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reception of the radio signal in the field. Unfortunately, workers are con- strained by equipment to only a few options. Any increase in received field signal strength from transmitter to a receiving radio is dependent on power output, efficiency of transmitting and receiving antennas, and the sensitivity of the receiving system. The other factors affecting reception are principally environmental and will be discussed in the case study. The type and size of battery used with radiotransmitters is an important consideration from the standpoint of size, weight, range, and life. For a given application, battery weight and size is directly proportional to range (power output) and the transmission life. Fish transmitters can be designed to operate for hours to years with a given battery, but the range (signal strength) decreases as transmitter life increases. Although range can be increased by using a larger battery, the weight and size increase may not be acceptable. If the theoretical longevity of a battery is obtained by dividing the current demand (drain) of the transmitter (milliamps) into the battery rated capacity (milliamp days), the resultant rating (life) of a radio module (transmitter, antenna, and battery) provides a useful guide for module selection. Battery life is also dependent on pulse duration and pulse rate of a transmitter, since these represent power output. The threshold sensitivity of the human ear indicates that a reduction of a pulsed tone not be reduced less than about 30 ms (Kolz and Johnson 1981), and tracking .is more difficult at pulse rates less than about 30 per minute. Most investigators use a chrono- graph (stopwatch) for determining pulse rates of transmitters; this method is simple and the gear dependable for field use. Although sophisticated "pulse interval timers" are available from industry, their high technology and potential oversophistication may offer no advantage at higher cost. Only a few types of transmitting antennas are suitable for monitoring fish. The two main types are straight whip and tuned loop. Whip antennas are generally small and omnidirectional. Although a whip antenna can be compressed in length by shortening it from 0.5 to 0.25 wavelength, or by coiling part of it, a loss in efficiency occurs. A decrease of the diameter of an antenna also reduces efficiency. The required length of the whip a;~tenna is dependent on the wavelength, and reducing the wavelength by raising the frequency will allow greater efficiency for a given antenna size. Unfortunately, a frequency increase lessens radio wave propagation through water; thus, the gain in efficiency of antenna operation is less at higher frequencies. Use of a loop (coiled) transmitting antenna or incorporation of the implant capsule as part of a dipole antenna have proven desirable for surgically implanted modules because of the necessity for compactness and the need to avoid protruding antenr,s. I~ may be possible to increase the radiation resistance (antenna efficiency) of some types of coiled antennas by increasing the length of the coil. However, for a "tuned" inductor type or other sophisticated antenna designs, the antenna may be part of a circuit, resonating at a certain mode. The tuned induc'~r types of antennas, therefore, may not .be made more efficient simply by increa::ing their lengths. Transmitting antennas must radiate radio signals in all directions. Receiving antennas should have a capability for receiving from all directions 140