Maria Zapetis – maria.zapetis@usm.edu
University of Southern Mississippi
118 College Drive
Hattiesburg, MS 39406

Jason Mulsow – jason.mulsow@nmmpfoundation.org
National Marine Mammal Foundation
2240 Shelter Island Drive, Suite 200
San Diego, CA 92106

Carolyn E. Schlundt – melka@peraton.com
Peraton Corporation
4045 Hancock Street, Suite 210
San Diego, CA 92110

James J. Finneran – james.finneran@navy.mil
US Navy Marine Mammal Program
Space and Naval Warfare Systems Center, Pacific
53560 Hull Street
San Diego, CA 92152

Heidi Lyn – hlyn@southalabama.edu
University of South Alabama
75 South University Boulevard
Mobile, AL 36688

Popular version of paper 2aAB8, “Bottlenose dolphin (Tursiops truncatus) vocal modifications in response to spectrally pink background noise”
Presented Tuesday morning, November 6, 2019, 11:00 AM in Shaughnessy (FE)
176th ASA Meeting, Victoria, British Columbia, Canada

You’re in the middle of a conversation when you walk out of a quiet building onto a crowded street. A loud plane flies directly overhead. You stop your car for a passing train. Chances are, you’ve experienced these kinds of anthropogenic (man-made) noises in your everyday life. What do you do? Most people raise their voice to be heard, an automatic reaction called the Lombard effect [1, 2]. Similarly, dolphins and other marine mammals experience anthropogenic noise — from human activities such as boat traffic and construction activities — and raise their “voices” to better communicate [3, 4]. Understanding the extent to which dolphins exhibit the Lombard effect and alter their vocalizations in the presence of man-made noise is important for predicting and mitigating the potential effects of noise on wild marine mammals.

In this study, bottlenose dolphins were trained to “whistle” upon hearing a computer-generated tone (Figure 1). After successful detection of a tone, dolphins typically produced a “victory squeal” (a pulsed call associated with success [5]). During tone-detection trials, the dolphins’ whistles and victory squeals were recorded while one of three computer-generated noise conditions played in the background (Figure 2). The dolphins responded to every background noise condition with the Lombard effect: as the noise frequency content and level increased, the dolphins’ whistles got louder (increased amplitude) (Figure 3). Other noise-induced vocal modifications were observed, such as changes in the number of whistle harmonics, depending on the specifics of the noise condition. Because this was a controlled exposure study with trained dolphins, we were not only able to exclude extraneous variables but were able to see how the dolphins responded to different levels of background noise. Control over the background noise allowed us to tease apart the effects of noise level and noise frequency content. Both properties of the noise appear to affect the parameters of dolphin signals differently, and may reflect an ability to discriminate within those properties independently.

Figure 1. Hearing test procedure with US Navy Marine Mammal Program dolphins in San Diego Bay, CA. [A] Each trial begins with the trainer directing the dolphin to dive underwater and position herself on a “biteplate” in front of an underwater speaker. [B] Once on the biteplate, the dolphin waits for the hearing test tone to be presented. When the dolphin hears the tone, she whistles in response. The researcher lets the dolphin know that she is correct by playing a “reward buzzer” out of another underwater speaker. The dolphin will often respond to the reward buzzer with a victory squeal before [C] coming up for a fish reward. The dolphin’s vocalizations are recorded from the hydrophone (underwater microphone) in a green suction cup just behind the blowhole.

Figure 2. Spectrogram examples of the four conditions. The [W] whistles, [RT] reward buzzers, and [VS] victory squeals of the dolphin in Figure 1 are labeled.

Figure 3. Whistle Amplitude across four conditions. Compared to the control condition (San Diego Bay ambient noise), both dolphins produced louder whistles in every noise condition.

1. Lombard, E. (1911). Le signe de l’élévation de la voix. Annales des Maladies de L’Oreille et du Larynx, 37, 101–119.
2. Rabin, L. A., McCowan, B., Hooper, S. L., & Owings, D. H. (2003). Anthropogenic noise and its effect on animal communication: an interface between comparative psychology and conservation biology. The International Journal of Comparative Psychology, 16, 172-192.
3. Buckstaff, K. C. (2004). Effects of watercraft noise on the acoustic behavior of bottlenose dolphins, Tursiops Truncatus, in Sarasota Bay, Florida. Marine Mammal Science, 20(4), 709–725.
4. Hildebrand, J. (2009). Anthropogenic and natural sources of ambient noise in the ocean. Marine Ecology Progress Series, 395, 5–20.
5. Dibble, D. S., Van Alstyne, K. R., & Ridgway, S. (2016). Dolphins signal success by producing a victory squeal. International Journal of Comparative Psychology, 29.

