A long-term acoustic survey of bat activity at remote islands, offshore structures, and coastal sites in the Gulf of Maine, Great Lakes, and mid-Atlantic Coast has helped reveal where and when bats fly offshore.

The multiyear study, funded by the Energy Department and conducted by environmental consulting and engineering firm Stantec, was the most extensive study on bat activity offshore conducted to date.

“This research helps us better understand patterns of bat activity offshore,” said Jocelyn Brown-Saracino, environmental research manager, DOE Wind Energy Technologies Office. “This study helps provide more certainty to offshore wind developers regarding  relative levels of bat activity likely to be experienced at a site depending on the region, distance to shore, and time of the year.”

The research effort used specialized monitoring equipment to detect echolocation calls of bats and document the presence of individual bat species and species groups at 36 coastline and offshore sites distributed across the three regions—some more than 20 miles offshore. Researchers deployed bat detectors on a variety of remote lighthouses, offshore towers, and weather buoys, as well as three research vessels from the National Oceanic and Atmospheric Administration.

Although bat activity was recorded at every site monitored, most sites had a relatively small number of bats, and their activity was correlated to several factors such as proximity to land and forested areas and seasonality. Average nightly wind speeds and temperatures varied among regions, with overall warmer temperatures and lower wind speeds in the mid-Atlantic and a wider range in temperature and wind speeds in the Great Lakes versus the other two regions.

The study identified several relationships that help predict when and where bats are most likely to be found offshore. For instance, offshore bat activity:

  • Is highest near heavily forested coastal areas or islands and lowest in areas with fewer trees
  • Lessens as distance from the mainland increases, although not as much in the Gulf of Maine where a large number of islands might mediate the effects of distance from the mainland
  • Correlates tightly with seasons of the year
  • Increases during periods of warmer temperatures and lower wind speeds
  • Peaks from July 15 through October 15, when most bat activity also occurs on land.

Stantec’s detection equipment recorded every bat call with a date-and-time stamp to enable connection with weather conditions at the time of the recordings. In total, the researchers detected 565,158 bat passes (defined as a series of two or more echolocation pulses) over the course of 17,730 detection nights. While all species of bats were documented offshore, Stantec researchers found that migrating species—eastern red, silver-haired, and hoary bats—more frequently occurred at greater distances from shore. Several species that typically hibernate, such as the big brown bat and those of the Myotis genus, were the most numerous bats found at sites on or near the coast.

“This information is critical in terms of predicting risk for bats at offshore wind farms,” said Steve Pelletier, Stantec’s principal investigator for the project. “If we better understand those periods when bats are likely to be present, and under what conditions relative to wind speeds and temperature, we can more effectively develop operational constraints that both reduce the risk to bats and promote efficient energy production. This is the biggest outcome of our work.”

For more study results, read Stantec’s final report to the Energy Department.