Acoustic tags turn monitored animals into targets

gray seals
CC BY 2.0 rp_naturephotography/Flickr

It starts out with good intentions: put acoustic tags on tuna, salmon, seals, sharks and eels to track them. But it can rapidly turn into a Pavlovian mess.

The tags, which operate on a sound frequency humans can't detect, could theoretically become a dinner bell for underwater predators. So researchers from the University of St. Andrews decided to investigate. They captured 10 grey seals in Scotland and presented them regularly with three types of boxes: ones with tagged fish, ones with untagged fish and ones with no fish at all. Before long, the seals showed a preference for the boxes with the tagged fish, where food would be guaranteed.

"Any animal who can perceive sound would be expected to be capable of learning associations between sound signals and food," researcher Amanda Stansbury, from the University of St. Andrews, told Discovery News. "This tells us that seals can exploit new sounds, such as fish tags, and use them to their advantage," she later told the BBC.

Most underwater predators use movement and smell to detect prey, but these tags could basically guarantee a meal in the real world once the predator has made the association. To test this, Stansbury and her team decided to put some tags in a box without fish, while the other boxes were empty. The seals followed the sound. The team then placed untagged fish in each of the boxes and left one box with only a tag (no fish) and once again, the seals went to the tag. Essentially, the seals had replaced their tendency to rely on smell with a tendency to rely on sound.

This is not the first case of tags making prey an easier target for predators. Juvenile salmon with acoustic tags tend to have lower survival rates than juvenile salmon with other types of tags. In other cases, tags have made animals more attractive to one another. For example, finches with colorful tags became more attractive to potential mates. So while tags are very useful for humans to track animals - particularly endangered animals - our interference is having an impact on the natural world.

"Our results... illustrate the importance of considering the auditory sensitivities of all animals in the environment when designing an acoustic tagging study," said Stansbury and her team.

Acoustic tags turn monitored animals into targets
Seals and other underwater predators are being conditioned to find - and eat - sea life