© 2005 International Council for the Exploration of the Sea
Estimation of harp seal (Pagophilus groenlandicus) pup production in the North Atlantic completed: results from surveys in the Greenland Sea in 2002
a Institute of Marine Research PO Box 6404, N-9294 Tromsø, Norway
b Science Branch, Department of Fisheries and Oceans, Northwest Atlantic Fisheries Centre PO Box 5667, St. John's, Newfoundland, Canada A1C 5X1
*Correspondence to T. Haug: tel: +47 776 09722; fax: +47 776 09701. e-mail: toreha{at}imr.no.
From 14 March to 6 April 2002 aerial surveys were carried out in the Greenland Sea pack ice (referred to as the "West Ice"), to assess the pup production of the Greenland Sea population of harp seals, Pagophilus groenlandicus. One fixed-wing twin-engined aircraft was used for reconnaissance flights and photographic strip transect surveys of the whelping patches once they had been located and identified. A helicopter assisted in the reconnaissance flights, and was used subsequently to fly visual strip transect surveys over the whelping patches. The helicopter was also used to collect data for estimating the distribution of births over time. Three harp seal breeding patches (A, B, and C) were located and surveyed either visually or photographically. Results from the staging flights suggest that the majority of harp seal females in the Greenland Sea whelped between 16 and 21 March. The calculated temporal distribution of births were used to correct the estimates obtained for Patch B. No correction was considered necessary for Patch A. No staging was performed in Patch C; the estimate obtained for this patch may, therefore, be slightly negatively biased. The total estimate of pup production, including the visual survey of Patch A, both visual and photographic surveys of Patch B, and photographic survey of Patch C, was 98 500 (s.e. = 16 800), giving a coefficient of variation of 17.9% for the survey. Adding the obtained Greenland Sea pup production estimate to recent estimates obtained using similar methods in the Northwest Atlantic (in 1999) and in the Barents Sea/White Sea (in 2002), it appears that the entire North Atlantic harp seal pup production, as determined at the turn of the century, is at least 1.4 million animals per year.
Keywords: abundance, aerial surveys, birth distribution, Greenland Sea, harp seal, pup production
Received 6 July 2004; accepted 23 July 2005.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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Three populations of harp seals, Pagophilus groenlandicus, inhabit the North Atlantic Ocean (Sergeant, 1991). Whelping occurs on the pack ice off eastern Newfoundland and in the Gulf of St. Lawrence (the Northwest Atlantic population), off the east coast of Greenland (the Greenland Sea or West Ice population), and in the White Sea (the Barents Sea/White Sea population). Relationships among the three North Atlantic populations of harp seals have been examined using cranial measurements (Yablokov and Sergeant, 1963), underwater vocalizations (Perry and Terhune, 1999), serum transferrins (Møller et al., 1966; Nævdal, 1966, 1969, 1971), blood serum proteins (Borisov, 1966), allozymes (Meisfjord and Nævdal, 1994), and DNA (Meisfjord and Sundt, 1996; Perry et al., 2000). These studies have revealed significant differences between the Northwest Atlantic population on the one side and the Greenland Sea and Barents Sea harp seal populations on the other, while no evidence of difference between the latter two was observed. Recent observations from satellite tagging experiments suggest that Greenland Sea and Barents Sea harp seals overlap in their feeding range during summer and autumn (June–October) in the northern Barents Sea (Folkow et al., 2004). Also, recaptures from tagging experiments, using traditional flipper tags, suggest that mixing of immature animals between the West Ice and Barents Sea populations may occur, but there is no evidence of mixing on the breeding grounds (Øien and Øritsland, 1995).
