To get big, grow slow

Whale shark females overtake the males to become the world’s largest fish.

This article is well suited to Biology students in years 2, 4, 5, 6, 7, and 10 who are learning about the lifecycle, adaptations and classification of native species as well as human impact on ecosystems.

 

Word Count / Video Length: 407 / 4:57 mins

Measuring whale sharks on Western Australia’s Ningaloo Reef. Credit: Andre Rereuka / AIMS

A decade-long study of whale sharks (Rhincodon typus) has determined that the biggest fish in the world are female.

While males of the species grow quite quickly before plateauing at an average of eight or nine metres, females proceed more slowly to around 14 metres. Some grow to 18 metres – about the size of a bendy bus on a city street.

“[E]ven though they’re big, they’re growing very, very slowly; it’s only about 20 or 30 centimetres a year,” says Mark Meekan from the Australian Institute of Marine Science (AIMS), who led the research reported in Frontiers in Marine Science.

He and colleagues visited Western Australia’s Ningaloo Reef for 11 seasons to track 54 whale sharks — a feat made possible by their unique “fingerprint” of spots that can be used to identify individuals – and made more than 1000 measurements using stereo-video cameras.

“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.”

Dr Mark Meekan, corresponding author, speaks about his research on whale sharks. CREDIT: AIMS

For the females, there are huge advantages to being big, Meekan says, not least the ability to carry lots of pups. “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.”

The findings also explain why gatherings of whale sharks in tropical regions are made up almost entirely of young males, Meekan says. “They gather to exploit an abundance of food so they can maintain their fast growth rates.”

Whale sharks are Western Australia’s marine emblem, and swimming with the iconic fish at Ningaloo Reef boosts the local economy to the tune of $24 million a year.

But they were listed as endangered in 2016.

Dr Meekan said the discovery has huge implications for conservation, with whale sharks threatened by targeted fishing and ships strikes.

“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.”

Dr Taylor said learning that whale sharks plateau in their growth goes against everything scientists previously thought.

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

Years: 2, 4, 5, 6, 7, 10

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Topics:

Biological Sciences – Ecosystems, Genetics, Lifecycles, Living Things

Additional: Careers, Technology

Concepts (South Australia):

Biological Sciences – Interdependence and Ecosystems, Diversity and Evolution, Form and Function