AEI lay summary of:
R. Williams, C.W. Clark, D. Ponirakis, and E. Ashe.  Acoustic quality of critical habitats for three threatened whale populations.  Animal Conservation (2013).

Innovative research along the coast of British Columbia has quantified the degree to which shipping noise is reducing the distance at which whale vocalizations can be heard.  This is one of the first studies to use recordings of actual ocean noise levels to examine how the “communication space” of whales is affected by shipping noise in an area where whale conservation is a priority.  Among its troubling findings is that endangered orcas are facing the highest levels of noise in areas that are legally designated as critical habitat, with communication space reduced to 25% or less even in average noise conditions; over the entire study region, the area over which orcas can hear each other can be reduced by 62% during average noisy conditions, and 97% during the noisiest times.  Humpback whales face nearly as large reductions in some key areas (though not formally designated critical habitat; and, notably, are showing signs of a tenuous recovery in some of the areas studied), while fin whales, who have louder calls than the other species, are only mildly affected by shipping noise.

(noise levels and communication space in median noise conditions)

Communication space (alternatively termed “effective listening area”) is a relatively recent introduction into scientific parlance; it’s a measure of the area within which a particular species can hear and be heard by others of its kind; both marine and terrestrial bioacousticians have begun using this framework to better understand the ways animals may be affected by increased background noise introduced by human activities, including shipping, roads, and airplane overflights.  Previously, small increases in background noise were commonly considered to cause only negligible impacts, since there is rarely a clear or consistent behavioral reaction.  However, many animals rely on hearing things at the edges of audibility (calls of their kin, the approach of predators, the presence of prey), and a significant reduction in an animal’s communication space can cause a need to use more energy hunting, or to be in a heightened state of alertness (and stress) to avoid predation.

In this study, recorders were deployed for 20-166 days in 12 locations, during various times from late May to early November. Some locations are subject to heavy shipping traffic (e.g., Haro Strait), and some are relatively remote with lighter daytime boat traffic (e.g. Kitimat). To quantify the reduction in communication space, the researchers looked at both the loudness of the calls and particular frequency ranges important to each species; fin and humpback calls are in the lower frequencies, while orcas utilize three different higher-frequency bands for different purposes (calls, whistles, and echolocation clicks). They then calculated the percent of communication area lost over various putative communication ranges, which also varied by species and call type. The one bit of good news for orcas was that their relatively loud echolocation clicks use much higher frequencies than are present in most shipping noise, so foraging is expected to be relatively unaffected within the kilometer or two over which the clicks are useful.

To put reduced communication space into perspective: An orca call that might normally be heard 8km away is potentially filling a communication space of 200 square kilometers, within which other orcas will hear the call. When ships are present, the average area over which this whale will be heard shrinks by 62%, to 75 square kilometers, while at the noisiest times, its call fills only 6 square kilometers, 3% of the area it would be heard in quiet conditions. A similar call that might normally be heard 4km away sees its area reduced from 50 square kilometers in quiet, down to 33 square kilometers in average noisy conditions and 4.5 square kilometers at the noisiest times.

A few caveats are in order. First, while we know that whale calls can be heard at long distances, we are less certain about the typical ranges that are actually used for communication (I might be able to hear someone’s voice from across a field, but I tend to have conversations with people that are much closer). “We really don’t have good information on the size of the habitat that the whales are using,” lead author Rob Williams told LiveScience. “You can say the whale’s acoustic space is being reduced by 50 percent or 80 percent, but that is relative to our best guess about the range the whales are using.” The researchers here considered relatively modest communication ranges, rather than the maximum extent of audibility in quiet conditions (e.g., orca calls may be heard as far as 25km away, while the authors considered ranges of 8km and less). Also, the data is presented as an average lost communication space during median noise (50% of the time it was louder, 50% of the time it was quieter), and as the average reduction during the noisiest 5% of the time. So, the worst-case reductions reported here are taking place about 5% of the time (presumably as ships are passing relatively close by). Likewise, the “average noisy” reductions represent medians of the many recording locations, rather than any particular noise level; this is a clear, statistically-valid way to present overall average impacts, but it doesn’t give us a very clear sense the actual experiences of individual whales (i.e., how many hours per day, or week does a whale in critical habitat around Haro Strait find its communication space shrunken to half or less of what it would be in quiet?) For example, note that in the graphic above, in several locations orcas experienced reductions to below 25% of normal communication space, even in average noisy conditions, whereas the overall average was 38%. Future studies will continue to dig into such details; this line of inquiry holds much promise, and it will be fascinating to see how it’s developed in the years to come.

The authors clearly recognize the need for more concrete ways to use this sort of data in forging management plans for species recovery and habitat protection. They note that managers may choose to assess the population consequences of chronic masking of calls by noise, and work to assure recovery targets are still achievable despite the increased noise levels. Alternatively, noise could be considered a “cost” or impact that is factored in to the establishment of critical habitat areas; if there is noise, such an area would need to be larger than if it is quiet. However regulators may choose to work with the emerging new wealth of noise data, the authors stress that “noise now has to be treated as a habitat-level stressor,” and that we need to begin to assess “ecosystem consequences of noise, much as we do for other anthropogenic stressors.”

Of particular importance in this study is that some of the areas where they found the lowest noise levels are under the threat of increased shipping, thanks to new oil and gas terminals and other developments. The authors note:

The quietest areas in our study (primarily mainland inlets) have substantial periods of quiet that are quieter than any recorded off the US east coast. As researchers consider experimental approaches to better quantifying ecosystem-level effects of noise, unusually quiet places like BC’s mainland inlets may present opportunities for conducting ocean noise or quieting experiments, or to consider novel conservation mechanisms to preserve acoustic wilderness sites…Today, throughout the Northern Hemisphere, these once-normal levels (of quiet) are becoming the exception rather than the rule.

The authors mention several policy initiatives now calling for reductions in ocean noise. Yet they conclude: “From a practical standpoint, identifying quiet areas and keeping them quiet will be easier than trying to remove sound sources from noisy areas.”

Related: See AEInews coverage of a similar study in Stellwagen Bank, which is host to ship traffic in and out of Boston, and of a study that used shipping records to model long-term sound levels throughout the coastal BC area also studied here.