Schools of fish living in the vast open ocean can be difficult to study, so researchers often use acoustic tags to monitor their movements and track their survival. But it turns out that scientists may have accidentally shot themselves in the foot because the tags could be inadvertently turning the fish into easy targets for predators.
According to a new study, which has been published in Proceedings of the Royal Society B, seals can quickly associate the pings emitted by the devices with the presence of an easy meal. The tags may therefore be creating a “dinner bell effect” in the wild, alerting seals to the presence of fish, which could be seriously compromising marine research.
Acoustic tags are often used in population studies in order to assess fish survival rates and stock health. These produce ultrasonic frequencies which are assumed to be imperceptible to the marked animal, but previous work has found that predators such as seals and sea lions are capable of detecting the signal.
To investigate whether seals could be learning to use this information to their advantage, scientists from the University of St Andrews studied ten juvenile grey seals that had never been in the sea nor been exposed to acoustic tags before.
They individually placed the seals in a pool containing 20 foraging boxes, all of which were empty except for one which contained a tagged fish and another which housed an untagged fish. Each seal was given the opportunity to explore the boxes in 20 different trials, in which the fish were randomly moved to different boxes each time. The seals didn’t find the tagged fish much faster than the untagged fish during the learning experiment, but after a few trials, the seals started to find the tagged fish after significantly fewer box visits than the untagged fish.
With each consecutive trial, the seals needed around 5% less time to find the box containing the tagged fish. Furthermore, the seals revisited boxes containing the tagged fish more frequently than any other box. This, the researchers say, demonstrated that the seals had learned to use the acoustic tag to locate food.
The team then conducted two control experiments to investigate the role that the acoustic signal was playing. For the first, acoustic tags were placed in one box, but no fish were present in any boxes. For the second, pieces of fish were placed in all boxes. Once again, in both of these experiments, the seals found the tagged box faster than the control box.
“This tells us that seals can exploit new sounds, such as fish tags, and use them to their advantage,” said lead researcher Amanda Stansbury. “We expect that other marine mammals are similarly able to use such information to find prey. Tagged fish may be more detectable by predators, which could affect the results of fish studies.”
Similarly, acoustic tags might be having negative impacts on sharks, as the pings could alert prey species such as seals of their presence. Thus, while these artificial sounds are benefitting one species, they could be negatively affecting another.