of an initial pilot study, using wireless hydrophone monitoring coded acoustic transmitters
(Lotek, CAFT Series) in the James River, Virginia indicated a maximum detection range of 750 m. It
should be noted that windy conditions produced choppy surface conditions on the river, likely
contributing to increased levels of background noise. It is recommended that study sites be
evaluated for ambient noise to determine the maximum detection range for that particular system
to ensure adequate coverage when deploying wireless hydrophones.
Combined Acoustic and Radio Transmitters. Combined acoustic and radio transmitters
(CART) are hybrid tags incorporating the best features of both radio and acoustic tags.
Combination tags are particularly useful in tracking the migration of anadromous fish species as
they move from marine to freshwater environments. These tags, manufactured by several
companies (e.g., Lotek Wireless, Sonotronics) are outfitted with a conductivity sensor to detect
the salinity of the water body around the fish and a microprocessor that can automatically switch
between acoustic and radio transmission modes. Both manual and automated tracking of CART
tags is possible. Currently over 100 individuals can be tracked on a single frequency. Weight in
water ranges from 12 to 37.7 grams for those models reviewed. Typical dimensions of CART
tags range from 60 to 105 mm in length and 16 to 18 mm in diameter. Tag life ranges from 1 year
for the smaller tags (13.5 g) to greater than 5 years for the largest (37.7 g) transmitters. Acoustic
frequencies range from 32 to 83 kHz, although most companies will typically manufacture tags
at one or two selected frequencies (e.g., Lotek, 65.5 and 76.8 kHz). Some manufacturers require
separate receivers to detect individual radio and sonic transmissions. Cost per single combined
acoustic radio transmitter can be as high as $450.
Sensors Used in Ultrasonic Transmitters. Acoustic transmitters can be equipped with a
variety of sensors, such as pressure (depth), temperature, velocity, heart rate, and accelerometers.
The data from these sensors are usually encoded in the repetition rate of the transmitted pulses.
Some transmitters can be outfitted with more than one sensor. Velocity sensors are used to
measure animal swimming speeds or water flow rates. Heart rate sensors can be used to measure
electrical signals generated during muscle contractions and therefore can be used to monitor
heart rate if the transmitter is placed near the tagged animal's heart. Accelerometers gather fine-
scale three-axis measurements of acceleration. Applications include measurement of tail beat
frequency and amplitude and mortality.
ANTENNAS, HYDROPHONES AND RECEIVER/DATALOGGERS
Antennas. The selection of receiving antennas is often not given adequate attention. Proper
antenna selection can mean the difference between receiving the necessary data and receiving no
signal at all. Selection of the appropriate antenna is important because only half of the transmitter
signal captured by the antenna is delivered to the receiver while the other half is re-radiated.
Matching the right antenna to its application can result in increased signal reception by as much
as threefold. Antennas can be divided into lightweight hand-held models and larger models that
may be fixed either on shore-based monitoring stations or survey vessels. Types of hand-held
antennas can be circle, oval, or diamond shaped. Hand-held antennas, such as the "Handi-Loop"
(available from AVM Instruments) measure only 6 x 10 cm. They provide excellent short-range
gain and directional accuracy in places where bulky Yagi antennas are impractical.