The Economist April 30th 2022 71
Science & technologyMarinebiologyNow hear this
Z
oologists routinely track animals,
from albatross to zebra, using global
positioningsystem (gps) tags which then
return their data via satellite. Marine biolo
gists have a harder time of it, though, be
cause seawater is infuriatingly opaque to
radio signals. This makes it impossible ei
ther to receive gpssignals or to transmit
any data collected back to base.
That does not stop people tagging sea
creatures. Data collected and stored in a tag
can either be sent to a satellite in bursts if
the species in question is one that comes to
the surface from time to time, or the tag
can be dispatched on a oneway trip to the
surface after a set period. A tag may also be
recovered if the animal carrying it is
caught by a fishing boat. (Fisherfolk are
typically paid a few hundred dollars per tag
returned to its home laboratory.)
None of these methods, though, keeps
accurate track of where the animal carry
ing the tag has been. For these and other
reasons, it would therefore be useful to
have a marine equivalent of gps. And one is
now being deployed. The Woods Hole
Oceanographic Institution, in Massachusetts, hopes to fill the seas with sonic bea
cons that will ape the role of gpssatellites
by broadcasting signals which can be used
to triangulate a receiver’s location when it
is underwater.Ping pong
The sea is divided into distinct layers that
have different temperatures and salinities.
During the second world war American na
val scientists showed that some of these
layers act as sonic waveguides. They
dubbed them “sound fixing and ranging”
(sofar) channels. Sound emitted in one of
these channels echoes between the layers
above and below, thus staying in the chan
nel—an effect comparable to the way light
is carried within an optical fibre. Thus con
strained, a sound wave can travel hundreds
of kilometres before it becomes too atten
uated to detect.The sofar transmitters deployed by
Woods Hole, as the institution is known
colloquially, are onetonne buoys moored
at an appropriate depth for the channel
concerned. Every 12 hours they broadcast a
32secondlong location signal known as a
pong. Pongs are so called because they are
similar to sonar “pings”, but of lower fre
quency (300hz rather than 18khz). In typi
cal conditions a pong can be picked up
1,000km away. By listening to the pongs
from several beacons a receiver can calcu
late its location.
Existing receivers for the two sofar
transmitters currently deployed are car
ried on freefloating instrument packs that
measure temperature, salinity and other
physical variables. But the plan is to deploy
two more transmitters this year, and more
in future years, with the aim of “ensonify
ing” the entire east coast of America, out as
far as the Sargasso Sea. At the same time,
people are working on shrinking the re
ceivers to the point where they can be at
tached easily to fish.
Godi Fischer of the University of Rhode
Island is one of those attempting this re
ceivershrinking. His latest offering, now
being tested on submersible drones called
gliders, is 24mm long, 11mm in diameter
and weighs just nine grams. That makes it
tiny enough to be attached to a range of
marine creatures. Even if the recipient is
not in a sofarchannel, Dr Fischer says
there is enough leakage from such chan
nels for it to pick up a signal at a distance of
perhaps 100km. Each tag costs just $200.An underwater gpsbased on sound waves could transform marine biology, help
fisherfolk catch the right species and improve knowledge of climate change→Alsointhissection
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