MARINE MAMMAL SCIENCE, 22(2): 394–412 (April 2006)
C 2006 by the Society for Marine Mammalogy
DOI: 10.1111/j.1748-7692.2006.00035.x
ABUNDANCE OF RINGED SEALS (PUSA HISPIDA)
IN THE FJORDS OF SPITSBERGEN, SVALBARD,
DURING THE PEAK MOLTING PERIOD
BJØRN A. KRAFFT
KIT M. KOVACS 1
MAGNUS ANDERSEN
JON AARS
CHRISTIAN LYDERSEN
Norwegian Polar Institute,
N-9296 Tromsø, Norway
E-mail: kit.kovacs@npolar.no
TORBJØRN ERGON
Centre for Evolutionary and Ecological Synthesis,
Department of Biology, University of Oslo,
0316 Oslo, Norway
and
USGS Patuxent Wildlife Research Center,
Laurel MD 20708–4017, USA
TORE HAUG
Institute of Marine Research,
9294 Tromsø, Norway
ABSTRACT
Ringed seal (Pusa hispida) abundance in Spitsbergen, Svalbard, was estimated
during the peak molting period via aerial, digital photographic surveys. A total of
9,145 images, covering 41.7%–100% of the total fast-ice cover (1,496 km2 ) of 18
different fjords and bays, were inspected for the presence of ringed seals. A total of
1,708 seals were counted, and when accounting for ice areas that were not covered
by images, a total of 3,254 (95% CI: 3,071–3,449) ringed seals were estimated
to be hauled out during the surveys. Extensive behavioral data from radio-tagged
ringed seals (collected in a companion study) from one of the highest density fjords
during the molting period were used to create a model that predicts the proportion
of seals hauled out on any given date, time of day, and under various meteorological
conditions. Applying this model to the count data from each fjord, we estimated that
a total of 7,585 (95% CI: 6,332–9,085) ringed seals were present in the surveyed
area during the peak molting period. Data on interannual variability in ringed
seal abundance suggested higher numbers of seals in Van Keulenfjorden in 2002
1
Corresponding author.
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KRAFFT ET AL: ABUNDANCE OF RINGED SEALS
395
compared to 2003, while other fjords with very stable ice cover showed no statistical
differences. Poor ice conditions in general in 2002 probably resulted in seals from
a wide area coming to Van Keulenfjorden (a large fjord with stable ice in 2002).
The total estimated number of ringed seals present in the study area at the time of
the survey must be regarded as a population index, or at least a minimum estimate
for the area, because it does not account for individuals leaving and arriving, which
might account for a considerable number of animals. The same situation is likely the
case for many other studies reporting aerial census data for ringed seals. To achieve
accurate estimates of population sizes from aerial surveys, more extensive knowledge
of ringed seal behavior will be required.
Key words: aerial survey, digital photography, abundance, ringed seal, Pusa hispida.
The ringed seal (Pusa hispida) is the most abundant marine mammal species in
Svalbard. It is a small seal (adults normally weigh 50–90 kg, Lydersen and Gjertz
1987), which is specially adapted to life in close association with sea ice (Smith and
Stirling 1975, 1978; Smith 1987). Both young and adult ringed seals of both sexes
maintain breathing holes usually in land-fast ice, and are able to access areas deep
into pack- and land-fast ice, which are unavailable to most other marine mammals.
In areas where enough snow accumulates, ringed seals dig lairs over some of their
breathing holes (Smith and Stirling 1975). These lairs are used for resting, and
adult females use them as a place to give birth. The lairs protect the animals from
harsh environmental conditions (Smith et al. 1991) and also provide some degree
of protection from predation (Smith and Stirling 1975, Smith 1976, Lydersen and
Gjertz 1986, Lydersen and Smith 1989, Furgal et al. 1996). Each adult ringed seal
has several lairs and breathing holes within its territory, and can move between these
structures in case of attacks from a predator. Subadult animals also use lairs, but do
not appear to be as exclusive about space use as adults; at least in terms of their use of
haul-out holes it is normal for several subadults to occur together. The peak pupping
period for ringed seals in Svalbard is during the first week of April (Lydersen 1995).
Following birth, the mothers nurse their single pups for about 40 d (Hammill et al.
1991). Toward the end of the nursing period in mid to late May, ringed seal adults
enter breeding condition and mating takes place. Following the breeding season,
ringed seals commence the molting period. All true seal species go through this loss
and replacement of hair and top layers of skin on an annual basis. During this process
the animals prefer to stay out of the water, on ice or land depending on the species.
In the air they can perfuse their skin with blood and thus accelerate the growth of
new hair with much less energy loss than if this process occurs in the water.
The availability of ringed seals on the surface of the ice during the molting period
provides the best opportunity to determine how many ringed seals occur in an area. To
estimate seal population sizes, surveys should be timed to coincide with those times
when the maximum proportion of a population is visible for counting (Erickson et al.
1993). For many seal populations this occurs during the birthing period when they
congregate in traditional areas. However, ringed seals occupy lairs at this time and are
not available for counting until later in the spring when they haul out on the surface
of the ice for their annual molt. For the Svalbard area, the peak of the molting period
for ringed seals occurs during the first part of June (Carlens et al. 2006, this issue).
