As the focus of renewable energy in the United States turns to offshore wind facility development, there is an increasing need for an understanding of potential noise impacts from this development on marine mammals. Pile-driving of offshore wind turbines produces loud, low frequency sound that can travel great distances and could potentially harm or disturb marine mammals. As a result, a critical first step is to understand the current baseline ambient noise levels and the spatiotemporal distribution of marine species that could potentially be impacted. In this study, the project partners conducted passive acoustic monitoring for three years to characterize underwater ambient noise levels and identify vocalizing marine mammal species within and around the Maryland Wind Energy Area (WEA).
The project partners collected three years of baseline data 12 – 60 km offshore of Maryland prior to construction and operation of an offshore wind energy facility. Two main types of sound recording devices that encompassed a range of frequencies were used to detect vocalizations from baleen whales (low frequencies) and odontocetes (high frequencies): the Marine Autonomous Recording Unit (MARU, or pop-up) sampling at 2 kHz and the C-POD (cetacean click detector), which monitors the 20 – 160 kHz frequency range. These were supplemented by additional acoustic recorders during select periods of the study at five sites to provide further information on mid-frequency sounds, such as dolphin whistling behavior. The use of a grid array design for the acoustic detection devices within the Maryland WEA facilitated localization of vocalizing whales to further understand spatial patterns of habitat usage.
Key findings of this study were:
- Baleen whales (North Atlantic right whale, fin, humpback and minke whale) were mainly detected during the November to May timeframe.
- Fin whales were the most frequently detected baleen whale species, but detections were most common offshore of the WEA.
- Humpback whales mainly occurred within and offshore of the WEA. Minke whales were only occasionally detected within our passive acoustic array.
- North Atlantic right whales were detected during every month of the year, but were most common during the November to April timeframe.
- Localized North Atlantic right whale calls indicated they migrated through and offshore of the Maryland WEA.
- Bottlenose dolphins were frequently detected year-round within and inshore of the WEA, except in February, whereas offshore sightings were limited to summer and fall. Common dolphins were detected offshore of the WEA from December to May. Within the Maryland WEA, a minimum of 700 individual bottlenose dolphins occurred within the detection range of our acoustic recorders during Summer 2016 to Summer 2017 based on analysis of their signature whistle calls.
- Harbor porpoises were detected from November to June with the peak between January and May. During the first year of the study harbor porpoises were most common within and offshore of the WEA, whereas in the second and third years they were detected more commonly within and inshore of the WEA.
- Sites along the eastern edge and offshore of the WEA had the loudest ambient noise levels, particularly within low frequency bands, suggesting shipping noise is a major contributor to the noise environment.
- Elevated ambient noise levels were associated with higher dolphin whistle frequencies and a less complex whistle contour.
There is substantial overlap between marine mammals and the Maryland WEA, but this varies seasonally. While the risk to endangered whales is lowest during the summer, the risk to bottlenose dolphins may be highest at this time, as they are most abundant in the summer time. The year-round occurrence of marine mammals offshore of Maryland will require decision-makers to consider the trade-off of the potential impacts to different species and assess approaches that will minimize population-level impacts to marine mammals of offshore wind development and other activities. Mitigation measures that focus on real-time monitoring for whales and minimizing harm and disturbance to bottlenose dolphins would help to reduce any potential negative effects of offshore wind farm construction and operation activities. The results of this study will help to inform regulators and resource managers so that appropriate protection and mitigation measures can be developed for future anthropogenic activities. Our baseline data can also be compared with continued passive acoustic monitoring during construction and operation of an offshore wind facility to determine marine mammal responses.