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OPEN

Received: 2 June 2017
Accepted: 21 September 2017
Published: xx xx xxxx

Long-term passive acoustic
recordings track the changing
distribution of North Atlantic right
whales (Eubalaena glacialis) from
2004 to 2014
Genevieve E. Davis1,2, Mark F. Baumgartner3, Julianne M. Bonnell1, Joel Bell4,
Catherine Berchok5, Jacqueline Bort Thornton4, Solange Brault2, Gary Buchanan6,
Russell A. Charif7, Danielle Cholewiak1, Christopher W. Clark7, Peter Corkeron1, Julien Delarue8,
Kathleen Dudzinski9, Leila Hatch10, John Hildebrand11, Lynne Hodge12, Holger Klinck7,13,
Scott Kraus14, Bruce Martin8, David K. Mellinger13, Hilary Moors-Murphy15, Sharon Nieukirk13,
Douglas P. Nowacek12,22, Susan Parks   16, Andrew J. Read12, Aaron N. Rice   7, Denise Risch17,
Ana Širović11, Melissa Soldevilla18, Kate Stafford19, Joy E. Stanistreet12, Erin Summers20,
Sean Todd21, Ann Warde7 & Sofie M. Van Parijs1
Given new distribution patterns of the endangered North Atlantic right whale (NARW; Eubalaena
glacialis) population in recent years, an improved understanding of spatio-temporal movements are
imperative for the conservation of this species. While so far visual data have provided most information
on NARW movements, passive acoustic monitoring (PAM) was used in this study in order to better
capture year-round NARW presence. This project used PAM data from 2004 to 2014 collected by 19
organizations throughout the western North Atlantic Ocean. Overall, data from 324 recorders (35,600
days) were processed and analyzed using a classification and detection system. Results highlight almost
year-round habitat use of the western North Atlantic Ocean, with a decrease in detections in waters
off Cape Hatteras, North Carolina in summer and fall. Data collected post 2010 showed an increased
NARW presence in the mid-Atlantic region and a simultaneous decrease in the northern Gulf of Maine.
In addition, NARWs were widely distributed across most regions throughout winter months. This study
1

