Quantifying Uncertainty in Aquatic Telemetry: Using Received Signal Strength to Estimate Telemetry Error

Telemetry error is not regularly considered in aquatic studies; however, when it is considered, it is generally treated as a static value despite high variation that can occur even within a single tracking event. We describe a simple procedure for using received signal strength (RSS) to estimate telemetry error. We recorded the RSSs of groundand airbased detections of large (11 g; 11 3 59 mm) and small (8 g; 11 3 43 mm) 172-MHz radio transmitters across a range of distances. Received signal strength was an excellent predictor of the distance to transmitter for ground tracking detections (large transmitter: r = 0.98; small transmitter: r = 0.97) and a fair predictor for aerial detections (large transmitter: r = 0.49; small transmitter: r = 0.57). We also manipulated transmitter antenna lengths and unexpectedly found that both transmitters performed better with shorter antennas relative to factory lengths. With calibrated models relating RSS to the distance to transmitter, RSSs from field-collected data can be used to approximate telemetry error and draw spatial confidence areas around location estimates for analysis and interpretation. During a concurrent movement study, we estimated telemetry error for 2,436 detections of fish at large. Ground tracking error estimates ranged from 1 to 131 m (median = 24 m), and aerial error estimates were most often less than 300 m but were as high as 1 km. The benefits of representing telemetry data as spatial confidence areas guided by the RSS of each detection are discussed. With appropriate caution, this method will provide a more robust alternative to the assumption that error is constant, negligible, or both. Active transmitters used in radiotelemetry and acoustic telemetry have played an important role in our understanding of fish movement, habitat use, and passage abilities, contributing greatly to both applied and basic fisheries research. They can be detected from long distances, in some cases greater than 1 km (Freund and Hartman 2002; Eiler 2012), making them ideal for longranging organisms and those found in environments not conducive to detection with other methods. Furthermore, high detection rates of active transmitters can reduce the sample size of individuals needed to estimate demographic parameters relative to passive transmitters (McMichael et al. 2010). Although the ability to detect organisms from long distances is advantageous, careful attention to spatial accuracy is required. Telemetry error is inevitable when using active transmitters to estimate locations of fish and includes two components. Animal location error is the difference between the true location of the animal and the estimated location, and mapping error is generated when translating locations to spatial coordinates (Rogers and White 2007). Global Positioning System (GPS) units have greatly reduced *Corresponding author: kurtcheim@gmail.com Received April 18, 2018; accepted August 29, 2018 North American Journal of Fisheries Management © 2018 American Fisheries Society ISSN: 0275-5947 print / 1548-8675 online DOI: 10.1002/nafm.10226