Underwater sound measurements of the Hawaii super ferry and the potential impact to humpback whales in the main Hawaiian Islands.

Abstract

Underwater measurements of the Hawaii Super Ferry were made with a vertical array of four hydrophones. The depths of the hydrophones were 3, 6.5, 10, and 20 m. The Alakai is a 107 m catamaran passenger and automobile ferry that has a waterjet propulsion system with no exposed propellers. The Alakai made three close proximity passes at speeds of 37, 24, and 12 kn on tracks that allowed for acoustic measurements of sounds radiating from the bow, broadside, and stern aspects. The track of the Alakai was recorded on global positioning system. The measurements were conducted while the ferry proceeded on normal scheduled trips, allowing for three measurements a day. All the sound pressure levels were referenced to 91 m (100 yards), the minimum legal approach distance of boats to humpback whales. Sound intensity increased with speed and was the highest at 37 kn in the broadside aspect (160 dB) and was 14 dB lower for both the bow and stern aspects. The sounds had a broadband noise quality with spectra that had a low frequency peak below 100 Hz and decreased continuously with frequency. The effects of the sounds will be discussed from a humpback whale perspective.