NOAA Northeast Fisheries Science Center, 166 Water Street, Woods Hole, MA, 02543, USA. 2University of
Massachusetts Boston, 100 Morrissey Blvd, Boston, MA, 02125, USA. 3Woods Hole Oceanographic Institution,
266 Woods Hole Road, Woods Hole, MA, 02543, USA. 4Naval Facilities Engineering Command Atlantic, Norfolk,
VA, 23508, USA. 5NOAA Alaska Fisheries Science Center, 7600 Sand Point Way N.E., Seattle, WA, 98115, USA.
6
New Jersey Department of Environmental Protection, Trenton, NJ, 08625, USA. 7Bioacoustics Research Program,
Cornell Lab of Ornithology, Cornell University, 159 Sapsucker Woods Road, Ithaca, NY, 14850, USA. 8JASCO Applied
Sciences, 32 Troop Ave, Dartmouth, NS B3B 1Z1, Canada. 9Dolphin Communication Project, Port St. Lucie, FL,
34985, USA. 10NOAA Stellwagen Bank National Marine Sanctuary, 175 Edward Foster Road, Scituate, MA, 02066,
USA. 11Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Drive MC 0205, La
Jolla, CA, 92037, USA. 12Nicholas School of the Environment, Duke University Marine Laboratory, 135 Duke Marine
Lab Road, Beaufort, NC, 28516, USA. 13Oregon State University, 2030 SE Marine Science Drive, Newport, OR,
97365, USA. 14New England Aquarium, Central Wharf, Boston, MA, 02110, USA. 15Fisheries and Oceans Canada,
Bedford Institute of Oceanography, 1 Challenger Drive, Dartmouth, NS B2Y 4A2, Canada. 16Syracuse University,
107 College Place, Syracuse, NY, 13244, USA. 17The Scottish Association for Marine Science (SAMS), Oban, PA37
1QA, Scotland, UK. 18NOAA Southeast Fisheries Science Center, 75 Virginia Beach Drive, Miami, FL, 33149, USA.
19
University of Washington, Applied Physics Laboratory, 1013 NE 40th Street, Seattle, WA, 98105, USA. 20Maine
Department of Marine Resources, West Boothbay Harbor, ME, 04575, USA. 21Allied Whale, College of the Atlantic,
105 Eden Street, Bar Harbor, ME, 04609, USA. 22Pratt School of Engineering, Duke University, Durham, NC, 27708,
USA. Correspondence and requests for materials should be addressed to G.E.D. (email: genevieve.davis@noaa.gov)
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demonstrates that a large-scale analysis of PAM data provides significant value to understanding and
tracking shifts in large whale movements over long time scales.
Understanding the distribution and movement patterns of marine mammals is essential for supporting their
conservation1,2. Large whales undertake some of the longest of mammalian migrations, with some species traveling over 10,000 kilometers annually3. Several species of baleen whales are known to migrate from productive
feeding grounds at higher latitudes in the summer to winter breeding grounds at lower latitudes4, although not all
individuals leave high latitudes each year5,6. The selective pressures driving these movements remain unresolved,
including the relative importance of environmental influences and predator avoidance7–9. Some species, such as
humpback (Megaptera novaeangliae) and gray (Eschrichtius robustus) whales, have relatively well-documented
migration routes to and from known wintering grounds10,11. The migratory movements of other species, for
example North Atlantic minke whales (Balaenoptera acutorostrata), have only been described recently12, while
those of others, like Antarctic blue whales (B. musculus intermedia), remain poorly known13.
North Atlantic right whales (NARW; Eubalaena glacialis) are one of the least abundant, but intensively studied, species of baleen whales14. Centuries of whaling resulted in their near extinction by the time protection was
introduced in 193515. Their abundance increased from approximately 350 in 199016 to 476 in 201017, but has since
shown evidence of decline18. The reasons for this decline are still unknown, but both widespread anthropogenic
impacts from fishing and shipping, and climatic changes are likely. Much of the substantial research program that
informs NARW conservation efforts is managed through the North Atlantic Right Whale Consortium (NARWC)
formed in 1986. The NARWC curates an extensive catalog of geo-referenced photo identifications of individual
whales, coupled with genetic, fisheries entanglement, and health-related data, that provides a detailed understanding of the status of the individuals comprising this species19.
These data come primarily from decades of research that has been focused in areas that NARWs are known to
use, including winter calving grounds off the coasts of Florida and Georgia, and more northerly feeding grounds
off the coasts of New England and Atlantic Canada15. Although the general distribution of NARWs seemed fairly
well known, recent surprises included discovering a potential mating ground in the Gulf of Maine in the winter20,21; acoustic detections and visual observations of whales in their historically-recorded habitats off Greenland
and Iceland15,22; or year-round presence in locations previously thought of as migratory corridors23–25. Sporadic
sightings of solitary NARWs in European waters26–28, such as the 131 day round-trip made by one individual
from U.S. waters to an old whaling ground off northern Norway29, demonstrate that our understanding of their
movement patterns remains incomplete.
Management to mitigate impacts of human threats to NARWs includes spatio-temporal measures, such as
Seasonal Management Areas (SMAs) or Areas to be Avoided (ATBA), within which ships are required (SMA)
or encouraged (ATBA) to reduce speed or avoid altogether during specified times when whales are known to
use these areas30,31. For this reason, it is important to gain a better understanding of the seasonal distribution
of NARWs, including how this distribution has changed in recent years. Since 2010, there has been a noticeable
shift in the distribution of NARWs. Whales appear to visit some regions much less frequently, such as the Bay of
Fundy, where they were regularly observed for at least two decades32. Likewise, there have been recent years with
substantially reduced sightings of NARWs in the broader Gulf of Maine33. On the other hand, the proportion
of the population that uses Cape Cod Bay, in the southern Gulf of Maine, appears to have increased as of late34.
Until the early 2000s, data collection on NARWs mainly consisted of visual effort from ship-based or aerial
surveys35. Visual surveys are limited by daylight, weather conditions, and the availability of suitable research
platforms at appropriate times and in appropriate locations36. Emerging technologies, such as PAM, provide new
ways to survey large areas. PAM can provide (1) continuous coverage of areas that are otherwise hard to observe
for species presence, (2) data on multiple species simultaneously, and (3) information on the acoustic habitat,
including natural and anthropogenic sounds36,37. Passive acoustic recorders deployed at different locations and
over multiple years can comprise a novel, persistent, large-scale monitoring network that is impossible to replicate
with other technologies. Such a network can provide detailed long-term information on multiple species to help
inform management38,39.
There have been numerous deployments of passive acoustic recorders along the eastern seaboard of North
America from Florida, USA to Nunavut, Canada25,40–42. These deployments were designed to answer a variety
of different individual research and management questions, but in combination, these data provide substantial
coverage of the continental shelves off the western North Atlantic Ocean since 2004. A collaborative program
across multiple institutions has brought these data together to assess the holistic occurrence of calling NARWs
throughout this area. Here, we show how these acoustic data demonstrate changes in the distribution of NARWs
across the eastern seaboard of North America during the past decade (2004–2014).