Estimating abundance and monitoring changes in population size is critical for the management of harp seals and to understand their role in the North Atlantic ecosystem. Harp seals are the most abundant pinniped in the North Atlantic, where they are the focus of the largest marine mammal harvest in the world. Although the three populations have historically been exploited and managed separately, the combined total reported harvest (conducted by Canada, Greenland, Norway, and Russia) in 2002 was approximately 450 000 animals (ICES, 2004). Thus, there is considerable interest in assessing the status and monitoring changes in abundance in all three populations in order to manage the respective harvests responsibly. In addition, knowledge of harp seal population size is one factor required in order to estimate the potential influence of this species on other marine organisms, including commercially important fish species. Harp seals are important predators, and may also play a role in structuring ecosystems, both in the Northwest (Hammill and Stenson, 2000; Bundy, 2001) and Northeast (Nilssen et al., 2000) Atlantic. For example, in Atlantic Canada waters harp seals accounted for over 80% of the estimated 4 million tonnes of fish and zooplankton consumed by all seal species in the area (Hammill and Stenson, 2000). Annual consumption by Barents Sea/White Sea harp seals was calculated by Nilssen et al. (2000) to be of a magnitude of 3.5 million tonnes, of which various fish species constituted well over 2 million tonnes – an amount comparable with the quantities of fish consumed annually by cod Gadus morhua, the main upper-level predator of the Barents Sea (Bogstad et al., 2000).
Due to uncertainties in the assumptions required when estimating abundance from catch-at-age data and sequential population models, total abundance of a population is estimated by fitting a population model using age-specific reproductive rates and catches to the independent estimates of pup production (e.g. ICES, 2004). Although mark-recapture techniques have been used previously (e.g. Bowen and Sergeant, 1983; Øien and Øritsland, 1995), the use of aerial photographic and visual surveys represents an alternative and industry independent approach to estimate harp seal pup production that has been used successfully both in the Northwest Atlantic (Stenson et al., 1993, 2002, 2003), in the Greenland Sea (Øritsland and Øien, 1995), and in the White Sea (Potelov et al., 2003; ICES, 2004). It is recommended that the comprehensive aerial surveys needed to provide estimates of current pup production should be conducted periodically, preferably with 5 or less years between two consecutive surveys, and that efforts should be made to ensure comparability of survey results (ICES, 1994, 2004; NAFO, 1995).
Recent estimates of harp seal pup production are available for the Northwest Atlantic (997 900, s.e. = 102 100, obtained in 1999; Stenson et al., 2003) and Barents Sea/White Sea (330 000, s.e. = 34 000, obtained in 2002; ICES, 2004) populations. Subsequent assessments, done by fitting population models to the independent estimates of pup production (e.g. Healey and Stenson, 2000; ICES, 2001, 2004), suggest that the total size of these two populations combined may now be approximately 7.5 million animals. There are no current estimates of harp seal pup production in the Greenland Sea. Greenland Sea harp seals were surveyed aerially in 1991, giving a combined estimate of 55 270 (CV 0.141) pups for four detected and surveyed patches (Øritsland and Øien, 1995). Obtaining updated information on pup production is needed to assess the status of the Greenland Sea harp seal population. Consequently, aerial surveys were carried out in 2002 during their whelping (pupping) period (March–April). The surveys were carried out using techniques similar to those developed and used previously to determine pup production for harp seals in the Northwest Atlantic (Stenson et al., 1993, 2002, 2003), in the Greenland Sea (Øritsland and Øien, 1995), and in the White Sea (Potelov et al., 2003; ICES, 2004). Standardization of methodology and interpretation of results were achieved by involving scientists from Canadian, Norwegian, and Russian institutions in both fieldwork and subsequent analyses of data. Although there were some small differences between surveys, the results from all areas are comparable and facilitate an updated "turn-of-the-century" estimate of the total pup production of harp seals in the entire North Atlantic based on sampling over a restricted time period (1999–2002).
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Reconnaissance surveys
Whelping concentrations were located using fixed-wing and helicopter reconnaissance surveys of areas historically used by harp and hooded seals in the Greenland Sea, mainly the pack ice areas along the eastern coast of Greenland between 67°30'N and 74°40'N (Figure 1). Surveys were carried out between 14 March and 5 April 2002 at altitudes between 800 and 1000 ft. Reconnaissance flights using the fixed-wing aircraft were generally flown as repeated systematic east–west transects spaced 10 nm apart, from the ice edge in the east into the dense drift ice closer to the Greenland shore. Due to ice drift and variation in pupping dates (mid to late March, see Øritsland and Øien, 1995), most areas were surveyed repeatedly to minimize the chance of missing whelping concentrations. Colour markers and VHF transmitters were deployed in major whelping concentrations to facilitate relocation and to monitor ice drift.
