A single fish, or even a school of them, swimming through the vast ocean isn’t an easy thing to keep track of, so scientists often rely on a little gadget called an acoustic tag. These tags give off ultrasonic “pings” that researchers pick up on hydrophones and other instruments and turn into digital data. By attaching them to or implanting them inside a fish, scientists can use them to monitor fish location, movement and survival rates.
Great care is always taken to make sure that these tags don’t hinder the animals that are being tracked, and fish don’t appear to be able to hear the pings. But that doesn’t mean other species can’t, and that might be a problem for both the fish and the scientists studying them.
Seals and sea lions have shown that they can hear fish tags’ pings from hundreds of feet away, but it wasn’t clear if the noises meant anything to them. Now, a new study suggests that the tags may be like little dinner bells for seals, helping them find tagged fish more easily and turning research subjects into easy prey.
Researchers from the UK, led by Amanda Stansbury, set up an experiment with 10 grey seals born on Scotland’s Isle of May. The seal pups were just three months old, didn’t associate sound with food, and had never even been in the ocean before. One by one, the seals were released into a long pool to find fish that the researchers had hidden in 20 boxes lining the sides. One box contained a tagged fish, one had an untagged fish in it, and the other 18 were empty. The pups searched around, poked their heads in the boxes and got a snack if they could find it.
After a few days—during which each seal got 20 turns in the pool—the seals all got faster at finding both types of fish. They not only reduced the time they needed to discover their prize, but also the number of boxes they had to check, even as the fish were moved from one hiding place to another. While the seals didn’t find the tagged fish much faster than the untagged ones, they did find them with fewer box checks and returned to the boxes with tagged fish twice as often as any of the other boxes. The researchers thought that the seals could have quickly learned to associate the tags’ sounds with food, but they might have also just been following their noses and not using sound that much during their searches.
To control for smell and other chemical cues, Stansbury and her team tried two variations of their original experiment. The first was a “tag only” trial where the seals explored the pool with just a fish tag, but no fish, in one box and all the others left empty. Even with no fish and no chemical cues to follow, the seals still checked the box with the tag in it for food faster than they did any of the other boxes. In the second “all fish” trial, the researchers filled all the previously empty boxes with fish that the seals couldn’t grab. With a tagged fish, an untagged fish, and 18 inaccessible fish all giving off odors in the pool, the seals again found the noisy fish faster than the silent one.
“Gray seals quickly learn that hearing pinging fish tags give away the location of a nice meal,” the researchers say, and they might rely on these sounds even more in the wild, where the scent of a live swimming fish would be harder to follow. This complicates things for scientists using acoustic fish tags. If tagged animals are more vulnerable to being eaten, that’s not only unfortunate for the fish, but could skew the data being collected and point researchers towards erroneous conclusions. There’s already some evidence that wild salmon wearing acoustic tags are easier pickings for predators and have lower survival rates than ones wearing silent tracking tags. And while noisy prey might make things easier for predators, a hunter that’s been tagged itself could alert its quarry and have a harder time finding something to eat. The researchers say that acoustic tagging is becoming more common in studies on sharks, and the tags could give them away to their seal prey (though shark researcher Michelle Jewell notes that the noisiness of seal colonies seems to leave them oblivious to the sounds of nearby tagged sharks).
Stansbury says that the results of the study show how important it is to consider all the effects, both good and bad, that an artificial sound might have in an environment. If acoustic tags alter the way predators and prey interact, that doesn’t mean that they’re no longer useful. Rather, finding this unintended consequence helps scientists fine tune their methods to save both themselves and the animals they’re studying a lot of trouble.