Marine Mammals, Sea Turtles, and Pile Driving

Written by Abbey Greene, Coastal Project Manager, National Sea Grant Offshore Wind Energy Liaison Team; Reviewed by Dr. Jim Miller, Ocean Engineering, University of Rhode Island’s Graduate School of Oceanography

Main Takeaways:

  • The marine mammal of highest concern when it comes to offshore wind development in the United States is the North Atlantic Right Whale.

  • In total, there are six kinds of protected species that the Bureau of Ocean Energy Management is monitoring when it comes to offshore wind development: Sea Turtles, North Atlantic Right Whales, Sperm Whales, Fin Whales, Blue Whales, and Sei Whales. 

  • There are different kinds of mitigation strategies that are used to minimize the effects on marine life, including time-of-year restrictions, protected species observers, and soft starts for pile driving.

  • More studies are needed to fully understand the impacts on marine mammals in the United States.

In recent years, the global expansion of offshore wind energy has resulted in an increased amount of impact pile driving, which is considered to be among the most intense sources of underwater noise. 

Abroad, studies have shown behavioral responses of harbor porpoises (Phocoena phocoena) and harbor seals (Phoca vitulina) that react to impact pile driving at considerable distance from the source. For both species, there is a risk of impaired hearing in the form of temporary threshold shift (TTS), and permanent threshold shift (PTS) is possible closer to the sound source and/or at relatively high acoustic doses. (Source)

In the United States, we have different species swimming in our offshore waters that need consideration when it comes to pile driving for offshore wind. In total, there are six kinds of protected species that the Bureau of Ocean Energy Management is monitoring: Sea Turtles, North Atlantic Right Whales, Sperm Whales, Fin Whales, Blue Whales, and Sei Whales. 

Observed behavioral responses in some marine mammals in association with pile driving include changes in swimming direction and/or speed, dive profiles, group movements, vocalizations (intentional sounds made by ocean life to communicate, navigate, and feed), and respiration rates. There are also concerns that avoidance responses to pile driving may cause displacement from key habitats and/or impact foraging, nursing, and/or mating activities, and therefore the overall fitness, of a marine mammal. (Source: Discovery of the Sound in the Sea)

Dr. James Miller explains to us that the species that has the most people worried about when it comes to offshore wind development off the east coast is the North Atlantic Right Whale. North Atlantic Right Whales have been listed as endangered under the Endangered Species Act since 1970. The latest preliminary estimate from NOAA suggests there are fewer than 350 remaining, with fewer than 100 breeding females. The number of new calves born in recent years has been below average.

Because this species migrates along the East Coast, construction of the Block Island Wind Farm was essentially scheduled around their migratory path and season. This resulted in the construction schedule being limited to days between April 1st and October 31st. Further, construction was only permitted during the day to ensure that, if a whale did enter the surrounding area, construction could be stopped. This kind of mitigation strategy is useful for the preservation of the well-being of marine life, along with others:

  • Time-of-Year Restrictions - Pile driving activities may be excluded during certain times of the year due to the presence of sensitive marine life during these periods. Time-of-year restrictions have been implemented for the North Atlantic right whale.

  • Protected Species Observers - Trained observers maintain an exclusion area for certain protected species around pile driving activities. For example, if a marine mammal or sea turtle is observed entering or within the relevant exclusion zones, pile-driving activity must be shut down and delayed.

  • Soft Start for Pile Driving - Gradual ramp up of hammer energy for impact pile driving that includes an initial set of strikes from the impact hammer at reduced energy, followed by a waiting period, with repetition of this process several times prior to initiation of pile driving.

  • Bubble Screens - Curtains of bubbles are generated by perforated pipes on the seafloor. These bubbles can dampen the sound from the pile driving by 10 dB.

Listen to the sound below to hear pile driving activity before and after a bubble curtain system is activated. Please note, this sound recording demonstrates the effectiveness of this mitigation option and is not meant to demonstrate how it is actually done (e.g., the bubble curtain would already be up and operational before any piling started in a real-world scenario)

Hear it

This video is of a sound recording of 36-inch diameter steel pipe (1” wall thickness) being driven by a diesel impact hammer at a water depth of 7.5m. Approximately midway through the recording, a dual-ring bubble curtain (400 cubic ft per minute flow rate, 75 psi air pressure) is activated. One should be able to hear a difference in the pile-driving sounds before and after the bubble curtain is activated. The sounds were recorded by a hydrophone in 20m of water, approximately 1m above the seabed. Courtesy of Connor, Heithaus, Berggren and Miksis.

The Bureau of Ocean Energy Management (BOEM) held a workshop to develop best management practices for Atlantic offshore wind facilities and marine protected species in 2018, and identified the following marine mammals and the challenges the offshore wind industry may pose to them:

Offshore wind development poses several potential threats to protected species, including: 

  • Vessel collision; 

  • Benthic habitat loss (potential feeding areas); 

  • Increased fishing pressure around structures, incidental capture/entanglement; 

  • Marine debris; 

  • Dredging/bottom-scouring or leveling; 

  • Spread of non-native species; 

  • Potential disruption of oceanographic features (i.e. surface currents); 

  • Water quality (habitat) degradation/contaminants; 

  • Noise; 

  • Displacement from or attraction to structures; and 

  • Energetic loss due to displacement. 

Animals that live in the ocean rely on sound to survive in their marine environment. If they are not able to hear as well, their communication range and ability to sense their surroundings will be impacted.

The National Environmental Policy Act outlines different kinds of mitigation measures to avoid the impact on marine species, including:

  • Avoiding the impact altogether by not taking a certain action or parts of an action; 

  • Minimizing impacts by limiting the degree or magnitude of the action and its implementation; 

  • Rectifying the impact by repairing, rehabilitating, or restoring the affected environment; 

  • Reducing or eliminating the impact over time by preservation and maintenance operations by the life of the action; and 

  • Compensating for the impact by replacing or providing substitute resources or environments. 

When it comes to North Atlantic Right Whales in particular, BOEM has developed mitigation measures specific to them around the pre-construction data collection phases in lease areas. (Figure 2). vAs of today, BOEM requires measures to avoid injury in marine mammals and sea turtles. However, they note that other effects may be worthy of consideration for future mitigation, such as “monitoring for stress hormones to gauge animal health, looking at secondary effects such as increased ship strikes, monitoring for cumulative effects, as well as conducting small-scale studies on sea turtles to explore health information.” (BOEM)

Learn even more:

  • How does pile driving differ for each kind of wind turbine foundation? (BOEM, pg. 35)

  • What is the reality of monitoring methods and protected species observers (PSO)? (BOEM, pg. 36)

  • What about sea turtles?(BOEM, pg. 38)
    There is ongoing research in this area.