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Figure 1. Radio signals from a transmitter, passing through the air water interface. Only a small portion of this radiated power actually crosses the air water interface and is available to the receiving system. However, if any "signif- icant" energy breaks through the interface, radio reception can occur at long ranges because of the rapid propagation of radio waves in air (Stasko and Pincock 1977). 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 decreases as transmitter life increases. Although range can be increased by using a larger battery, the weight and size increase may not be acceptable. The theoretical longevity of a battery may be obtained by dividing the current demand (drain) of the transmitter (milliamps) into the battery rated capacity (milliamp days). Transmission lives and sizes of some batteries are provided in Tables 1 and 2. In review, radiotelemetry is a better tool for monitoring the movement of fishes in certain aquatic environments, especially those in which use of ultrasonics may be infeasible. Ultrasonics has great value in quiet water and may be better for deep lakes, but radiotelemetry seems to be better for mon- itoring fish movement in rapidly flowing rivers. As stated by Priede (1980:110): "Radio signals.are not affected by air bubbles, water turbidity, or obstructions... the way that sonic signals are." Also, ultrasonic signals are not suitable for aerial. reception (e.g., by airplane) because the hydro- phone must be immersed in water to receive the ultrasonic signals. Although transmission through the air-water interface greatly reduces the apparent signal strength of radio waves, reception in air is possible and the signal strength is adequate for most applications. Radiotelemetry in high conductivity waters is marginal (Sinning 1979), and efforts must be taken to maximize the strength of the radio signal in the field. Increases in received field signal strength from a transmitter to a receiving radio is dependent on the efficiency of transmitting and receiving antennas. Available transmitter designs represent the state-of-the-art techno- logy, and lass of efficiency between transmitting and receiving antennas is largely dependent on fish behavior and water conductivity. RADIO TRANSMITTING ANTENNAS On]y a few types of transmitting antennas are suitable for monitoring fish movements. 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 is dependent on the wavelength, and reducing the wavelength by raising the frequency will allow greater efficiency for a given antenna size. Unfor- tunately, a frequency increase lessens the wave propagation through water;