Comparison of visual and acoustic detection rates for cetaceans off Washington.

From 2004–2010, we examined marine mammal occurrence off Washington acoustically and visually. Acoustic monitoring was conducted by Scripps Institution of Oceanography using high‐frequency acoustic recording packages (sample rate 80–200 kHz) in Quinault Canyon, part of the Navy’s Northwest Range. Visual surveys were conducted using small 6 m rigid‐hull inflatable boats covering out to the location of the HARPs during favorable weather year‐round. Visual surveys detected over 500 sighting of 12 species of cetaceans. Humpback whales were the most common baleen whale; harbor and Dall’s porpoise were the most common odontocete detected visually. Species detected acoustically included Pacific white‐sided and Risso’s dolphins, and beaked, killer, sperm, humpback, blue, and fin whales. While there were similarities in the detections by both methods there were also some Stark contrasts. Overall, there were more acoustic detections than visual sightings and these included sperm whales, which were detected frequently acoustically but never visually. For humpback whales, acoustic detections were highest in fall and winter (and low in summer), while visual sightings showed highest numbers in summer and early fall. These differences can be explained by the dive and calling behavior of these species and demonstrate how the strengths and weaknesses of these approaches complement each other.