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Estimates of abundance
Visual surveys
The number of pups present within the identified whelping patches was estimated by conducting systematic visual strip transect surveys, flown using a ship-based helicopter at an altitude of 30 m (100 ft). The helicopter was fitted with a satellite navigation system (GPS) and radar altimeter to ensure correct navigation and consistent altitude during surveys. The same two observers were used in all surveys, one always seated in the left and the other in the right rear seats. Their observational position in the helicopter was fixed to ensure standardization of strip widths. Each observer aligned marks on the window with a known distance on the ice. Additional marks, to indicate the horizon and bottom of the window, were used to maintain a constant position. The actual strip width was determined once the observers had undergone extensive practice runs and developed a consistent position. The strip widths, originally aimed to be 30–35 m for each observer, were calibrated against known distances on the ground after the surveys, and proved to be 30 and 34 m for the two observers, respectively. The observers counted all pups occurring within their stripe on each side of the aircraft, giving a total strip width of 64 m for the visual transects. Each pup observation was recorded directly into a laptop by individual observers and identified with a GPS location on the helicopter track. The transect began before a navigator, seated in the front, encountered seals and was terminated when no seals were seen on transect and were not observed outside the survey area. The direction of, and spacing between, transects depended on orientation and size of the concentration. Transects were carried out at right angles to the main orientation of the concentration. The subsequent data analyses were the same as used for the photographic surveys, assuming complete coverage along a transect (see Stenson et al., 1993).
Photographic surveys
Fixed-wing aerial photographic surveys were flown using a PA31 Piper Navajo fitted with the gyro mounted Leica RC 30 camera with 15.3 cm lens and AGFA PAN 200 aerographic black-and-white film. The surveys were mainly conducted at an altitude of 191 m (includes the entire Patch B), but due to low ceilings most transects were carried out at lower altitudes (some as low as 138.5 m) in Patch C. To avoid variations along transects, altitudes were monitored continuously during the entire photographic survey. The images covered areas varying from 284.1 x 284.1 m to 206.2 x 206.2 m per photo at altitudes of 191 m and 138.5 m, respectively. Each transect was allocated coverage according to flying altitude. Photos were taken along each transect at time intervals separated sufficiently to avoid overlap. The camera was turned on when seals were observed on a transect line, turned off if open water occurred for an extended period along a transect, and turned on when ice was encountered again. The photography on a transect line was finished when no seals were observed. Correct altitude and transect spacing were maintained using radar altimeter and a satellite navigation system (GPS).
Photographic counts
Positive prints were examined by two readers. Each frame was examined using an illuminated hand-lens (7–8x magnification). Readers examined a common series of photographs and compared seals identified with a reader with extensive previous experience. Once the cues used to identify seals were consistent among readers, all photos were read once. For each photograph the number and position of all pups were recorded on a clear acetate overlay.
After all photographs were read, the readers re-read a series of their photographs in sequence to determine if identifications had improved over the course of the readings (i.e. the "learning curve"). Photos were read until the second readings were consistently within 1% of the first. The original readings were replaced with the second readings up to this point. Additional photos were read subsequently to ensure that the first and second readings were consistent.
To correct for misidentified pups, 50 randomly selected frames from each whelping concentration were examined by four readers; the two original readers intended to read the entire material, plus two additional experienced readers. Individual seals identified by each reader were compared to determine a "best estimate" of the number of pups present, i.e. the number of pups present on a photograph agreed upon by the readers, after a comparison of multiple readings by the four different readers. Any pup that could not be positively identified was not included. Original counts (x) were regressed on the "best estimate" (y) to determine a correction factor for each survey and reader. Individual photo counts were corrected using the appropriate regression for each reader. The measurement error associated with variation about the regression was estimated, summed over transects to estimate the total measurement error for the survey, and added to the sampling variance, following Stenson et al. (2003).
For the reader who examined Patch B, the equation was: y = 0.9778x (s.e. = 0.01132); and for the reader who read Patch C, the equation was y = 1.1246x (s.e. = 0.01889). The different regression coefficients between the two patches reflect differences in flying altitude, weather condition, ice condition, and different readers.
