Antarctic-type blue whale calls recorded at low latitudes in the Indian ...

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Deep-Sea Research I 51 (2004) 1337–1346

Antarctic-type blue whale calls recorded at low latitudes in the Indian and eastern Pacific Oceans Kathleen M. Stafforda,*, DelWayne R. Bohnenstiehlb, Maya Tolstoyb, Emily Chappb, David K. Mellingerc, Sue E. Moorea a

National Marine Mammal Laboratory, 7600 Sand Point Way NE F/AKC4, Seattle, WA 98115, USA b Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Box 1000, Palisades, NY 10964-8000, USA c Cooperative Institute for Marine Resources Studies, Oregon State University/National Oceanic and Atmospheric Administration, Newport, OR 97365, USA Received 18 November 2003; received in revised form 28 April 2004; accepted 21 May 2004

Abstract Blue whales, Balaenoptera musculus, were once abundant around the Antarctic during the austral summer, but intensive whaling during the first half of the 20th century reduced their numbers by over 99%. Although interannual variability of blue whale occurrence on the Antarctic feeding grounds was documented by whalers, little was known about where the whales spent the winter months. Antarctic blue whales produce calls that are distinct from those produced by blue whales elsewhere in the world. To investigate potential winter migratory destinations of Antarctic blue whales, we examined acoustic data for these signals from two low-latitude locales: the eastern tropical Pacific Ocean and the Indian Ocean. Antarctic-type blue whale calls were detected on hydrophones in both regions during the austral autumn and winter (May–September), with peak detections in July. Calls occurred over relatively brief periods in both oceans, suggesting that there may be only a few animals migrating so far north and/or producing calls. Antarctic blue whales appear to use both the Indian and eastern Pacific Oceans concurrently, indicating that there is not a single migratory destination. Acoustic data from the South Atlantic and from mid-latitudes in the Indian or Pacific Oceans are needed for a more global understanding of migratory patterns and destinations of Antarctic blue whales. r 2004 Elsevier Ltd. All rights reserved. Keywords: Blue whale; Balaenoptera musculus intermedia; Bioacoustics; Vocalization behavior; Antarctic; Indian Ocean; Eastern tropical Pacific

*Corresponding author. Tel.: +1-206-526-6867; fax: +1-206-526-6615. . 0967-0637/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2004.05.007

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1. Introduction The Antarctic blue whale1 (Balaenoptera musculus intermedia) is the largest of three recognized sub-species. Prior to commercial whaling, these whales were found all around the Antarctic during the austral summer (Mackintosh, 1966). As the largest whale, this species was the preferred target of commercial whalers, resulting in the removal of roughly 99% of the population from the early 1900s until the mid-1960s, when the International Whaling Commission (IWC) banned blue whale harvest worldwide.2 Information on wintertime distribution of blue whales is scant. Because of the generally poor condition of animals harvested in the tropics, there was little whaling north of 30 S (Ingebrigtsen, 1929) and, after World War II, pelagic whaling was banned north of 40 S although whaling based from shore stations was still permitted (Chapman, 1974). Around the Antarctic, the greatest numbers of blue whales were killed in IWC Areas II, III and IV (Fig. 1) from December–February from 1926 to 1940 (Horwood, 1986). It was generally thought that these animals wintered in subtropical and tropical regions of the South Atlantic, Pacific, and Indian Oceans although specific regions were, and largely remain, unknown (Mackintosh, 1966; Yochem and Leatherwood, 1985; Perry et al., 1999). Although recovery of Discovery Mark tags and early whaling data suggested that there may have been discrete feeding stocks of blue whales in Antarctic waters (Hjort et al., 1932; Rayner, 1940; Mackintosh, 1946; Brown, 1954), other tag returns demonstrated that individuals were capable of long-distance longitudinal movements (at least 180 in one case), even over fairly short periods (Brown, 1954 1959, 1962; Mackintosh, 1966; Ivashin, 1971).

