Satellite observations of circulation features associated with a bowhead whale feeding ‘hotspot’ near Barrow, Alaska

Remote Sensing of Environment 115 (2011) 2168–2174

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Satellite observations of circulation features associated with a bowhead whale feeding ‘hotspot’ near Barrow, Alaska Stephen R. Okkonen a,⁎, Carin J. Ashjian b, Robert G. Campbell c, Janet T. Clarke d, Sue E. Moore e, Kevin D. Taylor a a

Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, Alaska 99775, USA Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, USA d SAIC, 14620 268th Ave. E., Buckley, Washington 98321, USA e NOAA/Fisheries Science & Technology-OERD2, 7600 Sand Point Way NE, Seattle, Washington 98115, USA b c

a r t i c l e

i n f o

Article history: Received 1 October 2010 Received in revised form 21 March 2011 Accepted 16 April 2011 Available online 11 May 2011 Keywords: Bowhead whales Fronts Synthetic aperture radar Oceanography Shelf break currents

a b s t r a c t Satellite images, along with oceanographic, meteorological, and whale aerial survey data, are used to illustrate aspects of ocean circulation associated with a bowhead whale feeding ‘hotspot’ near Barrow, Alaska. In response to weak winds, a strong front forms near the shelf-break along the southern edge of Barrow Canyon when the Alaska Coastal Current flows adjacent to the southern flank of Barrow Canyon or intrudes onto the western Beaufort shelf. This front is of particular local interest because it is indicative of aggregation and retention of zooplankton on the western Beaufort shelf and, as a result, is a locus for bowhead whales pausing to feed during their westward fall migration. Groups (4 or more individuals) of bowhead whales are primarily seen on the western Beaufort shelf following wind conditions that promote the formation of this front. © 2011 Elsevier Inc. All rights reserved.

1. Introduction Like the North Atlantic right whale, bowhead whales are planktivorous and require high concentrations of large copepods and/or euphausiids or krill to feed efficiently (Baumgartner et al., 2003; Baumgartner & Mate, 2003; Kenney, 2001; Lowry et al., 2004; Mayo & Marx, 1990). Convergences (or fronts) associated with canyons or other abrupt topographies are known, under certain conditions, to concentrate zooplankton and other marine organisms in sufficient abundance to be favorable feeding sites for planktivores and consumers of planktivores (see Genin, 2004 for a review). A ‘hotspot’ for bowhead whale feeding develops in late summer and early autumn of most years near Point Barrow, Alaska where Barrow Canyon incises the eastern end of the Chukchi shelf and abuts the western Beaufort shelf (Fig. 1). Here, bowhead whales may encounter enhanced feeding opportunities during their westward fall migration (Moore et al., 2010). According to a conceptual model proposed by Ashjian et al. (2010), eastward intrusion of Alaska Coastal Current (ACC) waters from Barrow Canyon (Okkonen et al., 2009) converge with Beaufort shelf waters, aggregating and trapping zooplankton (primarily krill that have been transported from the

⁎ Corresponding author. Tel.: + 1 907 283 3234. E-mail address: [email protected] (S.R. Okkonen). 0034-4257/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2011.04.024

Bering Sea (Berline et al., 2008) and subsequently upwelled onto the Beaufort shelf) adjacent to the southeastern edge of Barrow Canyon and on the inner western Beaufort shelf. In this paper, we use satellite images of the Barrow area ocean surface, Barrow wind records, ocean current measurements from the western Beaufort shelf, and sighting locations of bowhead whales from aerial surveys to refine and expand upon the Ashjian et al. (2010) conceptual model by showing that (1) favorable feeding conditions giving rise to a bowhead whale feeding hotspot on the western Beaufort shelf are generally associated with the occurrences of weak winds, east-northeastward-flowing currents near the southern edge of Barrow Canyon, and the presence of a front along the southern edge of Barrow Canyon and (2) less favorable feeding conditions are generally associated with moderate-to-strong winds from the east, northwestward-flowing currents near the edge of Barrow Canyon, and the absence of a front along the southern edge of Barrow Canyon. 2. Methods and data A low-profile, bottom-mounted oceanographic mooring, instrumented with an upward-looking 307 kHz Teledyne/RDI acoustic Doppler current profiler (ADCP) was deployed 14 km northeast of Point Barrow at 71.45°N 156.13°W in 15 m of water near the shelfbreak along the southern flank of Barrow Canyon (Fig. 1) during

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Fig. 1. Map of the Barrow study area. The blue asterisk indicates the location of the current meter mooring. The red line shows the location of the 40-m isobath, a representative location of the inshore base of the shelf-break front during weak wind conditions.