Measurement error associated with variation about the regression (Vphoto) was estimated for each photograph using:
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Survey analysis
Both visual and photographic surveys were based on a systematic sampling design with a single random start and a sampling unit of a transect of variable length. Data were analysed based on the methods outlined in Stenson et al. (1993, 2002, 2003) and summarized here.
The estimated number of pups for the i(th) survey is given by
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The estimates of error variance 
, based on serial differences between transects (Kingsley et al., 1985), were calculated as
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was summed over transects and multiplied by the weighting factor (ki) to estimate the total measurement error for the survey, and added to the sampling variance to obtain the variance of a given survey (Vi): |
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The total population was estimated as 
and its error variance 
, where I is the number of surveys.
Temporal distribution of births
To correct the estimates of abundance for pups that had left the ice or were not yet born at the time of the survey (Myers and Bowen, 1989; Stenson et al., 1993, 2002, 2003), it was necessary to estimate the distribution of births over the pupping season. This was done using information on the proportion of pups in each of seven distinct age-dependent stages. These arbitrary, but easily recognizable age categories were based on pelage colour and condition, overall appearance, and muscular coordination, as described for Northwest Atlantic harp seals by Stewart and Lavigne (1980):
- Newborn: Pup still wet, bright yellow colour often present. Often associated with wet placentas and blood-stained snow.
- Yellowcoat: Pup dry, yellow amniotic stain still persistent on pelt. The pup is lean and moving awkwardly.
- Thin whitecoat: Amniotic stain faded, pup with visible neck and often conical in shape, pelage white.
- Fat whitecoat: Visibly fatter, neck not visible, cylindrical in shape, pelage still white.
- Greycoat: Darker juvenile pelt begin to grow in under the white lanugo giving a grey cast to the pelt; "salt-and-pepper"-look in later stages.
- Ragged-jackets: Lanugo shed in patches, at least a handful from torso (nose, tail, and flippers do not count).
- Beaters: Fully moulted, weaned pups (a handful of lanugo may remain).
Prior to the survey, classifications of pup stages were standardized among observers to ensure consistency. To determine the proportion of pups in each stage on a given day, random samples of pups were obtained by flying a series of transects over the patch. Pups were classified from the helicopter hovering just above the animals. The spacing between transects depended on the size of the actual patch. Repeated classifications were obtained from each patch several days apart.
For each pup observed on staging flights, a set of randomized dates of birth were generated for each pup, taking 100 random draws from a normal distribution where the estimates of the mean and standard deviation of stage lengths were as given by Kovacs and Lavigne (1985). From these resampled estimates of birth date, all random draws born prior to midday of the day the survey (visual and/or photographic) was performed, were counted. For each survey of each patch, counts were made of the number of random draws "born" both before and after this date. A correction factor was derived by dividing the total number of randomized dates of birth by the number of randomized dates of birth that were estimated to have taken place prior to the survey. The original estimated number of pups from a survey was then multiplied by this proportion to give a corrected estimate of all pups born in each patch. An approximate estimate of variance for multipliers was obtained by estimating the range of birth estimates that occurred during the course of the survey (taken as 4 h). This variance estimator was incorporated into the overall variance following Stenson et al. (2003).
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Results |
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Identification of whelping areas
Two harp seal whelping concentrations, one small and one large, apparently in the very beginning of formation, were observed during a fixed-wing reconnaissance flight on 15 March in the area between 72°37'N–72°43'N and 10°03'W–12°22'W. Both patches were subsequently relocated during helicopter reconnaissance flights. A small patch (Patch A, Figure 1) was found on 17 March at an approximate mean position of 72°14'N/12°43'W and a VHF transmitter was deployed in the patch to monitor movements which were generally in a southwesterly direction. On 30 March, this patch was relocated at 70°52'N/14°11'W. A larger harp seal concentration (Patch B, Figure 1) was located on 20 March near position of 72°10'N/13°10'W. This patch also drifted to the southwest. On 5 April, a third harp seal whelping concentration (Patch C, Figure 1) was located during a fixed-wing reconnaissance flight between 69°00'N/19°52'W and 69°11'N/19°35'W. The known movement of the ice and other patches indicated that this group was separate from the previously identified patches. Very few harp seals were observed outside the whelping concentrations.
