Researchers from from UC Santa Barbara and the U.S. Geological Survey, along with members of California State University Long Beach and Central Michigan University, have found a new way to detect if large sharks — specifically great whites — had recently passed through an area.

The team’s research, which used environmental DNA to track populations of great white sharks off the coast of California, was published in the recent journal, Frontiers in Marine Science.

Environmental DNA is genetic material left behind by animals like sharks such as mucus, feces or shed skin that contains their genetic signatures, which can be parsed out and identified through genetic sequencing. Scientists can extract and amplify specific genes within the DNA fragments found in water samples, and determine if the DNA contained in those samples is from a specific species.

“One of the goals of this research is for a lifeguard to be able to walk down to the shore, scoop up some water, shake it and see if white sharks are around,” lead author Kevin Lafferty, a USGS ecologist and researcher with UCSB’s Marine Science Institute (MSI), said.

The results of the study found that after years of decline, the population of great whites was making a comeback. Researchers point to increased protections at both the state and federal level, the increase of marine mammals such as sea lions which the sharks depend on for prey and better fisheries management.

“However, white shark population recovery has co-occurred during a period when more people than ever before are using the coastal ocean for recreation, ultimately increasing the likelihood of interactions,” CSULB professor and study co-author Chris Lowe said. “While sightings of juvenile white sharks have risen considerably along California over the last eight years, there has been no dramatic increase in shark bites on people.”

Because eDNA can drift with currents, and sharks can swim long distances in the time it takes eDNA to degrade, the new approach only gives a rough idea about where sharks actually are at a particular moment. The researchers are optimistic that the technique will be able to help keep swimmers and surfers aware of potentially dangerous sharks in their vicinity.

“We can now sample eDNA along the coast to make better maps and seasons for white sharks,” Lafferty said, “And if we can do it for white sharks, we can do it for other marine species, too.”