New fish-tracking tags will alert researchers when their subjects have been eaten

Fisheries researchers tag fish with transmitters to learn more about where their study subjects move. The next generation of tags will let them know when the fish are moving through a predator’s digestive tract.

The tags are under development by Seattle-based Hydroacoustic Technology, Inc., which has been in the fish-tracking business for 20 years. It’s a big step forward in tagging technology, according to Caroline Mercado, media director at HTI.

“It’s been an evolutionary process for us,” Mercado said. “This is a whole new dynamic.”

Tagging studies have become a mainstay in fisheries research for learning where fish move throughout a habitat and, to a certain extent, how they behave. Researchers have a few ways to infer when a tagged fish has been eaten while carrying current-generation tags, which emit an acoustic signal that’s picked up by receivers deployed throughout the waterbody of interest. For example, if two tagged fish appear to swim right next to each other for an unnatural amount of time, they’re likely both in the same predator’s stomach.

But the new tags feature a mechanism that detects when a fish has been eaten and triggers a modified signal. The technology is patent-pending and Mercado couldn’t yet say how it works, but more information could be available this winter.

But it does work, according to Sam Johnston, a senior fisheries biologist with HTI. They’re still refining the consistency of the predation detection and working to cut down trigger times, though they’ve seen promising results in trials. Those included testing the triggers on tagged juvenile salmon that were fed to conditioned striped bass swimming in tanks.

Salmon smolts at the Seattle Aquarium (Credit: James Brooks, via Flickr/CC BY 2.0)

“They were essentially trained to come up and eat smolts on command,” Johnston said.

The striped bass and salmon smolts weren’t just convenient test subjects: They’re among the species that drove the development of predation-detecting tags. Low survival of young salmon migrating from West Coast rivers out to the ocean continues to be a problem, and predation by non-native striped and largemouth bass has been implicated as one of the causes. Fish tagging has already played an important role in learning more about how smolts act when they migrate downstream. Now it appears poised to help managers get a better handle on how many smolts are picked off by predators.

“We wanted to develop something where we could see what proportion of the fish actually are being eaten, as opposed to migrating downstream where they just disappear and we don’t know what happens to them,” Johnston said.

West Coast fisheries researchers aren’t the only ones interested in getting a better handle on predation rates. Charles Kruger, professor of aquatic ecology and conservation at Michigan State University and the director of the Great Lakes Acoustic Telemetry Observation System, says tags like these will be very important. Direct measures of mortality are rare, and scientists’ estimates of how many animals have died are usually based on how many are alive and left to count.

“Tags such as this would open a whole new world of questions that could be answered that never before could be answered,” he said in an email.