The Biggest Fish in the Ocean Are Girls, Study Reveals

Whale shark (Rhincodon typus), the world’s largest fish species. (Andre Rereuka / AIMS)

(CN) — Marine biologists revealed Tuesday that female whale sharks grow slower than males, but in the long run become significantly larger because males plateau once they reach adulthood.

The study, published in the journal Frontiers of Marine Science, is the first evidence of different growth rates between the two sexes, and proves that female whale sharks are the largest fish in the ocean.

Not whales at all, whale sharks have much more in common with them than with their shark family, including size and feeding habits. They have flat heads, a blunt snout and a unique pattern of spots and stripes along their backs similar to a human’s fingerprint. Their mouths are about 5 feet wide and house over 300 teeth, though these sharks are filter feeders so their dining consists of opening their mouths to let food, water and other materials in and expelling anything they don’t want to eat.

Study authors Mark Meekan, a fish biologist for the Australian Institute of Marine Science, and Brett Taylor, a marine scientist also with the institute found that while the male whale sharks grow rapidly, once they reach their adult size of 26-29 feet long, they slow down or stop growing altogether. Females grow much slower but in time they surpass the males, reaching an adult size of approximately 45 feet. The authors note that some of the largest whale sharks documented have reached a staggering 59 feet in length.

“That’s absolutely huge — about the size of a bendy bus on a city street,” Meekan said. “But even though they’re big, they’re growing very, very slowly. It’s only about 20 cm (7.87 inches) or 30 cm (11.8 inches) a year.”

From 2009 to 2019, the researchers observed 54 whale sharks, from Western Australia’s Ningaloo Reef as well as some aquariums, and tracked their growth over 11 seasons. They were able to see each individual’s progress with stereo-video camera technology, and Taylor said they recorded over 1,000 whale shark measurements.

“It’s basically two cameras set up on a frame that you push along when you’re underwater,” said Dr. Taylor. “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 research team conclusively found that not only do female whale sharks grow substantially larger than males — not uncommon with sharks — but that the two sexes grow differently. This is a significant discovery that Meekan called the very first evidence of its kind. He also added that this difference in size is likely an evolutionary advantage for the females to be able to carry all her offspring.

“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.”

Measuring whale sharks. (Andre Rereuka / AIMS)

Whale sharks typically enjoy solitude, though they are not territorial and will share feeding grounds, and are not aggressive towards humans so they are generally not bothered by divers. They do however seasonally gather in groups during plankton blooms to feed in places like the Ningaloo Reef, which attracts much eco-tourism. In fact, tourism is so prevalent during these seasons it generates $24 million annually to the local economy.

Unfortunately, human activity has caused the whale shark population to suffer and as a result they are now listed as endangered. Whaling activity and accidental captures are other major contributors to their depleting population health. 

As a result, Australia and other countries where the species frequents have enacted specific guidelines regarding interaction with them. This is also to prevent dissuading the sharks from returning to that feeding ground following too much human intervention, as they would struggle to find equally warm waters with enough food availability elsewhere.

“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,” said Meekan. The authors believe the findings from their study will contribute greatly to conservation causes for these creatures.

Furthermore, Meekan said the study explains why most of the whale sharks that gather in tropical regions consist of young males, as they need to consume more to sustain their accelerated growth rates before the plateau. 

“This paper has really rewritten what we know about whale shark growth,” Taylor concluded.

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