Aerial surveys for pack-ice seals
along the Ingrid Christensen and Princess Astrid Coasts, East Antarctica
R. Suresh Kumar 1 & J.A.
Johnson 2
1,2 Wildlife Institute of India, No. 18, Chandrabani, Dehradun, Uttarakhand248001, India
1 suresh.wii@gmail.com (corresponding author), 2 jaj@wii.gov.in
Abstract: We conducted
aerial surveys in the austral summer of 2009–2010 to count and record the
spatial distribution patterns of pack-ice seals hauled-out along the Ingrid
Christensen and Princess Astrid coast of East Antarctica. A total of 3,601 hauled-out seals were
counted from six aerial surveys totalling a length of
approx. 1,200km, with each survey lasting about two hours. Weddell Seal Leptonychotes weddellii was the most commonly sighted species
in both the areas surveyed (98.2%), and had an encounter rate of 2.9
seals/km. The other species
encountered during the survey were Crabeater Seal Lobodon carcinophaga(1.7%) and Leopard Seal Hydrurga leptonyx (0.03%). Group size of hauled-out Weddell Seals varied considerably and ranged
from solitary to maximum of 42 individuals. The median group size of Weddell seals hauled-out
along the Ingrid Christenson Coast was found to be significantly different
between the December 2009 and January 2010 survey. Further, along this coast Weddell Seals
were found hauled-out mainly close to the ice shelf and their spatial distribution
appeared to be influenced by the extent of sea ice in the area.
Keywords: Aerial census, Crabeater Seal, encounter rate, group size, Hydrurga leptonyx, Leptonychotes weddellii,
Leopard Seal, Lobodon carcinophaga,
spatial distribution, Weddell Seal.
doi: http://dx.doi.org/10.11609/JoTT.o3817.6230-8 | ZooBank: urn:lsid:zoobank.org:pub:9FEAE8D1-9BB7-46D3-91B7-2B3FEF85EE5E
Editor: Peter Boveng, NOAA Alaska Fisheries Science Center, Seattle, USA. Date
of publication: 26 August 2014 (online & print)
Manuscript details: Ms # o3817 |
Received 18 October 2013 | Final received 17 July 2014 | Finally accepted 21
July 2014
Citation: Kumar, R.S. & J.A. Johnson (2014). Aerial surveys for pack-ice seals along the Ingrid Christensen and
Princess Astrid Coasts, East Antarctica. Journal of Threatened Taxa6(9): 6230–6238; http://dx.doi.org/10.11609/JoTT.o3817.6230-8
Copyright: © Kumar & Johnson 2014. Creative Commons Attribution 4.0 International License. JoTT allows unrestricted use of this article in any medium,
reproduction and distribution by providing adequate credit to the authors and
the source of publication.
Funding: National Centre
for Antarctic and Ocean Research (NCAOR), Ministry of Earth Sciences.
Competing Interest: The authors
declare no competing interests.
Author Contribution: Equal contribution
from both the authors in carrying out field work and
manuscript preparation.
Author Details: R. Suresh Kumar is a scientist in the department of endangered species management
at WII and has been involved in field research on threatened and lesser known fauna since 1998. His research interests are in
understanding species distributions, their habitat requirements, assessing
their status and developing conservation strategies. And, his current research
activity includes the study of Olive Ridley Sea Turtle movements and migration
along the Odisha and Maharashtra coast of India using
satellite telemetry. Dr. J.A. Johnsonis a scientist in the department of habitat ecology at WII and mainly works on
aquatic resource management. His primary interests are stream fish ecology,
resource partitioning, stream community response on
impacts, aquatic habitat monitoring and conservation of threatened fish
species.
Acknowledgements: We thank the Director, National Centre for Antarctic and Ocean
Research (NCAOR), Ministry of Earth Sciences, Goa and Director, Wildlife
Institute of India (WII) for giving us the opportunity to participate in the
29th Indian Scientific Expedition to Antarctica. We extend our gratitude to Dr. Rajesh Asthana, voyage leader of the expedition, who was very
supportive of our work. We thank
Bjorn Frode Amundsen and Dante Johan Fontana,
helicopter crew for flying us safely and for their enjoyable assistance in
field. We also thank the captain
and crew of Ivan Papanin for all their support. Faculty colleagues Drs. S. Sathyakumar and K. Sivakumarprovided us the much needed guidance and encouragement. We also thank the two anonymous
reviewers for their comments, which helped in the preparation of the
manuscript.