Results

A total of 35,600 days of acoustic recordings were processed with the Low Frequency Detection and Classification
System (LFDCS)43, and subsequent NARW upcall detections were manually reviewed, resulting in 2,527 days
(7%) with confirmed NARW acoustic presence. NARWs were acoustically present along the entire eastern seaboard of North America from the western Scotian Shelf (region 3) to the waters off Jacksonville, Florida (region
10) throughout the winter months from late October through early April, with the exception of no detections on
the Scotian Shelf from December through February (Fig. 1). A decrease in detections was seen in summer months
in southern regions, reflecting the known movement of breeding individuals towards northern feeding grounds.
NARWs were also detected near Iceland and Greenland (region 2) from July-October (see previous analysis by
Mellinger, et al.22). Davis Strait (region 1) contained one day with possible NARW detections in both December
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Figure 1.  Weekly Presence Summary: Boxplots representing the number of days per calendar week with
confirmed North Atlantic right whale upcall acoustic presence in each region described in Fig. 4 and for
all years of the study (2004–2014). Horizontal lines within the boxes indicate the median, box boundaries
indicate the 25th (lower boundary) and 75th (upper boundary) percentiles, vertical lines indicate minimum and
maximum values, and black dots represent outliers. Grey blocks indicate weeks where no data were available for
that region.

and March. However, with high bowhead whale (Balaena mysticetus) and humpback whale calling also present,
NARW presence could not be confirmed with confidence. There were no NARW detections in any recordings
from Bermuda or the Caribbean (region 11), confirming the likelihood that NARWs do not currently venture
into the waters surrounding Bermuda and the Caribbean.
Average weekly acoustic presence was broken up into two time periods representing before (2004–2010) and
after (2011–2014) the described distribution shift starting in 2010 (Fig. 2)33. To test whether the occurrence
of right whales in regions differed over the two time periods, we ran a Generalized Linear Model (GLM; see
methods) with the number of days in which whale calls were detected as the dependent variable, and the time
periods (2004–2010; 2011–2014) and regions as independent variables, with their interaction effects included in
the model. There were too few data (in some time*region cells) available for Davis Strait, Iceland and Greenland,
Georges Bank, Cape Hatteras, and Bermuda and the Caribbean (regions 1, 2, 6, 9 and 11) for them to be included
in the model.
For all other regions, both factors and their interactions were significant (Table 1). Pairwise comparisons of
time periods across individual regions (run using phia::testInteractions) demonstrated differences between the
two time periods (Table 2), except for in Massachusetts Bay (region 5). Northern regions (3 and 4; Scotian Shelf
and Gulf of Maine) saw a reduction in calls in 2011–2014, but mid-Atlantic regions (7 and 8; southern New
England and the mid-Atlantic), and the Southeastern U.S. (region 10) saw increases (Table 3, produced using
phia:interactionMeans). We used the False Discovery Rate44 to adjust for alpha-value inflation.
Spatial distribution of NARW acoustic occurrence was summarized from 2004–2014 by seasons in Fig. 3.
Seasons were defined based on Roberts, et al.45 with November to February as Winter, March to April as Spring,
May to July as Summer, and August to October as Fall. Seasonal acoustic occurrence of NARWs reflected patterns
seen in the daily presence plots (Figs 1 and 2), with NARW presence along the entire coast in both winter and
spring seasons. Across all seasons, NARWs were detected from Cape Hatteras (region 9) to Nova Scotia (region 3),
highlighting the expansive habitat of NARWs for most of the year (Fig. 3).
The LFDCS, with a Mahalanobis Distance (MD) threshold of 3.0, missed days with NARW upcalls at an
estimated rate of 31%; if we used a threshold of one true upcall detection per day for daily presence, the rate of
missed days with NARW upcalls would have been 25%. Manually tallied upcalls from the Gulf of Maine analysis21
revealed that days where LFDCS detected true NARW upcalls had a median of 259 calls per day and ranged from
20–2770 calls per day, and days where LFDCS missed NARW upcalls had a median of 7 calls per day and ranged
from 1–66 calls per day. It is important to note that detection rates, and therefore missed detection rates, will be
highly variable due to variability in ambient noise levels, recorder types, habitat, bathymetry, and calling behavior
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Figure 2.  Weekly Presence Comparison from 2004–2014: Boxplots representing the number of days per
calendar week with confirmed North Atlantic right whale upcall acoustic presence in each region described
in Fig. 4 and for each time period of interest (2004–2010 and 2011–2014). Horizontal lines within the boxes
indicate the median, box boundaries indicate the 25th (lower boundary) and 75th (upper boundary) percentiles,
vertical lines indicate minimum and maximum values, and black dots represent outliers. Grey blocks indicate
time periods where no data were available for that region.

of the animals. However, this analysis indicates that at greater calling rates, indicative of higher calling activity and
potentially reflective of multiple calling individuals, daily NARW acoustic presence was most likely captured in
areas where they were vocalizing. Sporadic calls, which may be produced by lone individuals passing through an
area, were less likely to be detected in this analysis (note that lone animals can be missed by visual surveys, particularly aerial surveys, as well). Given the main goals of capturing broad-scale movements of the entire population with this analysis, failing to detect the presence of some individuals does not compromise the overall results.

