Less trade, quieter seas

Less trade, quieter seas. At frequencies between 10 and 100 Hz, the predominant sources of undersea sound are ships, whales, and earthquakes. To discover how much the total level depends on the number of ships, Megan McKenna of the Scripps Institution of Oceanography in San Diego, California, and her colleagues exploited two impromptu modulations in maritime traffic. The first was the Great Recession, which beset the US from December 2007 through June 2009. During that period, shipments into and out of the Port of Long Beach, the US’s second busiest, fell by 15%. The second modulation was regulatory. In July 2009 California mandated that ships passing within 24 nautical miles of its coast use low-sulfur fuel. Instead, ships began avoiding the coastal route through the Santa Barbara Channel. Meanwhile, the Scripps Whale Acoustic Lab has been using hydrophones like the one shown here to monitor the channel since 2006. From that stream of data, McKenna and her colleagues harvested sound levels at two frequencies, 40 Hz and 90 Hz, that are close to the peak of the mean ship spectrum, yet outside the whale communication spectrum. Correlating those levels with two measures of maritime trade—the number of ships plying the Santa Barbara Channel and the number of containers arriving at and departing Long Beach—the team found that one fewer ship transit per day reduced noise on average by 1 dB, a significant drop given the volume of traffic. The resultant effect on whales is unclear. (M. F. McKenna et al., J. Acoust. Soc. Am. 132, EL169, 2012.)