Simultaneous sightings and acoustic detections of sei whales (Balaenoptera borealis) are scarce, and there are few published data describing their vocalizations. Analysis of recordings from directional frequency analysis and recording sonobuoys in the presence of sei whales in the Southern Ocean in March 2013 identified both downsweep and upsweep calls.
Sound frequencies within all calls were between 34 and 87 Hz with an average call duration of 1.1 s. These very low-frequency sounds share characteristics with sei whale calls recorded near the Hawaiian Islands and off Cape Cod in winter and summer, respectively, but are the first documented sei whale calls in the Southern Ocean that are clearly less than 100 Hz.
1. INTRODUCTION
Sei whales occur throughout the Southern Hemisphere, predominantly offshore, migrating between low-latitude tropical and subtropical regions in the winter and temperate and subpolar latitudes in summer (Leaper et al., 2008; Reilly et al., 2008). Miyashita et al. (1995) report summer distribution of sei whales in the Southern Hemisphere to be mainly between 40°S and 50°S in the South Atlantic and southern Indian oceans and between 45°S and 60°S in the South Pacific.
There were relatively few sightings of sei whales from the International Decade of Cetacean Research – Southern Ocean Whale and Ecosystem Research (IDCR-SOWER) circumpolar cruises south of 60°S (Branch and Butterworth, 2001) where the search effort for these cruises was concentrated.
In International Whaling Commission (IWC) Management Area V, the region where the sei whales in this study were detected, sei whale catch data show that nearly 17 000 animals were taken between 1914 and 1977 (Leaper et al., 2008).
Few recordings of vocalizations attributed to sei whales have been corroborated with visual sightings, and their repertoire is therefore poorly understood. Knowlton et al. (1991) recorded vocalizations in the presence of sei whales off Nova Scotia in August and September that comprised two phrases of frequency modulated sweeps between 1.5 and 3.5 kHz.
These shared similar frequency characteristics with the long bursts of pulses with a peak energy of 3 kHz recorded by Thompson et al. (1979) between Nova Scotia and Newfoundland. McDonald et al. (2005) recorded vocalizations in the presence of sei whales off the Antarctic Peninsula in February that comprised broadband, tonal, and frequency modulated sounds between 100 and 600 Hz with durations of 1–3 s. McDonald et al. noted distinctive frequency “stepping” in many of the tonal and frequency swept calls. Calls recorded by Gedamke and Robinson (2010) off East Antarctica in the summer also had frequency stepping within a similar frequency range, and they attributed those calls to sei whales on the basis of the results of McDonald et al., although there were no corroborating sightings of sei whales.
Rankin and Barlow (2007) described vocalizations from sei whales near the Hawaiian Islands in November that comprised two types of calls: Downsweeps from 100 to 44 Hz with a duration of 1.0 s and downsweeps from 39 to 21 Hz with a duration of 1.3 s. Baumgartner et al. (2008) recorded vocalizations using autonomous ocean gliders off Cape Cod in May that were significantly associated with visual observations of sei whales. These sounds were similar to those recorded by Rankin and Barlow (2007): Downsweeps ranging from 82 to 34 Hz over 1.4 s.
These occurred most often as single calls, but pairs and triplets were also detected. Newhall et al. (2012) were able to successfully localize calls with similar properties to those recorded by Baumgartner et al. (2008) that they attributed to sei whales. They found less variability among calls from the same location than among calls from different locations, suggesting some acoustic differentiation between individuals might be possible, at least in the short term.
During the 2013 Southern Ocean Research Partnership (SORP) Antarctic Blue Whale Project (ABWP) cruise, two sei whale sightings were made south of the subantarctic Auckland Islands, in the New Zealand Exclusive Economic Zone. Vocalizations associated with these sightings were recorded using directional frequency analysis and recording (DIFAR) sonobuoys. This enabled a description of the characteristics of sei whale calls in this area of the Southern Ocean.
