Detection Distances of Selected Radio and Acoustic Tags in Minnesota Lakes and Rivers

In freshwater, the optimal choice between radiotelemetry or acoustic telemetry has often been unclear because the combined effects of tag power, conductivity, tag depth, and antenna type on radio tag detection distances have not been quantified. To enable more informed decisions regarding the best telemetry methods to use at particular study sites, we measured maximum detection distances of an acoustic tag and two different 48-49-MHz radio tags over a wide range of conductivities, depths, and acoustic conditions in Minnesota lakes and rivers. Radio tag detection distances increased with increasing tag power. decreased with increasing conductivity, and generally decreased with increasing depth. Detection distances measured with a Yagi antenna were typically about double those measured with a loop antenna. Maximum detection distances of the radio tags were predicted by the equation log(e)R = 13.69 - 0.005771.C - 0.006575.D + 0.1044.P + 0.7275.A - 0.001302.C.D - 0.00008208.C.P (where R = maximum detection distance, m; C = surface conductivity, mu S/cm, at ambient temperature; D = tag depth, m; P = tag output power, dB relative to 1 mW; and A = antenna type [1 = Yagi, 0 = loop]). Acoustic detection distances were less variable overall than radio detection distances but still differed substantially between and within water bodies. We saw no consistent relationship between acoustic detection distance and tag depth, but detection distance was negatively affected by ambient noise. Our model indicates that a 48-49-MHz radio tag can be detected with a boat-mounted Yagi antenna at a depth of 50 m or more if conductivity is low enough, but the maximum detection depth decreases drastically as conductivity increases. The equation above can be used to predict whether detection distances of radio tags similar to the ones we tested will be adequate for a particular study.