Summary of Some Research Developments by Dr. John Ehrenberg in Fisheries Sonar and Acoustic Telemetry over the Last 3 Decades

Abstract

Over the last 30 years, we have been fortunate to work closely with Dr. John Ehrenberg in the fisheries sonar and acoustic telemetry research fields. With John's passing in the fall of 2018, we felt it was important to honor John's research with this publication. We were involved in multiple research topics with John including: 1) fisheries sonar and telemetry system detectability; 2) characterization of acoustic tag signal types and the estimated detection ranges of these signal types; 3) techniques for studying the behavior of fish in a fixed location; 4) theoretical estimation of position accuracy of acoustic fish tags; and 5) the development of an acoustic tag for sensing and detecting tagged fish predation. This presentation includes summaries of these research topics. For every application of fisheries sonar sampling technique there are several elements that when combined, affect the probability of detecting fish. They broadly include the following: 1) sampling environment; 2) hydroacoustic echo sounder sampling parameters; and 3) the behavior and physiology of fish being monitored. Examples will be presented demonstrating the effect of each of these parameters on the ability to detect fish using fisheries sonar. Recent advancements in the implementation, deployment and analysis of acoustic tag systems include techniques for optimally locating the receiving hydrophones to minimize location errors, the development of acoustic signal waveforms that provide unique target identification, accurate location estimates and optimize detection ranges, as well as the development of tracking algorithms that associate and track the multiple returns from an individual fish. These various techniques will be described. Acoustic telemetry systems are often used to study the movement of fish in a region of interest. A method has been developed for predicting the accuracy of the position estimates provided by acoustic tag systems. This approach provides a method for the direct calculation of the position error as a function of hydrophone geometry, standard deviation of the signal arrival times, and inaccuracies in the assumed sound velocities. This method is independent of the algorithm used to determine the position solution. Multiple acoustic-tag signal-encoding schemes have been implemented for tag systems. The relationship between the various characteristics of acoustic signals transmitted by the tags and the tag-system performance that can be achieved will be presented. Implemented tag signal types impact the ranges at which tags can be detected and uniquely identified, the positional accuracy, as well as the number of unique codes that can be identified. Pulse-repetition period tag encoding schemes have been demonstrated to provide superior tag detection range performance relative to schemes employing binary-encoded bits as part of the transmitter signal. The parametric results presented will assist investigators in their selection of the type of acoustic tags or tag parameters needed to achieve the objectives of individual fisheries acoustic telemetry studies. Acoustic tags are also often employed to estimate fish survival along a migration route. These studies assume that the acoustically tagged fish is alive as the fish passes a detection site. However, if tagged fish are preyed upon by other fish, the tag continues to operate, thus providing incorrect data for estimating survival. An acoustic tag has been developed to detect predation events and change its signal so that the collected data indicates the occurrence of predation. Examples of predation events signal will be presented.