7979

• UCREFRP Catalog Number: 7979
• Author: Tyus, H. M.
• Title: Fish Radiotelemetry
• USFW Year: 1982.
• USFW - Doc Type: Theory And Application For High Conductivity Rivers.
• Copyright Material: NO

Radiotelemetry
THEORETICAL CONSIDERATIONS
TYPES OF TELEMETRY
Radiotelemetry has been used to monitor movements and other behavior of
terrestrial animals for many years. Earlier workers did not use radiotelemetry
for studying fish movement because it was believed that.radiotransmission
through water would be too poor to be effective. Most of this early work
involved use of ultrasonics (Stasko and Pincock 1977). The suitability of
using both techniques is evaluated below.
Ultrasonic Telemetry
Ultrasonic transmissions of high frequency sound waves have been success-
ful in many aquatic systems. Frequencies employed are usually between 30 and
200 kHz, which are not audible to the human ear. Ultrasonics have an advantage
over radiotransmissions for tracking fish under certain conditions. For
example, ultrasonic transmission range is not greatly affected by dissolved
solids. Also, a hydrophone (receiving collector) is generally more directional
than a conveniently sized radio antenna due to the shorter acoustic wavelengths
and the slower sound velocity. The slower velocity allows the pulse interval
to be used in computing the distance travelled from the transmitter to the
receiver (Stasko and Pincock 1977).
There are, however, several disadvantages of ultrasonic tracking. For
example, the detection of ultrasonic signals require that the receiving hydro-
phone be immersed in water. This makes tracking difficult with ice cover and
also eliminates the use of aircraft. Aircraft use may be essential in a large
river system, especially if numerous tagged fish are being tracked.
Ultrasonic telemetry is markedly influenced by water temperature, turbu-
lence, and sediment load. Temperature affects the velocity of ultrasonic
emmissions and a thermal discontinuity may reflect ultrasonic energy away from
the hydrophone (Sinning 1979). Entrained air from waves, boat propellers, and
turbines in dams may cause enough noise to mask ultrasonic reception, as does
the movement of bottom sediments caused by stream currents. The only known
use of ultrasonics in the Green River was marginally successful due to water
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Wydowski 1980).
Radiotelemetry involves the generation (by a transmitter) and propagation
of radio waves (electromagnetic radiation) through water. The transmitted
energy must then cross the air-water interface and be received by an antenna
operating in air. Although radio frequencies of 10 kHz to 50 MHz have been
experimented with in water, frequencies between 40 and 160 MHz are usually
used.
The propagation of radio waves through any medium, in this case water, is
inversely proportional to the frequency. Thus, radio waves of high frequency
travel a given distance with a greater loss of power than an emission of lower
frequency. The nature of the change in propagation with frequency is approxi-
mately logarithmic (Lortsdale 1967; Lonsdale and Baxter 1968). Radio signals
are attenuated (diluted) more rapidly in water than air, and the amount of
attenuation is inversely proportional to the conductivity of the water (Weeks
et al. 1977). For this reason, a considerable loss of signal strength is
expected when radio waves are propogated through fresh water of high conduc-
tivity. Radio transmission in salt water is virtually impossible.
The behavior of radio energy at the air-water interface (Fig. 1) is an
important consideration for radiotelemetry. Energy contacting the interface
is reflected unless the angle of incidence is less than 6° (Weeks et al.
1977). Radiation of the energy that passes this interface produces a signal
with. the apparent signal source being a circle on the surface of the water
(Priede 1980).
The preceding explanation does not account for the reflection of radio
energy from~the water surface to the bottom and back through the surface, or
radiation from anything other than a smooth surface. However, these additional
effects result in a signal being emitted from the surface in a pattern dif-
ferent from that predicted by theory. Sinning (1979) noted that the U.S.
Navy, which experimented with subsurface radio emissions for over 30 years,
uses a surface antenna for submarines. This is evidence of the difficulties
with long distance underwater radio transmissions yet to be solved.
Factors, other than the propagation of radio transmissions through water,
must also be considered. The most obvious one is the strength of the emitted
radio signal. The range of a signal is directly proportional to the power of
the transmitter. Power radiated by the transmitter is a function of the
efficiency of both the transmitter and the transmitting antennas. These
efficiencies are typically 65 to 70% for underwater applications when the
radiated transmitter output is contrasted with battery power ehtering the
transmitter (Sinning 1979).
Antenna efficiencies (Terman 1955) can be lower (less than 1% to 25%)
i;han--transma~te~-P#ficienciES.-~nlhe~-lnss~sf_effisSencies_for_transmitters_ _ _
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and antennas are combined, antenna efficiency may range from less than 1% to
18% of the battery power.