August–September of both 2008 and 2009 (1815 ADT Aug. 22 —1830 ADT Sept. 8, 2008 and 1445 ADT Aug. 22—1800 ADT Sept. 14, 2009). A mooring deployed at this location in 2007 was lost, presumably due to bottom scouring by sea ice, and never recovered. Consequently, the relatively brief mooring deployment periods in 2008 and 2009 reflect multiple operational constraints: the desire to acquire measurements during the open water season when bowhead whales are often observed in the Barrow area, exposure to sea ice hazards is minimized, and the need to recover the moorings before the onset of the local fall whale hunt and impending freeze up. The moored ADCPs acquired water column velocity measurements in 0.5-m bins at 15-minute intervals. Current measurements at each time step were verticallyaveraged and tidal currents, estimated using a least squares fit of the data to M2, N2, S2, K1, O1, and P1 tidal constituents, were removed. The resulting non-tidal currents were then sub-sampled at hourly intervals for comparison with hourly wind data. Wind speed and direction measurements at Barrow were obtained from the Climate Monitoring and Diagnostics Laboratory (ftp.cmdl.noaa. gov/met/hourlymet/brw/) for years 1982–2006 and from the Atmospheric Radiation Measurement website (http://www.archive.arm.gov) for years 2007–2010. An hourly interval time series was generated for use as a working data set. Relationships between winds, whale sighting locations, and currents are based on average wind speeds and directions from the 12-hour periods preceding the whale and current observations. The 12-hour averaging period is a representative time scale for zooplankton accumulation on the western Beaufort shelf (Ashjian et al., unpubl.). Synthetic aperture radar (SAR) images of the Barrow area, acquired by the Canadian Space Agency Radarsat-1 and European Space Agency ERS-2 satellites, were obtained from the Alaska Satellite Facility. ENVI software was used to geocode the radar images. Visibleband images of the Barrow area acquired by the NASA Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) were obtained in geocoded format from the NASA/GSFC MODIS Rapid Response System website (rapidfire.sci.gsfc.nasa.gov/subsets/index.php?subset = AERONET_Barrow). The 40-m isobath, a representative delineation of the shelf-break front during weak wind conditions (see Fig. 7, Okkonen et al., 2009), is overlaid on each satellite image. Bowhead whales were counted during line transect aerial surveys conducted from September–October 1982–2009 as part of the

Bowhead Whale Aerial Survey Project (BWASP; Monnett & Treacy, 2005). This time period coincides with the annual westward migration of the whales from their summering grounds in the Canadian Beaufort Sea to the Bering Sea. The Barrow study area represents a subset of BWASP data; surveys were conducted annually over a much larger geographic area so survey effort in the study area varied among years. Sighting data were limited solely to those collected while on transect because they were collected systematically over the 28-year span and represent the most robust and least biased data subset. Surveys were flown in Grumman Turbo Goose and deHavilland Twin Otter aircraft at 305 to 460 m altitude, maintaining a speed of 220 to 300 km h− 1. From 1982 to 2008, transects were oriented approximately north–south and located between randomly determined start and end points. Transect design was altered in 2009 for the area 157–158°W, with transects positioned perpendicular to the coast (Clarke & Ferguson, 2010). Two primary observers maintained a continuous watch for marine mammals, one on each side of the aircraft, while a third observer/data recorder entered data into a computer for each sighting and whenever survey conditions changed, or every five to ten minutes. Data routinely logged when bowheads were seen included time, altitude of aircraft, position, sea state, ice cover, visibility, inclinometer angle (to determine distance from the trackline), number of whales, initial animal heading and apparent behavior. Whales within five body lengths of one another were logged as belonging to the same group. A whale more than five body lengths from the nearest whale was logged as an individual whale. Of the logged data, only the times, locations, and numbers of whales were used in this study. Additional details of the survey protocol are provided elsewhere (e.g., Monnett & Treacy, 2005; National Marine Mammal Laboratory et al., 2010). 3. Results 3.1. Wind-current regimes A histogram of wind directions from the long-term (1982–2009) record of mid-August/mid-September winds at Barrow (Fig. 2, top panel, narrow black bars) shows that prevailing winds are from the eastern quadrant. While the prevailing winds associated with the mid-August/mid-September 2008–2009 current meter deployment

