If you have a toddler, or have encountered one in the past year, you’ve probably heard the song “Baby Shark.” Somehow, every child seems to know the song, but scientists actually know very little about when and where sharks give birth. The origins of these famous baby sharks are still largely a mystery.
Many of the larger iconic shark species, such as great whites, hammerheads, blue sharks, and tiger sharks, travel hundreds or thousands of miles of ocean each year. Because they range over such a wide area, much of their lives, including their reproductive habits, remain a mystery. Scientists have struggled to understand exactly where and how often sharks mate, how long they gestate, and many aspects of the birthing process.
I’m a PhD student studying shark ecology and reproduction, and I’m part of a team of researchers hoping to answer two important questions: Where and when do sharks give birth?
Innovation is needed
Until very recently, the technology to answer these questions didn’t exist. But marine biologist James Sulikowski, a professor and research advisor at Arizona State University, has changed that. He’s developed a new satellite tag called Birth-Tag with the help of technology company Lotek Wireless. He has no stake in the company. Our team is working to use this new satellite tag to reveal where and when tiger sharks give birth, providing a proof of concept for how scientists can do the same for other large shark species.
The Birth Tag is a small, egg-shaped device that we implant into the womb of a pregnant shark, where it will remain immobile and hidden among the fetal sharks throughout pregnancy. This type of tag has never been used in sharks before, but similar implanted tags have been used with great success for decades to locate the birthplaces of land mammals such as deer. When a tagged mother shark gives birth, the tag will be expelled with the babies and swim to the surface of the sea. When it senses dry air, the tag transmits its location to a passing satellite, which then transmits that location and transmission time back to our lab. Once we download that information, we know where and when that shark gave birth.
After years of fine-tuning this new technology, we launched the first phase of the study in December 2019 and began deploying tags. Once the study was approved by the Institutional Animal Care and Use Committees at both Arizona State University and the University of Miami, as well as the Bahamian government, we set out to find tiger sharks. To do this, our team of researchers from the Sulikowski Shark and Fish Conservation Laboratory and Shark Research and Conservation Program at the University of Miami, led by marine biologist Neil Hammerschlag, took to the crystal-clear waters of Tiger Beach off the coast of Grand Bahama Island to tag tiger sharks.
Close-up with an apex predator
Tiger Beach is a popular spot for female tiger sharks in many different life stages, including large gravid individuals. These gravid females may be congregating in the warm, calm waters of Tiger Beach to seek shelter and accelerate their gestation.
The abundance of pregnant sharks in this small area makes finding one much easier, but catching a shark over 10ft long and bringing it to the boat is no easy task. We fish for sharks using drum lines and it can take several hours to safely catch one of these powerful creatures, pull them in by hand and secure them to the side of the boat.
When we catch a female tiger shark, we first take some length and girth measurements to get an idea of her overall health and to see if she is sexually mature. Then we check for bite marks, which could be evidence of a recent mating event.
Once we have gathered this basic information, we turn it upside down to put it into a trance-like state called tonic immobility. Tonic immobility is a natural reflex that causes a state of physical immobility in many sharks. This keeps the powerful shark calm and still for the most exciting part of the study, which is where my experience comes in, which is contraception.
Waiting
Just like human ultrasounds, we use a mobile ultrasound machine to tell if a shark is pregnant. I put on goggles that allow me to see everything the ultrasound sees, lean over the edge of the boat, and place the probe on the belly of the upturned shark. The image is often blurry at first because of the water splashing over the shark and into the boat. I slowly move the probe along its belly while the team holds the shark still. Then, if it is pregnant, something magical happens.
Before my eyes, wriggling tiger shark cubs appear, up to 40 of them tightly packed together in their mothers’ wombs. The image also appears on a screen held by another crew member on the boat, and everyone cheers as they gather around to get a glimpse into the secret world of the unborn sharks. We watch as they pump fluid through their still-developing gills and watch in fascination as they wiggle around, unaware that something extraordinary is happening in the world. Once we have enough data on the approximate size of the cub – which gives us an idea of how far along the pregnancy is – it’s time to tag the mother shark.
While holding the probe as steady as possible to visualize the shark’s internal anatomy, Dr. Sulikowski retrieves the Birth Tag and carefully inserts it into the uterus through the urogenital opening using a specially designed applicator. No surgery is required, and the tagging procedure is completed in a few minutes. Once the tag is inside the uterus, we stand the shark upright to wake it up and release it back into the open ocean. I am filled with hope as I watch it gracefully swim away to continue its pregnancy, with a Birth Tag hidden among its unborn offspring.
Solving the mystery
Last December, we placed the first Birth Tags on three pregnant tiger sharks. Gestation for tiger sharks is thought to last 12-16 months, but researchers have no hard data. Since these tagged sharks ranged from recently mated to mid-gestation, an added bonus of this study is that it could help improve gestation length estimates for this species.
Although we work in the Bahamas, a shark sanctuary where it is illegal to kill sharks, tiger sharks migrate extensively. Therefore, any sharks tagged will likely spend time in unprotected waters outside the Bahamas, where they must navigate carefully to avoid interacting with fishing gear. Tiger sharks are considered near threatened by the International Union for Conservation of Nature and their population is currently declining. The data we obtain from this first round of tagging will provide us and policymakers with information that can inform future protections for this species.
We are currently waiting to receive notification from our online ARGOS satellite system that one of our sharks has given birth. When that happens, we will be the first in the world to know in near real time where and when tiger sharks are giving birth.
Many shark species are at risk of extinction, and understanding their reproductive cycles is key to effectively protecting these ecologically important and beautiful creatures. Using Birth Tag, we are on the verge of unlocking this information about tiger sharks, and hopefully we will show that this can be done for many more species.
We are planning expeditions to deploy more Birth Tags in the future, but for now, we will continue to sing “Baby Shark” as we patiently wait for our first glimpse into the private lives of these incredible creatures.
This story has been updated to remove the Birth Tag photo.
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This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and trusted analysis to help you understand our complex world. By Hannah Verkamp Arizona State University
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Hannah Verkamp is a Research Associate in the Sulikowski Shark and Fish Conservation Laboratory at Arizona State University. The research is funded by the Herbert W. Hoover Foundation.