Characterizing the soundscape in deep water off Hawaii

The deep-water environment off the coast of Kona, Hawaii is not well explored, but likely heavily impacted by anthropogenic activities such as fishing, fish farming and other vessel activity. To characterize the soundscape we used High-frequency Acoustic Recording Packages (HARPs) to record acoustic data year-round at a 200 kHz sampling rate in deep-water (~500-700 m). Data were collected for 10 years (2007-2017) and were analyzed using a suite of detectors and classifiers, requiring varying amounts of human input, providing a time series for sounds coming from biological, anthropogenic and physical sources. Based on this data set we determined that the soundscape in this location is dominated by signals generated by humans and by odontocete cetaceans, generally alternating on a diel cycle. During daylight hours the dominant sound sources are vessels and echosounders, with strong signals ranging from very low frequencies (20 Hz) to higher frequencies (80 kHz and above), while during nighttime the clicks and whistles from odontocetes dominate the soundscape between 5 and 90 kHz. There are periodic intrusions of other signals, such as winter-resident humpback whales that produce calls in lower frequencies (200 Hz-2 kHz), occasionally present sperm whales that generate loud echolocation clicks in the 2-20 kHz range, and regular detections of Kogia spp. echolocation clicks, which are relatively low amplitude, but dominate the otherwise quiet frequencies between 90 and 130 kHz. There are relatively few signals that vary on a seasonal basis, which is not surprising given the tropical latitude, and deep-water environment. The long time series spanning over a decade allows us to characterize the dynamic nature of this location, and to begin to identify changes in the soundscape over time, which may help with longer-term protection of natural resources and effective management of human activities in this ecologically important area.