For figures, images, tables -- click here
Introduction
Five of the 18
extant species of true or earless seals of the family Phocidae:
the Southern Elephant Seal Mirounga leonina, Weddell Seal Leptonychotes weddellii, CrabeaterSeal Lobodon carcinophaga,
Leopard Seal Hydrurga leptonyxand Ross Seal Ommatophoca rossii occur exclusively in the southern
Hemisphere. They have a circumpolar
distribution around the Antarctic Continent and also occur a little further
north in the southern Ocean and on sub-antarcticislands (Folkens et al. 2008). Being amongst the dominant top predators
in the Antarctic ecosystem they likely play an important role (Bowen 1997),
however the role of marine mammals in aquatic ecosystems is poorly understood (Yodzis 1994, 2001; Bowen 1997; Mackinsonet al. 2003; Morissette et al. 2006).
The population
status of the Antarctic pack-ice seals except for the Southern Elephant Seal
until recently was unknown and, therefore, a continent-wide census under the
Antarctic Pack Ice Seals (APIS) program was initiated by the Scientific
Committee on Antarctic Research Group of Specialists on Seals (Southwell et al. 2012). In the 19th and early 20thcenturies, the Southern Elephant Seal was hunted almost to extinction, while
the other species were free from commercial exploitation (Shirihai2008). These other species escaped
by virtue of their range being in dangerous and ice-filled seas (Testa & Siniff 1987). All of these five species of seals are
currently protected under the Antarctic Treaty and there has been no commercial
sealing in Antarctica since the 1950’s, though there have been Norwegian and
Russian experimental commercial seal harvests since that time. Due to the present widespread occurrence
and large population size globally all of the Antarctic pack-ice seals are
classified as Least Concern by the IUCN.
In recent times,
loss of sea ice due to climate change has been reported or anticipated to have
an impact on marine mammals including pinnipeds (see Learmonth et al. 2006). Loss of suitable breeding and resting
habitats of pack-ice seals due to loss of sea ice, coupled with changes in food
web dynamics may negatively impact seal populations, specifically the Crabeater and Weddell seals (Siniffet al. 2008; Costa et al. 2010; Forcada et al.
2012). Considering the potential
threat to Antarctic pack-ice seals it becomes important to monitor their
habitats and track population trends.
The National
Centre for Antarctic and Ocean Research (NCAOR), Ministry of Earth Sciences,
India, has been undertaking annual expeditions to the Antarctic continent since
1981. As part of NCAOR’s “Long-term
monitoring of wildlife and their habitats in Antarctica”, primarily focussed in areas where the Indian research stations are
located, regular counts of sea birds, seals and other marine mammals are
conducted during the austral summer through aerial and ship based surveys (Sathyakumar 1998; Bhatnagar &Sathyakumar 1999; Hussain& Saxena 2000; Sathyakumar& Sivakumar 2009). Here we report the findings of an ice
seal survey carried out during the austral summer of 2009–2010 along the
Ingrid Christenson and Princess Astrid coasts in East Antarctica as part of the
29th Indian scientific expedition to the continent.
Material and Methods
Study area
Seal surveys were
conducted near two Indian research stations: Bhartiand Maitri in Antarctica (Image 1). The first station visited was Bharti located in the LarsemannHills along the Ingrid Christenson Coast, East Antarctica. There, surveys were carried out during
December 2009 and January 2010 within the Prydz Bay
between 74–77 0E longitudes. This coastline is characterisedby coastal hills and ice shelves including that of the Amery Ice Shelf, and
several rocky islands. Fast ice
stretched to over 25km offshore until the third week of January 2010, and
gradually declined in its extent, and later became entirely ice free. The second station, Maitri,
located in the Princess Astrid Coast was visited in February 2010, where
surveys were carried out between 11–14 0E longitudes. Here the coastline was completely unlike
that of the Prydz Bay region, and is primarily
composed of the Fimbul and Lazarevice shelf margins. During the time
of visit to the site, most of the area surveyed was open water and pack ice.
Methods
Aerial surveys
using a single-engine Eurocopter AS 350 B2 were
conducted for pack-ice seals at both the study sites. Due to limitations in the availability
and operational capacity of the helicopter no systematic design could be
followed for the surveys. In the Prydz Bay region, two surveys were made in each of the two
months surveyed, while at the Fimbul Ice Shelf site
only two aerial surveys were made in February 2010. Using the ship as the base, surveys were
made to the east and to the west of the region, and each trip covered a
distance of about 200 to 250 km, lasting on average two hours. Each flight followed a pattern of first
flying along the contour of the coastline covering on average 100km, then
flying out over the fast ice and up to the pack ice edge, and then surveying
all along the pack ice edge and over fast ice before returning to the
ship. Censuses of Weddell Seals in
particular are recommended to be carried out after
1430 and before 1700 hr local time, especially in themoulting season, which is from January to March (Lake
et al. 1997). While our study
partly overlapped this period we could not conduct surveys during the
recommended time due to logistic difficulties, and all surveys were made
between 0900 and 1100 hrs local time.