Estimation of the size of local ringed seal populations has been conducted using a
variety of methods including: ship-based surveys (McLaren 1961); land-based counts
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MARINE MAMMAL SCIENCE, VOL. 22, NO. 2, 2006
(Smith 1973a); extrapolation from breathing hole or lair densities found during dog
surveys (Hammill and Smith 1990, Lydersen et al. 1990, Lydersen and Ryg 1991,
Smith and Lydersen 1991); acoustic monitoring (Stirling et al. 1983, Calvert and
Stirling 1985); using the size of polar bear (Ursus maritimus) populations and their
estimated energy requirements in terms of numbers of ringed seals they would need
to consume (Stirling and Øritsland 1995); and the most common method, aerial
surveys (McLaren 1966; Burns and Harbo 1972; Smith 1973b, 1975; Helle 1980,
1986; Finley et al. 1983; Kingsley et al. 1985; Jensen and Knudsen 1987; Lunn et al.
1997; Frost et al. 2004).
Even during the peak of the molting period there will always be a proportion of
the seals that are in the water and thus not accessible for counting on the ice surface.
Many studies have shown that the fraction of time ringed seals spend hauled out varies
considerably with location, season, time of day, and various meteorological factors (e.g.,
Smith 1973b, Finley 1979, Smith and Hammill 1981, Kelly and Quakenbush 1990,
Kingsley and Stirling 1991, Härkönen and Lunneryd 1992, Harwood and Stirling
1992, Heide-Jørgensen et al. 1992, Lydersen et al. 1993, Lydersen and Hammill
1993, Gjertz et al. 2000, Born et al. 2002, Moulton et al. 2002). Studies of haul-out
activity of ringed seals in the Svalbard area, based on a combination of visual surveys
of seals on the ice surface and activity of VHF-tagged individuals, have revealed a
clear diurnal and seasonal patterns and also significant effects of temperature and wind
(Carlens et al. 2005, this issue). These natural sources of variation in the number of
seals on the surface must be taken into account when doing population assessments.
The purpose of the present study was to estimate the number of ringed seals in the
fjords of Spitsbergen, Svalbard. This was accomplished by (1) counting seals hauled
out on the ice during the peak molting period using aerial digital-photographic
images in combination with (2) the development of a model that accounts for seals
in the water at any given time based on novel analyses of data collected during a
companion study (Carlens et al. 2005, this issue).
METHODS
The aerial digital photographic surveys were conducted from 9 to 21 June 2002
and 2003 on Spitsbergen, Svalbard, Norway (∼77◦ –80◦ N, 10◦ –20◦ E; Fig. 1). All
fjord and bay areas containing fast ice at this time of year (Areas 1–18; Fig. 1) were
included in the surveys. Areas 1–16 were surveyed in 2002, while Areas 17 and 18
were surveyed in 2003. Additionally Areas 2, 3, and 13 were repeated in 2003 to
provide interannual comparative data. All flying was conducted between 0900 and
2100. The aircraft used was a Piper Seneca II (PA 34). Two camera houses (Hasselblad 555 ELD) outfitted with backpacks (H-20 Phase One A/S, 2000 Frederiksberg,
Denmark), containing CCD chips designed to capture digital images, were mounted
through the floor of the aircraft. The cameras were connected to two laptop computers
with external hard disks. Orientation and positions of the line transects to be flown
were determined prior to each flight using specialized software (Snapplan and Snapshot) from Track Air, Aerial Survey Systems (B.V. Boortorenweg 20, 7550 Hengelo,
Netherlands). Snapplan was used to plan the photo flights because this software deals
well with the specifications of the camera, survey altitude, and precalculated GPS
positions. GPS positions were exported into the flight control program Snapshot.
Snapshot was used to control the Tracker External Camera Interface (TECI-3) box;
which contained a built-in 12 channel GPS system. TECI-3, in combination with
KRAFFT ET AL: ABUNDANCE OF RINGED SEALS
397
Figure 1. Map of Spitsbergen, Svalbard, showing the fast-ice distribution (dotted areas)
at the times the aerial surveys were conducted. Fast ice was found in Hornsund (Area 1), Van
Keulenfjorden (Area 2), Van Mijenfjorden (Area 3), Fridtjovhamna (Area 4), Tempelfjorden (Area 5), Billefjorden (Area 6), Skansbukta (Area 7), Dicksonfjorden (Area 8), EkmanNordfjorden (Area 9), Borebukta (Area 10), Ymerbukta (Area 11), St. Jonsfjorden (Area 12),
Kongsfjorden (Area 13), Krossfjorden (Area 14), Raudfjorden (Area 15), Liefdefjorden (Area
16), Sorgfjorden (Area 17), and Lomfjorden (Area 18). All these fast-ice areas were included
in the survey.
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MARINE MAMMAL SCIENCE, VOL. 22, NO. 2, 2006
Figure 2. Map showing the transect lines flown over the land-fast ice in Van Mijenfjorden
(Area 3) and Fridtjovhamna (Area 4) in June 2003. Each small square on the transect lines
corresponds to a digital image covering 336 × 336 m. All land-fast ice in Fridtjovhamna and
48.2% of the ice in Van Mijenfjorden were covered, resulting in 26 and 2,235 digital images,
respectively. (The ice-covered area northeast in Van Mijenfjorden was not surveyed because
this is a very shallow, muddy area that is unsuitable for ringed seals).