1 B.m.intermedia has sometimes been referred to as the ‘true’ blue whale versus the ‘pygmy’ blue whale in the southern hemisphere. However, we prefer the term ‘Antarctic’ blue whale as suggested by Rice (pers. comm.) since all blue whales are ‘true’ blue whales. 2 Despite this ban, Soviet whaling vessels illegally harvested at least 550 Antarctic blue whales from 1966 to 1973 (Zemsky et al., 1995).

Almost 40 years ago, Mackintosh (1966) addressed what he called ‘‘the problem of the breeding grounds’’ for blue whales. To wit: because little whaling occurred north of 40 S, there was no wintertime pelagic whaling fleet to ‘‘discover’’ whales in the open ocean; there were few pelagic shipping routes in the Southern Ocean; and because blue whales were believed to be widely dispersed in tropical and subtropical latitudes, the effort required to find and delineate wintering grounds was nearly insurmountable. Arguably, with regards to sighting data, little has changed since then (Clapham et al., 1999). Recently, acoustic data have been used to infer migration and wintering locations for blue whales in the northeast Pacific (Stafford et al., 1999a). Further, geographic variation has been demonstrated for blue whale calls recorded in ocean basins throughout the world, as summarized by Mellinger and Clark (2003). For example, blue whale calls differ substantially between the eastern and western North Pacific, the eastern South Pacific, the North Atlantic, and the Indian Oceans (Cummings and Thompson, 1971; Edds, 1982; Thompson and Friedl, 1982; Alling et al., 1991; Ljungblad et al., 1998; Stafford et al., 2001). Somewhat surprisingly and despite great differences in longitude of locations of available recordings from the Southern Ocean, the same call type has been recorded for blue whales around the Antarctic during IWC- Southern Ocean Whale Ecosystem Research (SOWER) cruises (IWC, 2003, Fig. 1). Antarctic-type blue whale calls consist of repeated series of a single call unit (Ljungblad et al., 1998, Fig. 2). The unit begins with a tone around 28 Hz that lasts about 10 s and then sweeps sharply down to about 20 Hz over only 2 s and ends with another, slightly frequency modulated 10 s tone from about 20 to 18 Hz (Ljungblad et al., 1998). A series may contain from one to a dozen or more closely spaced call units followed by a period of silence greater than that between the closely spaced calls. In this paper, we present acoustic data from low-latitude hydrophones in the eastern tropical Pacific (ETP) and Indian Ocean basins that suggest blue whales which feed around the

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Fig. 1. Locations of hydrophones and locations around the Antarctic where Antarctic-type blue whale calls have been recorded. IWCSOWER locations where Antarctic-type calls have been recorded are shown as ; Indian Ocean hydrophone locations are shown as ’; ETP hydrophones where many Antarctic-type blue whale calls were recorded are shown as %, a few calls , or no calls $. Regions where large numbers of blue whales were killed are designated with hatching and regions where fewer than 10 animals were killed are indicated by . IWC Whaling area boundaries are shown as dark lines with area names in bold.

Fig. 2. Spectrogram of a series of Antarctic-type blue whale calls recorded at Cape Leeuwin on 17 May 2002 in the Indian Ocean (spectrogram parameters: 512 point FFT length, 87.5% overlap, Hanning window, for a filter bandwidth of 0.7 Hz).

Antarctic during the austral summer may winter in different ocean basins.

2. Methods Acoustic datasets from two regions were examined for the presence of Antarctic-type blue whale calls (Fig. 2). The first dataset was from a longterm (May 1996–November 2002) deployment of six autonomous hydrophones in the ETP arrayed north to south at latitude 8 S, 0 , and 8 N along longitudes 95 W and 110 W (Fig. 1, Fox et al., 2001). These instruments were moored to the seafloor and floated at a depth near the axis of the deep sound channel (Urick, 1983). The moorings were recovered for data retrieval and redeployed