2. METHODS
The ABWP cruise in 2013 used DIFAR sonobuoys to detect, localize, and track Antarctic blue whales. Sonobuoys were deployed from research vessel FV Amaltal Explorer throughout the cruise, including during the passage to and from New Zealand and the main study area near the pack ice north of the Ross Sea and opportunistically close to sightings of species other than blue whales (Miller et al., 2013). A dedicated visual survey was also conducted by a team of experienced observers from the open-air flying bridge and the enclosed bridge, using naked eye and binoculars. On 11 March on the return leg of the survey, two apparently solitary sei whales were seen by the visual observers in sea states of Beaufort 3–4. The sightings were separated by a distance of 11 km and occurred approximately 160 km south of the Auckland Islands. There were no other cetaceans seen in the 2 days before the first sei whale sighting or for nearly 20 h after the second. After the first sighting, the course of the vessel was altered to confirm species identification.
The vessel approached to within 150 m of the whale, and it was clearly observed through several surfacing bouts.
Two DIFAR 53 F sonobuoys with an omnidirectional pressure sensor were deployed during this time. One had been deployed opportunistically before the first sighting occurred; the second was deployed in response to the first sighting but before the second sighting. Sonobuoy signals were received by VHF radio onboard the research vessel, digitized, recorded, processed, and the DIFAR bearings to calls resolved as described by Miller et al. (2013). Where the same signal was received on both sonobuoys, the time of arrival difference was also measured to determine the hyperbolic surface describing possible locations of the vocalizing whale. Recordings were analyzed using Adobe audition 3.0 and pamguard (www.pamguard.org; Gillespie et al., 2008).
For analysis and measurement, recordings were resampled from an original recording sample rate of 48 000 samples per second to a sample rate of 6000 samples per second to facilitate processing. Spectrograms were generated by fast Fourier transform (4096 FFT size, 97% overlap, Hamming window, 1.46 Hz frequency resolution, 682.67 ms time resolution, 21.33 ms time step size).
The high frequency, low frequency, frequency change, peak frequency duration, and inter-call interval of each call were measured. Only a small number of calls was recorded, and measurements were made from all calls. However, ship and ambient noise were present during the recordings, and the resulting low signal to noise ratio of many of the calls precluded further analysis beyond the measurement of these parameters.
Sound frequencies within all calls were between 34 and 87 Hz with an average call duration of 1.1 s. These very low-frequency sounds share characteristics with sei whale calls recorded near the Hawaiian Islands and off Cape Cod in winter and summer, respectively, but are the first documented sei whale calls in the Southern Ocean that are clearly less than 100 Hz.
1. INTRODUCTION
Sei whales occur throughout the Southern Hemisphere, predominantly offshore, migrating between low-latitude tropical and subtropical regions in the winter and temperate and subpolar latitudes in summer (Leaper et al., 2008; Reilly et al., 2008). Miyashita et al. (1995) report summer distribution of sei whales in the Southern Hemisphere to be mainly between 40°S and 50°S in the South Atlantic and southern Indian oceans and between 45°S and 60°S in the South Pacific.
There were relatively few sightings of sei whales from the International Decade of Cetacean Research – Southern Ocean Whale and Ecosystem Research (IDCR-SOWER) circumpolar cruises south of 60°S (Branch and Butterworth, 2001) where the search effort for these cruises was concentrated.
In International Whaling Commission (IWC) Management Area V, the region where the sei whales in this study were detected, sei whale catch data show that nearly 17 000 animals were taken between 1914 and 1977 (Leaper et al., 2008).
Few recordings of vocalizations attributed to sei whales have been corroborated with visual sightings, and their repertoire is therefore poorly understood. Knowlton et al. (1991) recorded vocalizations in the presence of sei whales off Nova Scotia in August and September that comprised two phrases of frequency modulated sweeps between 1.5 and 3.5 kHz.
