Scientists look to DNA to help track white sharks

As white shark sightings increase in the Gulf of Maine, researchers are developing new tools for tracking them, shedding light on the movement of white sharks in the future and advancing the use of environmental DNA.
Wed, 07/26/2023 - 8:45am

In 2020, a swimmer was killed by a white shark off Harpswell Cove. The encounter was unusual — Maine’s first fatal shark attack — but the media was abuzz. As white shark sightings have increased in the Gulf of Maine, especially around Cape Cod, locals have been asking why this is happening and what it could mean for them. What has been missing from those conversations, though, is rigorous science on how to track white sharks — and forecast where they might appear in the future.

Researchers at Bigelow Laboratory for Ocean Sciences are trying to answer those questions. They’re using cutting-edge techniques to understand the movements of sharks and other marine species, both now and in the future as a changing climate continues to reshape life in the ocean.

“We’re interested in understanding species on the move, ranging from kelps — a foundational species of our rocky coastline — all the way up to the large fish using those kelp forests, and that includes white sharks,” said Senior Research Scientist Doug Rasher.

One of the tools that can help illuminate those species movements is environmental DNA, the floating bits of DNA that organisms shed and secrete into their environment, which can be detected in water samples.

Bigelow Laboratory is co-leading a five-year, state-wide, multi-institutional initiative with the University of Maine to unlock the potential of this eDNA to advance understanding and management of coastal ecosystems. This tool would be particularly useful for species that can’t be easily detected by other means, such as new arrivals to an ecosystem or large but rare species like white sharks.

A major challenge, though, is that water moves.

“When you detect an organism through its genetic signature, you don’t know exactly where that DNA came from because it’s floating around in water that’s moving with the currents and tides,” Rasher said. “You can’t easily know if it’s from right here an hour ago or several kilometers up the coastline a few days ago.”

The nuances of interpreting eDNA are particularly sensitive when it comes to a species as prone to misunderstanding as white sharks. That’s where Kyle Oliveira comes in.

A UMaine doctoral student working at Bigelow Laboratory, Oliveira’s research focuses on developing detection methods for white sharks. Part of his work is quantifying how much eDNA white sharks shed into the environment, as well as how that eDNA moves and degrades, to better understand how eDNA can be used as a tool for monitoring sharks.

“Each species is going to be different in terms of how much eDNA they shed, which also changes with how active an organism is, and what happens to that eDNA once it’s shed,” Oliveira said. “We’re trying to understand all of that while at the whims of the ocean.”

On several occasions since 2021, Oliveira has gone out off Cape Cod after white shark sightings with the Atlantic White Shark Conservancy, a nonprofit group that runs shark tours and education programs. He plans to go out again this summer. At each sighting, the team drops three buckets into the water, each outfitted with a float and GPS receiver.

The buckets are used to triangulate and follow a defined parcel of water through which a shark recently passed. The researchers take samples of water over time at different depths and locations across the parcel. Tracing the genetic signature of the shark across the parcel shows how eDNA spreads, sinks, and eventually fades away.

With a better understanding of those patterns, beach managers may eventually be able to use eDNA as another tool for tracking sharks as they move along the coast.

“EDNA is not a silver bullet for asking questions about the distribution of organisms in nature,” Rasher said. “But when you combine it with other forms of information, like acoustic and visual monitoring, it can tell us about facets of the ecosystem we’re not otherwise seeing.”

Part of the challenge with studying white sharks, actually, is that there are already so many different methods for detecting them, each with their own limitations and advantages. To develop a complete picture, then, of the movements of white sharks in the Gulf of Maine and how it might change over time, Oliveira needs to blend those datasets together with advanced mathematical models.

“If you have a lot of the same type of data, you just need more processors. But when you have lots of different kinds of data, that’s a challenge you need a human to figure out,” said Nick Record, Director of Bigelow Laboratory’s Tandy Center for Ocean Forecasting and one of Oliveira’s advisors. “The goal is to merge together these different types of measurements to say something coherent about where white sharks are, why they’re there, and where they might be going in the future.”

To that end, the team is trying to merge different datasets to create unified maps of where sharks have been sighted and the kinds of environmental conditions they seem to prefer. That will give them a sense of the “environmental niche,” as Record described it, or ideal habitat of sharks across the region. From there, they can use climate models to begin to predict where sharks are likely to be found in the future as the Gulf of Maine continues to warm.

Understanding those movements will be essential for protecting commercially-important species — and the species they feed on — and managing potential increases in human-wildlife interactions as the climate shifts. So, using complex models to forecast the distribution and range of species in the future is a central element of much of the work in the Maine-eDNA project.

“An integral part of our project is to develop near-term forecasts because we need to turn the data we’re collecting into tools for stakeholders,” Rasher said. “We want the data we’re collecting to be useful to people beyond the academic community.”

Last month, Record released a survey asking the public for their input on what kind of forecasts people would like to see in the future for Maine’s marine life — from microbes to sharks and whales.

“There’s a joke in ecology that counting fish is like counting trees, except that fish are invisible and they move around,” Record said. “But a lot of our ecological theory comes from counting things that are fixed on land, and that just doesn’t apply well in the ocean.”