Scientists’ tags on fish may be leading seals to lunch

To study fish or invertebrates in the sea, scientists often mark individual animals with acoustic tags that make soft, ultrasonic pinging noises. The marked swimmer can’t hear that noise, but other animals can — specifically predators, such as grey seals. And now a new study finds that the seals may hear those tags as a dinner bell that’s calling them to their next meal.

Amanda L. Stansbury of the University of St Andrews in Fife, Scotland, and colleagues worked with three female and seven male captive seals. They set up a foraging experiment consisting of 20 boxes placed at regular locations along the bottom of the wall of a long pool. Eighteen boxes were empty, one had fish in it, and the last had fish plus an acoustic fish tag. Each seal was then given 20 trials to find a fish. The results of the study were published November 19 in the Proceedings of the Royal Society B.

Over time, the seals became better at finding a box with the fish. And in the later trials, they found the fish in the box with the tag more easily than the one with the fish alone.

To confirm that the seals had learned to associate the sound of the pinging tag with food, the researchers altered their experiment. This time, they placed the tag in one box and left the other 19 empty. The seals quickly homed in on the tagged box. Then the researchers placed fish in all the boxes, though only two boxes had fish accessible to the seals. One of those boxes also had an acoustic tag. Again, the seals got to the fish in the tagged box faster.

The seals were probably using smell or other chemical cues to find the fish, but they figured out that the sound of the tags could also lead them to a meal, the researchers conclude. Exactly how this would play out in the wild isn’t clear — smell might not be as important to find swimming fish, for instance — but the results suggest that the tags could be making it easier for predators to find marked fish.

There is already evidence that the tags affect some species negatively. Juvenile salmon, for instance, have lower survival rates when tagged, and predator species have found it more difficult to hunt when marked with an acoustic tag. If tagging makes species more vulnerable to getting eaten, that’s not only bad for the tagged animals but also for scientists who depend on those tags to make accurate assessments of fish stocks, the researchers note.

Changing the frequency of the tags might make them invisible to some predator species, such as grey seals, but other animals, like cetaceans, may still be able to use them as an easy marker for where to find lunch.