Discussion

This study on acoustic presence of NARWs demonstrates this species’ high mobility and broad geographic range.
It also shows that the habitats NARWs frequent can change over time and that they are often distributed widely
across their entire range. Previous understanding of NARW movements assumed the majority of the population migrated between the calving grounds in winter months and northern feeding grounds in summer months,
where visual survey effort was therefore concentrated27,46. Little was known about the whereabouts of whales that
did not frequent these habitats, or the exact timing and location of other important areas. This study demonstrates nearly continuous year-round NARW presence across their entire habitat range, particularly north of
Cape Hatteras, suggesting that not all of the population undergoes a consistent annual migration. It is possible
that the non-migrating whales could be mobile individuals occupying broader areas throughout the year, like
some subspecies of blue whales in the Antarctic47, or individuals that do not migrate annually, like a portion of
the east Australian population of humpback whales5. Our data clearly demonstrate that NARWs occur along the
entire eastern seaboard of North America for most of the year, even if that distribution has shifted within the past
decade.
The acoustic data supports NARW distributional changes that have been recently observed during visual
surveys. Furthermore, the acoustic data provide additional insights into where NARWs are located at times of
the year when the poor weather and lack of light make visual surveys highly restricted (i.e., late fall to early
spring). NARWs appear to have shifted from previously prevalent northern grounds, such as the Bay of Fundy
and greater Gulf of Maine (regions 3 and 4), to spending more time in mid-Atlantic regions year-round (regions
7 and 8)25,48. In addition, improved visual survey effort in Canadian waters shows that NARWs have also been
sighted more frequently further north in areas such as the Gulf of Saint Lawrence35. Acoustic data are currently
being processed to allow further evaluation of this region, but were not available for this study. This is an area of
extreme importance, with at least 10 linked NARW deaths this year alone49. Moving forward, it is crucial to have

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Figure 3.  Seasonal Occurrence Maps: The number of days per season with confirmed North Atlantic right
whale (NARW) upcall acoustic detections, summarized for all available recordings locations (2004–2014).
Filled orange circles indicate NARW acoustic presence, and circle size indicates the number of days with NARW
acoustic detections during a season. White dots indicate recorder locations with no NARW acoustic presence
for any year during that season. Figure produced with ArcGIS 10.3.1 (http://www.arcgis.com); background map
credits: Esri and GEBCO.
better available coverage for past and future monitoring efforts in such important regions where entanglement
and ship strike threats are prominent. Likewise, we know of past NARW occurrence in historical habitat areas
such as Greenland and Iceland, with acoustic presence in 200722, and it would be valuable to understand current
usage of this region and confirm the northern extent of their range with more recent data. Therefore, this type of
long-term, large-scale approach using PAM is invaluable for helping to track presence, and changes in presence,
of key species to understand (1) how they have shifted their distribution and (2) whether these shifts puts them at
increased risk from anthropogenic threats.

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Response: # days with
NARW presence

Chi-Square

Degrees of Freedom

P-value

timePeriod

21.57

1

3.41e-06***

Region

542.81

5

<2.2e-16***

timePeriod:Region

489.70

5

<2.2e-16***

Table 1.  Results of the Poisson Generalized Linear Model (GLM) testing whether the occurrence of North
Atlantic right whales in the regions differed over two time periods (2004–2010; 2011–2014). The number
of days in which whale calls were detected is the dependent variable, and the time periods and regions are
independent variables, with their interaction effects included in the model. Davis Strait, Iceland and Greenland,
Georges Bank, Cape Hatteras, and Bermuda and the Caribbean (regions 1, 2, 6, 9 and 11) are excluded from
the model due to insufficient data in some time*region cells. For all other regions, both factors and their
interactions were significant. Table 1 shows the overall GLM results between the two time periods (timePeriod)
and region (Region). (***) indicates significance of P < 0.001.

Region

Value

Degrees of Freedom

Chi-Square

P-value

A-B: 3

5.07

1

136.43

<2.2e-16***

A-B: 4

3.19

1

28.31

1.24e-07***

A-B: 5

1.03

1

0.07

0.78

A-B: 7

0.18

1

99.70

<2.2e-16***

A-B: 8

0.18

1

59.67

2.25e-14***

A-B: 10

0.46

1

42.21

1.23e-10***

Residuals: 42

Table 2.  Results from the Poisson Generalized Linear Model (GLM) testing between the two time periods
(A–B) for each region separately, using the False Discovery Rate44 to correct for alpha-value inflation. Pairwise
comparisons of time periods across individual regions were run using phia::testInteractions69. (***) indicates
significance of P < 0.001.

