Long-term Variability in Spinner Dolphin Acoustic Activity Measured at Two Resting Bays off Oahu, Hawaii

Spinner dolphins (Stenella longirostris) frequent the Waianae coast off western Oahu as part of their daily behavioral routine of foraging at night on offshore, mesopelagic prey and resting during the day in shallower, inshore waters. Dolphin resting habitats are in close proximity to human activities—particularly swim-with dolphin tours—which raises concern for the potential disturbance of their crucial resting behavior. Makua Beach and Kahe Point are two well-studied spinner dolphin resting areas on the Waianae coast. Recent deployments of bottom-moored acoustic recorders have sampled the soundscape at both locations intermittently from 2016 to 2019. Acoustic data were also collected from both sites in 2009. These historical data provide the opportunity to quantify the difference between soundscapes and dolphin acoustic activity over time at these locations. Data subsets were taken from March 30^th – May 31^st of 2009 and 2017 for Makua Beach and from March 30^th – May 31^st of 2009 and 2019 for Kahe Point. Daytime recordings (0500 h – 2000 h) from these deployments were manually scanned for dolphin signals, and dolphin acoustic activity was quantified using an index based on the amount and type of calls (whistles, clicks, or burst pulses) in each recording. Acoustic activity was then used as a proxy to examine variability in the spinner dolphins’ use of the two resting habitats over an approximately 8-10-year period. Additionally, the variation in ambient noise was quantified by the mean sound pressure level in 1-octave frequency bands for each deployment. Determining the variation in dolphin acoustic activity and ambient noise between years and across sites can inform management of spinner dolphins’ potential to adapt to changing coastal environments.