Snapshot, automatically activated the two digital cameras, when predefined positions
were reached. Two parallel images were shot every fifth second from an altitude of
2,400 ft (∼730 m), with 80-mm lenses; each image thus covered an area of 336 ×
336 m of the surface. The aircraft was flown at 110 knots (∼200 km/h). At this
height and speed, there was no overlap between the images. Parallel transects were
flown, starting in the innermost parts of each fjord or bay and ending outside the
fast-ice edge (Fig. 2). The airplane’s position, angle of inclination, track, and altitude
during the time when the images were taken was stored in World Geodetic System
(WGS-84) format.
Image processing software (Light Phase Image Capture, Phase One A/S, 2000,
Frederiksberg, Denmark), was used to manually inspect the digital photographic
material. Using this software, the images were classified into five categories: (1) ice
images, in which the entire photo covered only fast ice or ice comprised of large floes,
(2) ice–land images, in which the photo included areas with fast ice connected to
land or glacier fronts, (3) ice–land–water images that included areas with fast ice or
floe ice, land or glacier, and seawater, (4) ice–water images that included areas with
fast ice or floe ice and seawater, and (5) land–water/water/land images that did not
include fast ice or floe ice. All image types that included fast ice (categories 1–4) were
enlarged and inspected in detail for the presence of seals. The software also enabled
us to enlarge the digital images and thus improve our ability to detect hauled out
seals. Initially, a large number of digital images (n = 1,000) were inspected by two
readers for the presence of seals in order to examine potential reader biases. Seals
were easily detected on the digital images. The variation in the number of ringed
KRAFFT ET AL: ABUNDANCE OF RINGED SEALS
399
seals detected between the two readers was only 1.6% and therefore no attempt was
made to correct for reader bias. The area of the fast ice that was covered during the
photographic survey was calculated based on the classification of the images, known
GPS positions from each image, and by utilizing ArcGIS software (ESRI Arc View,
Ver. 8.3, Redlands, CA).
Counts from the aerial surveys were used to estimate the total number of seals
that were hauled out on the fast ice in each fjord. In each fjord i a fraction q i was
covered by r i images from the aerial survey, and the number of seals y ij on each
image j
was counted ( j = 1, . . . , r i ). Letting K i be the total number of observed seals
i
y ij ), the total abundance of seals hauling out on the fast ice in a given
(K i = rj=1
fjord (Areas 1–18, Fig. 1) was estimated as Ŷi = K i /q i . Since the survey covered a
large proportion of the fast ice in each fjord (42%–100%; Table 1), and the images
were systematically dispersed over the entire ice (see Fig. 1), we treated the images
as a random sample of the entire fast ice in the fjord, and hence used the standard
estimator for the sampling variance when samples are drawn from a finite population
without replacement (Williams et al. 2002).
2
ˆ i
ri
− ri
,
v
ar(Ŷi ) =
qi
qi
where ˆ 2 is the empirical variance in number of seals between the surveyed images
r i
ˆ i2 =
(y − ȳ i )/(r i − 1).
j=1 ij
In order to obtain an estimate of the total abundance of seals (on and below the
ice) in each fjord, the estimated number of seals hauled out on the ice was divided by
an estimate of the proportion of seals that were on the ice surface during the time of
survey. In order to accomplish this, detailed behavioral data were required. The raw
data were collected in a companion study (Carlens et al. 2006, this issue). Carlens
et al. is partly based on the behavior patterns of 24 ringed seals equipped with VHF
tags, which were monitored every 0.5 h around the clock via an automatic receiving
station. In the present study we used the raw data from these animals to construct a
model to predict the proportion of the seals present in a fjord, which were hauling out
on the fast ice at a given date, hour of the day, and under given weather conditions
(temperature, sunlight, and wind speed). Generalized linear mixed models, with
a binomial error and a logit-link, were fitted using the “glmmPQL” function in
the MASS Library of S-Plus 6.2 (Venables and Ripley 2002). In order to account
for autocorrelation in repeated observations of proportions of the tagged seals that
were hauling out on the ice, checked at 0.5 h intervals by the automatic logging
station, a first-order autoregressive correlation structure at the level of the residuals,
together with a random between-day variance component was included in the model.
The effect of date (season), hour of the day, temperature, wind speed, and intensity of
sunlight was examined as fixed-effect predictors in candidate models. Assumptions of
linearity were assessed by exploratory GAM-plots (S-Plus 6.2, Insightful Corporation
2001) and by examining the residuals of fitted models. There was a relatively strong
nonlinear effect of both date (season) and hour of the day. The proportion of VHFtagged seals hauling out on the ice was highest in the beginning of the study (early
May), and then declined from late May onward. We modeled this with a threshold
model and estimated the time at which a linear (on a logit scale) decline started by
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Area no.