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Wind Speed (m s-1) Fig. 2. (Top panel) Histogram of wind directions for mid-August/mid-September 1982–2009 (narrow black bars); histogram of wind directions for mid-August/mid-September 2008–2009 (light gray bars correspond to wind speeds b5 m s− 1, dark gray bars correspond to wind speeds ≥5 m s− 1). (Lower left panel) Scatterplot of wind-current relationships at the mooring location during 2008–2009 mid-August/mid-September deployment periods. (Right panel) Histogram of depth-averaged current directions (light gray bars correspond to currents associated with wind speeds b 5 m s− 1, dark gray bars correspond to currents associated with wind speeds ≥ 5 m s− 1).

periods (light gray and dark gray bars) are also from the eastern quadrant, winds from the east-southeast occurred relatively more frequently and winds from the northeast occurred relatively less frequently compared to the long-term wind record. A multi-color scatterplot (Fig. 2, lower left panel) depicts the association between hourly depth-averaged currents recorded by ADCP moorings during the 2008–2009 field seasons at the southeastern edge of Barrow Canyon and 12-hour average contemporaneous winds. The histogram of these current directions (Fig. 2, lower right panel) has two broad peaks; one peak is centered on currents directed to the east-northeast and a second peak is centered on currents directed toward the north-northwest. Taken together, the histogram of wind directions, histogram of current directions, and scatterplot suggest two principal generalized wind-current regimes: (Regime 1) when winds are weak (purple-blue-teal diamonds), regardless of wind direction, shelf-break currents most often flow toward the eastnortheastern quadrant onto the Beaufort shelf or along the shelfbreak (see Fig. 1) and (Regime 2) when winds from the eastern quadrant are moderate-to-strong (green–yellow–red diamonds), shelf-break currents most often flow toward the north-northwest quadrant off the Beaufort shelf into Barrow Canyon. The average wind speeds associated with wind-current regime 1 (blue box) and regime 2 (red box) are 4.4 m s− 1 and 6.6 m s− 1, respectively, suggesting that, in the present context, a reasonable wind speed threshold between weak winds and moderate-to-strong winds is ~ 5 m s− 1.

3.2. Satellite images of circulation features 3.2.1. Regime 1 — weak winds A MODIS Aqua false color image acquired 23 Aug 2008 (Fig. 3A) shows the Barrow area ocean surface a day after the current meter mooring was deployed near the edge of Barrow Canyon. This is the only date during the 2008 and 2009 current meter deployment periods for which skies were clear over Barrow Canyon and a visibleband image of the local study area was obtained. Average winds were from the north-northeast and weak (0.3 m s− 1) and currents at the mooring site were to the east at 14 cm s− 1. A filamentous suspended sediment plume extends from Point Barrow east-northeastward ~70 km along the shelf-break bordering Barrow Canyon after which it turns back to the west. This filamentous plume is the relict of an extensive sediment plume (see Fig. 4A) that was originally driven southwestward around Point Barrow and along the Alaskan Chukchi coast by prolonged strong winds from the east-northeast. The boundary between the sediment plume and clearer water in Barrow Canyon provides a visual signature of the front between canyon and shelf waters. The southeastward indentation of this front suggests intrusion of waters from Barrow Canyon onto the western Beaufort shelf. Although not concurrent with the oceanographic mooring deployment periods, satellite images acquired when free-floating sea ice is present can better illustrate the existence and locations of fronts/

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Fig. 3. (A) MODIS Aqua false color image acquired 23 August 2008, (B) SAR image acquired on 23 October 2007, (C) MODIS Aqua quasi-true color image acquired on 13 July 2009, and (D) MODIS Aqua quasi-true color image acquired 24 July 2010. The location of the 40-m isobath is shown as a red or white line. The annotated pink arrow identifies the wind speed and direction. The annotated blue arrow identifies the current speed and direction.