on a semi-annual to annual schedule. From May 1996 to November 1999, the hydrophones recorded continuously with a 100 Hz sample rate (SR) and 8-bit A/D resolution. From November 1999 to November 2000, the hydrophones recorded with a 250 Hz SR and 8-bit A/D resolution) and after November 2000, with a 250 Hz SR and 12-bit A/D resolution). All instruments had a flat (73 dB) frequency response in the 15–40 Hz range of interest. The second dataset (January 2002–April 2003) was recorded by hydrophones of the International Monitoring System (IMS), deployed in support of the Comprehensive Test-Ban Treaty and designed to detect the sounds generated by explosions that may be carried out at or below the ocean surface. The IMS data used in this study were made available in support of a broader project of Indian Ocean noise characterization, under contract from the Defense Threat Reduction Agency. Each IMS station consisted of a set of three hydrophones deployed in a triangular configuration (triads) with approximately 2 km spacing between instruments. Due to the close instrument spacing, data were analyzed from only one instrument in each triad. Data were analyzed from two stations located near Diego Garcia (British Indian Ocean Territory), one B180 km to the northwest (6.3 S, 71.0 E; here called Diego Garcia North) and one B25 km



to the south (7.6 S, 72.5 E; here called Diego Garcia South) of the island atoll, and a third station located B100 km to the southwest of Cape Leeuwin, Australia (34.9 S, 114.1 W) (Fig. 1). The deployment of these instruments was similar in concept to those deployed in the ETP, with each moored to the seafloor and the hydrophone floated near the sound channel axis. The IMS stations recorded continuously with a 250 Hz SR and 24bit A/D resolution, and all instruments had a flat (73 dB) frequency response from 8–100 Hz. Unlike the autonomous instruments deployed in the Pacific, the data were transmitted via cable to shore and were available in near-real time. Data were scanned for Antarctic-type calls using the automatic detection method of spectrogram correlation (Mellinger and Clark, 2000; Mellinger, 2001). This involved specifying frequency contours and durations that matched average characteristics of known Antarctic-type blue whale calls. For this study, we used contours that consisted of the following: a 9 s long 27.7 Hz tone, a 1 s long frequency decrease from 28 to 19.5 Hz and a 10 s long tone that decreased to 18.8 Hz. Additionally, we employed spectrogram noise equalization with a time constant of 40 s to eliminate continuous interfering tones such as those from ships. These parameters worked well for data from both the ETP and the Indian Ocean. We set the detection threshold fairly low, which allowed us to detect many Antarctic-type blue whale calls but also resulted in many false detections. Detections were saved as individual sound files that were assessed visually (KMS) to distinguish correct and incorrect detections. To estimate the time and frequency characteristics of these calls, measurements were made in a spectrogram (2.56 s frame and FFT length, 87.5% overlap, Hanning window, for a filter bandwidth of 1.6 Hz) of the start and end frequencies and the durations of the different parts of the calls. Number of calls in a series, inter-call interval, and inter-series intervals were measured for long series of high intensity calls. To avoid biasing of results by days in which there were many more calls measured, daily averages of call characteristics were first computed and then overall mean values of the daily averages were computed.

3. Results 3.1. Detections Two types of Antarctic calls were detected in the Indian Ocean and ETP: the call first described by Ljungblad et al. (1998) and a shorter call that is only the first, higher-frequency, part of this call. The shorter call type (a 27 Hz tone) is also the most common one recorded in the presence of blue whales in the Ross Sea (Rankin, pers. comm.). Call characteristics from the ETP for 1996 are provided by Stafford et al. (1999a); briefly, these sounds began with a 28 Hz, 9 s tone that swept to a frequency-modulated (FM) tone that began at 21 Hz and changed at 19 Hz over 11 s. For the Indian Ocean, 782 total calls from 7 days were measured. The number of calls in a series averaged 6.573.4 (range 1–16). Although the repetitive series of calls tended to be of either one call type or the other, there were series that contained both call types. The longer call and the 27 Hz tone call were measured separately. The longer calls consisted of a 27.870.1 Hz tone that swept slightly downward to 27.370.1 Hz and lasted 7.870.5 s, decreased abruptly in frequency to 20.170.7 Hz over 2.270.2 s, and ended with a 7.171.4 s FM sweep to 19.170.9 Hz (n ¼ 6). Total duration was 15.572 s. Average time from the end of one call to the beginning of the next in a series was 48.572.8 s. The 27 Hz tone calls began at 27.770.1 Hz, swept to 27.270.04 Hz and lasted 9.070.7 s on average. Antarctic blue whale calls were recorded seasonally both in the ETP and in the Indian Ocean (Figs. 3 and 4). Calls were detected only during the austral autumn and winter (May–September) in both oceans. False detections included signals from earthquakes, pygmy and southeastern Pacific blue whales (Cummings and Thompson, 1971; Stafford et al., 1999b; McCauley and Cato, 2003), and shipping noise. The number of calls detected is likely an underestimate of the total number of calls recorded because faint calls or calls that were masked by other noise were probably not detected. Because ocean noise levels are highest during the austral winter within the frequency range of the Antarctic-type calls, the dearth of calls during