Temporal distribution of births
Estimates of the proportion of pups in each developmental stage were obtained from Patches A and B. Systematic east–west staging transects (spaced 1–3 nautical miles apart) were flown over Patch A on 17, 19, 21, and 30 March, and over Patch B on 22, 24, 27, and 29 March and on 2 April (Table 1). Prior to the visual and photographic surveys, newborns (stage 1) were absent while yellowcoats (stage 2) occurred in low numbers in the patches. No newborns were observed after the surveys were flown. The majority of pups present during and immediately after the estimation survey periods were thin whitecoats (stage 3) in Patch A and fat whitecoats (stage 4) in Patch B.
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The 100 random draws for each sighted pup resulted in 60 700 estimates for dates of pup births in Patch A and 625 800 estimates for Patch B. From these estimates of birth date, all random draws born prior to midday on the days the surveys were run were counted. The number of random draws "born" just after midday on the day of the survey was counted also, and the midpoint of these used as the time when the visual survey occurred. This resulted in a correction of an additional 16.9% (Patch A) and 0.8% (Patch B) of the pups being born after the visual survey. No correction was necessary on the day of the photographic survey of Patch B. An approximate estimate of variance for multipliers was obtained by estimating the range of birth estimates that occurred during the course of the survey (taken as 4 h). This variance estimator was incorporated into the overall variance following Stenson et al. (2003).
The applied method only looks at the possibility of births after the survey, and does not account for the possibility that pups have left. Pups do not appear to leave the area before they get to the ragged jacket and beater stage. Younger pups appear to be on the ice most of the time – some may move among ice pans, but they tend to travel on the surface where they are seen by the visual and photographic readers. A small number of ragged jackets were observed in Patch B, which suggests that a few could have gone into the water. However, the shape of the birth curve, given the timing of harp seal pups' moulting, suggested that if any pups left the whelping patch prior to either survey, the number would be minor.
No staging was carried out in Patch C and therefore no correction could be attempted.
Visual surveys
Systematic visual strip transect surveys were flown over Patch A on 20 March (Figure 1). The whelping concentration occupied an area between approximately 71°41'N–71°50'N and 11°40'W–12°50'W. Eighteen east–west transects were flown spaced 0.5 nautical mile apart. A total of 277 harp seal pups was counted on transects within a 64-m-wide strip (Table 2). This gave a total estimated number of 4008 (s.e. = 706). Including the correction for the temporal distribution of births, the number of pups in Patch A estimated from the visual survey was 4686 (s.e. = 707) during the survey.
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Patch B was surveyed on 28 March when the patch occupied an area between approximately 71°13'N–71°43'N and 14°38'W–16° 30'W (Figure 1). Sixteen east–west transects were flown spaced 2 nautical miles apart. In total, 1416 pups were counted on the 64-m-wide transect strip (Table 3). The estimated number of pups in Patch B was 81 955 (s.e. = 16 711). Including the correction for the temporal distribution of births, the number of pups in Patch B estimated from the visual survey was 82 615 (s.e. = 16 711).
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Visual surveys were not flown over harp seal Patch C.
Photographic surveys
Harp seal whelping Patch A was not surveyed photographically. A survey of Patch B (occupying an area between 70°52'N–71°25'N and 14°44'W–16°38'W) was successfully completed on 29 March (Figure 1). Twenty transects were flown in an east–west direction, spaced 2 nautical miles apart (Table 4). A total of 5220 pups was counted on the 521 exposures obtained. Correcting for reader errors, but not for pups born outside the photographs along a transect, pup production was estimated to be 66 545 (s.e. = 13 534).
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The harp seal whelping Patch C was surveyed with photographic strip transects on 6 April in relatively difficult weather conditions (Figure 1). However, 14 east–west transects, spaced 1 nautical mile apart, were flown over the whelping patch, which covered an area between 69°01'N–69°14'N and 19°06'W–19°51'W. A total of 321 exposures was taken, and 1282 pups were counted (Table 5). Including the correction for reader errors, but not for pups born outside of photographs along a transect, a total of 11 166 (s.e. = 1202) was estimated to have been born.
