Measuring and understanding receiver efficiency in your acoustic telemetry array

Methods to evaluate receiver performance within acoustic telemetry arrays are needed to quantitatively determine which receivers are the most important to maintain. A recently developed approach, the Receiver Efficiency Index (REI), expresses the proportion of transmitter activity from throughout an array that occurs at each receiver location within it. The components of this composite index equally weight the proportion of detections, individual tags, and species from the entire array that were detected at a given receiver and then adjusts for the proportion of time that each receiver was deployed. In this study, we evaluated receivers in a long-term (8+ years) telemetry array deployed in Gray's Reef National Marine Sanctuary (GRNMS) located off the coast of Georgia (southeastern USA). Specifically, we explored the causes of fluctuations in the index over time, evaluated correlations between the REI and its component measures, and determined the sensitivity of the composite to individual components. Additionally, we examined the fish assemblages detected at each receiver over time using non-metric multidimensional scaling (nMDS) to assess how REI scores vary across fish communities. Results indicate that receiver importance varied through time. Correlations between the REI and each of its components were all positive and significant, and the REI was robust to exclusion of any one component. Sites with similar fish assemblages had very different REI values in GRNMS, but not when we reexamined the data from an array in Florida that was used to develop the REI. These findings suggest that decisions regarding which species groups (e.g. resident versus transient), time periods (e.g. seasonal versus inter-annual), or even components of the REI (e.g. duration of deployment) to include during analysis can have strong effects on the interpretation of receiver importance. We also recommend that the REI be coupled with species assemblage analyses such as nMDS to better understand how fish assemblages differ at sites with similar REI scores.