Two observers
along with the pilot searched for hauled-out seals on either side of the helicopter. The helicopter was flown at a constant
speed on average 90 knots and at an altitude of approx. 80m. The pilot and the observer seated next
to him reported the seal sightings, the species, and group size, while the
observer seated behind the pilot recorded the data. The observer in the front
also estimated visually the perpendicular distance of the seal from the
track-line and marked the location on a hand-held GPS at every sighting. To take counts of seals in a large
congregation the helicopter was slowed down. On a few occasions when the seal species
could not be identified or the number of seals in a large congregation could
not be counted, a photograph of the same was taken. The photographs were later examined in
camp to identify and or count the number of seals recorded at that point. Following Erickson et al. (1993) all
seals occurring within 20m of each other were considered as a group.
Analysis
A general summarisation of the seal sightings as encounter rates was
made since the survey effort was low and not allocated in a manner that would
support design-based estimates of density and distribution. The seal sighting
data were segregated according to the sighting location such as shelf-ice/edge,
fact ice and pack ice.
Results
Seal survey
A total of six
aerial surveys were flown during this study, totallinga length of approx. 1200km, comprising four surveys in the PrydzBay region totalling 800km, and two surveys along theFimbul Ice Shelf totalling400km. Except for the Southern Elephant
Seal and Ross Seal all the other species of pack-ice seals were recorded during
the aerial surveys (Image 2).
Encounter rate
A total of 3645
seals that included Weddell, Crabeater and Leopard
seals were counted during the six aerial surveys; of these in the Prydz Bay region alone 3273 seals were counted (see Table
1a & 1b). A total of 41 seals in the Prydz Bay and three
in the Fimbul Ice Shelf sites could not be
distinguished as to whether they were Weddell or CrabeaterSeals. Weddell Seals were the most
commonly sighted species in both the areas surveyed (97.0%), and had an
encounter rate of 2.9 seals/km. This was followed by the sightings of CrabeaterSeals (1.7%) and a single record of a Leopard Seal (0.03%). Ross Seals were not encountered during
the aerial surveys, however a pair was seen once from the ship en route to the Fimbul Ice Shelf, hauled out on fast ice.
The encounter rate
of Weddell Seals in Prydz Bay (4.0 seals/km) was much
higher compared to that of the Fimbul Ice Shelf site
(0.8 seals/km). Within Prydz Bay the encounter rate of Weddell Seals differed
between the months of survey; there 2.7 seals/km were
recorded during December while in January 6.1 seals/km were encountered. Also, Weddell Seal encounters for the
two months combined were greater in the western shelf (5.4 seals/km) than in
the eastern shelf (2.2 seals/km). The encounter rate of Weddell Seals also differed with respect to
location and sea ice conditions. Maximum sightings occurred along the
shelf-ice/edge with an encounter rate of 6.8 seals/km, while over the fast ice
and pack ice 2.3 and 0.1 seals/km were recorded respectively.
Group size
A total of 668
Weddell Seal groups with sizes ranging from solitary to maximum of 42
individuals were sighted during the surveys in the PrydzBay site. Maximum numbers of
Weddell Seals were observed solitary (n = 237) and accounted for 35.5 %
of all groups sighted. More
solitary individuals were sighted during surveys in January (40.8 %) than in
December (27.7 %) in the Prydz Bay (Fig. 1). A higher number of groups with >5
individuals was seen in January (n = 103) as
compared to December (n = 86).
Discussion
The high number of
Weddell Seal encounters during this expedition in both Prydzbay and the Fimbul Ice Shelf site is because the
survey tracks there were almost entirely near the coast of Antarctica, where
the species is known to largely inhabit (Lugg 1966; Stirling 1969a; Kooyman 1975; Testa & Siniff 1987). The presence of fast ice does not
necessarily limit Weddell Seal occurrence as they can maintain breathing holes
by abrading the sea ice with their canine teeth (Stirling1969a). Moreover, in inshore fast
ice areas, Weddell Seals use ice breaks or cracks caused by tidal action and
glacial movement for breathing, hauling out and pupping (Stirling1969a). This was typical of the
surveyed site in the Prydz Bay, which is located far
inshore and with smaller bays sheltered by the rocky slopes of the Larsemann Hills, and is possibly the reason why more
Weddell seals were encountered there. In the western Ross Sea counts of Weddell Seals in a square mile of pack
ice were reported to be low when compared to a similar track length in fast ice
where more than 100 seals were recorded (Stirling1969), consistent with the species’ preference for fast ice during the summer.