Hornsund
Van Keulenfjorden
Van Mijenfjorden
Fridtjovhamna
Tempelfjorden
Billefjorden
Skansbukta
Dicksonfjorden
Ekman-Nordfjorden
Borebukta
Ymerbukta
St. Jonsfjorden
Kongsfjorden
Krossfjorden
Raudfjorden
Liefdefjorden
Sorgfjorden
Lomfjorden
Total
Area name
20 June
12 June
12 June
12 June
13 June
13 June
13 June
14 June
11 June
15 June
15 June
15 June
16 June
20 June
20 June
20 June
11 June
11 June
Date
1000–1800
1400–2100
1400–2100
1400–2100
1330–1600
1330–1600
1330–1600
1240–1540
1400–2000
1130–1600
1130–1600
1130–1600
1200–1430
1000–1800
1000–1800
1000–1800
0900–1330
0900–1330
Flight time
0
12
12
12
10
10
10
15
0
15
15
15
20
5
0
0
10
20
Wind
speed
(knots)
1
2
2
2
2
4
2
4
4
1
1
1
1
0
−2
−2
−5
−7
Temperature
(◦ C)
39.6
72
390
2.7
49
107
0.5
155
316
34
3.5
53
45
7
16
3
38
165
1,496.3
60.6
41.7
46.2
100
53.1
56.1
100
74.2
46.5
73.5
100
62.3
53.3
100
65.6
50.0
44.7
42.4
422
335
2,117
30
559
254
9
691
1,734
221
46
269
257
199
376
69
782
775
9,145
139
187
204
8
268
143
0
78
262
39
14
62
86
22
128
13
22
33
1,708
Fraction of
No. of No. of seals
Area of fast ice
total fast-ice images counted on
covered with images cover (%) inspected
images
(km2 )
(q)
(r)
(K)
Table 1. Data from an aerial digital photographic survey of ringed seals on Spitsbergen, Svalbard, June 2002 and 2003, showing areas included in
the survey, number of images inspected with the resulting number of ringed seals.
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MARINE MAMMAL SCIENCE, VOL. 22, NO. 2, 2006
KRAFFT ET AL: ABUNDANCE OF RINGED SEALS
401
searching for the threshold point that minimized the deviance of the model. The
proportion of seals hauling out on the ice peaked at mid-day, and this effect was
adequately modeled with a second-order polynomial (on a logit scale).
Model selection with respect to the fixed effects of temperature, wind speed, and
sunlight was based on conditional F-tests as recommended by Pinheiro and Bates
(2000); we settled on a model including only a linear effect of wind speed. Subsequently, we used the predictions from the model fitted to the VHF data ( p̂ i ) together
with the aerial survey estimates (Ŷi ) to estimate the total number of seals present
(on and below the ice) in each fjord, N̂i = Ŷi / p̂ i . The variance of logit( p̂ i ) was estimated as the prediction error variance based on the fixed effects plus the estimated
between-day variance (i.e., variance of the predictions at a random day rather than an
average day). Using the ∂-method (Morgan 2000) we then obtained
p̂ i ) = (1 − p̂ i ) p̂ i Var(logit(
p̂ i )),
Var(
and
N̂i ) =
Var(
1
p̂ i
2
Ŷ
Ŷi ) + − i
Var(
p̂ i2
2
p̂ i )
Var(
= N̂ 2 (CV2 (Ŷ) + CV2 ( p̂ )),
where CV2 (x) = Var(x)/x 2 . The
m total number of seals in all (m) fjords surveyed
N̂i , and the variance of N̂tot was estimated in
was then estimated as N̂tot = i=1
accordance with the ∂-method as
Y 0
Var( N̂tot ) = a
a ,
0
p
where a is a row vector of the partial first derivatives of N̂tot with respect to the Ŷi ’s
and the p̂ i ’s, a = [1/ p̂ i , · · · , 1/ p̂ m , −Ŷ/ p̂ i2 , · · · , Ŷ/ p̂ m2 ], and where ΨY is
a diagonal matrix with the variances of the Ŷi ’s and Ψp is the variance–covariance
matrix of the p̂ i ’s. The reported confidence intervals of N̂’s are based on the assumption
of a normal distribution on a log scale (±2 standard errors on a log scale).
RESULTS
The fast ice in the 18 fjords and bays included in this study constituted an area of
1,496 km2 (Fig. 1, Table 1). The fraction of the fast ice area that was covered with
digital images varied between locations, from 41.7% to 100% (Table 1). A total of
9,145 images (69% of total number of images taken) from these 18 fjords and bays
were classified into categories 1–4 and inspected for the presence of hauled out ringed
seals. A total of 1,708 ringed seals were detected on these images (Table 1), and when
the ice areas that were not covered with images were accounted for a total of 3,254
(95% CI: 3,071–3,449) ringed seals (Table 2) were estimated to be hauled out on the
ice during the survey. The model designed to account for the proportion of ringed
seals in a fjord, which were hauled out on the ice during the time of the aerial survey
is presented in Figure 3 (see Methods for a detailed description). Applying this model
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MARINE MAMMAL SCIENCE, VOL. 22, NO. 2, 2006
Figure 3. Fitted model of the proportion of VHF-tagged seals hauling out on the ice
depending on date (panel A), time of the day (B), and wind speed (C). Each panel shows
the fitted predictions (solid line) using the mean values of the two other predictor variables.