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Estimate of total pup production for 2002
Although a photographic estimate was available for Patch B, it was incomplete since it did not account for pups born between the photographs along the transect and therefore was negatively biased. Therefore, combining visual survey estimates of Patches A and B and the photographic estimate of Patch C resulted in a total estimate of pup production of 98 467 (s.e. = 16 769, CV = 17.0%, Table 6).
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The survey used methods comparable with those applied in previous surveys performed for harp seal assessments in the Northwest Atlantic in 1990, 1994, and 1999 (Stenson et al., 1993, 2002, 2003), in the Greenland Sea in 1991 (Øritsland and Øien, 1995), and in the White Sea in 1998, 2000, and 2002 (Potelov et al., 2003; ICES, 2004). Additionally, the survey and analysis methods were standardized with researchers who have carried out previous harp seal surveys to ensure that the results were comparable. The estimate presented here (98467, s.e. = 16769) is slightly negatively biased owing to the lack of correcting for non-overlapping photographs and areas where the camera was turned off during the photographic survey of Patch C. Along transects, the camera was turned off in areas with open waters, i.e. where there was no habitat (ice) and, therefore, no seals. Unfortunately, the exact areas where the camera was turned off were not properly recorded, so the estimate could not be corrected for pups present on ice between photographs while the camera was still on. However, given the relatively small number of seals present in this whelping patch, this correction is not likely to change the total estimate significantly. Similarly, although the late date of the survey suggests that most of the pups had been born before the survey of Patch C, where no staging was performed, it is possible that some pups had left the ice. Therefore, the estimate from Patch C may be further underestimated slightly.
The photographic survey of Patch B was also negatively biased for the same reasons as Patch C. Since a complete visual survey was available, the photographic survey was not used. However, applying a range of reasonable correction factors to the photographic estimate resulted in estimates that were consistent with the visual survey results.
In general, surveys of ice-breeding seals are designed to obtain a minimum of one good estimate for each whelping patch. In this study, good visual coverage was obtained for two patches (A and B) and a photographic survey of Patch C. Which technique is the most useful is not known until the survey is actually done. When more than one survey is available for a patch, and there is no obvious reason to reject one, a combined estimate is obtained from the average, weighted inversely by their variance. This approach is similar to harp seal surveys in the Northwest Atlantic where visual and photographic methods have contributed differently, depending upon the logistics and conditions in a particular year (see Stenson et al., 1993, 2002, 2003). Given the uncertain and extremely variable conditions (such as unpredictable weather and ice drift) usually encountered when harp seal pups are to be surveyed, preparations must always be made to carry out both methods wherever possible. Results will subsequently determine which has the most influence in a given survey.
The Greenland Sea stocks of harp seals have been subject to commercial exploitation for centuries (Iversen, 1927; Nakken, 1988; Sergeant, 1991). Knowledge of the Greenland Sea catches in the 18th and the first two-thirds of the 19th centuries, performed by Dutch, British, German, and Danish ships, is poor. Norwegian sealers appeared for the first time in the Greenland Sea in 1846, and have subsequently participated with increased effort. Exploitation levels reached a historical maximum in the 1870s and 1880s when annual catches of harp seals (pups and adults) varied between 50000 and 120000 (Iversen, 1927). This overexploitation appears to have driven the stock to an all time low, and the competition for a limited supply of seals in the 1870s resulted in the disappearance of all non-Norwegian fleets (Sergeant, 1991). It was evident that the catch levels in the 1870s were higher than the stock could sustain, and some regulatory measures (mainly designed to protect adult females) were introduced in 1876 (Iversen, 1927). In the first decades of the 20th century, the annual harp seal catches varied between 10000 and 20000 animals. An increase to around 40000 seals per year occurred in the 1930s (Iversen, 1927; Sergeant, 1991). After a 5-year pause in the sealing operations during World War II, total annual catches quickly rose to a post-war maximum of about 70000 in 1948, but then followed a decreasing trend until quotas were imposed in 1971 (Sergeant, 1991; ICES, 2004). From 1955 to 1994 a minor part of the catches was taken by the Soviet Union/Russia, and the total annual catches have varied between a few hundreds to about 17000 from 1971 to present (ICES, 2004).
