Why grey seals are loving tracking tags

A team of researchers in Scotland have found that the acoustic tags that scientists attach to fish to track populations are also being used by predators like grey seals to locate potential meals.

What if a device that's used by scientists to help protect fish might actually be putting them in harm's way?

Marine researchers use acoustic tags to track fish populations, but the scientists might not be the only ones listening. According to a study published Wednesday in the journal Royal Society Proceedings B, the grey seal, a predator of a number of fish species, can use the tags' high frequency sounds to locate its next meal.

Scientists have long known that some tracking methods can influence other animals: tadpoles marked with a dye that stains their skin are more vulnerable to predators, and finches with certain colored tracking rings can appear more attractive to potential mates. A team of scientists from the University of St. Andrews and University of Cumbria suspected that this would be the case for auditory signals as well.

The researchers wanted to find out if grey seals actually use these acoustic tags to their benefit when foraging for food. The tags emit a high-pitched sound – one believed to be imperceptible to their wearers and likely annoying to humans. For the seals, however, the sound proved to be the equivalent of a dinner bell.

"We might think it is an unpleasant noise," Amanda Stansbury, postgraduate student at the University of St. Andrews and an author of the paper, told the Monitor. "They learn it means food."

For their experiment, the team used ten grey seals from the Isle of May off of mainland Scotland. The seals had never spent time in the open sea, though, and primarily lived in the Sea Mammal Research Unit at the University of St. Andrews. Dr. Stansbury and her colleagues built twenty foraging spots that sat at the bottom of a pool. Each one consisted of a box with a bucket attached to the front for a seal to poke its head into and investigate the contents of the box.

The first experiment consisted of placing a tagged fish, in this case herring, into one box and an untagged fish into another box. If the seals showed a preference for the box with the tagged fish, the scientists reasoned, that would suggest that the seals are able to hear the tag and can associate it with food.

But it didn't work out that way. The seals quickly learned to locate both the tagged and untagged herring, probably by relying on scent instead of sound.

So the team devised two more experiments. In the first, they put an acoustic tag – no fish attached – in one of the boxes, while the rest were empty. In the second experiment, the researchers repeated the initial experiment, but added fish pieces that were out of reach of the seals to the eighteen boxes that were empty the first time around.

In both cases, the research group found that the seals learned to use the auditory cues from the acoustic tag. When all fish were absent, the presence of the acoustic tag on its own led to a decrease in time needed to visit the box. And when there was fish in every box, the researchers saw a significant reduction in the time that a seal needed to retrieve the tagged fish. 

For both these situations, the seals were unable to rely on smell because there were no fish or because there were too many fish. Using their noses might be seals' first inclination, but the acoustic tags may help them to search faster or provide another level of assurance that prey is nearby.

These findings suggest that marine biologists should reevaluate their use of acoustic tags, says Stansbury. The way to do this is to get a better understanding of the role tags are playing in their owners' environments.

"The tags might be fine, but we need to evaluate the ecosystems that the animals are in," says Stansbury.

There already exists evidence in the wild that acoustic tags can be detrimental to their wearers: scientists found lower survivorship rates for acoustically tagged juvenile salmon than for the salmon that had tags making no sound. Some members of Stansbury's team are now working on an acoustic tag they believe will actually deter predators like the grey seal. In the meantime Stansbury plans to continue evaluating the extent that tracking devices affect the lives of aquatic animals.

"It's pretty complicated with marine species because they travel quite large distances," Stansbury says, "so they can be interacting with quite a few different areas and quite a few species."