Going off the deep end: pitfalls of acoustic telemetry beyond scuba depths

Mr Stephen Scherrer¹, Brendan Rideout², Giacomo Giorli³, Eva-Marie Nosal², Kevin Weng⁴

¹Department of Oceanography, University Of Hawaii Manoa, Honolulu, United States, ²Department of Ocean and Resources Engineering, University of Hawaii at Manoa, Honolulu, HI, USA, Honolulu, USA, ³National Institute of Water and Atmospheric Research Ltd., Wellington, New Zealand, ⁴Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, USA

Abstract:

As oceans warm, some species are moving to deeper waters and the use of acoustic telemetry is also expanding beyond surface waters accessible by divers. Using coded acoustic transmitter tags and passive receivers to track mobile aquatic organisms is an established method for collecting long-term movement records. With few exceptions, application of this method has occurred in relatively shallow environments. We deployed these systems at depths between 75-500 m to track demersal snapper and, in the process, revealed a number of challenges associated with application in deeper settings.

We found that performance characteristics of these systems differ between shallow and deep environments. In shallow settings, the probability of detecting a transmission decreases with increased distance between the tag and receiver. However, in deeper water (~200 m) receivers also fail to reliably detect transmissions from tags at short distances. The relative arrival times of the direct acoustic signal and multipath signals can produce close proximity detection interference (CPDI), or colloquially, the doughnut effect, allowing individuals to go undetected. Failure to account for CPDI can lead to problematic biases when estimating residency, using state space models, or when multiple receivers form an overlapping array. We have developed tools for predicting the extent to which CPDI occurs in different environments.

Additionally, there are a number of biological considerations when tagging deep-water teleosts. Barotrauma and predation in particular can have appreciable effects on post-tagging survivorship. We will discuss our experience with different release protocols and recommendations for maximizing survivorship in future deep-water tagging studies.

Biography:

Stephen is a student interested in tools to understand ecology responses to management.

His research is focused on a complex of deepwater fishes and their fishery around the Hawaiian Archipelago.

In the process he developed a number of software tools to design sensor arrays and interpret their performance.

Today he will be presenting on the perils of using acoustic telemetry below scuba depths.