Available knowledge of both previous and more recent abundance of Greenland Sea harp seals has been rather restricted. Based on catch per unit effort analyses and mark-recapture pup production estimates, it has been assumed that the population may have increased since the early 1960s, although direct evidence is limited (Ulltang and Øien, 1988; Øien and Øritsland, 1995). From 1977 to 1991, about 17000 harp seal pups were tagged in a comprehensive mark-recapture experiment in the Greenland Sea (Øien and Øritsland, 1995). Based on this experiment, pup production was estimated to be 40000–50000 in 1980. Updates of the mark-recapture estimates indicated pup production in 1991 of 67300 (95% CI 56400–78113) (NAFO, 1995). Aerial surveys performed in 1991 suggested a minimum pup production of 55000 (cv 0.14) in this year (Øritsland and Øien, 1995). The present estimate, obtained 11 years later, suggest that pup production is higher than the 1991 estimates, although it is important to remember that caution should be taken when comparing estimates made by different methods, as they are subject to different biases. Interestingly, the point estimate we obtained (98500) is within the confidence intervals of the estimate of pup production for 2000 (76700; 95% CI 48000–105000) derived from a population model, tuned to the previous pup production estimates based on mark-recapture data (ICES, 2001).
Using preliminary results from the 2002 survey, the Joint ICES/NAFO Working Group on Harp and Hooded Seals (WGHARP) estimated the total stock size and assessed the impact of various levels of annual harvest on future population size (ICES, 2004). They estimated a current population of 349000 (95% CI 319000–379000) one year of age and older, and they also predict that the population will continue to increase under the current harvest regime of very small annual removals. Confirming these results will have implications both for future management of the stock and for the understanding of its role in the Greenland Sea and Barents Sea ecosystems.
Assuming that the estimates of the mean and standard deviation of pup stage length were as given by Kovacs and Lavigne (1985), results from the staging flights over Patches A and B suggest that the majority of harp seal females in the Greenland Sea whelped between 16 and 21 March in 2002. This is in accordance with observations made in the Greenland Sea in 1991, whereas in 1990 it appears that the breeding may have peaked 5–7 days later (Øritsland and Øien, 1995). Variations from year to year in peak pupping has been observed also for harp seals in the Northwest Atlantic, where pupping generally peaks earlier than in the Greenland Sea (see Stenson et al., 2003). Earlier pupping than in the Greenland Sea is also observed for the White Sea stock of harp seals (Potelov et al., 2003).
By combining this estimate with those obtained for other populations in recent years (1999–2002) provides the first estimate of total pup production for North Atlantic harp seals. The most recent (1999) estimate of harp seal pup production from the Northwest Atlantic was 997900 (s.e. = 102100, Stenson et al., 2003), while an estimate of 330000 (s.e. = 34000) pups was obtained in 2002 for the Barents Sea/White Sea (ICES, 2004). The present 2002 estimate from the Greenland Sea (98500, s.e. = 16800) shows that this population is the smallest, and that the North Atlantic harp seal pup production, as determined at the turn of the century, is in the order of 1.4 million animals per year (1.426 million with an s.e. of 109500).
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Acknowledgements |
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The authors thank I. Ahlqvist, B. Bergflødt, L. Lindblom, N. E. Skavberg, pilots, engineers, and camera operators, and the crew on board RV "Lance" – without their assistance and enthusiasm during fieldwork the surveys would not have been completed. We are especially grateful to A. K. Frie, L. Lindblom, D. Wakeham, and A. P. Golikov for reading the photos, and to J. Lawson who provided the map. Participation in surveys as well as subsequent analyses and publication of results by G. B. Stenson was secured by funding by the Norwegian Council of Research, project no. 146573/120.
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Footnotes |
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1 Current address for P. J. Corkeron: Bioacoustics Research Program, Cornell Laboratory of Ornithology, 159 Sapsucker Woods Road, Ithaca, NY 14850, USA.
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References |
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