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Fig. 4. Number of calls detected (bars and left Y-axis) and percentage of days with detections (symbols and right Y-axis) at Indian Ocean hydrophones by month from January 2002—April 2003. Cape Leeuwin detections are shown in black; Diego Garcia South detections in gray; and Diego Garcia North in white.

the summer months cannot be attributed to seasonal variability in ambient noise (Tolstoy and Bohnenstiehl, 2002). Because only calls that were well above ambient ocean noise levels were detected, it was assumed that the animals producing these sounds were probably less than 100 km from the individual instruments. 3.2. Eastern tropical Pacific Ocean The detection of Antarctic-type blue whale calls in the ETP showed both interannual and geographic variation (Fig. 3). Most detections were from the southernmost (8 S, 95 W and 8 S,

110 W) hydrophones in 1996 and 2002. There were only 74 total calls from 44 different days (1.7 calls/day) detected from the four hydrophones on and north of the equator. The hydrophone at 8 S 95 W had two peaks in Antarctic-type blue whale call detections. Beginning on 11 July 1996, over 1000 calls were detected in just 8 days (125 calls/ day), while starting on 12 July 2002, 650 calls were detected in 6 days (108 calls/day). Call detections for the other years were much lower but were always in austral winter months (June–September), with peak detections in July. The hydrophone at 8 S 110 W had many fewer detections than the more eastern hydrophone. The greatest number of



detections in any month at that site was 23 detections over 3 days (7.7 calls/day) in July 2001. At 8 S 95 W, however, detections were all during the austral winter. No data were available for this hydrophone from February 1998 to May 1999 due to instrument failures. 3.3. Indian Ocean Antarctic-type blue whale calls were detected at all three Indian Ocean (IO) hydrophone stations (Fig. 4). Calls were detected at Cape Leeuwin from May through November 2002. The greatest number of detections was in May and July 2002, with a sharp drop in June. At Diego Garcia South, 90% of the total calls (725 of 809) detected were in July 2002, with very few from November to May. Call detections peaked at Diego Garcia North in May and June and then tapered off sharply from July to September. Calling occurred in discrete bouts rather than uniformly over time. For instance, at Cape Leeuwin, the 671 calls detected from this site in July were from only 4 days (167.8 calls/day) and over half of the calls detected in July (531 of 725) at Diego Garcia South were from a single 36-hour period (354 calls/day). The difference in call detection data between the two instruments is probably due to their placement on opposite sides of the Diego Garcia atoll.

4. Discussion Brown (1962) proposed that each of the oceans communicating with the Southern Ocean—the Indian, South Pacific, and South Atlantic—were home to at least one or more breeding populations of the blue and fin whales that summered in the Antarctic. The location and seasonality of Antarctic-type blue whale calls in this study provide information as to possible migratory and wintering grounds for Antarctic blue whales in the first two of these oceans. Concurrent acoustic data from the ETP and IO were available only in 2002. However, the detection of Antarctic-type blue whale calls in these two oceans that year shows that Antarctic-type blue whales occupy at least these two basins in austral winter. Wintering

