Influence of thermal stratification and storms on acoustic telemetry detection efficiency: a year-long test in the US Southern Mid-Atlantic Bight

Abstract

Background: The detection efficiency of ultrasonic transmitters is seasonally variable, requiring long-term studies to evaluate key environmental features that mask, alter speed, bend, or reflect transmissions. The US Southern Mid-Atlantic Bight shelf is characterized by a strong summer thermocline capping remnant winter water, known as the Cold Pool, and a well-mixed water column in other seasons. To investigate the effects of interactions between temperature stratification and storm-induced noise on transmission detectability, we conducted a year-long range test of 69-kHz acoustic transmitters in the bottom waters of the US Southern Mid-Atlantic Bight. We used generalized additive models and cross-validation to develop and evaluate a predictive model of detection efficiency and visualize variability in detection distance throughout the year of deployment. Results: The most-predictive model contained the effects of temperature stratification and ambient noise, predicting that stratification results in a 33% increase in detectability and 56% increase in detection distance. The model had an overall error rate of 17.1% and an 18.7% error at a distance of 800 m, predicting 17% detectability at median ambient noise when the water column was not stratified and > 50% when the difference between surface and bottom temperatures was greater than 4.2 °C. The distance at 50% detectability increased with the formation of the Cold Pool during spring, increasing by nearly 300 m over 3 days. All seasons were associated with storm-induced reductions in overall detectability and distance at 50% detectability. Conclusion: Thermal stratification within the Southern Mid-Atlantic Bight increases bottom water ultrasonic transmitter detection distance and reduces the impact of surface noise. This effect leads to a seasonal increase in detection distance from the late-spring through the summer. To our knowledge, this study is the first to report and quantify an increase in detection range as a result of temperature stratification, likely due to placing transmitters and receivers on the same side of a strong thermocline.