Said in Context: A Complicated Reef Soundscape as a Medium for Acoustic Telemetry

Acoustic telemetry is the passing of information across open ocean using high-frequency transmissions, but the environment between transmitter and receiver can dramatically change signal detection rates.Shallow coastal reef environments vary on predictable time scales, with nocturnal and seasonal snapping shrimp behavior driving background noise. A combination of gliders, moored transceivers, hydrophones, and a surface buoy were used during this 2020 experiment to define the physical and acoustic environment as a medium for telemetry signals to provide context for collected detections. A large portion of the variability in detection efficiency was found to be controlled by background noise levels: the snapping shrimp activity at night and in warmer waters resulted in less successful transmission detections, while efficiency improved during the daytime and during colder months. Wind appeared to have a strong enabling effect, lowering high-frequency noise and increasing detection efficiency, suggesting either a change in noise created (biotic, shrimp behavior) or a change in noise lost (abiotic, attenuation). Snapping shrimp activity was quantified using passive recordings of the benthic environment. Surface bubble loss was calculated to estimate how much noise was lost at the air-sea interface as winds increased, and this loss was often a significant source of attenuation. Snapping shrimp were unaffected by increased winds, but the noise they created was attenuated at the ocean surface. Previous attempts at modeling acoustic propagation were simplistic, prioritizing speed, and did not account for the aforementioned attenuation or background noise. By including estimates of realistic noise and signals strength, the transmission models were improved and are more accurate representations of the reef environment. The wind-driven attenuation of background noise was the largest predictor of detectability. Effects from strong wind lowered detection efficiency when background noise was low and static, but significantly increased detectability in environments where wind lowered the detection threshold. This work can aid acoustic telemetry users working near coastal reefs. Reefs are biologically important habitats, but noise-creating species like snapping shrimp can hinder research efforts. Accounting for variable levels of background noise and signal strength helps temper expectations and add context to collected detection data.