convergence zones associated with Regime 1 conditions. For example, a Radarsat-1 image acquired on 23 October 2007 (Fig. 3B) when average winds were weak (4.2 m s− 1) and from the northeast clearly shows a frontal boundary along the southern edge of Barrow Canyon separating open water over the canyon from grease/new ice that is largely confined to the western Beaufort shelf. A second front, extending southeastward from Point Barrow and lying roughly parallel to the Beaufort coast, separates more consolidated sea ice from less consolidated sea ice. Other representative examples in which the existence and locations of fronts/convergence zones on the western Beaufort shelf are revealed by free-floating sea ice are provided in MODIS images acquired on 13 July 2009 (Fig. 3C) and 24 July 2010 (Fig. 3D). The 13 July 2009 image, acquired when average winds were weak (2.3 m s− 1) and from the east, shows a front along the southern flank of Barrow Canyon between the comparatively ice-free western Beaufort shelf and the northeastwardflowing, ice-laden ACC. The 24 July 2010 image, also acquired when winds were weak (4.1 m s− 1) and from the east, shows that sea ice has been displaced from Barrow Canyon by the northeastward-flowing ACC resulting in a well-defined front between open water over the canyon and ice-covered water on the Beaufort shelf. A band of concentrated sea ice lying parallel to the Beaufort coast indicates the presence of a secondary front similar in location to that observed in Fig. 3B.

3.2.2. Regime 2 — moderate-to-strong winds from the east A MODIS Aqua false color image acquired 16 Aug 2008 (Fig. 4A) shows ocean surface conditions after three days of sustained moderate-to-strong winds from the east-northeast. The distribution of suspended sediments indicates that this wind event has driven western Beaufort inner shelf waters southwestward around Point Barrow along the Alaskan Chukchi coast and, in so doing, disrupted the shelf-break front along the southern edge of Barrow Canyon. Free-floating sea ice can also serve as a tracer for surface currents responding to strong winds from the east. A Radarsat-1 image acquired on 30 October 2007 (Fig. 4B; i.e. one week after the Fig. 3B image) during strong winds (7.1 m s− 1) from the east shows the grease/new ice that was previously confined to the western Beaufort shelf has been pushed northwestward, off-shelf across Barrow Canyon. The fronts along the southern edge of Barrow Canyon and parallel to the Beaufort coast that were present during weak wind conditions (see Fig. 3B) are no longer present. An ERS-2 image acquired on 22 October 2009 (Fig. 4C) shows grease/new ice being blown southwestward around Point Barrow by very strong (16.6 m s− 1) winds from the northeast. A front along the southern edge of Barrow Canyon is absent. Well-defined fronts are also absent from the canyon edge and Beaufort coast in a MODIS image acquired on 28 July 2005 during moderately strong winds (6.2 m s− 1)

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Fig. 4. (A) MODIS Aqua false color image acquired 16 Aug 2008, (B) SAR image acquired on 30 October 2007, (C) SAR image acquired on 22 October 2009, and (D) MODIS Aqua quasitrue color image acquired 28 July 2005. The location of the 40-m isobath is shown as a red or white line. The annotated pink arrow identifies the wind speed and direction.

from the east (Fig. 4D). The image shows free-floating sea ice to be dispersed across the western Beaufort shelf and across the mouth of Barrow Canyon. Representative satellite images presented in this section (Fig. 3A–D) showed that suspended sediments and sea ice are concentrated along fronts and retained on the western Beaufort shelf when winds are weak. If the associated convergent current regime also concentrates and retains zooplankton in sufficient quantities to support efficient feeding by groups of whales, then aerial observations of whale groups should be more common over the shelf when winds are weak. Conversely, if, as indicated by Fig. 4A–D, moderate-to-strong winds from the east do not promote current convergence and attendant accumulation and retention of zooplankton on the western Beaufort shelf, then aerial observations of whales groups should be less common during these windier conditions. These wind–whale relationships are investigated in the next section.

3.3. Whale sightings A total of 548 aerial survey sighting locations representing 1564 bowhead whales were recorded while on transect within the Barrow study area (70.8–72.0°N, 153.0–158.0°W) during September–October 1982–2009. Of these, 422 locations representing 1287 whales were associated with average wind speeds of less than 5 m s− 1. The average whale group size (3.05 whales) associated with these observations is a convenient threshold to partition the observations into small groups (1–3 whales) and large groups (≥4 whales). During weak wind conditions, small groups (Fig. 5A) were observed both on the shelf (237 groups; 328 whales) and in deeper waters seaward of the 40-m isobath (123 groups; 172 whales). However, large groups (Fig. 5B) were much more likely (53 of 62 groups; 730 of 787 whales) to be observed in shallow (b40 m) shelf waters than in deeper waters when winds were weak. When winds from the east were moderate-

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Fig. 5. (A) Locations of small groups (1–3 whales) sighted by BWASP observers during weak winds (b5 m s− 1) September–October, 1982–2009. (B) Locations of large groups (4 or more whales) sighted by BWASP observers during weak winds (b5 m s− 1) September-October, 1982–2009.

