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...