Susanna B Blackwell

Publications (49) View all

• Source

  Available from: Daniel E Crocker

  Article: Diving behaviour and foraging location of female southern elephant seals from Patagonia

  C. CAMPAGNA, B. J. LE BOEUF, S. B. BLACKWELL, D. E. CROCKER, F. QUINTANA
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  ABSTRACT: Our aim was to describe the free-ranging diving pattern and to determine the location of foraging of pregnant female southern elephant seals, Mirounga leonina, from Peninsula Valdes, Argentina. This colony is unusual in two respects: it is removed from deep water by a broad shallow shelf (345–630 km wide), and colony numbers have been increasing in recent years in contrast to numbers from other southern hemisphere colonies that are stable or in decline. Microprocessor controlled, geolocation-time-depth recorders were deployed on four females, recording a total of 15,836 dives (270 dive days) during the period February to April, 1992. Departing seals crossed the continental shelf quickly (54–5–62–1 h) and did not show signs of foraging until reaching deep water, due east of the colony in the South Atlantic Ocean. Diving was virtually continuous (93% of the time underwater) with overall mean (±S.D.) rates of 2.5±0.2 dives/h, mean dive durations of 22.8 ± 7.1 min (maximum dive duration = 79 min) with 1.6±0.6min surface intervals between dives, and dive depths of 431±193m (maximum dive depth = 1,072 m). The diving pattern of females from Patagonia is similar to that of seals from colonies where numbers are decreasing (Macquarie stock) or are stable (South Georgia Island). Our subjects did not, however, feed in or south of the Antarctic Polar Front, or in cold waters along the Antarctic coast, where seals from declining or stable colonies forage.
  Journal of Zoology 03/2009; 236(1):55 - 71. · 2.04 Impact Factor
• Conference Proceeding: Computer-aided classification of bowhead whale call categories for mitigation monitoring

  D. Mathias, A. Thode, S.B. Blackwell, C. Greene
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  ABSTRACT: Since 2001 Directional Autonomous Seafloor Acoustic Recorders (DASARs) have been used to localize and record bowhead whale (Balaena mysticetus) calls during their annual migration. In 2007 DASARs were deployed at 35 locations over a 280 km swath in the Beaufort Sea, during seismic exploration activities (Fig. 1), in order to monitor potential changes in the animals' location and/or acoustic activity during the seismic activities. The large amount of acoustic data generated (about 50 days per DASAR) motivated the development of computer-aided methods to assist in detecting and classifying bowhead whale calls. Bowhead whale calls can be classified in various ways. Here, we divide calls into six categories: (1) upsweeps, (2) downsweeps, (3) constant calls, (4) u-shaped and (5) n-shaped undulated calls, and (6) complex calls, a catch-all category that covers both frequency-modulated calls with multiple inflections, and amplitude-modulated calls such as warbles, growls, and other such sounds. In addition, walrus and bearded seal calls can produce similar call features in a spectrogram, yielding a total of eight classification categories. The frequency range, duration, and fine structure of individual calls vary considerably even within each category, creating difficulties when using simple matched- filtering or spectrogram correlation methods. A manually reviewed test dataset was assembled, containing examples from each call category, arranged by signal-to-noise ratio (SNR) in 5 dB bins, ranging from 5 to 40 dB. The dataset was then used to test several methods for extracting relevant parameters from the signal for subsequent classification. Contour tracing methods that estimate frequency bandwidth, inflection points, and duration were examined, as well as other boundary descriptors that utilize standard image segmentation techniques. An optimization procedure was then used to determine appropriate decision boundaries for optimum statistical classifiers.
  New Trends for Environmental Monitoring Using Passive Systems, 2008; 11/2008
• Source

  Available from: Daniel E Crocker

  Article: Respiration and heart rate at the surface between dives in northern elephant seals.

