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The real-time impact of ship noise on dolphin calls

Fouda, L., Wingfield, J. E., Fandel, A. D., Garrod, A., Hodge, K. B., Rice, A. N., & Bailey, H. (2018). Dolphins simplify their vocal calls in response to increased ambient noise. Biology letters, 14(10), 20180484.

Figure 1: Fungi, the resident bottlenose dolphin of Dingle, Ireland. This picture was taken July 2018 by A.M. Hartwell.

Have you ever been having a conversation in a loud room and feel like you need to almost yell short phrases to get any point across? That is kind of what I imagine it is like for the Bottlenose dolphins (Figure 1; Latin name Tursiops truncates) described by Fouda and colleagues at University of Maryland and Cornell University in an open access paper in Biology Letters.

Dolphins use noise to communicate.  Communication can become difficult when there are too many other noises going on at the same time and in the same place. These noises are both natural (i.e. wave motion, fish in the water column) and created by humans (i.e. shipping, recreation, dredging, sonar). Biologists have long recognized that background noise can impact the reproductive and foraging behavior of dolphins, as well as the pitch of their whistles.

However, a gap in knowledge about how dolphins alter their whistles  in real-time still remained.  To fill the gap, Fouda et al., 2018 focused their research on understanding how the complexity of bottlenose dolphin whistles’ changed in real-time in response to the background noise in their environments.

Dolphin whistles and ambient noises were recorded for 3 months in the summer of 2016 near shipping channels approximately 18.5 miles off the coast in Delaware Bay, Western North Atlantic.  The sound was captured with a Wildlife Acoustic Recorder that sampled noises in 2-minute intervals each time a certain pitch sound was detected.

Back at the lab, researchers reviewed the data to identify whistles.  For each whistle recorded, they looked at how specific characteristics, such as the duration, starting pitch, ending pitch, and the change in pitch, changed during high levels of background noise.

The background noise associated with each whistle was determined from the 2-seconds of recorded data immediately before the whistle started.   Overall background noise was determined from all 2-minute recordings in the 3-month deployment period.

In the 3 months of deployment, a total of 200 whistles were identified, each lasting between .07 and 1.17 seconds.   In that time period, the background noise was over the USA Marine mammal regulatory threshold for behavioral disruption 11% of the time.

Fouda et al., 2018 found that the whistle characteristics varied depending on the pitch of the background noise. The greatest impact came from the noise that was outside of the range of pitches that the dolphins use for their whistles.  This observation suggested to researchers that the dolphins modify their calls to be higher pitched, simpler and shorter (Figure 2), as opposed to the calls being lost due to interference with the background noise.

Figure 2:
Screenshot from Fouda et al., 2018 with added annotation. Box A (left) is a whistle recorded when background noise was low and Box B (right) was recorded with background noise was high. The dolphin whistles are circled in yellow. The X axes are highlighted in green to show that they are different scales (2 seconds vs. 0.7 seconds). In Box A, the whistle is longer, starts at a lower frequency, and has a greater change in frequency between the start and finish of the whistle compared to the whistle Box B.

Researchers suspect that the less complex whistles carry less information compared to more complicated whistles.  More research is needed to establish that validity of that hypothesis and what implications it could have on dolphin pods.

Dolphins are not alone in their adaption to noisy surroundings. Bats, primates, Beluga Whales, and birds all change their communication in response to too much background noise. Understanding the long-term sustainability of these animals ability to communicate with each other with altered calls is important for establishing meaningful protections for them as greater levels of noise are introduced into their surroundings.



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