Blacktip Shark Migration and the Mystery of the Missing Females

Thousands of male blacktip sharks just a few feet off the coast of Florida

Graduate student Beth Bowers and professor Stephen Kajiura at Florida Atlantic University are shark scientists.

Their experiment hopes to answer a lingering question in the marine science community: each year 10,000 blacktip sharks aggregate off the coast of southern Florida. Why do they migrate? Why are there so many? And why are they all male?

We met up with the scientists at their lab to discuss shark aggregations in Florida, sharks as a signal for climate change, and chasing your dreams as a young scientist.

Florida Atlantic University is known for its marine sciences research and teaching.

Florida Atlantic University is an hour’s drive north of Miami. It’s a public university of 30,000 students, and is known for research in marine sciences.

It’s also close to several main diving hubs. Jupiter, Florida is only 30 minutes away, and just four miles from the coast is the gulf stream — a deep underwater current that starts in the Gulf of Mexico and shoots north. It’s what influences Florida’s climate and creates a vibrant intersection of ocean ecosystems.

This project has been tracking the blacktip sharks commonly found in the waters off Florida. Scientists have long studied their annual migrations, where entire populations of male sharks break off from the females before the mating season. The male sharks aggregate in huge numbers further north in search of food.

The hypothesis for why the sharks separate involves the resources available, but, no one really knows.

For the past two years, Beth and Stephen have been tagging sharks to identify and monitor their movements across the Atlantic Ocean.

“The last time anybody described the range of the species they labelled Cape Hatteras as a northern parameter, as a boundary, They said the sharks only go to Cape Hatteras, North Carolina. Within our first ten transmitters, we had two go to Long Island, New York. So, we blew that out of the water.”
Long lines used to catch the sharks are repaired in the lab outside of the field season.
Stephen showing some empty holders of the transmitters used to tag the sharks.

It turns out, the range of the migration of blacktip sharks is stretching further and further north, and in bigger numbers.

For the past several years, the range of the migration of blacktip sharks is stretching further and further north, and in bigger numbers. It turns out, it’s very closely correlated to the water temperature.

That’s not to say that rising ocean temperatures is what’s causing the shark migrations to move northwards. It’s extremely difficult to pinpoint the exact causative relationship between the shark movement and climate change. However, a wealth of other studies have shown that rising water temperatures promote plankton growth, and certain lab-based experiments with invertebrates show better fitness at certain temperatures (you’re prone to die if the water temperature is too high).

Even more astounding is that you can take the past 70 years worth of data and compare it with predictions based on the temperature parameters.

“They can increase water temperatures and what scientists have found is that as temperatures increase, various marine species will move to higher latitudes to stay within their preferred temperature.”

All of this leads to a need for more data. That’s why Beth is raising funds to purchase more shark transmitters to gather data. With the additional transmitters, they hope to tag females sharks to also answer the question of where the females go before the mating season.

A radio tag receiver can detect when a tagged shark swims by and record that data.

The lab also has receivers ready to be deployed out into the ocean. When a shark with a transmitter tag swims close to a receiver, it will record the sharks identity, the time, water temperature, and a whole host of other metadata that will be useful for painting a picture of who, what, when, where, and why for that particular shark.

In cooperation with several park authorities, they’ll be looking to set up receivers hooked up to batteries and a cell signal. When a shark swims by, the receiver will be able to transmit that data point to a computer far away. With enough receivers spread around the Florida coast, it will create a real-time map of the populations of the blacktip sharks. Not just sharks, they’ll be able to track any marine creature that’s been tagged by other scientists too.

Stephen and Beth plan to make this data pipeline freely available to the public, but they need funding for more transmitters first.

Understanding more about this species of sharks is one small piece of a giant shifting puzzle that is our ocean ecosystem. But, this is a potential game-changing dataset waiting to tell us more.

“What is the outcome of this study? Well, the blacktip shark itself is very prone to stress. It is known as a very delicate shark, and so you can kind of make a leap from that to say that this is not gonna be one of the hardiest species that you’re looking at in the face of global climate change. If they are able to migrate and adapt successfully, you can then apply that to a lot of other endothermic apex predators like other sharks. And say, they’re okay and gonna do just fine. Or hey, the blacktips are dying off because now they have nowhere to go. Then you can directly say we need to start putting some kind of umbrella conservation on certain areas and species.”

Want to learn more? Listen to our full interview with Beth Bowers and Stephen Kajiura.

Check out their project on Experiment and help fund the research to receive the results and findings of their shark study.

This story is part of the Experiment Shark Challenge Grant, celebrating elasmobranch research and crowdfunding.