Determining a historic baseline of anthropogenic noise in spinner dolphin resting bays along the Kona Coast of Hawaii

Abstract

On March 11th, 2011, the 9.03 magnitude Tohoku earthquake struck off the coast of Japan and caused a tsunami event that crossed the Pacific. Using this event opportunistically to assess how it altered anthropogenic ambient noise in the waters of four bays of Hawaii (Kauhako, Honaunau, Kealakekua and Makako Bays), valuation of what the currently unmeasured baseline noise levels were prior to human existence or influences was attempted. Though a baseline has not yet been found, the results presented will assist in furthering our understanding of oceanic soundscapes and can aide in future soundscape models and research. Once found, the historic baseline will provide information that will help in referencing acceptable levels of anthropogenic noises in future policy-making decisions in Hawaii and elsewhere. Data was recorded on bottom-mounted archival hydrophones for years 2011-2013 in all four bays at 4-minute intervals with 30-second durations. A two-week window (March 4-18th) was completely analyzed for the years 2011 and 2012 (2012 being the control) in efforts to find an associated drop in noise level around the tsunami event. Values for typical bay sounds were found by averaging a one year span of data (January 8, 2011-January7, 2012) for 4 different categories: whales present, dolphins present, both present, and neither present (presence determined through acoustic data interpretation) in all 4 bays. Had a significant drop in noise level been found within the 2011 two-week time period, a comparison between it and the typical noise level under that biological category and bay could have illustrated how humans are currently influencing the oceanic soundscape in the area of study.