  B J Le Boeuf, D E Crocker, J Grayson, J Gedamke, P M Webb, S B Blackwell, D P Costa
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  ABSTRACT: All underwater activities of diving mammals are constrained by the need for surface gas exchange. Our aim was to measure respiratory rate (fb) and heart rate (fh) at the surface between dives in free-ranging northern elephant seals Mirounga angustirostris. We recorded fb and fh acoustically in six translocated juveniles, 1.8-2. 4 years old, and three migrating adult males from the rookery at Año Nuevo, California, USA. To each seal, we attached a diving instrument to record the diving pattern, a satellite tag to track movements and location, a digital audio tape recorder or acoustic datalogger with an external hydrophone to record the sounds of respiration and fh at the surface, and a VHF transmitter to facilitate recovery. During surface intervals averaging 2.2+/-0.4 min, adult males breathed a mean of 32.7+/-5.4 times at a rate of 15. 3+/-1.8 breaths min(-)(1) (means +/- s.d., N=57). Mean fh at the surface was 84+/-3 beats min(-)(1). The fb of juveniles was 26 % faster than that of adult males, averaging 19.2+/-2.2 breaths min(-)(1) for a mean total of 41.2+/-5.0 breaths during surface intervals lasting 2.6+/-0.31 min. Mean fh at the surface was 106+/-3 beats min(-)(1). fb and fh did not change significantly over the course of surface intervals. Surface fb and fh were not clearly associated with levels of exertion, such as rapid horizontal transit or apparent foraging, or with measures of immediately previous or subsequent diving performance, such as diving duration, diving depth or swimming speed. Together, surface respiration rate and the duration of the preceding dive were significant predictors of surface interval duration. This implies that elephant seals minimize surface time spent loading oxygen depending on rates of oxygen uptake and previous depletion of stores.
  Journal of Experimental Biology 12/2000; 203(Pt 21):3265-74. · 3.00 Impact Factor
• Article: Automated detection and localization of bowhead whale sounds in the presence of seismic airgun surveys.

  Aaron M Thode, Katherine H Kim, Susanna B Blackwell, Charles R Greene, Christopher S Nations, Trent L McDonald, A Michael Macrander
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  ABSTRACT: An automated procedure has been developed for detecting and localizing frequency-modulated bowhead whale sounds in the presence of seismic airgun surveys. The procedure was applied to four years of data, collected from over 30 directional autonomous recording packages deployed over a 280 km span of continental shelf in the Alaskan Beaufort Sea. The procedure has six sequential stages that begin by extracting 25-element feature vectors from spectrograms of potential call candidates. Two cascaded neural networks then classify some feature vectors as bowhead calls, and the procedure then matches calls between recorders to triangulate locations. To train the networks, manual analysts flagged 219 471 bowhead call examples from 2008 and 2009. Manual analyses were also used to identify 1.17 million transient signals that were not whale calls. The network output thresholds were adjusted to reject 20% of whale calls in the training data. Validation runs using 2007 and 2010 data found that the procedure missed 30%-40% of manually detected calls. Furthermore, 20%-40% of the sounds flagged as calls are not present in the manual analyses; however, these extra detections incorporate legitimate whale calls overlooked by human analysts. Both manual and automated methods produce similar spatial and temporal call distributions.
  The Journal of the Acoustical Society of America 05/2012; 131(5):3726-47. · 1.55 Impact Factor
• Article: Call localization of marine mammals using directional autonomous recorders.

  Susanna B Blackwell, Charles R Greene, Christopher S Nations, Trent L McDonald, Aaron Thode, A Michael Macrander
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  ABSTRACT: Directional sensors of low-frequency acoustic waves have been used by navies in sonobuoys for submarine detection and localization for decades. Composed of an omnidirectional pressure sensor and two horizontal directional elements sensitive to particle motion, they provide information for determining the relative bearing to a sound source without ambiguity. They were adapted for bowhead whale monitoring in the mid-1980s and have been used in autonomous seafloor acoustic recorders since 2000. Applied to the coastal Beaufort Sea north of Alaska, fall bowhead migration was observed in detail during 2007 and 2008, providing a wealth of information on variability in the migration paths of calling whales and the influence of industrial sounds on the locations of calling whales. For example, many calls were detected within about 30 km of seismic survey activities, where received sound pressure levels from airgun pulses were often greater than 140 dB per 1 muPa. Also, seismic activities were correlated with statistically significant shifts in the whales' distance from shore, either offshore or inshore. However, interpretation of the results was challenged by the difficulty in distinguishing between a whale that ceases calling and a whale that deflects away from the study area. [Work supported by Shell Oil Co.].
  The Journal of the Acoustical Society of America 05/2009; 125(4):2647. · 1.55 Impact Factor