’s largest fish, growing to maximum known sizes of 18 m (59 feet) total length. A new study led by University of Western Australia marine biologists has found that male whale sharks grow quickly, before plateauing at an average adult length of about 8-9 m (26.2-29.5 feet), and that female whale sharks grow more slowly but eventually overtake the males, reaching an average adult length of about 14 m (46 feet).

Meekan et al found evidence that male whale sharks (Rhincodon typus) attain asymptotic sizes of around 8.5-9 m at Ningaloo Reef, which suggests that females are more likely to reach the largest sizes (over 12 m) observed in the species. Image credit: Andre Rereuka / Australian Institute of Marine Science, University of Western Australia.

Meekan et al found evidence that male whale sharks (Rhincodon typus) attain asymptotic sizes of around 8.5-9 m at Ningaloo Reef, which suggests that females are more likely to reach the largest sizes (over 12 m) observed in the species. Image credit: Andre Rereuka / Australian Institute of Marine Science, University of Western Australia.

“Whale sharks have been reported up to 18 m long. That’s absolutely huge — about the size of a bendy bus on a city street,” said Dr. Mark Meekan, a fish biologist in the Australian Institute of Marine Science at the University of Western Australia.

“But even though they’re big, they’re growing very, very slowly. It’s only about 20 or 30 cm (7.9-11.8 inches) a year.”

In the study, Dr. Meekan and colleagues collected yearly length measurements of whale sharks at Ningaloo Reef, Western Australia, from 2009 to 2019, during the peak of the annual whale shark aggregation, typically in the first week of May each year.

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.

“We recorded more than 1,000 whale shark measurements using stereo-video cameras,” explained Dr. Brett Taylor, a marine scientist in the Australian Institute of Marine Science at the University of Western Australia.

“It’s basically two cameras set up on a frame that you push along when you’re underwater.”

“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.”

The study also included data from whale sharks in aquaria.

“It is the first evidence that males and female whale sharks grow differently,” Dr. Meekan said.

“For the females, there are huge advantages to being big. Only one pregnant whale shark had ever been found, and she had 300 young inside her.”

“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 discovery has huge implications for conservation, with whale sharks threatened by targeted fishing and ships strikes,” he added.

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

“The finding also explains why gatherings of whale sharks in tropical regions are made up almost entirely of young males,” Dr. Meekan said.

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

“Learning that whale sharks plateau in their growth goes against everything scientists previously thought,” Dr. Taylor said.

“This paper has really re-written what we know about whale shark growth.”

The findings were published in the journal Frontiers in Marine Science.

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Mark G. Meekan et al. Asymptotic Growth of Whale Sharks Suggests Sex-Specific Life-History Strategies. Front. Mar. Sci, published online September 16, 2020; doi: 10.3389/fmars.2020.575683

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