Seals eat evidence of marine research findings
Electronic tags were meant to track movement of fish but also alerted
When scientists began tagging wild fish to track their movements, the
researchers thought they were the only ones listening to the acoustic
Unfortunately for the shoals fitted with the devices, experts had not
reckoned on the intelligence of hungry seals that learnt to interpret
the noise tags as "dinner bells", beckoning the predators straight to a
tasty fish meal.
In fact, the quick-learning seals have become so adept at picking up
the signals, and realising they meant food was around, that academics
fear their attempts to study the movement and behaviour of the tagged
fish could have been skewed to a "profound" extent, ruining their
Studies monitoring the movements of salmon and lingcod have already
suffered disproportionate losses of juveniles, and scientists believe
further research may be compromised unless a different type of tag is
Acoustic tags are increasingly being used by researchers to monitor
shark populations. But there is a risk, at least for the smaller species
and the young fish, of the subjects becoming "more detectable by prey
species such as seals", said Amanda Stansbury, of the University of St
Andrews. She said that experiments in conjunction with the University of
Cumbria had provided "concrete evidence" of the so-called dinner-bell
Ten young grey seals born on the Isle of May in the Firth of Forth
were used in experiments to test whether they learnt to associate the
ultrasound frequencies of the tags with food.
All were taken to the university's Sea Mammals Research Unit before
they had swum in the sea.
They were released back into the wild before their first birthdays.
In the experiments the speed with which they found herring hidden in
boxes was measured along with the frequency with which they sought boxes
containing tags compared with those just baited with fish.
"Seals revisited boxes with the acoustic tag significantly more often
than boxes which initially contained untagged fish," the researchers
wrote in the journal Proceedings of the Royal Society B: Biological
"Seals learnt the relevance of the acoustic cues and adjusted their
foraging strategy to revisit profitable foraging spots.
"Our findings present a novel way of looking at anthropogenic
[man-made] noise that illustrates how animals exploit cues when they
Warning that the adaptation could be costly to studies using tags to
monitor fish populations they said: "Research agencies worldwide invest
significant resources in acoustic tagging studies to assess fish stocks
and determine survival rates.
"As acoustic tags could make a fish more vulnerable to predation,
tagging can lead to erroneous conclusions in such studies.
"This concern is supported by observations of decreased survivorship
rates for acoustically tagged juvenile salmon compared with those with
similar tags that produce no sound signal. "Similarly, tagged predator
species may experience a decrease in foraging success. Acoustic tags are
becoming more widely used on sharks and could make them more detectable
by prey species such as seals."
- The Independent