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

Michael H P O’Brien,David H Secor

doi:10.1186/s40317-021-00233-3

Michael H P O’Brien, David H Secor

Open Access

https://doi.org/10.1186/s40317-021-00233-3

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Abstract

BackgroundThe 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.ResultsThe 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.ConclusionThermal 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.

Highlights

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
Like multilaminar flow [14], internal waves [11], local fronts [10], and stratification [15] variably reflect, attenuate, or increase both signal and noise propagation through the water column according to their strength and origin. To evaluate both seasonal- and event-scale changes in detection range within a key migration corridor for elasmobranchs, fishes, and turtles [16, 17], we evaluated telemetry detection ranges over a 12-month period in a dynamic shallow shelf system: the Southern Mid-Atlantic Bight (SMAB)
Mid‐Atlantic cold pool The study area remained well-mixed (ΔT ≈ 0) from December 19, 2017 until mid-April 2018; thereafter vernal heating and a reduction in high-wind events resulted in a gradual rise in thermal stratification and subsequent Cold Pool formation in nearshore and mid-shelf waters (Fig. 2)

Summary

Introduction

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. 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. As the prevalence of biotelemetry in long-term monitoring of fish migration and habitat use increases, so too should the evaluation of temporally varying detection probability. At a given distance and transmission strength, detectability is a function of signal loss due to spreading and sound attenuation in water. Summer temperature effects dominate over salinity impacts in marine shelf environments due to the comparatively larger temperature range

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