Fig. 6. (A) Locations of small groups (1–3 whales) sighted by BWASP observers during moderate-to-strong (≥ 5 m s− 1) winds from the east, September–October 1982–2009. (B) Locations of large groups (4 or more whales) sighted by BWASP observers during moderate-to-strong (≥ 5 m s− 1) winds from the east, September–October 1982–2009.

to-strong, small groups (Fig. 6A) were seen to be more or less evenly distributed between shelf waters (20 groups; 27 whales) and deeper waters (21 groups; 22 whales), whereas large groups (Fig. 6B) were not observed at all in deep waters and rarely observed in shallow shelf waters (3 groups; 93 whales). The different distributions of small and large groups of whales between shallow shelf waters and deeper waters during the two wind conditions suggest that aggregations of whales in large groups on the western Beaufort shelf are a behavioral response to locally enhanced feeding conditions on the shelf. These favorable feeding conditions result from convergence of the ACC with Beaufort shelf waters that promotes the aggregation and retention of whale plankton prey (copepods, krill) on the western Beaufort shelf.

directed onto the Beaufort shelf or along the canyon edge in response to weak winds. However, when moderate-to-strong winds from the eastern quadrant drive shelf waters off-shelf into Barrow Canyon, this shelf-break front is absent. Additionally, aerial survey data indicate that large groups of (four or more) bowhead whales are more likely to be seen on the western Beaufort shelf when winds are weak and less likely to be seen when winds from the eastern quadrant are moderateto-strong. Accordingly, it is reasonable to infer that the presence of this shelf-break front is both an indicator and promoter of better feeding opportunities for bowhead whales on the western Beaufort shelf. While the preceding analyses indicate that the Barrow Canyon shelf-break front contributes to the potential efficacy of the Barrow area bowhead whale feeding hotspot, Ashjian et al. (2010) point out that energetically-efficient feeding opportunities available to bowhead whales near Barrow will vary year-to-year depending also on upstream conditions: (1) the northward volume transport of Pacific Water, (2) the quantity, growth, and survival of euphausiids during the northward transit of Pacific Water, and (3) the transit time of such water and plankton from the Bering Strait to Barrow.

4. Discussion Satellite images, supported by ocean current and wind measurements, indicate that a shelf-break front is present along the southeastern edge of Barrow Canyon when shelf-break currents are

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There are biases within the BWASP dataset that potentially affect analyses. Larger group size may be indicative of feeding behavior, but could also be an artifact of the data collection process. Sightings of individual bowhead whale in the earlier years (1982–1989) were often lumped together because of limitations of the computer used to record data. As computer systems improved, so too did the capability to record individual sightings instead of lumping them together. Additionally, BWASP sighting data generally represent periods of lower wind speeds. Aerial surveys can only be effectively conducted under sufficiently low wind conditions (b8.7 m s− 1, 17 kts) because sighting conditions are compromised with higher sea states. Therefore, it is possible that whale distributions and group formation may differ at higher wind speeds for which aerial observations are impractical. Despite these limitations and potential biases, our analyses and results identify and illustrate the meteorological and oceanographic conditions that create a favorable feeding environment for bowhead whales. This predictive capability may be useful for developing mitigation strategies for resource development activities and anticipated increases in vessel traffic in the Barrow area. Finally, it is instructive to note that had our analyses of associations between whales, oceanography, and meteorology been limited to only those years for which there are oceanographic data from the western Beaufort shelf (2005–2009), our present understanding of the mechanism driving the Barrow area bowhead whale feeding hotspot would be much more tenuous and subjective. The preceding analyses illustrate the utility and value of decadal-length time series such as the BWASP and meteorology data sets for addressing ecosystem mechanics and variability. Acknowledgments This publication resulted from research sponsored by the Cooperative Institute for Alaska Research with funds from the National Oceanic and Atmospheric Administration (NOAA) under cooperative agreement NA08OAR4320751 with the University of Alaska and the Cooperative Institute for Climate and Ocean Research with funds from NOAA under cooperative agreement NA17RJ1223 with the Woods Hole Oceanographic Institution. Support was also provided by the Minerals Management Service (MMS) through Interagency Agreements 0106RU39923 and M08PG20021 between the National Marine Fisheries Service and MMS. MODIS images were provided by NASA/GSFC, MODIS Rapid Response. SAR data were processed by the Alaska Satellite Facility. Meteorological data were obtained from the Atmospheric Radiation Measurement (ARM) Program sponsored by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental

