Acoustic interaction of humpback whales and recreational fishing vessels in a temperate fjord - Measurements in Rivers' Inlet, British Columbia

Underwater environments are acoustically complex and highly variable. Natural processes, biological organisms and human activities all emit sound, at various levels and over broad frequency ranges. To facilitate ocean resources management and mitigate impacts of anthropogenic noise, areas where these sources interact in the space, time and frequency domains need to be identified. This paper presents analyses of field measurements taken on the west coast of Canada in August/September 2008, along the temperate fjord of Rivers’ Inlet (British Columbia). Humpback whales (Meganoptera novaeangliae) use this fjord as a feeding ground, sharing it with recreational fishing craft mostly consisting of small boats (20 m or less). Measurements were taken over several days at regularly-spaced locations in deep water from the mouth of the fjord inward, using a broadband (0.1 – 30 kHz effective bandwidth) hydrophone deployed 10 m deep. Analysis focused on the joint use of acoustic bandwidths by humpback whales and recreational fishing vessels. Recorded spectra of humpback whale vocalizations and vessel noise overlapped considerably in the range 0.1 – 1 kHz. At the distances recorded (> 1 km) vessel noise produced a mean increase of 4 dB above background levels in this range. Ambient noise levels are also discussed briefly with reference to previous studies.