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as well as functioning directionally. The only practical approach to this problem with existing technology is to employ two separate antennas. For fish work, the omnidirectional straight (or whip) antenna can be used to receive signals from all directions. For directional tracking, either a loop or Yagi antenna may be used. The loop antenna resembles a hoop on a stick. The Yagi type consists of several short cross pieces mounted perpendicular on a vertical pole (Figure 2). Figure 2. Radio signals from an implanted fish to receiving systems illus- trating different types of antennas. A 0.25 wavelength simple whip antenna is about 1.5 m long (at 50 MHz). The whip antenna usually rests on some object on the ground plane. Aground reference plane placed perpendicular to the whip antenna aids in efficiency (Tyus 1982) especially if the antenna is mounted on a high stand. The whip antenna is more sensitive than a loop of comparable size, but less sensitive than a Yagi. Its advantages are simplicity of design, low cost, and ease of mounting. This antenna has the least air resistance and, for this reason, is preferred for aircraft use. The convenience of a whip antenna makes it more desirable than a Yagi for fish work, and the loss in efficiency is slight. Unlike terrestrial applications, directional antennas are not necessary for aircraft tracking in rivers for two reasons: (1) rivers provide their own boundaries, and (2) all microhabitat studies require follow-up by boat when habitat data are recorded from more precise fish locations. Once a fish has been located (Figure 2) by an omnidirectional whip antenna, loop and Yagi antennas are used to locate the signal source by rotating the antenna until the null (or minimum) reading for signal strength is detected. For the Yagi antenna, the gain (efficiency) increases with an increasing number of elements in the antenna, although the rate of increase becomes less with each element added. Each element of the 0.5 wavelength 141