Comparison of High-frequency Echolocation Clicks (likely Kogia) in Two Simultaneously Collected Passive Acoustic Data Sets Sampled at 200 kHz and 320 kHz

Several species of odontocetes, including those in the genus Kogia, produce high- frequency (> 100 kHz) echolocation clicks. To compare the detection performance of High-frequency Acoustic Recording Packages (HARPs) for these signals, two instruments were deployed simultaneously on the same mooring off the Kona coast of the Island of Hawai’i. One instrument sampled at 200 kHz, while the other sampled at 320 kHz. The instrument sampling at 320 kHz recorded the full frequency content of high-frequency clicks and formed the basis for a comparison of relative click detection rates in the data sampled at 200 kHz, where clicks were aliased and truncated at the Nyquist frequency of 100 kHz. We compared a variety of detection metrics, including the number of high-frequency clicks, the number of encounters including high-frequency clicks, and the number of days with high-frequency click detections within each data set. This comparison revealed that the 200 kHz sampled data set contained 69% of the high-frequency click encounters relative to the 320 kHz sampled data set, representing 89% of days with high-frequency click detections. The vast majority of long-term passive acoustic data sets collected as part of the Pacific Islands Passive Acoustic Network are sampled at 200 kHz and this comparison suggests that measures of high-frequency click occurrence at the scale of days or longer are likely unbiased. Researchers should be cautious using 200 kHz sampled data to infer fine-scale (e.g. hourly) occurrence, as such measures are more likely to be biased by other factors that may influence high-frequency click detection on a similar scale.

Merkens KP, Oleson EM. 2018. Comparison of high-frequency echolocation clicks (likely Kogia) in two simultaneously collected passive acoustic data sets sampled at 200 kHz and 320 kHz. NOAA Tech Memo. NMFS-PIFSC-74, 21 p. doi:10.25923/jjss-er34.