Sathyakumar & Sivakumar (2009) encountered 7.2 Weddell seals/km during
January 2009 in the Prydz Bay, and 1.2 seals/km
during February 2009 in the Fimbul Ice Shelf area,
which is similar to our observations of this study. The lower encounter rate of Weddell
seals at the Fimbul site recorded during 2009 and the
current study is also perhaps related to the time of survey. The visit to this site was in late
summer when much of the fast ice cover was reduced to open water, resulting in
poor detectability of seals.
Within the Prydz Bay site the higher encounters of Weddell Seals in the
western shelf than in the east during this study appeared to be as a result of
availability of fast ice in the area. Much of the fast ice cover along the eastern shelf was reduced to open
water earlier than the western parts. In a span of a month the fast ice cover
in the entire area observed in mid-December of approx2500km2 reduced to approx. 600km2 in January, much of
which remaining in the western parts. Along the western parts the break-away ice
sheets from the Amery Ice Shelf provided hauling sites for seals, which likely
lead to the crowding of Weddell Seals there and thus resulted in higher
encounters. Along the Princess
Martha Coast seal densities were reported to increase as the amount of pack ice
diminishes with the advance of summer (Bester et al. 1995).
Weddell seals are
known to form large aggregations in breeding colonies (Stirling1969a), but females are reported to be solitary after giving birth; the pupping season being late September to early November
(Smith 1965; Stirling 1969a). By the end of November, Weddell Seal
females are known to wean their pups (Lake et al. 1997), which may be a reason
for the high number of solitary Weddell Seals seen here. Further, during January, the progressing
austral summer in Prydz Bay resulted in sea ice melt
and innumerable cracks in the fast ice became available for seals to haul-out
along, leading to higher number of solitary Weddell Seal sightings. Large aggregations of up to 42
individuals were found in the Prydz Bay area when
there was a single tidal crack along a vast stretch of thick fast ice (Image
3). These groups may not represent
any sociality and are probably a result of limited number of natural openings
in the fast ice.
The low occurrence
of other species such as Crabeater and Leopard Seal
particularly in the Prydz Bay area during this study
is possibly because these seals are inhabitants of the pack-ice edge. Their absence may also be related to
their seasonal movements associated with breeding and feeding. CrabeaterSeals are predominantly krill feeders and use pack ice to haul out upon
(Gilbert & Erickson 1977; Kooyman 1981; Nordøy et al. 1995; Bengtson& Cameron 2004). Krill is
typically abundant near pack-ice and occurs in
proximity to the 1000m isobaths around the Antarctic coast during summer (Ichii 1990; Hosie et al.
2000). Consistent with this Southwell et al. (2005),
using a presence-ocean depth model predicted CrabeaterSeal distribution to be in a band extending northwards by 1.5–5.00latitude from the continental shelf-break. The Prydz Bay area is located much further
south from the predicted distribution range of CrabeaterSeals and thus, may have resulted in our low encounters of the species. Similarly, Leopard Seals are believed to
breed on the outer fringes of the pack ice in summer, when they are solitary
and sparsely distributed (Erickson et al. 1971; Sniff & Stone 1985). In the adjoining VestfoldHills in the Prydz Bay encounter rates of Leopard
Seals during austral spring and summer surveys in 1992 and 1993 were only 0.06
and 0.14, and seals were reported confined to the very outer edge of the fast
ice (Rogers & Bryden 1997).
Since our survey
effort was only minimal no conclusions could be made, though it appears that
seal abundance particularly within the Prydz Bay is
related to the extent of fast ice cover available in the area. It is possible that more hauled-out
seals may have been encountered had the aerial surveys been conducted mid-day
as recommended by Lake et al. (1997), and if more frequent surveys were
conducted within and between seasons. We, therefore, recommend that future surveys in the area take into
account the time of the aerial surveys and adopt standardized, quantitative
methods for design or model-based estimates such as those used by Southwell (2005), Conn et al. (2013), and ver Hoef et al. (2014).
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