Ultrasounds on Whale Sharks Deepen Mysteries of This Big Fish

Whale sharks are the ocean’s biggest fish and perhaps its biggest mystery. With the goal of unlocking secrets about female reproduction, new methods were developed to conduct ultrasounds on free-swimming whale sharks.

But the results weren’t exactly what researchers had expected or hoped. The findings, published recently in the journal Endangered Species Research, quash the leading scientific theory that the exceptionally large females with bulging bellies that show up annually in the Galapagos Marine Reserve in Ecuador and near St. Helena Island are pregnant. They aren’t.

It’s a head scratcher, really. We know a lot about male whale sharks, but little about females because they don’t congregate the way males do. We had hoped to confirm that the females were pregnant so we could track their movements and locate important pupping grounds.

The research, led by Okinawa Churaumi Aquarium, in collaboration with Marine Megafauna Foundation, Galapagos Whale Shark project and Universidad San Francisco de Quito in Ecuador, included developing underwater ultrasound technology which was coupled with conducting blood draws, a technique developed by Georgia Aquarium on free-swimming whale sharks.

“To our surprise, none of the sharks were pregnant.”

One of the greatest secrets that whale sharks hold is where they give birth. Truly little is understood about their reproductive lifecycle because only one pregnant female – caught by a commercial fishing boat in 1995 – has ever been examined. Births have never been recorded.

Whale shark populations are declining, and the species is at risk of extinction. The best hope for protecting and conserving them is understanding them – their movements, the habitats they need protected, and what is posing the greatest threat to survival.

That is one reason Georgia Aquarium is shifting their research focus from Galapagos and St. Helena to Taiwan. The only pregnant female ever recorded was caught in Taiwanese waters, and much more needs to be known about the biodiversity, abundance, and migration dynamics of sharks in Taiwan.

The Aquarium, National Taiwan Ocean University (NTOU) and Taiwan’s Fisheries Research Institute (TFRI) recently entered a 10-year conservation program with Georgia Aquarium that will compile baseline data on multiple marine species living off Taiwanese shores. The research will include placing receivers onto fishing trap nets throughout Taiwan’s coastal waters and acoustic tags onto individual animals to track where they travel and for how long.

“There is so much for us to learn about whale sharks, and this partnership has great potential to help us uncover details about the whale sharks who visit these waters,” Dove said. “We are excited to embark on research in this region. There is much to learn and contribute to conservation knowledge for not only whale sharks, but other threatened shark and ray species as well.”