Research, Climate and Environmental Sciences Division. The Barrow Arctic Science Consortium provided logistic support at Barrow, both before and during the project. We thank the Barrow Whaling Captains Association, the Alaska Eskimo Whaling Commission, the City of Barrow, and the North Slope Borough for their continuing support of this project. We recognize Chuck Monnett for his longstanding support and advocacy for BWASP and numerous observers, Dave Rugh in particular, who participated in the collection of BWASP data since 1982. We also thank the reviewers and Megan Ferguson, BWASP/COMIDA Program Coordinator, for providing comments on an earlier draft of this manuscript. Finally, we are grateful to Captain Bill Kopplin and co-Captain Randy Pollock of the R/V Annika Marie who contributed to the success of the oceanographic field work for this project. References Ashjian, C. J., Braund, S. R., Campbell, R. G., George, J. C., Kruse, J., Maslowski, W., Moore, S. E., Nicolson, C. R., Okkonen, S. R., Sherr, B. F., Sherr, E. B., & Spitz, Y. (2010). Climate variability, oceanography, bowhead whale distribution, and Iñupiat subsistence whaling near Barrow, AK. Arctic, 63(2), 179–194. Baumgartner, M. F., Cole, T. V. N., Campbell, R. G., Teegarden, G. J., & Durbin, E. G. (2003). Associations between North Atlantic right whales and their prey, Calanus finmarchicus, over diel and tidal time scales. Marine Ecology Progress Series, 264, 155–166. Baumgartner, M. F., & Mate, B. R. (2003). Summertime foraging ecology of North Atlantic right whales. Marine Ecology Progress Series, 264, 123–135. Berline, L., Spitz, Y. H., Ashjian, C. J., Campbell, R. G., Maslowski, W., & Moore, S. E. (2008). Euphausiid transport in the western Arctic Ocean. Marine Ecology Progress Series, 360, 163–178. Clarke, J.T. & Ferguson, M.C. (2010). Aerial Surveys of Large Whales in the Northeastern Chukchi Sea, 2008–2009, with Review of 1982–1991 Data. Paper SC/62/BRG13 presented at the International Whaling Commission Scientific Committee Meetings, Morocco, June 2010. 18 pp. Genin, A. (2004). Bio-physical coupling in the formation of zooplankton and fish aggregations over abrupt topographies. Journal of Marine Systems, 50, 3–20. Kenney, R. D. (2001). Anomalous 1992 spring and summer right whale (Eubalaena glacialis) distributions in the Gulf of Maine. Journal of Cetacean Research Management (Special Issue), 2, 209–223. Lowry, L. F., Shefffield, G., & George, J. C. (2004). Bowhead whale feeding in the Alaskan Beaufort Sea, based on stomach contents analyses. Journal of Cetacean Research & Management, 6, 215–223. Mayo, C. A., & Marx, M. K. (1990). Surface foraging behaviour of the North Atlantic right whale, Eubalaena glacialis, and associated zooplankton characteristics. Canadian Journal of Zoology, 68, 2214–2220. Monnett, C., & Treacy, S. D. (2005). Aerial surveys of endangered whales in the Beaufort Sea, Fall 2002–2004. 153pp. OCS Study MMS 2005–037. Anchorage AK: USDOI, MMS, Alaska OCS Region. http://www.boemre.gov/alaska. Available from Moore, S. E., George, J. C., Sheffield, G., Bacon, J., & Ashjian, C. J. (2010). Bowhead whale distribution and feeding near Barrow, Alaska in late summer 2005–06. Arctic, 63, 179–194. National Marine Mammal Laboratory, NMFS, NOAA. (2010). BWASP and COMIDA Observer Manual - 2010. (unpublished, available from Clarke). 135pp. Okkonen, S. R., Ashjian, C. J., Campbell, R. G., Maslowski, W., Clement-Kinney, J. L., & Potter, R. (2009). Intrusion of warm Bering/Chukchi waters onto the western Beaufort Shelf. Journal Geophysical Research, 114, C00A11. doi:10.1029/2008JC004870.