Connection Between Mammals Incidents and Geophysical Survey in Nantucket Sound, USA
On December 19, 2012, NMFS received an application from CWA for the taking of marine mammals incidental to high resolution survey activities. NMFS determined that the application was adequate and complete on December 31, 2012.
CWA plans to conduct a high resolution geophysical survey in Nantucket Sound, Massachusetts. The survey would occur during daylight hours over an estimated 109-day period beginning in April 2013. The following equipment used during the survey is likely to result in the take of marine mammals: shallow-penetration subbottom profiler and medium-penetration subbottom profiler. Take, by Level B harassment only, of individuals of five species is anticipated to result from the specified activity. This is basically an extension of the authorization issued on January 1, 2012 for survey activities that were not completed under the previous IHA. CWA’s survey activities will not change from what they originally proposed in their 2011 IHA application. However, the geotechnical portion of the survey was completed in 2012 and will not be continued during the 2013-2014 season.
Acoustic stimuli (i.e., increased underwater sound) generated during operation of the shallow-penetration and medium-penetration subbottom profilers may have the potential to cause short-term behavioral disturbance for marine mammals in the survey area. This is the principal means of marine mammal taking associated with these activities. NMFS does not expect take to result from collision with survey vessels because they will be moving at relatively slow speeds (3 knots) during seismic acquisition and there is not a high density of marine mammals within Nantucket Sound. It is likely that any marine mammal in the vicinity would be able to avoid the vessel.
Description of the Specified Activity
CWA plans to conduct a high resolution geophysical survey in order to acquire remote-sensing data around Horseshoe Shoal which would be used to characterize resources at or below the seafloor. The purpose of the survey is to identify any submerged cultural resources that may be present and to generate additional data describing the geological environment within the survey area. The survey will satisfy the mitigation and monitoring requirements for “cultural resources and geology” in the environmental stipulations of the Bureau of Ocean Energy Management, Regulation, and Enforcement’s lease. The survey is part of the first phase of a larger Cape Wind energy project, which involves the installation of 130 wind turbine generators on Horseshoe Shoal over a 2-year period. The survey will collect data along predetermined track lines using a towed array of instrumentation, which will include a side scan sonar, magnetometer, shallow-penetration subbottom profiler, multibeam depth sounder, and medium-penetration subbottom profiler. Survey activities will not result in any disturbance to the sea floor.
Dates and Duration
Survey activities are necessary prior to construction of the wind turbine array and are scheduled to begin in the spring of 2013, continuing on a daily basis for up to 5 months. Survey vessels will operate during daytime hours only and CWA estimates that one survey vessel will cover about 17 Nautical miles (31 kilometers) of track line per day. Therefore, CWA conservatively estimates that survey activities will take 109 days (28 days less than what was expected under the 2012 IHA). However, if more than one survey vessel is used, the survey duration will be considerably shorter. NMFS is issuing an authorization that extends from April 1, 2013, to March 31, 2014.
Survey vessels are expected to depart from Falmouth Harbor, Massachusetts, or another nearby harbor on Cape Cod. In total, the survey will cover approximately 110 square kilometers (km2). This area includes the future location of the wind turbine generators–an area about 8.4 km from Point Gammon, 17.7 km from Nantucket Island, and 8.9 km from Martha’s Vineyard–and cables connecting the wind park to the mainland. The survey area within the wind park will be transited by survey vessels towing specialized equipment along primary track lines and perpendicular tie lines. Preliminary survey designs include primary track lines with northwest-southeast orientations and assume 30-meter (m) line spacing. Preliminary survey designs also call for tie lines to likely run in a west-east orientation covering targeted areas of the construction footprint where wind turbine generators would be located. The survey area along the interconnecting submarine cable route includes a construction and anchoring corridor, as part of the wind farm’s area of potential effect. The total track line distance covered during the survey is estimated to be about 3,432 km (as opposed to the 4,292 km included in the 2012 IHA).
Multiple survey vessels may operate within the survey area and will travel at about 3 knots during data acquisition and approximately 15 knots during transit between the survey area and port. If multiple vessels are used at the same time, they will be far enough apart that sounds from the chirp and boomer will not overlap. The survey vessels will acquire data continuously throughout the survey area during the day and terminate survey activities before dark, prior to returning to port. NMFS believes that the likelihood of a survey vessel striking a marine mammal is low considering the low marine mammal densities within Nantucket Sound, the relatively short distance from port to the survey site, the limited number of vessels, and the small vessel size. Vessel sounds during survey activities will result from propeller cavitation, propeller singing, propulsion, flow noise from water dragging across the hull, and bubbles breaking in the wake. The dominant sound source from vessels will be from propeller cavitation; however, sounds resulting from survey vessel activity are considered to be no louder than the existing ambient sound levels and sound generated from regular shipping and boating activity in Nantucket Sound (MMS, 2009).
Press Release , April 03, 2013; Image: nantucketmarinemammals