Unraveling the mystery of when and where sharks give birth

If you have a young child, or if you found one in the last year, you have almost certainly experienced the Song “Baby Shark”. Somehow every child seems to know this song, but in reality scientists know very little about it. where and when do sharks give birth. The origins of these famous baby sharks are still a mystery.

Many iconic large shark species – such as great white sharks, hammerhead sharks, blue sharks and tiger sharks – traverse hundreds or thousands of kilometers of ocean every year. Because they are so wide-ranging, much of shark life, including their reproductive habits, remains secret. Scientists have struggled to figure out precisely where and how often sharks mate, the length of their gestation and many aspects of the birth process.

I am a Ph.D. 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?

In need of innovation

Until very recently, the technology did not exist to answer these questions. But marine biologist James Sulikowski, a professor at Arizona State University and my research mentor, changed that. He developed a new satellite tag called Birth-Tag with the help of technology company Lotek wireless. He has no stake in the company. Using this new satellite tag, our team is working to find out where and when tiger sharks give birth and is demonstrating a proof of concept for how scientists can do the same for other species of large sharks.

The birth tag is a small egg-shaped device that we insert into a pregnant shark’s uterus, where it will remain dormant and hidden among the fetal sharks throughout the pregnancy. This type of tag has never been used on sharks, but similar deployed tags have been used to discover the birthplaces of land mammals such as deer for decades with great success. When a tagged mother shark gives birth, the tag will be expelled along with the babies and float to the sea surface. After detecting dry air, the tag transmits its location to a passing satellite, which then sends the location and time of transmission back to our laboratory. Once we download this information, we will know where and when that shark gave birth.

After years of improving this new technology, we launched the first phase of the study in December 2019 and began implementing the tags. After the study was approved by the Institutional Animal Care and Use Committees at Arizona State University and the University of Miami, as well as the Bahamian government, we set out in search of some tiger sharks. To achieve this, our team of researchers from Sulikowski Shark and Fish Conservation Laboratory and the Shark Research and Conservation Program from the University of Miami, 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 an apex predator

Tigre Beach is a hot spot for female tiger sharks from many different stages of life, including pregnant women. These pregnant females may be gathering in the warm, calm waters of Praia do Tigre to take refuge and accelerate their pregnancy.

The high number of pregnant sharks in this small area makes finding one much easier, but catching and bringing a 10-foot shark to the boat is no easy task. We fish for sharks using batteryand it can take several hours to capture, hand pull, and safely secure one of these powerful creatures to the side of the boat.

After we capture 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 are evidence of a recent mating event.

After we gather this basic information, we turn her upside down to induce her into 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 mighty shark calm and still during the most exciting part of the investigation, the part where my experience comes into play: the pregnancy test.

Waiting

Just like ultrasounds used on humans, we use a mobile ultrasound machine to find out if a shark is pregnant. I put on a pair of glasses that allow me to see everything the ultrasound sees, I lean over the side of the boat and place the probe on the shark’s abdomen upside down. The picture is often confusing at first, as the water splashes over the shark and up onto the boat. The team holds the shark still while I slowly maneuver the probe along its belly. So if she is pregnant, something magical happens.

Writhing baby tiger sharks, about 40 of them packed tightly inside 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 get a peek into the secret world of unborn sharks. We spy on them as they pump fluid through their still-developing gills and watch in awe as they move around, blissfully unaware that something extraordinary is happening out there in the world. Once we have enough data on the approximate size of the offspring – which gives us an idea of ​​how advanced the pregnancy is – it’s time to tag the mother shark.

While I hold the probe as still as possible to maintain a view of the shark’s internal anatomy, Dr. Sulikowski takes the birth tag and uses a custom applicator to carefully insert it into the uterus through the urogenital opening. No surgery is required, the marking procedure is completed in a matter of minutes. Once the tag is inside the uterus, we turn the shark upright to wake it up and release it back into the open sea. I am filled with hope as I watch her swim gracefully to continue her pregnancy, with a birth tag hidden among her unborn children.

Solving the mystery

Last December, we implanted the first birth tags into three pregnant tiger sharks. For tiger sharks, pregnancy is believed to last 12-16 months, but researchers have little hard data. Since these tagged sharks ranged from recent mating to mid-gestation, an added bonus of this study is that it may help refine estimates of pregnancy length for this species.

Although we work in the Bahamas, a shark sanctuary where is it illegal to kill sharks, tiger sharks migrate extensively. As such, each tagged shark will likely spend time outside the Bahamas, in unprotected waters, where it will have to navigate carefully to avoid interacting with fishing gear. Tiger sharks are considered almost threatened by the International Union for Conservation of Nature and their populations are currently in decline. The data we obtain 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 to receive a notification from our online website ARGOS satellite system this 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 species of sharks are threatened with extinction, and understanding their reproductive cycles is fundamental to the effective conservation of these beautiful and ecologically important creatures. Using the birth tag, we are about to unlock this information about tiger sharks and hope to show that this can be done for many more species.

We’re planning future expeditions to implant many more birth tags, but for now we’ll 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 was republished from The conversation, an independent, nonprofit news organization that brings you trusted facts and 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.