Time Period

Region

Adjusted Mean Standard Error of Link

2004–2010

3

18.89

0.07

2011–2014

3

3.73

0.12

2004–2010

4

20.85

0.05

2011–2014

4

6.53

0.21

2004–2010

5

20.71

0.07

2011–2014

5

20.06

0.09

2004–2010

7

2.12

0.17

2011–2014

7

12.06

0.04

2004–2010

8

1.18

0.21

2011–2014

8

6.40

0.07

2004–2010

10

4.81

0.10

2011–2014

10

10.40

0.06

Table 3.  Values for each region and time period individually from the Poisson Generalized Linear Model
(GLM). Results were produced using phia:interactionMeans69.

It remains unclear if these observed distribution shifts are due to environmental or anthropogenic effects, if
they are a response to short-term changes in the environment, or part of a longer-term cycle in which NARWs
shift their distribution. With recent studies finding the Gulf of Maine is the fastest warming body of water in the
world50, it is not surprising to see distributional changes across marine species. We suspect further changes in
distributions will occur as water temperatures continue to rise, forcing movements towards both favorable oceanographic conditions and food sources elsewhere. Regardless of the factors influencing these changes in distribution, it is critical for management strategies to reflect new threats that may arise for this species as they move into
regions outside of existing management areas51.
The purpose of this study was to provide baseline information of NARW distributions across their current
range. Here, PAM data was used for distributional analysis and assumes homogeneity in the probability of upcalls
across all regions. We caution that this is not necessarily the case, as known calling rates vary among demographic
groups, such as quieter mother-calf pairs52, resulting in lower detection probabilities. Call rates are also lower
with certain behaviors, such as foraging and logging53, and therefore it is reasonable to suspect different calling
rates over the different habitats and seasons summarized in this study. Additional variation exists across all the
regions, with detection probabilities likely varying with different acoustic habitats, bathymetry and recorders.
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Figure 4.  Recorder Locations: Locations of available passive acoustic recorders used for this study spanning from
Bermuda and the Caribbean (bottom right map inset) to the northernmost locations in Davis Straight (top left
map inset). Yellow points indicate the locations of recorders available from 2004–2010; black points indicate the
locations of recorders available from 2011–2014; and pink points indicate locations of recorders available for both
time periods. Red boundaries outline the designated regions, which were defined by the historical distribution
patterns of North Atlantic right whales across their range. Region numbers correspond to the following geographic
areas: 1. Davis Strait; 2. Iceland and Greenland; 3. Scotian Shelf; 4. Northern Gulf of Maine; 5. Massachusetts Bay;
6. Georges Bank; 7. Southern New England; 8. Mid-Atlantic; 9. Cape Hatteras; 10. Southeastern U.S.; 11. Bermuda
and the Caribbean. Figure produced with ArcGIS 10.3.1 (http://www.arcgis.com); background map credits: Esri
and GEBCO.

Risch, et al.12 show that ambient noise levels varied across three sites included in this study (Massachusetts Bay,
mid-Atlantic, and Southeastern U.S.; regions 5, 7, and 10) over each season, affecting detection probabilities and
ranges; while Rice, et al.54 provide detailed results on the varying acoustic environment at 10 different sites within
the NARW range. These studies highlight the challenges with large-scale datasets, however, further analysis into
detection ranges were beyond the scope and goal of this study. Thus, this work used an average range of detection
distances based on published studies55,56. As such, it provides a minimum estimate of NARW presence, with the
understanding that in some regions the detection ranges may have been slightly more constricted or wider than
the average used.
This study integrates data from a suite of smaller-scale studies focused on the fine-scale occurrence of NARWs
and other species. Comparing our results with these smaller-scale studies such as Hodge, et al.48, Bort, et al.21,
Salisbury, et al.25, Mellinger, et al.22, Morano, et al.23, and Whitt, et al.57, there are time periods with discrepancies
between the NARW acoustic detections presented in our study and these previous studies, with the previous
studies finding right whale calls in some months that we did not find confirmed detections. This is likely due to
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Figure 5.  Recording Effort: Figure indicating the proportion of year with available passive acoustic recordings
in each region (see Fig. 1). Years are split into quarters from January 2004 to December 2014. Black indicates at
least one recorder present for the entire quarter year for that region, lighter gray indicates a portion of that time
period with recordings, and white indicates no available acoustic data for that region and time period.

