Performance of acoustic telemetry receivers in shallow estuarine systems

Acoustic telemetry is becoming a popular technique to investigate movements, habitat use, and behavior of coastal fishes. Yet, few studies have considered the effect of spatio-temporal variance in receiver performance in the interpretation of their results. We conducted long-term (2 year duration) range tests in a shallow (< 2.5 m) Louisiana estuary as part of a telemetry study on spotted seatrout. Receivers (Vemco,VR2Ws) were positioned at various distances (100-500m) from stationary fixed-delay acoustic transmitters; physicochemical and meteorological variables were continuously monitored. Elevated wind speed and turbidity associated with the passage of weather systems (e.g., cold fronts and tropical storms) caused stochastic, rapid deterioration in receiver performance. Environmental conditions also exerted more temporally stable (and predictable) controls on receiver performance. A Fast-Fourier-Transform analysis indicated a strong diel periodicity in hourly detection efficiencies, which were significantly higher at slack tide than at mid-ebb or mid-flood. These results demonstrate that false negatives can be prominent in telemetry studies, whereby a lack of detections is due to reduced detection range instead of actual fish behavior (i.e., emigration from the study site). We discuss the implications of dynamic detection ranges for designing receiver arrays and analyzing telemetry data, particularly habitat use periodicities and migration rates.