grounds, then, are apparently widely dispersed, as has been suggested by numerous authors (Mackintosh et al., 1929; Harmer, 1931; Mackintosh, 1946). However, it may be that not all animals migrate northwards, since some blue whales were killed in the Antarctic during the austral winter (Harmer, 1931) and Antarctic-type calls have been detected during the winter months from recorders near the Antarctic Peninsula ! ˇ et al., 2004). (Sirovic The seven-year time series from the ETP is dominated by detections in July of 1996 and 2002 at 8 S 95 W. Like the IO detections, these were clustered over a few days. Both of the ETP instruments are relatively far offshore (B1500 and 3000 km from continental South America), and the difference in the number of detections of two orders of magnitude between the two may represent an offshore decrease in animals similar to that seen in the calls of other species (Stafford et al., 1999a). In the IO, the pattern of call detection off Cape Leeuwin, with detection peaks in May and July and a large drop in June, suggests that animals may move north past the Cape in May en route to lower latitudes and south past the Cape in July towards Antarctic waters. This interpretation agrees with the data at Diego Garcia North, which shows the greatest number of detections in May and June but a decrease in July. The data from Diego Garcia South, however, are somewhat equivocal in that the single July peak might support the hypothesis of animals moving south from Diego Garcia North, but there is no acoustic evidence of animals moving north in May past this location. In both the IO and the ETP, the large number of detections over relatively few days suggests that either there are relatively few animals that are very vocal over short time periods, or that many animals are producing fewer calls at roughly the same time. In the ETP, the number of detections and days with detections were comparatively few and varied substantially by year. The idea that Antarctic-type blue whales might call less often in the winter than they do during the summer does not seem particularly tenable since blue whale calls recorded elsewhere in the world occur as often, if

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not more so, at low latitudes than at high latitudes (Stafford et al., 1999a). Furthermore, if the repetitive series of sounds produced by Antarctic blue whales serve as a male song display as has been suggested by some authors (McDonald et al., 2001; Clark and Ellison, 2004), then we should expect more calls to be detected during winter months. Antarctic blue whales may not often migrate as far north as the latitudes of the instruments (Harmer, 1928; Kellogg, 1928; Mackintosh, 1972) resulting in the paucity of calls described here. Kellogg (1928) reported that blue whales were found in the vicinity of South Georgia throughout the winter. Although blue whales were once abundant off South Georgia, less than a dozen were seen in these waters during ship- and shorebased observations from 1979 to 1997 (Moore et al., 1999). The same scarcity of visual observations is true of the Antarctic Peninsula (Horwood, 1986; Kasamatsu et al., 1996; Moore et al., 1999) although acoustic data from this region indicate that blue whales frequent this area (IWC, 2003; ! ˇ et al., 2004). There are no acoustic data Sirovic from the South Georgia area to indicate whether blue whales still occur there. If blue whales migrate only to sub-tropical latitudes in the IO, one would expect more calls to be detected at the Cape Leeuwin hydrophone (32 S) than at the Diego Garcia hydrophones (6 S–8 S). This possibility could be investigated by deploying hydrophones at higher latitudes, but in the same longitude. Alternatively, animals could be closer to the coasts than our instruments, which, with the exception of Cape Leeuwin, are far from any continental shoreline (Mackintosh, 1966). A third possibility, and what we think the most likely, is that most Antarctic blue whales migrate northward into the South Atlantic Ocean (Wheeler, 1946). Roughly three times as many blue whales were killed north of 40 S in the eastern Atlantic (9969) as in the Indian Ocean (3245) and two times as many as in the eastern Pacific Ocean (4976) during the winter season (Mizroch and Rice, pers. comm.). In a comparison of cetacean communities between the ETP and the western IO, Ballance and Pitman (1998) noted that blue whales in both