a difference in detectors and analysis methodology. In order for this study to include and process such a large
acoustic dataset, it was necessary to use an automated detector, LFDCS, and manually review these detections
at a coarser level than may have been done in the smaller-scale studies. Our review of the LFDCS performance
with a MD threshold of 3.0 at three sites found that 31% of days with right whales present were missed when
compared to a full manual evaluation of the data, and that days with lower calling rates tended to be missed more
often. Therefore, our results represent a minimum presence compared with these more detailed studies, and it is
possible that NARWs have higher occurrence in some areas than is reported here. The missed detection rate could
be reduced by lowering our classification threshold (i.e., increasing the maximum MD), but at the cost of increasing the time required to manually screen each detection, since increasing the maximum MD will result in more
detections that must be reviewed by an analyst. As with all detection systems, there is a balance between accuracy
and processing time that must be considered when choosing a detector within the context and scope of any study.
This is one of the first comprehensive, long-term passive acoustic studies to investigate an entire habitat range
for a marine mammal at this temporal and spatial scale, and is made possible only by the cooperation and collaboration of an extensive research community. Even in areas where data were collected for alternate purposes,
the combined contributed recordings provided crucial information to assess both the acoustic occurrence and
changing distribution of the NARW population. This analysis demonstrates what can be accomplished for other
poorly understood species, and encourages broad research collaborations in the future. All contributing data sets
were combined for the common goal of understanding the distribution of a critically endangered species facing
extreme threats from anthropogenic and environmental influences. In planning future large-scale studies, standardization of acoustic recorders and methods should be considered to improve the quality of datasets.
PAM is a powerful, cost-effective, long-term monitoring tool that can give a better understanding of temporal
trends and reveal range expansion, decline, or distribution shifts in populations, as well as interannual changes.
This information can be used to direct science and management to focal areas of interest. Most importantly, in
an ocean where conditions are changing rapidly, adaptive management is needed to identify and protect areas
that are crucial for species on the brink of extinction. Potential ways forward include setting up real-time passive
acoustic monitoring systems (see NEPAN39), or thinking beyond the traditional means of classifying NARW
critical habitat as static, confined areas. This is especially relevant when considering the mobile nature of this
species whose distribution patterns may still be changing. It is imperative to continue effective surveys and timely
conservation efforts to ensure the recovery of this endangered species.

Methods

Data collection.  Passive acoustic data collected for a multitude of different research projects and goals were
combined to examine a decade-long acoustic record of the spatial and temporal occurrence of NARWs throughout the western North Atlantic Ocean. This large area was divided into 11 regions, covering the main historical
areas where the existing NARW population has been found since the late 1600s, ranging from Florida, USA to
Cape Farewell, Greenland58. Recorders were assigned a region based on the biological and geographical importance of the area (Fig. 4). Region 3 was broken up to include subregion 3A, representing the Gulf of St. Lawrence:
an area having an increase in NARW sightings and research effort over the last few years, but insufficient acoustic
data available to contribute to this study. Data were examined across two time periods (2004–2010 and 2011–
2014) representing before and after the observed distribution shift in 2010. Due to the ad hoc nature of this largescale collaborative project, available data are patchy, a few recorders were duty cycled, and there are some regions
or time-periods with no available data (Fig. 5).
Five types of bottom-mounted passive acoustic recorders were deployed from 2004 through 2014 (See
Supplementary Table 1): the High-frequency Acoustic Recording Package (HARP)59, the Marine Autonomous

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Figure 6.  A spectrogram example produced by the Low Frequency Classification and Detection System,
showing four North Atlantic right whale upcalls with their corresponding pitch tracks (black and colored lines).
Warmer colors on the selected (colored) pitch track indicate high amplitudes of sound, while cooler colors
indicate lower amplitudes.

Recording Unit (MARU)55, the Autonomous Multichannel Acoustic Recorder (AMAR)60, the National Oceanic
and Atmospheric Administration’s (NOAA) Pacific Marine Environmental Laboratory’s (PMEL) Moored
Autonomous Hydrophones (HARU)61, and the Guardbuoy (http://geospectrum.ca/guard-buoy). Data collected from each of these recorders varied from a minimum of 25 days to a maximum of 2 years (Supplementary
Table 1). Some recorders (59 out of 324) were duty cycled, ranging in recording from 12–95% of the time, while
most (265 out of 324) recorded continuously. The spatial configuration in which they were deployed varied from
single units to lines and arrays of recorders; the configuration was determined by the original goal of the specific
research project in question (Fig. 4). The majority of recordings were sampled at 2 kHz, with some ranging up to
250 kHz. All recordings were low-pass filtered and decimated to 2 kHz for analytical consistency across data in
order to make them comparable.
Maximum detection ranges for NARWs can vary considerably, depending on recording equipment, location,
and environmental conditions, as well as call type and behavioral context62, but are estimated to range from 8 to
16 km55,56. Consequently, single recorders were selected for analysis from array configurations with units spaced
less than 8 km apart, to ensure full coverage of the area while minimizing duplicative detections across recorders.
We focused our analyses on data collected between January 2006 to December 2014, with the exception of data
collected in 2004 and 2005 in the Bay of Fundy, Emerald Basin, and Roseway Basin, Canada, since these were the
only long term recordings available for these areas that had previous well-known occurrence of NARWs. Data
from a total of 324 recorders were analyzed, comprising 35,600 recording days of data. All government funded
acoustic data are publicly available upon request from the data owner.

