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Scientists Gain Insights into the Diet of Endangered Whale Shark

The elusive whale shark Rhincodon typus is the largest fish in the world and has often been regarded as an enigma by scientists ever since its first discovery 183 years ago. But now, scientists at the University of Tokyo have gained insights into the diet of the vulnerable filter-feeders by performing health checks similar to those we experience at the doctors.

Each year, swathes of tourists and scuba divers travel to tropical countries in the attempt to catch a glimpse of the elusive whale shark. Little is known about so many aspects of the species with juveniles being rarely observed and mating and birthing has never been recorded. But what is known is that the ocean giants are experiencing a decline in population size that has landed them on the International Union for Conservation of Nature (IUCN) red list for endangered species.

This has led to scientists such as Alex Wyatt, a project researcher at the Atmosphere and Ocean Research Institute, University of Tokyo, to conduct research that aims to develop a greater understanding of whale shark health with the hope that it will aid in future conservation efforts. “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 decided to analyse the growth and diet of eight whale sharks off the coast of Japan. They took a commonly used method of looking at diet called stable isotope analysis which involves looking at the ratio of stable forms of elements found within the tissues and combined it with blood tests in order to give a full health check.

"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 results of their study published in the journal Ecological Monographs found that according to the blood test results, several of the wild whale sharks may have not eaten for weeks or months and 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 can become as limited as our blood tests suggest," said Wyatt.

The researchers now hope that implementing the full health check method will contribute to the management and assessment of other elusive and highly migratory species in the future.

By Ellis Moloney

Journal Reference: Wyatt, A., Matsumoto, R., Chikaraishi, Y., Miyairi, Y., Yokoyama, Y., & Sato, K. et al. (2019). Enhancing insights into foraging specialization in the world's largest fish using a multi-tissue, multi-isotope approach. Ecological Monographs. doi: 10.1002/ecm.1339

Funding was provided by the Japan Society for the Promotion of Science (JSPS) to A. S. J. Wyatt (Grants # 11F01075 and 15F15904) and the Japan Science and Technology Agency to T. Nagata (JST CREST Grant # JPMJCR13A4).

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