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TRACKING animals through water is a tall order, but scientists are constantly
developing new techniques for remote observation. There are now several
well-tried ways to follow fish, each suitable for different purposes. Acoustic
transmitters are used mostly in the sea; while VHF radiotransmitters are
suitable only in fresh water. UHF radio beacons that can be monitored by
satellite are suitable only for large animals.

The commonest method of tracking a fish is to fit it withan acoustic
pinger, typicallya small cylinder about 15 millimetres in diameter and between
30 and 100 millimetres long, depending on the size of the battery. The pinger
can be attached externally, or it can be inserted into the fish’s stomach
or implanted in the body cavity. The pinger emits regular pulses of ultrasound,
every 1 to 2 seconds. The frequency of the pulse is usually between 30 and
100 kilohertz – above the range of sensitivity of most aquatic animals.

The tracker picks up the signals with a small hand-held directional
hydrophone linked to a battery-powered receiver. The receiver converts the
high-frequency signal to one within the audible range for humans, which
the operator monitors on headphones. To locate the animal, the operator
rotates the hydrophone to find the strongest signal. This simple system
can be used from small boats, on foot from the river bank or from ships
at sea. The only requirement is that the hydrophone is suspended in the
water with ‘line of sight’ to the transmitter.

The range of the signal depends on many factors, but if conditions are
good the signal can travel as far as a kilometre. Changes in salinity or
temperature from one place to another reduce the range. Suspended solids
or air bubbles in the water also shorten the range by refracting the signal.

Noise in the water has a similar effect. Snapping shrimps are notorious
sources of noise in some parts of the world. And while tracking sea snakes
in the Bay of Panama I have heard dolphins trying to imitate the pulse of
our acoustic transmitters. Higher frequencies, inaudible to dolphins, are
absorbed to a greater extent by sea water. When choosing a frequency to
work at, the scientists must make a trade-off between greater absorption
at higher frequencies and interference by noise at the lower end of the
range.

The basic system of acoustic pinger and hydrophone has served well in
a wide range of studies, tracking sea snakes, young turtles, lobsters and
many species of fish.

In the 1960s, John Kanwisher of the Woods Hole Oceanographic Institution
in Massachusetts developed acoustic transmitters that telemeter physiological
information coded by varying the pulse rate of the transmitter. In a famous
experiment with Francis Carey and others at Woods Hole, Kanwisher used a
transmitter that senses temperature to demonstrate that certain species
of tuna maintain a high body temperature.

Since then, researchers in a number of laboratories have developed transmitters
that monitor heart beat, salinity and the concentration of dissolved oxygen
in the water. In Britain, a team from the University of Aberdeen, the North
West Water Authority and the Water Research Centre used telemetry to study
the effects on salmon of low concentrations of dissolved oxygen in the water
around sewage outfalls flowing into the Irish Sea.

One step on from a simple pinger is a transponder that emits a pulse
when ‘interrogated’ by sonar. Using this principle, scientists at the Lowestoft
Fisheries Laboratory have developed the most sophisticated of all sea-going
fish tracking systems, which shows the sea floor as well as the position
of the fish. Such a system avoids the need for transmitters that sense depth.

All these systems demand that the operator is vigilant the entire time.
To overcome this problem, researchers have looked to automation. If the
target animal occupies a limited area, or ‘home range’, the answer is to
set out hydrophones on the seabed: it is possible to calculate the position
of the fish from the time lag in the signals at each hydrophone.

A microcomputer can do this automatically, so giving the position of
the fish every second, night and day – something a human operator can never
hope to achieve. Biologists have tracked juvenile cod in the sea lochs of
Scotland and herbivorous fish off the coast of California in this way.

In fresh water, radio signals are better than acoustic signals. Acoustic
transmitters are virtually useless in noisy, fast-flowing rivers, and radio
transmitters have replaced them almost entirely in research on salmon. Radio
waves propagate up through the surface of the water and can be picked up
by a receiving antenna in the air. The antenna can be held by hand, mounted
on a vehicle or on an aircraft. Radio tracking can be much less labour intensive
than acoustic tracking: an aircraft, for example, can fly along a river
and quickly pinpoint fish.

Normally, VHF pulsed radio transmitters are used. With different pulse
rates and different frequencies, scientists can identify many individual
fish in the river. Radiotransmitters use less power than acoustic transmitters
of equivalent range and small tags can last as long as a year.

Automatic logging stations are now a vital part of these studies. They
consist of receivers set up at intervals along the riverbank which automatically
scan all the frequency channels and identify the fish. The receivers are
linked to microcomputers, which log the fish as they pass each station.

Following the long-distance migrations of fish at sea remains difficult.
Even at shallow depths sea water effectively blanks out radio waves. So
for most fish only acoustic systems are suitable. And ships and their crews
cannot stay at sea forever. For animals that come to the surface of the
sea, researchers have had some success with special UHF radio beacons that
can be located by receivers on board satellites. The satellite provides
continuous worldwide coverage and can track many individually identified
animals at the same time. However, the animal must be on the surface when
the satellite passes over.

Moreover, the lightest radio beacon weighs at least 100 grams and most
weigh a hefty 1 or 2 kilograms. So far, the only fish tracked this way is
the basking shark, a species that weighs 7 tonnes and hardly notices the
extra weight. Conveniently, it feeds near the surface on sunny days. Biologists
are now collecting data on manatees off Florida, and are looking at techniques
for tracking turtles and whales.

The main advantage of tracking systems is that they allow us to observe
animals that are otherwise inaccessible. An extreme example is the grenadier
fish, a scavenger that lives on the sea floor of great ocean basins at depths
of around 5000 metres. At the University of Aberdeen, we have developed
an acoustic pinger that will work at the great pressures at these depths.

With Ken Smith, of the Scripps Institution of Oceanography in Southern
California, we have duped grenadiers on the floor of the Pacific into swallowing
acoustic tags in bait. The tags then allowed us to follow their movements.
Instruments on the sea floor record the movements of the fish, and we retrieve
the instruments – and the data – after an appropriate time.

It is virtually impossible to bring grenadiers to the surface alive
in order to study them. The change in pressure is inevitably fatal. The
ingestible transmitter is providing more information on the behaviour of
these deep-sea fish than is available for many species that live in shallow
water.

Dr I. G. Priede is in the department of zoology at the University of
Aberdeen.