Detection and classification of NARW calls.  All acoustic data were processed using the Low Frequency
Detection and Classification System (LFDCS)43 which creates conditioned spectrograms (Fig. 6) using the
short-time Fourier transform with a data frame of 512 samples and 75% overlap resulting in a time step of 64
ms and frequency resolution of 3.9 Hz. After tracing contour lines, or “pitch tracks”, through tonal sounds, the
program uses multivariate discriminant analysis to classify the pitch tracks into call types. Calls were classified
based on a user-developed call library; our library included four North Atlantic baleen whale species: NARW,
fin (Balaenoptera physalus), sei (B. borealis), and humpback whales. Here, we focused only on the detections
classified as NARW calls, specifically the low-frequency modulated upsweep known as the upcall. The upcall is
a contact call used throughout the NARW range, produced by all ages and both sex classes, and is therefore the
most reliable call to use for determining right whale presence53,55.
The call library described in Baumgartner and Mussoline43 was expanded and improved for this analysis to
include a wider variety of examples of NARW upcalls and increase detection probability. Each detection was
assigned a MD, which measures the deviation of a detection from the assigned call type (see Baumgartner and
Mussoline43 for a more complete description). A lower MD indicates a closer match to the assigned call type. All
NARW upcall detections with a MD less than or equal to 3.0 (after Baumgartner, et al.63) were manually screened
by experienced analysts to determine which were correctly classified. For an ideal call type in the LFDCS (i.e.,
the seven attributes used in the discriminant function analysis are multivariate normal), 75% of actual calls will
have a MD of 3.0 or less63; we chose this threshold to make the laborious process of manual screening manageable at the expense of sometimes missing genuine right whale upcalls (see below). This approach ensured that
false detections were eliminated. The high degree of variability in NARW upcalls and the overlap with other
species’ vocalizations, such as upsweeps produced by humpback whales, necessitated this extra manual step in
data processing63.
For continuous data, a given day was marked as having NARWs present if three or more true upcall detections
were found. Three upcalls were used to establish presence in order to be conservative and confident in stating
NARW presence (we also conducted all analyses using a criterion of one upcall per day to indicate daily right
whale presence, but neither our results nor conclusions changed). For duty-cycled data, the criteria was dropped
to one true upcall detection signifying NARW presence so that presence was not underestimated due to a lower
probability of recording vocalizing animals64. Weekly NARW acoustic presence per recorder was then summarized as the number of days per calendar week with daily presence.
To test whether the occurrence of right whales in regions differed over the two time periods, we ran a
Generalized Linear Model (GLM) in R 3.4.165, using the libraries ggplot266, MASS67, car68, and phia69. This had the
number of days in which whale calls were detected as the dependent variable, and the time periods (2004–2010;
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2011–2014) and regions as independent variables, with their interaction effects included in the model. As the
call data were counts, we ran the GLM with a Poisson distribution with log-link. The number of recording days
was multiplied by the duty-cycle to correct for non-continuous data. Because effort (the number of days during
which recorders were present) varied across time and region, we included the log of the number of days during
which recorders were present plus 1 (as for some time*region cells, there were no recorders present) as an offset
in the model.
The model formula in R was:
nDaysWithWhales ~ timePeriod ∗ Region, family = ‘poisson’,
offset = log(nDaysRecordings + 1)

Detector evaluation/missed detection rate.  Detector performance was quantified to evaluate whether

the missed detection rate for upcalls resulted in underestimating full days of NARW presence in our analysis.
Three MARUs, manually reviewed for upcall presence in previous studies, were selected and used as ground-truth
datasets to compare to our findings for days with NARW acoustic presence. These sites included year-round
recordings from the Gulf of Maine (region 1); southern North Carolina (region 10); and Georgia (region 10)21,48.
Selected days from each recorder were manually screened for NARW upcalls. Every third day of the Gulf of Maine
recording was viewed and all upcalls for those days were counted (for detailed results on this analysis, see Bort,
et al.21). Output from a detector70 run on North Carolina and South Carolina units was manually reviewed for
daily presence of at least one NARW upcall (for detailed results on this analysis, see Hodge, et al.48). The resulting
data were combined and used to generate a ground-truthed dataset of days with NARW presence; this was then
compared to the number of days estimated to have NARW presence based on the manually screened output of
the LFDCS system.