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of these areas might track localized areas of high seasonal productivity to take advantage of feeding opportunities in low latitudes. Based on this, we hypothesize Antarctic blue whales are more likely to be found in the eastern, rather than western, South Atlantic, in the Benguela Current region, and may occur as far north as Cabo Frio, where year-round upwelling occurs. However, this hypothesis will remain untested without acoustic data from the western Pacific and the eastern South Atlantic Oceans. The South Atlantic is the one ocean basin from which no long-term acoustic recordings have been made and we strongly suggest that efforts be made to rectify this omission. Lastly, interannual variation in call detections may be due to interannual variation in migratory behavior. Commercial whalers noticed year-toyear differences in the abundance and distribution of whale species in the Antarctic (Mackintosh et al., 1929; Hjort et al., 1932). Based on the catch composition, some years were considered ‘‘fin whale years’’ while others were called ‘‘blue whale years’’ (Mackintosh et al., 1929; Hjort, 1933). These differences were thought to be related to ice and current conditions that drive the abundance of krill (Hjort et al., 1932; Ruud, 1932; Hjort, 1933). In the eastern Pacific, Antarctic-type blue whale calls were recorded in regions that are known habitat for pygmy blue whales (B. m. brevicauda) and in which other blue whale call types have been recorded. The ETP is an area that is occupied by blue whales year-round (Reilly and Thayer, 1990; Palacios, 1999). An examination of acoustic data from the array discussed here has revealed Antarctic, northeastern Pacific (Thompson et al., 1996) and Chilean (Cummings and Thompson, 1971) type blue whale calls (Stafford et al., 1999a). In addition to this acoustic overlap, sighting and whaling data indicate that both Antarctic and pygmy blue whales are found off Peru and Chile but do not intermingle (Aguayo, 1974; Berzin, 1978; Ichihara, 1981; Donovan, 1984). Kills and sightings of blue whales off Peru peaked in the austral summer (December–March; Harmer, 1928; Donovan, 1984), which is opposite the acoustic detection pattern of Antarctic-type calls but coincides with the period in which Chilean-type



calls were recorded (Stafford et al., 1999a). This suggests that there are two distinct populations of blue whales in the southeast Pacific: those that feed in the Antarctic during the summer and those that remain in the Subantarctic during this time. These two populations produce distinctly different calls. The status of blue whales in the Indian Ocean is unclear. They have been sighted throughout the IO, both north and south of the equator, generally during the North West Monsoon season (November–March), but seldom in large numbers (Leatherwood et al., 1984; Alling et al., 1991; Kasuya and Wada, 1991; Robineau, 1991; Small and Small, 1991). It has been suggested that animals seen off Sri Lanka and the Maldives may be a resident population of the northern IO (Alling et al., 1991; Ballance and Pitman, 1998). Whether these are contiguous with blue whales sighted in the Arabian Sea, which have been proposed as a separate stock based on Japanese sighting data (Kasuya and Wada, 1991), is unknown. Our acoustic data show that Antarctic blue whales occur in the IO during months in which there are no current sighting data, and that the IO is a potential wintering ground for some of these whales. Based on the seasonal detection of these calls in the austral winter, it may be that illegal Soviet whaling in the IO, which occurred from late October to mid-December, did not take Antarctic blue whales (Mikhalev, 2000). Interpretation of the call detections reported here underscores our ignorance of blue whale distribution during the season in which reproductive activity is likely taking place. Understanding this part of blue whale life history is critical to support the recovery of this endangered species. This leaves us with two final caveats, one from the past and the other for the future: first, the acoustic/migratory behaviors discussed here may not be at all representative of Antarctic blue whales. This population has undergone a severe reduction in numbers, and it is possible that the current population is less than 1% of the prewhaling population. Our relatively few detections may simply mirror the fact that few whales are left. Secondly, the influence of climate warming on the Antarctic ecosystem with the concomitant decrease in sea ice and krill (De La Mare, 1997;

Nicol et al., 2000) could influence, and may already have influenced, the distribution and reproductive capability of blue whales in the Antarctic and elsewhere.

Acknowledgements This work was performed while K.M. Stafford held a National Research Council Research Associateship Award at the National Marine Mammal Laboratory, Alaska Fisheries Science Center. This work was partially supported by and IMS hydrophone data have been made available through the United States Air Force under contract DTRA 01-01-C-0070. The information released in this manuscript conforms to the conditions set forth in this contract. This work was additionally supported by Office of Naval Research grants N00014-03-1-0099 and N0001403-1-0735 to D.K. Mellinger. D. Rice, D. Palacios and three anonymous reviewers provided helpful comments on earlier drafts of this manuscript. Thanks to C. Fox of NOAA and R. Dziak of Oregon State University and the NOAA/Pacific Marine Environmental Laboratory projects office for access to the Pacific data. This is LDEO contribution number 6606 and PMEL contribution number 2706.

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