Aaron M. Thode athode@ucsd.edu
Scripps Institution of Oceanography, UCSD, La Jolla, California 92093, USA

Susanna B. Blackwell, Katherine H. Kim, Alexander S. Conrad
Greeneridge Sciences, Inc., 90 Arnold Place, Suite D, Santa Barbara, California 93117, USA

Popular version of paper 1a2b3c
Presented Monday morning, 1pAO, Arctic Acoustical Oceanography II

We live in a world full of ever-changing noise with both natural and industrial origins.  Despite this constant interference, we’ve developed several strategies for communicating with each other.  If you’ve ever attended a busy party, you’ve probably found yourself shouting to be overheard by a nearby companion, and maybe even had to repeat yourself a few times to be understood.

Figure 1: Spectrogram of whale calls; sound file attached 4x normal speed

Whales are even more dependent on sound for communicating with each other.  For example, each autumn bowhead whales, a species of baleen whale, migrate along the Alaskan North Slope from their Canadian summer feeding grounds towards the Bering Sea.  During this voyage, they make numerous low-frequency sounds (50-200 Hz) that are detectable on underwater microphones, or “hydrophones,” up to tens of kilometers away.  There are many mysteries about these calls, such as what type of information they convey, and why their frequency content seems to be shifting downward with time (Thode et al., 2017) for unknown reasons.  Nevertheless, scientists generally agree that bowheads use these sounds for communication.

Changing conditions in the Arctic have encouraged more human industrial activity in this formerly remote region.  For example, over the past decade multiple organizations have conducted seismic surveys throughout the Arctic Ocean to pinpoint oil-drilling locations or establish territorial claims. The impulsive “airgun” sounds generated by these activities could be detected over distances of more than 1000 km (Thode et al., 2010).

Figure 2: Spectrogram of seismic airgun signals along with bowhead whale calls; sound file attached 4x normal speed

Previous work by our team has found that bowhead whales double their calling rate whenever distant seismic signals are present (Blackwell et al., 2015).  But what consequences, if any, could this behavioral change have on the long-term health of the bowhead population?

To answer this question my colleagues at Greeneridge Sciences Inc. and I have studied how bowhead whales respond to natural changes in noise levels, which during the summer and fall are caused primarily by wind in the relatively ship-free waters of the North.  We found that, like humans at a party, whales can respond in two ways to rising noise levels.  They can increase their loudness, or “source level,” and/or they can increase the rate at which they produce calls.  Measuring this effect is challenging, because whenever background noise levels increase it becomes difficult to detect weaker calls, an effect called “masking”.  Because of masking, as noise levels rise one might measure a decrease in calling rate as well as an apparent increase in call source levels, even if a whale population didn’t actually change their calling behavior.

To solve this problem our team deployed multiple groups of hydrophones that allowed machine learning algorithms to localize the positions of over a million whale calls over the eight years of our study.  We then threw away over 90% of these positions, keeping only calls that were produced at close range to our sensors, and thus wouldn’t become masked by changes in noise levels.  Continuing the party analogy, we effectively only listened to people close to us, so we could still detect whispers along with shouts.

We found that whales tried to increase their source levels as noise levels increased, but when noise levels became high enough (75% of maximum noise levels encountered naturally) the whales didn’t call any louder, even as noise levels continued to rise (Figure 3).

Figure 3: Relationship between background noise level and whale calling level, for calls made within 3.5 km of a sensor.

Whales do, however, keep increasing their call rates with rising noise levels.  We found that a 26-dB (400-times) increase in noise levels caused calling rates to double, the same rate increase caused by seismic airguns (Figure 4).

Figure 4: Image of relationship between whale calling rate (over ten minutes) and background noise level.

This work has thus allowed us to place bowhead whale responses to human disturbance in a natural noise context, which eventually may assist us in evaluating the long-term impact of such activities on population growth.

References

Blackwell, S. B., Nations, C. S., McDonald, T. L., Thode, A. M., Mathias, D., Kim, K. H., C.R. Greene, J., and Macrander, A. M. (2015). “The effects of airgun sounds on bowhead whale calling rates:  evidence for two behavioral thresholds.  ,” PLoS One 10, e0125720.

Thode, A. M., Blackwell, S. B., Conrad, A. S., Kim, K. H., and Michael Macrander, A. (2017). “Decadal-scale frequency shift of migrating bowhead whale calls in the shallow Beaufort Sea,” The Journal of the Acoustical Society of America 142, 1482-1502.

Thode, A. M., Kim, K., Greene, C. R., and Roth, E. H. (2010). “Long range transmission loss of broadband seismic pulses in the Arctic under ice-free conditions,” J. Acoust. Soc. Am. 128, EL181-EL187.