Innermost stippled lines show 95% CI (±2 SE on a logit scale) of the predictions of the mean,
and outermost dotted lines show uncertainty in the predictions for a random
day (including
both uncertainty in the mean and random day-to-day variation; i.e., ±2 SE2 + ˆ 2 ) where ˆ 2
is the estimated random between-day variance component). The distribution of the predictor
variables in the calibration data is shown in the bottom of each panel, and the distributions
of the variables in the aerial survey data are shown in the top of the panels. Panel D shows
deviance residuals plotted against fitted predictions for each day (daily coefficients accounted
for). The predicted proportion of seals hauling out before 23 May is, on a logit scale, −1.18
(SE = 0.23) + 0.30 (SE = 0.04) × h − 0.011 (SE = 0.001) × h2 − 0.038 (SE = 0.011) ×
wind speed. After 23 May the predictions decline by 0.031 (SE = 0.005) on a logit scale per
ˆ was estimated to be 0.24 (95%
day. The between-day standard deviation on a logit scale ()
CI: [0.16, 0.37]) and the correlation between observations at the half-hour intervals was 0.63
(95% CI: [0.59, 0.66]). There was no evidence of overdispersion (dispersion scale estimated
to 0.93, but fixed to 1 in the final fit).
to adjust the estimated numbers of seals hauled out on the ice in each fjord, so that
estimates of the total abundance of seals in the fjords (on and below the ice) were
obtained, resulted in a total estimate of 7,585 (95% CI: 6,332–9,085) ringed seals
in the study area (Table 2).
The average density of ringed seals hauled out on the ice in the study area was
1.4 per km2 , but densities varied substantially, ranging from locations with no seals
(e.g., Skansbukta, Area 7) to regions with 8.0 seals per km2 (e.g., Raudfjorden,
Area 15; Table 3). The highest numbers of hauled out seals were found in Van
Area name
Hornsund
Van Keulenfjorden
Van Mijenfjorden
Fridtjovhamna
Tempelfjorden
Billefjorden
Skansbukta
Dicksonfjorden
Ekman-Nordfjorden
Borebukta
Ymerbukta
St. Jonsfjorden
Kongsfjorden
Krossfjorden
Raudfjorden
Liefdefjorden
Sorgfjorden
Lomfjorden
Total
Area no.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
229 (33)
449 (47)
442 (28)
8 (0)
505 (30)
255 (20)
0 (0)
105 (7)
563 (37)
53 (6)
14 (0)
100 (10)
161 (29)
22 (0)
195 (23)
26 (17)
49 (11)
78 (13)
3,254 (94)
Estimated no. of
seals hauled out
on the fast ice (SE)
(Ŷ)
3.50 (0.51)
2.60 (0.27)
0.52 (0.03)
2.96 (0)
5.47 (0.33)
1.34 (0.10)
0 (0)
0.50 (0.03)
0.83 (0.05)
1.15 (0.13)
4.00 (0)
1.18 (0.12)
1.91 (0.34)
3.14 (0)
8.00 (0.94)
4.33 (2.83)
0.58 (0.13)
0.20 (0.03)
Estimated
density of
hauled-out seals
km−2 (SE)
48.4 (6.6)
39.1 (6.7)
39.1 (6.7)
39.1 (6.7)
44.2 (6.8)
44.2 (6.8)
44.2 (6.8)
39.2 (7.1)
52.1 (6.3)
38.6 (7.1)
38.6 (7.1)
38.6 (7.1)
33.6 (7.5)
43.8 (6.6)
48.4 (6.6)
48.4 (6.6)
45.0 (6.7)
36.0 (7.6)
Proportion seals
estimated hauled
out (SE)
( p̂ × 100)
474 (94)
1,148 (231)
1,131 (207)
20 (4)
1,142 (187)
577 (99)
0
268 (52)
1,082 (149)
137 (29)
36 (7)
258 (55)
480 (137)
50 (8)
403 (72)
54 (36)
109 (29)
216 (57)
7,585 (685)
Estimated total
no. of seals in
area (SE)
( N̂ = Ŷ/ p̂ )
182–394
821–1,425
89–211
25–52
169–394
271–851
37–68
282–576
14–204
64–187
127–367
6,332–9,085
319–703
767–1,718
784–1,631
15–29
822–1,585
409–813
95% CI
Table 2. Estimated numbers of ringed seals hauled out on the fast ice and total number of seals present (including animals not hauled out) in the
various areas included in the aerial digital photographic survey on Spitsbergen, Svalbard, June 2002 and 2003.
KRAFFT ET AL: ABUNDANCE OF RINGED SEALS
403
Area of fast ice covered with images (km2 )
Fraction of total fast ice covered (%) (q)
No. of images inspected (r)
No. of seals counted on images (K)
Estimated no. of hauled out seals on fast ice (SE) (Ŷ)
Estimated density of hauled out seals km−2 (SE)
Proportion seals estimated hauled out (SE) ( p̂ × 100)
Total no. of seals in area (SE) ( N̂ = Ŷ/ p̂ )
95% CI
Relative difference − [95% CI] = (( N̂2003 − N̂2002 )/ N̂2002 )
Variable
2003
72
183
41.7
46.5
335
911
187
117
449 (47)
252 (19)
2.60 (0.27)
0.64 (0.05)
39.1 (6.7)
50.5 (6.4)
1,148 (231)
499 (73)
767–1,718
372–669
−0.56 [−0.73, −0.28]
2002
Van Keulenfjorden
(Area 2)
2003
390
465
46.2
48.2
2,117
2,235
204
193
442 (28)
401 (25)
0.52 (0.03)
0.42 (0.03)
39.1 (6.7)
51.7 (6.3)
1,131 (207)
774 (106)
784–1,631
588–1,019
−0.30 [−0.55, 0.08]
2002
Van Mijenfjorden
(Area 3)
2003
45
63
53.3
100
257
676
86
187
161 (29)
187 (0)
1.91 (0.34)
2.97 (0)
33.6 (7.5)
51.6 (6.4)
480 (137)
362 (45)
271–851
283–463
−0.25 [−0.58, 0.36]
2002
Kongsfjorden
(Area 13)
Table 3. Area coverage, percentage of ice covered, number of images inspected, resulting numbers of ringed seals, estimated number of ringed seals
hauled out, estimated density of seals, proportion hauled out, estimated total numbers of seals present. CI, and interannual differenced for various areas
that were included in the digital aerial photographic survey on Spitsbergen, Svalbard, both in June 2002 and 2003.
