Tidal Variability of Acoustic Detection

Acoustic telemetry is an essential tool for ecological and behavioral analysis of fish and marine mammals in fresh and sea water. However, the accuracy of acoustic telemetry is a challenging problem in that acoustic detection is affected by various environmental factors that govern sound speed and propagation: density, density stratification, mixing, and the environmental processes that control them. In this paper, we focus on tidal variability in acoustic range and detection using data from a summer 2014 experiment at Gray's Reef National Marine Sanctuary in which an array of stationary receivers was deployed with co-located tags for 53 days. We assess the potential effect of signal collision, which occurs when multiple tag transmissions interfere and cannot be detected by the receiver, and find that signal collision may occur more frequently than predicted. Number of hourly detections at each receiver is found to vary significantly in space and in time, and tag/receiver pairs oriented alongshore have more detections than cross-shore pairs. The principal lunar component (M2), the tidal constituent that contributes the majority of current variance, is found to explain variability in detections. Maximum detections occur when currents are aligned with the semi-major axis (oriented cross-shore), while no strong directional correlation is found when detections are least likely. This result may give us clues about how important tidal variability can be in determining acoustic range relative to other processes that govern acoustic propagation. Better understanding of detection range and its variability in time can improve array design and data interpretation.

I. Introduction

Over several decades, acoustic telemetry has been used to monitor fish and marine mammals in freshwater and oceanic environments [1]. Acoustic telemetry uses acoustic receivers to monitor for the presence of fish tagged with transmitters attached to inside or outside skin of fish (depending on size), and records both time stamps of detection and identification numbers of transmitters. Ecological and behavioral analysis of telemetric data has revealed important new information about life cycles of marine species [2], [3]. However, the accuracy of telemetry is dependent on the detection range, or the maximum distance over which receivers are able to detect transmitters/tags. If detection range is unknown, telemetry does not inform scientists about where the detected animal fish is located. Moreover, detection range with spatial and temporal variability is affected by a great number of environmental factors such as density, density stratification, and mixing, which are mediated by wind, buoyancy input, and other effects [1], [4].