Researchers using sound-emitting tracking tags to follow fish for scientific research may not be doing the finny creatures any favors, instead creating a "dinner bell" effect that's attracting seals to an easy meal, a study finds.
British scientists working with captive gray seals found the seals quickly learned to associate the sound of the tracking tags with the possibility of a quick and easy fish feast.
Ten young seals with no previous experience of the acoustic tags were placed in a pool containing 20 boxes containing fish, some tagged and some not.
"The seals found the tagged fish in fewer box visits than the untagged fish in later trials, demonstrating the learned use of the acoustic tag to locate food," the researchers reported in the journal Proceedings of the Royal Society B.
Marine scientists routinely make use of acoustic tags in mark-and-recapture research to analyze fish species survival and the health of fish stocks. The tags emit ultrasonic frequencies believed to be imperceptible to fish they're attached to.
It turns out what's imperceptible to the fish may not be to their predators, the researchers found.
While previous studies have suggested such signals are audible to predators, the new research is the first to uncover a "learned association between a signal and food leading to a 'dinner bell' effect," they said.
"Our study shows how important it is to know the sensory abilities of other animals in the area to prevent any unintended effects of tagging," says study leader Amanda Stansbury of the University of St. Andrews in Scotland.
There could be a reverse corollary at work when scientist tag predators for research purposes, the researchers suggest; those predators could suffer if their prey pick up the "pings" of the tags and manage to evade to approaching predator.
Marine researchers have been tagging both prey and predator species, including fish such as salmon and tuna and other species including seals, sharks, squid, eels and turtles.
"All tagging studies rely on the basic assumption that tags have no significant impact on marked individuals," the researchers wrote, noting that their study suggests that basic assumption may need to be reconsidered.
Stansbury cites existing evidence that juvenile salmon fitted with acoustic tags have poorer survival rates than those carrying tags that produce no sound, like satellite or radio tags, and suggests that could be a result increased predation of the acoustically tagged salmon.
"Our results... illustrate the importance of considering the auditory sensitivities of all animals in the environment when designing an acoustic tagging study," the researchers concluded.