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MARINE MAMMAL SCIENCE, VOL. 22, NO. 2, 2006
KRAFFT ET AL: ABUNDANCE OF RINGED SEALS
405
Keulenfjorden (Area 2), Van Mijenfjorden (Area 3), Tempelfjorden (Area 5), and
in Ekman-Nordfjorden (Area 9), each of which had more than 1,000 individuals
(Table 2).
In order to study annual variation in ringed seal abundance, three large areas
known to be used by ringed seals during the molt, Van Keulenfjorden (Area 2),
Van Mijenfjorden (Area 3), and Kongsfjorden (Area 13) were surveyed both in June
2002 and June 2003 (Table 3). Based on comparisons of confidence intervals between
years, the number of seals inhabiting Van Keulenfjorden was significantly higher in
2002 compared to 2003, while the two other areas showed no statistically significant
difference in seal abundance between the two years (although the difference between
years was potentially large, Table 3).
DISCUSSION
Photographic surveys have been conducted on a variety of marine mammal species
(e.g., Stenson et al. 2003). But the use of digital photography during aerial surveys
of marine mammals is a relative recent technique that to our knowledge has only
been used for narwhal (Monodon monoceros) surveys in Greenland (Heide-Jørgensen
2004). The advantage of any photographic technique, over more traditional visual
surveys, is that observer biases (perception biases) can be avoided. Such biases are a
significant source of uncertainty in many population surveys (see Frost et al. 2004).
Digital images are superior to analogue images in that they are cheaper to produce
and are more user friendly in terms of storage and analysis due to a host of analytical
tools (various software applications) that have been created for use with these sorts of
images.
In the present study we assumed that all ringed seals on the digital images were
detected. The image processing software (Light Phase Image Capture) enabled the
readers to adjust enlargement, clarity, and light condition on each image to maximize
readability. This also made it easy to separate bearded seals (Erignathus barbatus) from
ringed seals, based mainly on size and haul-out location. The difference between
readers (1.6%) in the number of ringed seals found when two independent readers
inspected the same 1,000 images was deemed small enough that this source of variance
would not have a significant impact on the precision of the final results. A similar
finding was made regarding the digital image readings in a narwhal survey (<1%,
Heide-Jørgensen 2004). Our aerial survey was flown at 2,400 ft (∼730 m), which is
so high that the noise from the aircraft should have no impact on the behavior of the
seals. Previous studies have shown only minor responses by hauled out ringed seals
to survey-plane flights performed at 300 ft (∼90 m) (Richardson et al. 1995, Born
et al. 1999). Another potential disturbance factor that could impact the presence of
ringed seals on the ice is snowmobile traffic that at times can be relatively extensive
in parts of the study area. However, snow and ice conditions during spring 2002 and
2003 were such that the snowmobile season had ceased well before early June when
the aerial surveys took place. We are therefore confident that our aerial survey reflects
a precise, and representative, picture of the number of ringed seals in our study area.
The average density of ringed seals hauled out on the fjord-ice in the present study
was 1.4 seals km−2 , which is quite similar to densities reported from other fjord
habitats in the Arctic (Eastern Baffin Island: 1.7 seals per km2 Finley et al. (1983);
Kong Oscars Fjord: 1.0 seals per km2 Born et al. (1998); Scoresby Sund: 2.0 seals
per km2 Born et al. (1998)). However, as in the present study, other ringed seal
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MARINE MAMMAL SCIENCE, VOL. 22, NO. 2, 2006
surveys have also reported large amounts of variation in seal densities within their
study areas (Stirling et al. 1977, Helle 1980, Härkönen and Heide-Jørgensen 1990,
Lunn et al. 1997). In Spitsbergen, we found the density of ringed seals varied from
0.0 to 8.0 seals per km2 among the 18 fjords and bays that were surveyed. These
differences are likely the result of one or more biotic or physical factor(s). Proximity
to an ice edge, where the density of prey is often high, is a factor that has been shown
to influence ringed seal density (Hammill and Smith 1989). During the molting
period, ringed seals, similar to most seal species, have a reduced food intake or do
not eat at all (Ryg et al. 1990), but most of the adult breeding population reside
in the fjords during the entire winter season (Lydersen 1998). They are thought to
be territorial during this period (Smith and Hammill 1981) with adult animals at
least defending underwater territories with associated breathing holes and lairs. The
winter and perhaps especially the early spring, which is the nursing period for the
adult females, is energy demanding (Lydersen 1995). Prey availability during this
period is therefore likely to be a determinant of the densities of ringed seals in a
given fjord area. This factor’s influence might extend into the molt since at least the
breeding animals likely stay in their home areas until the molt is over.