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Acknowledgements

We thank Daniel Woodrich, Taylor Broadhead, Margaret Daly, and Alyssa Scott for help in data analysis for this
project. We thank Chris Pelkie, David Wiley, Michael Thompson, Chris Tessaglia-Hymes, Lance Garrison, and
Anurag Kumar for help with project planning, field work support, and data management. For all the support and
advice, thanks to the NEFSC Protected Species Branch, specifically Dana Gerlach, Annamaria DeAngelis, Heather
Heenehan, Jenni Stanley, Josh Hatch, Eric Matzen, Chris Tremblay, and Samara Haver. Thanks to the many field
and crew teams on all of the ships that helped in the numerous deployments and recoveries. This research was
funded and supported by many organizations, specified by projects as follows: Data recordings from region 1
were provided by K. Stafford and this research effort was funded by the National Science Foundation #NSFARC 0532611. Region 2 data were provided by D. K. Mellinger and S. Nieukirk, funded by National Oceanic
and Atmospheric Agency (NOAA) and the Office of Naval Research (ONR) #N00014–03–1–0099, NOAA
#NA06OAR4600100, US Navy #N00244-08-1-0029, N00244-09-1-0079, and N00244-10-1-0047. Region 3A data
were provided by D. Risch, funded by NOAA and Navy N45 programs. Region 3 data were provided primarily
by H. Moors-Murphy, the Whitehead Lab of Dalhousie University for data from the eastern Scotian Shelf and
Emerald Basin. A. Cogswell, J. Bartholette, A. Hartling, and the crew of vessel CCGS Hudson provided logistical
support and assistance with the eastern Scotian Shelf instrument deployments. Eastern Scotian Shelf and Roseway
Basin Guardbuoy data were supported through the Department of Fisheries and Oceans, Oceans and Coastal
Management, Species at Risk Management, and Strategic Program for Ecosystem-Based Research and Advice
(SPERA) funds. Emerald Bank and Roseway Basin data were provided by D. K. Mellinger and S. Nieukirk, funded
by NOAA. Region 4 data were provided by S. Parks, funded by NOAA and Cornell University and E. Summers,
S. Todd, J. Bort Thornton, A. N. Rice, and C. W. Clark, funded by Maine Department of Marine Resources,
NOAA #NA09NMF4520418, and #NA10NMF4520291. Region 5 data were provided by S. M. Van Parijs, D.
Cholewiak, L. Hatch, C. W. Clark, D. Risch, and D. Wiley, funded by National Oceanic Partnership Program
(NOPP), NOAA, and Navy N45. Region 6 data were provided by S. M. Van Parijs and D. Cholewiak, funded
by Navy N45 and Bureau of Ocean and Energy Management (BOEM) AMAPPS (Atlantic Marine Assessment
Program for Protected Species) program. Region 7 data were provided by A. N. Rice, H. Klinck, A. Warde, B.
Martin, J. Delarue, and S. Kraus, funded by the New York State Department of Environmental Conservation,
Massachusetts Clean Energy Center, and BOEM. Region 8 data were provided by G. Buchanan, and K. Dudzinski,
funded by New Jersey Department of Environmental Protection and the New Jersey Clean Energy Fund as well
as by A. N. Rice, C. W. Clark, and H. Klinck, funded by the Bioacoustics Research Program at Cornell University
and BOEM. Region 9 data were provided by J. E. Stanistreet, J. Bell, D. P. Nowacek, A. J. Read, and S. M.Van Parijs,
funded by NOAA and the Naval Facilities Engineering Command (NAVFAC). Region 10 data were provided
by L. Garrison, M. Soldevilla, C. W. Clark, R. A. Chariff, A. N. Rice, H. Klinck, J. Bell, D. P. Nowacek, A. J. Read,
J. Hildebrand, A. Kumar, L. Hodge, and J. E. Stanistreet, funded by NAVFAC, BOEM, NOAA, and NOPP. Region
11 data were provided by C. Berchok as part of a collaborative project led by the Fundacion Dominicana de
Estudios Marinos, Inc. (Dr. Idelisa Bonnelly de Calventi), with funding support from The Nature Conservancy
(Elianny Dominguez), by D. Risch, funded by World Wildlife Fund, NOAA, and Dutch Ministry of Economic
Affairs, and by A. Širović and J. Hildebrand, funded by NOAA (with support from Dr. Jason Gedamke).

Author Contributions

S.M.V.P., G.E.D., and M.F.B. designed the study. M.F.B. created the detection software. G.E.D. and J.M.B.
conducted the acoustic analysis, and P.C. conducted the statistical analyses. J.B.T. and A.N.R. provided analysis
for detector evaluation. G.E.D., S.M.V.P., and P.C. wrote the manuscript, and G.E.D. prepared the figures. J.B.,
C.B., J.B.T., G.B, R.A.C., D.C., C.W.C., J.D., K.D., L.Ha., J.H., L.Ho., H.K., S.K., B.M., D.K.M., H.M., S.N., D.P.N.,
S.P., A.J.R., A.N.R., D.R., A.S., M.S., K.S., J.E.S., E.S., S.T., A.W, and S.M.V.P. all provided the acoustic data. M.F.B.,
P.C., C.B., J.B.T., G.B., R.A.C., D.C., J.D., K.D., J.H., H.K., S.K., D.K.M., H.M., S.N., S.P., A.N.R., D.R., A.S., M.S.,
K.S., J.E.S., S.T. and S.M.V.P. all provided extensive edits to the manuscript. All authors reviewed and edited the
manuscript.

Additional Information

Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-017-13359-3.
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