Whale Sharks Eat Their Veggies Too

Blood tests reveal that whale sharks off the coast of Okinawa consume significant amounts of plants and algae.

AsianScientist (Jan. 23, 2019) – Whale sharks, the world’s largest fish, likely endure periods of starvation and may eat more plants than previously thought, say scientists at the University of Tokyo, Japan. Their findings are published in the journal Ecological Monographs.

Whale sharks are filter-feeding, soft-bodied fish that travel tropical oceans in search of their microscopic food. They can grow up to 12 meters long and weigh 21 metric tons, about as long as a public city bus and as heavy as three African elephants. Despite their conspicuous size, many details of whale sharks’ lives in the open ocean remain a mystery, even 183 years after they were first discovered.

In the present study, researchers led by Mr. Alex Wyatt at the Atmosphere and Ocean Research Institute of the University of Tokyo devised a health check for whale sharks based on a blood test.

“Whale sharks are one of the most exciting organisms to encounter for tourists and scientists alike, not just due to their sheer size, but also their grace and beauty. It is a privilege to unveil some of the mystery surrounding their lives,” said Wyatt.

The researchers drew approximately ten milliliters of blood from each of the whale shark’s pectoral fins. They then analyzed the blood samples immediately on board a research ship and examined the different forms—or isotopes—of carbon and nitrogen inside biopsied whale shark tissues using specialized laboratory equipment.

“Similar to blood tests performed when you visit the doctor, we are able to assess the health of whale sharks based on the contents of their blood,” said Wyatt. “We combine blood tests and tissue isotope analyses to create an accurate health check for the animals.”

The complete health check was performed on eight wild whale sharks that were being freed from accidental entanglement in fishing nets off the coast of Okinawa, Japan. According to blood test results, several of the wild whale sharks may have not eaten for weeks or months.

Although groups of whale sharks at coastal sites have been seen eating a range of prey, from tiny krill and fish eggs up to small fish and squid, the scientists found that all sharks tested showed signs of eating significant amounts of plants and algae.

“This is a somewhat surprising and controversial finding, since whale sharks are generally assumed to feed strictly on higher levels of the food chain. However, some whale sharks have been found with seaweed in their stomachs and eating plants might make sense if feeding opportunities become limited, as our blood tests suggest,” said Wyatt.

The health check and diet discoveries from wild Okinawan whale sharks may not be the same for other species, or possibly even other whale shark populations. However, Wyatt hopes that the complete health check approach will be used to investigate the possibility of starvation, individual foraging specialization and unexpected food choices in a wide variety of other threatened marine animals.

The article can be found at: Wyatt et al. (2019) Enhancing Insights Into Foraging Specialization in the World’s Largest Fish Using a Multi‐tissue, Multi‐isotope Approach.


Source: University of Tokyo; Photo: K. Sato/Okinawa Churashima Foundation Research Center.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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