When it comes to extinction, no animal is considered immune. Take, for instance, the biggest fish in the world that may be disappearing quickly - or hopefully just hiding in the ocean.

Australian researchers discovered that although there are still whale sharks, many seem to have disappeared in the warm waters near India or instead found, although still not as often, in the eastern side of the Pacific.

What remain are the small ones, the babies and kids, that measure around 23 feet. When you come to think of it, the size is still enormous, but they are still young and thus may not be able to reproduce.

"Where are they? We urgently need to know ... We need to have the big mums and big dads to keep the species going," said Ana Sequeira, the study's lead researcher.

To answer the question, Sequeira and her colleagues traveled to Nangaloo Reef, a part of a World Heritage site in Western Australia. The coast is teeming with whale sharks, which tend to move in groups.

Attaching a Crittercam, a small device equipped with a camera that can be placed on animals, the researchers were able to carefully follow the activities and behavior of the whale sharks in their natural habitat.

Based on the information they obtained from the recordings, as well as the data that go back to as far as 10 years ago, they learned that the sharks' sizes have been shrinking. A typical whale shark seen in the area is only 20 feet long, which may indicate that it is still young. Moreover, there's a decrease of around 10 feet on the average size of whale sharks observed from the 1990s to the early 2000s.

Majority of these giant adult whale sharks were last seen almost eight years ago, although the females are still around in Mexico and Galapagos Island.

What happened to the whale sharks then? There are many possible stories. One, whale sharks are hunted especially for their meat. It's also possible that the ban on overfishing may mean the babies are still taking time to grow, which is naturally slow.

However, the researchers are crossing their fingers that the adult whale sharks simply prefer the deeper parts of the ocean unlike the young ones. Otherwise, they may truly be doomed.

The team is planning to conduct more studies and set up a more reliable tracking program for these marine creatures.

"Not knowing the whereabouts of adult whale sharks still presents a challenge for understanding their conservation status," said Mark Meekan, co-author.

The study is published in Royal Society Open Science.

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