Polar bears and arctic foxes (Alopex lagopus) are ringed seal predators that potentially
could have impacts on the number of seals present on the sea-ice surface during our
survey. However, neither of these predators was seen on the digital images that were
analyzed. Arctic foxes would have been difficult to detect due to their small size
and cryptic coloration, however, they mainly prey on new-born ringed seal pups
that are available only during a short period about 2 mo before the survey was
conducted. Other possible disturbance factors could be caused by humans. However,
as mentioned above, the ice conditions did not allow for snowmobile traffic, there
were no ice-breaking vessels in any of the fjords or bays at the time of the survey, and
most of the surveyed areas are remote places far from human settlements.
Probably the most important physical factors determining ringed seal densities are
the stability of the sea ice and the presence of sea-ice structures around which snow
accumulates, and in which the seals dig lairs. The latter condition is particularly
important for breeding. The date of ice formation is also likely important to how
much snow accumulates. Such conditions vary from year to year (Smith and Lydersen
1991), but in general, the most stable areas of fast ice are in the inner parts of the
fjords, and the fjords that tend to be good for breeding in Svalbard, where relatively
little snow falls, are those that have active glaciers calving into them. The pieces of
ice from the glaciers (calves) become frozen into the fjord ice and accumulate snow
around them. The presence of outer islands also serves to protect the sea ice deep
within some fjords keeping the inner-fjord ice stable later into the season (e.g., Van
Mijenfjorden and Kongsfjorden in Svalbard).
Van Mijenfjorden (Area 3) has a very stable ice cover due to the presence of a large
island at the mouth of the fjord that protects ice inside it from wave action. However,
there is very little glacier activity in this area and it has previously been classified as
a poor breeding habitat (Lydersen et al. 1990) due to the low number of places where
it is possible for the seals to construct lairs. During the present survey we found a
seal density in this area of only 0.5 seal per km2 while areas such as Tempelfjorden
(Area 5) and Raudfjorden (Area 15) which have stable ice and contain active glaciers,
have seal densities above 5.0 seals per km2 (Table 2). Other physical factors that
may contribute to the observed densities are variation in weather parameters such
as temperature and wind, but these factors are reasonably well accounted for in our
modeling of correction factors.
KRAFFT ET AL: ABUNDANCE OF RINGED SEALS
407
Van Keulenfjorden (Area 2), Van Mijenfjorden (Area 3), and Kongsfjorden (Area
13) were surveyed both in 2002 and 2003 to explore potential interannual variation
in the numbers of seals using specific areas for molting. It is known that at least
some adult ringed seals return to the same fjords for breeding in consecutive years,
and that adult animals display strong territoriality in the breeding season (Smith
and Hammill 1981, Lydersen and Kovacs, unpublished data). If philopatry is a
general phenomenon, one would expect that a given area would contain a similar
number of seals each year. This was the case for Kongsfjorden and Van Mijenfjorden,
however, Van Keulenfjorden had significantly more ringed seals in 2002 compared
with 2003 (see Table 3). The ice situation in Kongsfjorden and Van Mijenfjorden
were more similar between the two years while it differed substantially between
these same years in Van Keulenfjorden. Ice data from the Norwegian Meteorological
Institute show that in 2002 the sea ice in the Van Keulenfjorden formed later, broke
up earlier and was generally much less abundant compared to 2003. The ice cover
in Kongsfjorden and Van Mijenfjorden is generally less affected by many of the
conditions that determine when ice breaks up elsewhere, because islands protect
the ice from current and wave action from the west in both of these fjords. Van
Keulenfjorden on the other hand has no such protection and the ice cover here varies
substantially on an annual basis, mainly as an effect of wind, waves and currents. In
a poor ice-year like 2002, the small bays south of Van Keulenfjorden are probably
ice free and thus Van Keulenfjorden might attract seals from these areas that are
searching for a molting platform. Biological differences, in abundance, between years
could have been masked by the broad confidence intervals around the estimates
for Kongsfjorden and Van Mijenfjorden. However, a visual aerial survey of ringed
seals in Van Mijenfjorden and Van Keulenfjorden conducted in 1986 (Jensen and
Knudsen 1987), during the same period as in the present study produced an estimate
remarkably similar to our estimate in 2002. The ice conditions in 1986 strongly
resembled those we experienced in 2002. This indicates that the numbers of seals
using these fjords for molting appears to be quite similar to two decades ago. A ground
survey performed in Van Mijenfjorden during the breeding season in 1986 estimated
that the total breeding population was approximately 125 animals (Lydersen et al.
1990), which is considerably lower than the numbers estimated in this study during
the molt (1,131 SE 207, Table 3). However, it is not surprising that this fjord that
offers so little breeding habitat, but is stable late into the season, is more important
for molting than for breeding.
In this study, we surveyed 16 of the areas in 2002 and the remaining two areas
(Sorgfjorden Area 17 and Lomfjorden, Area 18) in 2003 (in addition to repeating
Areas 2, 3, and 13; see above). Both Sorgfjorden and Lomfjorden have stable ice
cover with little interannual variation due to the fact that they face northeast and are
hence protected by land and polar pack-ice against wave action, so we felt confident
that the number of seals occupying these two fjords was likely to be quite similar
between the two years. Another factor that justifies merging the 18 areas into one
survey, relates to whether or not the area is a good breeding habitat. The breeding
fraction of a population using a given area is likely more stable than the transient
part of the population that moves in just for molting. Ringed seals that use an area
for breeding prefer to stay in the inner parts of the fjords where the sea ice breaks
up last. Seals that use an area only for molting do not need to travel far into the
ice, and therefore would likely haul out closer to the ice edge. If we consider survey
results for Van Mijenfjorden and Kongsfjorden that both have a stable ice cover but
where Van Mijenfjorden is considered to be a poor breeding habitat (Lydersen et al.
