Dude, Where's my Transmitter? Probability of Radio Transmitter Detections and Locational Errors for Tracking River Fish

To simulate radio-location tracking of fish in a lotic system, we deployed 148-MHz radio transmitters and measured signals from three altitudes over a known location. We examined detection distance, location error, and detection probability for two transmitter types at seven transmitter depths. We found that transmitter type, altitude, flight direction, and depth affected both detectability of transmitters and accuracy of locations. Maximum observed detection distance was 5,614 m for the small-type transmitters at an altitude of 300 and 14,508 m for large-type transmitters at an altitude of 600 m. Detection distances declined rapidly with increasing transmitter depth for both transmitter types. Locational errors ranged from 8 to 842 m (x over bar = 177 m; SE = 15.2) and were biased with flight direction. Detection probabilities declined with increasing transmitter depth and with increasing number of scanned frequencies. Scanning five frequencies, at the optimal flight altitude for a given transmitter type, resulted in nearly a 50% loss of detection probability at a 5-m depth and a 90% decrease at a 7-m depth. We recommend that researchers model their probability of detection a priori, all transmitters transmit on a single frequency, and a receiver altitude of 300 m should be maintained.