Can you hear me now?
In the ocean’s murky depths, sound is critical to life’s basic functions. Whales, dolphins, seals, and even some fish, are social animals and use sound to communicate. Sounds are used to locate food—or avoid becoming food, attract mates, navigate, and otherwise make sense of their environment. Unfortunately, human-caused sounds are flooding the ocean. These sounds interfere with natural sounds and disrupt marine mammal behavior.
In and around Glacier Bay National Park and Preserve, marine noise comes primarily from fishing vessels, commercial shipping, and cruise ships (or other tour vessels. Energy development, research, military testing, and ship traffic have all led to a noisier underwater experience for wildlife.
Shipping is one of the most pervasive sources of ocean noise today, effectively producing an ever-present din that leads to acoustic masking. Acoustic masking is what happens when man-made sounds interfere with an animal’s ability to detect, recognize, or understand sounds of interest (e.g., mating, navigation, feeding, and others). Shipping vessels’ low frequency matches that of several marine animals, including seals, sea lions, and fish, and it increasingly interferes with marine animal daily activities, leading to hearing loss, stress, difficulty feeding, mating disruption, strandings, and even death.
The marine environment is a critical resource for Glacier Bay National Park and Preserve. We study marine acoustics and marine animal behaviors to better understand how they are connected. By understanding the changes taking place, we can make better management decisions that protect marine wildlife.
Predicting the acoustic exposure of humpback whales from cruise and tour vessel noise in Glacier Bay, Alaska, under different management strategies
A study based on the Acoustic Integration Model (AIM©) predicts that humpback whales in Glacier Bay are exposed to less underwater noise from large vessels when there are slower ships, fewer ships, or ships that are scheduled to arrive an hour apart.
Creating opportunities for visitors to enjoy Glacier Bay is an important part of the National Park Service’s mission, and the cruise and tour vessels that are authorized to bring visitors into Glacier Bay play an important role in meeting that mission. At the same time, underwater noise from these and other types of motorized vessels can affect humpback whales and other marine life by decreasing the distance over which they can communicate and detect predators and prey. Park managers must do their best to understand the effects of these vessels to protect the underwater sound environment when making management decisions.
We used a software package called the Acoustic Integration Model (AIM©) to simulate whale and vessel movement in order to estimate the maximum and cumulative levels of cruise ship and tour vessel noise, as well as the amount of quiet time, that whales in the park would experience under different management scenarios.
First, we distributed 1,201 simulated humpback “whales” throughout Glacier Bay in areas they are known to frequent. These “whale animats” were programmed to behave like real humpbacks (dive times, dive depths, swim speeds, and other known characteristics). Then we added a track for each cruise ship or tour vessel. Using existing knowledge about how loud each vessel is and how sound travels in the bay, AIM created a “sound footprint” for each vessel as it traveled through the bay past each “whale.” AIM then estimated the received sound level for each “whale” every 15 seconds for the entire day. In each simulation, we varied the number, speed (13 knots = “slow” vs. 20 knots = “fast”), and timing of cruise ships while keeping the number, speed, and timing of smaller tour vessels the same. One of the measures that AIM produced was the cumulative sound exposure level, or CSEL, which represents the total amount of noise that a whale is exposed to over the entire day.
A key finding from this study was that cruise ship speed was the dominant factor affecting how much noise “whales” were exposed to in Glacier Bay. In fact, the median CSEL values from two “slow” ships were lower than from just one “fast”
ship. On days with fewer or slower cruise ships, the noise from the three daily tour vessels, which are much smaller than cruise ships, was more important to the bay’s total underwater soundscape.
Another practical and important finding was that even though slower cruise ships produce longer exposure times (that is to say, they pass by more slowly), they produce much lower CSEL than faster cruise ships. The difference was substantial. The “whales” in simulations with cruise ships traveling at 13 knots were exposed to CSELs that were three times lower than when ships traveled at 20 knots. Even in cases where the ship was only a few decibels quieter at a slower speed, CSELs were lower, even though the ship’s transit past the “whale” takes much longer.
Maximizing Noise-free Intervals
Synchronizing cruise ship arrival times had little effect on CSEL, but did decrease the cumulative amount of time that the “whale” was exposed to ship noise. It also created quiet periods of time that may benefit whale communication or behavior. However, managers must take into account the noise from other types of vessels using the bay, which are smaller, but more numerous (including charter boats, private yachts, skiffs, and National Park Service vessels). These other vessel classes may create underwater noise during planned cruise ship “quiet periods,” albeit at much lower levels than cruise ships produce.
Overall, these results suggest that the most effective way to reduce humpback whale exposure to underwater noise from cruise and tour vessels is to reduce cruise ship speed or numbers. Adjusting ship schedules may also be beneficial by allowing longer relatively quiet intervals between ship noise events.
The work summarized here was made possible by the cooperation of tour vessels and cruise ships who voluntarily allowed park scientists, in cooperation with acousticians with the U.S. Navy, to measure the underwater noise produced by each vessel at different speeds. Model results like these are an important first step toward understanding the acoustic effects of management decisions and protecting park resources while allowing visitors to enjoy the park.
Frankel, A. S. and Gabriele, C. M. 2017. Predicting the acoustic exposure of humpback whales from cruise and tour vessel noise in Glacier Bay, Alaska, under different management strategies. Endangered Species Research 34:
Underwater acoustic ecology metrics in an Alaska marine protected area reveal marine mammal communication masking and management alternatives
Vessel-generated underwater noise can affect humpback whales, harbor seals, and other marine mammals by decreasing the distance over which they can communicate and detect predators and prey. Emerging analytical methods allow marine protected area managers to use biologically relevant metrics to assess vessel noise in the dominant frequency bands used by each species. Glacier Bay National Park (GBNP) in Alaska controls summer visitation with daily quotas for vessels ranging from cruise ships to yachts and skiffs. Using empirical data (weather, AIS vessel tracks, marine mammal survey data, and published behavioral parameters) we simulated the movements and acoustic environment of whales and seals on 3 days with differing amounts of vessel traffic and natural ambient noise. We modeled communication space (CS) to compare the area over which a vocalizing humpback whale or harbor seal could communicate with conspecifics in the current ambient noise environment (at 10-min intervals) relative to how far it could communicate under naturally quiet conditions, known as the reference ambient noise condition (RA). RA was approximated from the quietest 5th percentile noise statistics based on a year (2011) of continuous audio data from a hydrophone in GBNP, in the frequency bands of whale and seal sounds of interest: humpback “whup” calls (50–700 Hz, 143 dB re 1 μPa source level, SL); humpback song (224–708 Hz, 175 dB SL), and harbor seal roars (4–500 Hz, 144 dB SL). Results indicate that typical summer vessel traffic in GBNP causes substantial CS losses to singing whales (reduced by 13–28%), calling whales (18–51%), and roaring seals (32–61%), especially during daylight hours and even in the absence of cruise ships. Synchronizing the arrival and departure timing of cruise ships did not affect CS for singing whales, but restored 5–12% of lost CS for roaring seals and calling whales, respectively. Metrics and visualizations like these create a common currency to describe and explore methods to assess and mitigate anthropogenic noise. Important next steps toward facilitating effective conservation of the underwater sound environments will involve putting modeling tools in the hands of marine protected area managers for ongoing use.
Gabriele, C. M., D. W. Ponirakis, C. W. Clark, J. N., Womble, and P. Vanselow. 2018. Underwater acoustic ecology metrics in an Alaska marine protected area reveal marine mammal communication masking and management alternatives. Frontiers in Marine Science 5:270.