Average levels and power spectra of ambient sound in the habitat of southern resident orcas

We used a pair of hydrophones to monitor sound pressure levels (SPL, dB re 1 μPa) and frequency spectra of ambient sounds in Haro Strait during 18 months (April 2004 November 2005). Half-hour average SPL in the broad frequency band 0.1-15 kHz ranges from minimum (background) levels of 95 dB to maximum levels of 130 dB. The average SPL over 17 months is 115 dB. The broadband sound field is dominated by noise from large vessels (commercial ships). They increase two-second average SPL ~20-25 dB for 10-30 minute periods about 20 times per day, thereby elevating the 17-month average SPL ~20 dB above background levels. Smaller vessels (motor boats) raise two-second SPL about as much as ships (15-20 dB) but do so for shorter periods and at higher frequencies (10-20 kHz). Their prevalence during summer afternoons explains why the average SPL during summer days (118 dB) is 3 dB higher than during summer nights, winter days, or winter nights.