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MARINE MAMMAL SCIENCE, VOL. 22, NO. 2, 2006
1990) while Kongsfjorden is a good breeding habitat (Lydersen 1998), we find that
87% of all seals (same numbers in 2002 and 2003) were found in the outer half of
Van Mijenfjorden, while the corresponding number for Kongsfjorden was 13%. The
aerial survey showed that Sorgfjorden and Lomfjorden had 27% and 36% of their
ringed seals distributed in their outer parts, respectively, suggesting that these fjords
are relatively good breeding areas and likely to have relatively stable numbers of seals
from year to year.
This study has shown that (1) about 7,585 (95% CI: 6,332 – 9,085) ringed seals
are present in the molting grounds on Spitsbergen during the peak of molting; (2) the
density of hauled out ringed seals in Spitsbergen is relatively similar with densities
of ringed seals in other Arctic fast-ice habitats; (3) densities vary substantially from
area to area within Spitsbergen as a consequence of various biotic and physical factors,
where sea-ice quality and prey availability are likely to be among the most important
factors; and (4) data on interannual variability in ringed seal density indicate that
fjords and bays with relative stable ice conditions contain quite stable numbers of
animals, while other fjords show considerable variation in the number of animals
present during the molt. However, despite the fact that (1) this aerial digital photographic survey was performed at an altitude high enough not to bias the results by
frightening the hauled out seals, (2) the survey was performed during the optimal period to visually detect the largest proportion of the population of ringed seals, (3) the
results from the interpretation of the digital images were reliable and of high quality,
and (4) we have adjusted for animals that were in the fjords but not visible from the
air using extensive behavioral data, the total number of ringed seals estimated in this
study is probably still far below the real population size. This is due to the fact that
the number of animals in the molting areas, even during the peak molting period,
represents a currently unknown fraction of the whole population. In the ringed seal
behavior study by Carlens et al. (2006, this issue), the number of tagged animals
declined while the total number of seals (based on hourly visual counts each day) was
still increasing. This does not compromise the modeling of the correction factor for
seals in the water at the time of the survey. However, it indicates that only a fraction
of the population is present in an area during any given time period.
In the present study it was estimated that 7,585 (95% CI: 6,332–9,085) ringed
seals were present in the molting grounds of Spitsbergen during the peak of the
molting period. In a previous study of ringed seals in Svalbard, the annual production
of pups in the Spitsbergen area was estimated to be about 8,000 (Smith and Lydersen
1991). This study was based on the knowledge of ice distribution, ice type, and
densities of birth lairs found during dog surveys in various types of ice. For harp
seals (Pagophilus groenlandicus) that have similar life history traits to ringed seals
the population size is normally modeled to be about five times pup production
(ICES 2004). Applying this factor to the ringed seal pup estimate one would expect
the Spitsbergen ringed seal population to be about 40,000 seals. This method for
estimating ringed seal pup production is a very crude method, despite being very labor
intensive and requiring specially trained dogs and handlers and it might overestimate
the population size; however, it gives some insight into the order of magnitude that
the in- and out-flux of ringed seals through the molting period might represent. For
many seal species it is normal that different age and sex groups molt at different times
(Daniel et al. 2003, Kirkman et al. 2003, Reder et al. 2003). For harbor seals (Phoca
vitulina) in Svalbard subadults molt before the adult animals and adult females start
the molt before adult males (e.g., Reder et al. 2003). A similar progression could be
expected to occur for ringed seals. However, the 24 VHF-tagged animals included
KRAFFT ET AL: ABUNDANCE OF RINGED SEALS
409
pups of the year, subadults and adults of both sexes, and the tagged animals that left
the area having finished molting were a heterogeneous group. This indicates that
there is a significant in- and out-flux of ringed seals of various age and sex groups
from the molting grounds. As a consequence this study does not claim to produce an
estimate for the population size of ringed seals in Spitsbergen, although we believe it
does give an accurate estimate of the number of ringed seals present in the molting
grounds during the peak of this process.
Ringed seals are a species that is difficult to survey; they spend a lot of time underwater when at sea, and spend a lot of time under snow (in lairs) when using ice to
haul out during winter and early spring. They are only available for visual or photographic counting at reasonable densities during their annual molt. Aerial surveys at
the time of peak molting are certainly the most straightforward methodological tool
we have to assess ringed seal populations. But to achieve accurate assessments of total
population size based on aerial surveys, much more extensive knowledge regarding
ringed seal behavior is required.
ACKNOWLEDGMENTS
Electronic equipment, a pilot, the plane(s) and a data operator were hired from COWI A/S
(cowi@cowi.dk). Financial support for this project was provided by Store Norske Spitsbergen
Grubekompani A/S, the Norwegian Research Council, and the Norwegian Polar Institute.
We thank Odd Harald Hansen for advice regarding the use of ArcGIS software and Dr. Tom
Smith for his helpful review.
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Received: 14 February 2005
Accepted: 27 October 2005