Tags Could Make Fish 'Easy Prey' For Seals

Tags that help researchers track the survival of fish may actually be making these animals more vulnerable to marine predators with hypersensitive hearing, such as seals. That would then make these tags little more than tiny "dinner bells," and could seriously skew survival data.

That's at least according to a study recently published in the journal Proceeding of the Royal Society B, which details how even seals unfamiliar with tagging are quickly drawn towards fish who boast an acoustic tag.

This was demonstrated in 20 trials, in which young seals were exposed to a pool containing 20 boxes with fish hidden inside. Over and over again, the seals had far greater success finding tagged fish than they did finding those that were untagged, suggesting that these acoustic tags were letting the sensitive animals home in on potential prey.

"Our results... illustrate the importance of considering the auditory sensitivities of all animals in the environment when designing an acoustic tagging study," the researchers wrote.

Acoustic tagging is widely used in capture-and-release studies intended for tracking aquatic populations. Unlike with sharks, dolphins, or whales, fish are often far too small to be able to swim with a GPS tag strapped to their back. Likewise, even large fish species swim too deep for satellites to consistently pick up the GPS ping, making tracking difficult.

Acoustic trackers, however, are far easier to design and are much smaller. Tagged to these fish, these trackers repeatedly send an ultrasonic frequency through the waters that receivers can then measure to determine the distance and position of a fish.

However, while fish appear unbothered by this signal, the sensitive ears of predators like seals may be able to pick up on it, leading to a "dinner bell effect," as the researchers call it.

Because the tags are often used to track average fish mortality, these tags may be leading to skewed data, meaning the situation of a "ringing" meal with fins is certainly not average.

"We showed that acoustic tags... aid prey detection, potentially increasing predation of tagged animals and potentially skewing study finding," the study authors wrote. "When introducing artificial sound sources into an environment, it is important to take into consideration all potential effects on local species, both detrimental and beneficial."