Researchers in the Galapagos have successfully completed ultrasounds on free-swimming whale sharks, and taken blood samples from adult whale sharks for the first time ever in the wild.
The incredible results allowed them to see and identify reproductive organs, such as the ovaries, and even developing follicles. These technologies hold promise for finally unlocking the mystery of breeding in the world’s largest sharks.
A team of global whale shark experts, comprised of scientists and conservationists from the Galapagos Whale Shark Project (Ecuador), Galapagos National Park (Ecuador), Okinawa Churashima Foundation (Japan), University of San Francisco/Galapagos Science Center (Ecuador) and the Marine Megafauna Foundation (USA), has just returned from a two-week expedition to Darwin Island, in the far north of the Galapagos Archipelago.
This remote volcanic island is one of the few places where huge adult female whale sharks, up to 14m (45 ft) in length, are commonly seen each year. The main aim of the expedition was to assess the sharks’ reproductive state.
Jonathan R. Green, the expedition leader and founder of the Galapagos Whale Shark Project, notes: “Almost nothing is known about the reproduction of these giant sharks. After I first saw these huge female whale sharks in the far north Galapagos, I realised that this was a great opportunity to learn more. We’ve been able to put together an experienced team to research sharks in this remote area, one of the world’s most isolated dive sites.”
Dr Simon Pierce, an expedition member from the Marine Megafauna Foundation, explains further: “Whale shark breeding is a mystery. Only one pregnant shark has been physically examined so far, back in 1995 in Taiwan. That ‘megamamma’ shark had 304 little whale shark eggs and pups inside, all less than 60 cm in length.”
The team conducted scans using a 17kg ultrasound system in a waterproofed case. Whale sharks have tough protective skin, more than 20cm thick on some individuals, so the 30cm penetration of the ultrasound waves proved a challenge – not to mention the difficulty of carefully checking the whole belly area of a gigantic shark while it is swimming. Dr Matsumoto had to use a propellor system mounted on his air-tank to keep up with the sharks.
“We use some interesting technology anyway, but working with the Okinawa team was something else”, commented Dr Pierce. “I felt cool by association. We saw dive groups a couple of times at the site, and I can only imagine what they thought – why is that guy diving with a briefcase? And a jetpack?”
Dr Matsumoto reports that the initial results were promising: “We confirmed the presence of follicles in the ovaries but none of the images captured embryos or egg capsules inside the uterus. These adult female sharks we saw at Darwin Island might be on their way to mate further offshore. I am confident that we can judge the sexual maturity, and probably also determine the pregnancy of whale sharks in the field, using the underwater ultrasound”.
The researchers attached satellite-linked tags to the sharks to track their onwards movements. Professor Alex Hearn from the University of San Francisco/Galapagos Science Center explains: “We’ve tagged whale sharks in Galapagos before, but there are lots of predatory sharks at Darwin and they often try to eat the tags, which can rip them out of the whale sharks almost immediately. To reduce early tag loss, we tried a different method on this trip, clamping the tags to the tip of the dorsal fins. All tags are transmitting well, so we should get great information on where these sharks swim over the months to come.”
Project member Dr Alistair Dove, from Georgia Aquarium, notes that these tags could document some amazing behaviours: “Whale sharks are already known to be the deepest-diving of all fish. The current depth record is 1,928 m – well over a mile – set by a juvenile whale shark. Larger, older animals can generally dive deeper than young smaller ones, so perhaps we will challenge that record.”
Kiyomi Murakumo, from Okinawa Churashima Foundation, successfully collected blood samples from six adult sharks – no easy job. Her colleague, Dr Ryo Nozu, analyzed the results immediately following the trip: “Sex steroid hormone levels in the blood are an excellent way to monitor reproduction in individual sharks. This study measured levels of estradiol, progesterone and testosterone of wild, adult female whale sharks for the first time in the world. Estradiol could be associated with follicular development, and progesterone could be involved in ovulation and pregnancy. Over time, as we sample more whale sharks, we can build up a complete picture of their reproductive cycle by combining the blood sampling with the ultrasonography.”
Jonathan R. Green added: “These big female sharks are not going to give up their secrets easily. One thing is clear: there’s a lot of work still to do to understand the reproductive processes of this endangered species. However, this trip proved that it is possible to research their breeding in the wild. We’ll continue to hone our techniques and build upon this knowledge, as we need to understand these enigmatic sharks and protect them through their life cycle.”
Image © Simon J Pierce