I’ve just finished reading what must be the most exciting research paper I’ve seen this year, barely nudging out a similar paper addressing terrestrial noise impacts. A small group of researchers, with Chris Clark of Cornell as the lead author, took a giant step forward in addressing the impacts of ocean noise on the communication ranges of whales. They came up with a clear and strikingly rigorous set of new metrics that will allow researchers and ocean planners to have a much more practical picture of how numerous noise sources combine to create cumulative impacts on acoustic habitat. The new approach centers on the “Communication Space” of individual animals, as well as groups, and provides an intuitively obvious way to both imagine and assess the effects of ocean noise – measuring the area in which an animal can hear or be heard by others of its species.
My formal “lay summary” of this paper is reprinted in full below the fold, and I encourage anyone with a deep interest in ocean noise to read through that five-paragraph overview, or to download the paper yourself. The key takeaway for those of you with a more casual interest in these issues is that in the test case that they used to illustrate their new approach, the researchers found that shipping noise has dramatically different impacts on different species, even though all three species they studied are low-frequency communicators. In the area off Boston Harbor that they investigated, the critically endangered right whale is by far the most affected by shipping noise: on a day when two ships passed through the area (the average is often six), right whale Communication Space was reduced by an average of 84% over the course of the day, with several hours in which they could hear and be heard in an area less then 10% of that which would be expected without shipping nearby. Since right whales call back and forth to find each other as they form groups for feeding, this is truly worrying (though the key question of how a reduced communication range actually affects animals remains unanswered). Fin whales and humpbacks were far less dramatically affected, with their Communication Spaces reduced by just 33% and 11% respectively.
These first examples focus on the effects of low-frequency shipping noise on low-frequency communication by large whales, but this approach can easily be used to address mid- or high-frequency noise sources (sonars, airguns) and higher frequency animal sounds such as those used for echolocation, opening a vast and exceedingly useful new doorway for biologists and ocean managers, as well as the general public, to appreciate the impacts of human sounds in the sea. (click through for complete lay summary)
AEI lay summary:
Clark, Ellison, Southall, Hatch, Van Parijs, Frankel, Ponirakis. Acoustic masking in marine ecosystems: intuitions, analysis, and implication. Mar Ecol Prog Ser, Vol. 395: 201-222, 2009.
This has to be one of the most exciting papers I’ve seen this year; reading it was downright joyous, especially in contrast to decidedly unsatisfying previous attempts to address cumulative impacts of ocean noise (mostly undertaken by large committees and resulting in vague and complex conceptual approaches that are nearly impossible to implement). By contrast, here we have, at last, a clear and strikingly rigorous approach to assessing the impacts of ocean noise sources on the communication of whales. As promised in the introduction, the authors “present an analytical paradigm to quantify changes in an animal’s acoustic communication space” and “a metric to quantify the potential for communication masking.” While of course, much uncertainty remains, both in some of the specific terms used in these metrics and in assessing the biological impact of reduced communication space, the tools provided here will open a vast and exceedingly useful new doorway for biologists and ocean managers.
Researchers will find much to work with in the suite of new metrics and relatively simple functions used to calculate them, including factors that reflect “ancient ambient” conditions as well as “present ambient” and “present noise sources”, a more nuanced twist on signal-to-noise ration called the “recognition differential”, acknowledgement of subtle biological inputs such as “signal processing gain”, and “potential communication space” for a sender, and for a receiver hearing multiple other animal senders….this is just a hint of the power of these combined metrics, and surprisingly, most of the functions are really quite simple, though they do of course interact in complex ways. The present examples focus on the effects of low-frequency shipping noise on low-frequency communication by large whales, and incorporate differences in the specific frequency ranges used by the three species addressed; the model can easily be used to address mid- or high-frequency noise sources and higher frequency animal sounds such as those used for echolocation.
For the rest of us who are not designing new studies, though, the power of this approach is equally clear, and can readily inform both public awareness and policy-making. Extending the nascent idea of “communication space” (which has recently been introduced in terrestrial ecosystems by researchers at the National Park Service and Colorado State) into ocean habitats, we are presented with an intuitively obvious way to both imagine and assess the effects of ocean noise – measuring the area in which an animal can hear or be heard by others of its species. As its first application, this paper presents data from ongoing work at Stellwagen Bank National Marine Sanctuary offshore from Boston Harbor. There, a network of hydrophones is collecting soundscape data that allows researchers to both pinpoint and track the locations of whales and ships, along with the actual received sound levels throughout an extended area. By plugging this data into their new equations, the researchers are able to, for the first time, quantify the effects of shipping noise on local populations of fin, humpback, and right whales.
The results are both fascinating and sobering. Thanks to slightly different frequency bands used by each species, and the lower source level of right whale calls, the impacts of shipping noise on these three species vary substantially. On the day used for this initial analysis, two ships passed through the region (an average of six ships has been noted here). On a low-traffic day with two ships, the Communication Space for humpbacks were reduced by an average of 11%, fins by 33%, and right whales by a staggering 84%. For most of the day (from 5am until 5pm), right whale Communication Space was diminished by 88-98%. While we do not know how this vastly shrunken communication range affected the whales, the authors note that “we do know that these whales counter-call and use these episodes of calling to find each other and to aggregate, so one immediate cost is the loss of opportunities to form social groups. Right whales form aggregations during mating and during feeding, so one likely cost is the loss of mating and feeding opportunities.”
Back to the science-advancement side of things, the authors stress that this metric provides “a critical missing link to the major current dilemma of assessing noise impacts” that can, for the first time, “quantify biological cost within an ecological framework.” They note that this model is a key step toward giving some practical form to the previous PCAD (Population Consequences of Acoustic Disturbance) model proposed by the National Research Council in 2005. There is still uncertainty in some key variables. Most strikingly, we don’t know how far these species actually communicate; while we know they can be heard for tens to hundreds of kilometers, limited studies have only confirmed communication exchanges in the range of 20km, which is used in this model for now (if actual communication ranges are longer, as is entirely possible, then reductions in Communication Space would be much greater, especially for the fin and humpback whales). The authors note it was 1971 when Payne and Webb first raised the question of whether shipping might reduce long-range communication for some large whales, and that “that hypothesis was ignored for a quarter of a century.” They conclude: “As the planet’s dominant species, humans have choices to make. In the case of the ocean’s acoustic ecological habitat, the choices we are making now have profound implications for the future of marine mammals. It is our opinion that the right choice cannot wait another 3 decades.”