If you have a toddler, or have encountered one in the past year, you’ve almost certainly heard the song “Baby Shark.” Somehow, every child seems to know the song, but scientists actually know very little about where and when 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—cross hundreds or thousands of miles of ocean each year. Because they have such a wide range, much of the life of sharks, including their breeding habits, remains a mystery. Scientists have struggled to understand where and how often sharks mate, how long their gestation period is, and many aspects of the birthing process.
I am a PhD candidate studying shark ecology and reproduction. I am part of a research team that aims to answer two important questions: Where and when do sharks give birth?
Need for innovation
Until recently, the technology to answer these questions didn’t exist. But marine biologist James Sulikowski, a professor at Arizona State University and my research mentor, changed that. He developed a new satellite tag called the Birth-Tag with the help of the technology company Lotek Wireless. He has no interest in the company. Using this new satellite tag, our team is working to track where and when tiger sharks give birth and demonstrate 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 place in the uterus of a pregnant shark, where it remains dormant and hidden among the fetal sharks for the duration of the pregnancy. This type of tag has never been used on sharks before, but similar implanted tags have been used for decades with great success to track birth locations in land mammals, such as deer. When a tagged mother shark gives birth, the tag is ejected along with the babies and floats to the ocean surface. Once it senses dry air, the tag transmits its location to a passing satellite, which then relays that location and time of transmission back to our lab. Once we download this information, we know where and when that shark gave birth.
After years of refining this new technology, we launched the first phase of the study in December 2019 and began deploying the tags. After 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 so, our team of researchers from the Sulikowski Shark and Fish Conservation Lab and the University of Miami Shark Research and Conservation Program, led by marine biologist Neil Hammerschlag, traveled to the crystal clear waters of Tiger Beach on Grand Bahama Island to tag tiger sharks.
Up close with a top predator
Tiger Beach is a hotspot for female tiger sharks of various life stages, including large pregnant specimens. These pregnant females may congregate in the warm, calm waters of Tiger Beach to seek refuge and accelerate their pregnancy.
The high number of pregnant sharks in this small area makes it much easier to find one, but catching and bringing a shark over 10 feet to the boat is no easy task. We fish for the sharks with drumlines and it can take several hours to safely catch, hand-reel and secure one of these powerful creatures next to the boat.
Once we catch a female tiger shark, we first take several length and girth measurements to get an idea of her general health and to see if she is sexually mature. We then check for bite marks, which indicate a recent mating event.
After gathering this basic information, we turn her upside down to induce a trance-like state called tonic immobility. Tonic immobility is a natural reflex in many sharks that induces a state of physical inactivity. This keeps the powerful shark calm and still for the most exciting part of the workup, the part where my experience comes in: the pregnancy check.
Assuming
Just like the ultrasounds used on humans, we use a mobile ultrasound machine to find out if a shark is pregnant. I put on a pair of goggles that allow me to see everything the ultrasound sees, lean over the side of the boat and place the probe on the belly of the upside-down shark. The image is usually blurry at first because water splashes over the shark and onto the boat. The team holds the shark still while I slowly maneuver the probe along her belly. And when she is pregnant, something magical happens.
Wriggling baby tiger sharks, up to 40 of them packed tightly together in their mother’s womb, appear before my eyes. The image also appears on a screen held by another team member on the boat, and everyone cheers as they gather to peek into the secret world of unborn sharks. We spy on them as they pump fluid through their still-developing gills, and we watch in awe as they wiggle around, unaware that something extraordinary is happening in the world outside. Once we have enough data on the estimated size of the offspring—giving us an idea of how far along the pregnancy has gone—it’s time to tag the mother shark.
As I hold the probe as still as possible to keep the shark’s internal anatomy in view, Dr. Sulikowski takes the Birth-Tag and uses a specially designed applicator to gently insert it into the uterus via the urogenital opening. No surgery is required, the tagging procedure is completed in a matter of minutes. Once the tag is in the uterus, we turn the shark upright to wake her up and release her back into the open ocean. I am filled with hope as I watch her gracefully swim away to continue her pregnancy, a stowaway Birth-Tag hidden among her unborn offspring.
Solving the mystery
Last December, we placed the first Birth-Tags on three pregnant tiger sharks. Tiger sharks have an estimated gestation of 12-16 months, but researchers have little hard data. Given that these tagged sharks ranged from recently mated to mid-gestation, an added benefit of this study is that it may help refine estimates of gestation length for this species.
Although we work in the Bahamas, a shark sanctuary where it is illegal to kill sharks, tiger sharks are highly migratory. As such, any tagged shark will likely spend time outside the Bahamas in unprotected waters where they must navigate carefully to avoid contact with fishing gear. Tiger sharks are considered Near Threatened by the International Union for Conservation of Nature and their populations are currently declining. The data we gain from this first round of tags will provide us and policymakers with information that can inform future protections for this species.
We are currently waiting for a notification from our online ARGOS satellite system that will alert us 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 give birth.
Many shark species are threatened with extinction and understanding their reproductive cycles is essential to effectively protect these ecologically important and beautiful creatures. With the help of the Birth-Tag we are on the verge of unlocking this information for tiger sharks and hopefully showing that this can be done for many more species.
We are planning future expeditions to deploy many more Birth-Tags, but for now we will just have to keep singing the “Baby Shark” song as we patiently wait for our first glimpse into the private lives of these incredible creatures.
This story has been updated to remove a photo of the birth tag.
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This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and reliable analysis to help you understand our complex world. It was written by: Hannah Verkamp, Arizona State University
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Hannah Verkamp works for Arizona State University as a Research Associate in the Sulikowski Shark and Fish Conservation Lab. The research is funded by the Herbert W. Hoover Foundation.