Acoustic monitoring of harbor porpoises in Hood Canal, WA: Efforts to improve detection of a highly cryptic cetacean

Abstract

Harbor porpoises (Phocoena phocoena) are the smallest, most abundant cetaceans occurring within the waters of the US Navy’s Northwest Training and Testing Ranges. The ability to detect harbor porpoise and monitor their presence over time is essential to assessing potential impacts of Navy activities. This can be difficult due to their highly cryptic nature exhibited in their surface behavior (vessel avoidance) and acoustic repertoire (producing only high-frequency clicks). This presents significant challenges for both visual and acoustic (PAM) survey methods. Here, we discuss the Navy’s efforts to improve harbor porpoise monitoring capability using an automated acoustic detection system. We tested multiple detection algorithms and evaluated their performance in identifying and extracting harbor porpoise clicks from acoustic recordings collected in Hood Canal, WA. Using this detection data combined with data collected from concurrent visual surveys of Hood Canal, we explored how acoustic and visual methods may be used in concert to improve harbor porpoise monitoring. Hood Canal provides a unique opportunity to do this, as it is the only known US Navy range with a resident population of harbor porpoise. These efforts will also inform future behavioral response studies needed to measure the impact of Navy sounds to individual animals.