These shared similar frequency characteristics with the long bursts of pulses with a peak energy of 3 kHz recorded by Thompson et al. (1979) between Nova Scotia and Newfoundland. McDonald et al. (2005) recorded vocalizations in the presence of sei whales off the Antarctic Peninsula in February that comprised broadband, tonal, and frequency modulated sounds between 100 and 600 Hz with durations of 1–3 s. McDonald et al. noted distinctive frequency “stepping” in many of the tonal and frequency swept calls. Calls recorded by Gedamke and Robinson (2010) off East Antarctica in the summer also had frequency stepping within a similar frequency range, and they attributed those calls to sei whales on the basis of the results of McDonald et al., although there were no corroborating sightings of sei whales.
Rankin and Barlow (2007) described vocalizations from sei whales near the Hawaiian Islands in November that comprised two types of calls: Downsweeps from 100 to 44 Hz with a duration of 1.0 s and downsweeps from 39 to 21 Hz with a duration of 1.3 s. Baumgartner et al. (2008) recorded vocalizations using autonomous ocean gliders off Cape Cod in May that were significantly associated with visual observations of sei whales. These sounds were similar to those recorded by Rankin and Barlow (2007): Downsweeps ranging from 82 to 34 Hz over 1.4 s.
These occurred most often as single calls, but pairs and triplets were also detected. Newhall et al. (2012) were able to successfully localize calls with similar properties to those recorded by Baumgartner et al. (2008) that they attributed to sei whales. They found less variability among calls from the same location than among calls from different locations, suggesting some acoustic differentiation between individuals might be possible, at least in the short term.
During the 2013 Southern Ocean Research Partnership (SORP) Antarctic Blue Whale Project (ABWP) cruise, two sei whale sightings were made south of the subantarctic Auckland Islands, in the New Zealand Exclusive Economic Zone. Vocalizations associated with these sightings were recorded using directional frequency analysis and recording (DIFAR) sonobuoys. This enabled a description of the characteristics of sei whale calls in this area of the Southern Ocean.
2. METHODS
The ABWP cruise in 2013 used DIFAR sonobuoys to detect, localize, and track Antarctic blue whales. Sonobuoys were deployed from research vessel FV Amaltal Explorer throughout the cruise, including during the passage to and from New Zealand and the main study area near the pack ice north of the Ross Sea and opportunistically close to sightings of species other than blue whales (Miller et al., 2013). A dedicated visual survey was also conducted by a team of experienced observers from the open-air flying bridge and the enclosed bridge, using naked eye and binoculars. On 11 March on the return leg of the survey, two apparently solitary sei whales were seen by the visual observers in sea states of Beaufort 3–4. The sightings were separated by a distance of 11 km and occurred approximately 160 km south of the Auckland Islands. There were no other cetaceans seen in the 2 days before the first sei whale sighting or for nearly 20 h after the second. After the first sighting, the course of the vessel was altered to confirm species identification.
The vessel approached to within 150 m of the whale, and it was clearly observed through several surfacing bouts.
Two DIFAR 53 F sonobuoys with an omnidirectional pressure sensor were deployed during this time. One had been deployed opportunistically before the first sighting occurred; the second was deployed in response to the first sighting but before the second sighting. Sonobuoy signals were received by VHF radio onboard the research vessel, digitized, recorded, processed, and the DIFAR bearings to calls resolved as described by Miller et al. (2013). Where the same signal was received on both sonobuoys, the time of arrival difference was also measured to determine the hyperbolic surface describing possible locations of the vocalizing whale. Recordings were analyzed using Adobe audition 3.0 and pamguard (www.pamguard.org; Gillespie et al., 2008).
For analysis and measurement, recordings were resampled from an original recording sample rate of 48 000 samples per second to a sample rate of 6000 samples per second to facilitate processing. Spectrograms were generated by fast Fourier transform (4096 FFT size, 97% overlap, Hamming window, 1.46 Hz frequency resolution, 682.67 ms time resolution, 21.33 ms time step size).
The high frequency, low frequency, frequency change, peak frequency duration, and inter-call interval of each call were measured. Only a small number of calls was recorded, and measurements were made from all calls. However, ship and ambient noise were present during the recordings, and the resulting low signal to noise ratio of many of the calls precluded further analysis beyond the measurement of these parameters.
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