Distribution of ringed seals in the southeastern

Distribution of ringed seals in the southeastern Beaufort Sea during late summer LOISA. HARWOOD' Department of Zoology, University of Alberta, Edmonton, Alta., Canada T6G 2E9 AND

IANSTIRLING

Can. J. Zool. Downloaded from www.nrcresearchpress.com by Texas A&M University on 06/07/13 For personal use only.

Canadian Wildlife Service, 5320 122 Street, Edmonton, Alta. , Canada T6H 3S5 and Department of Zoology, University of Alberta, Edmonton, Alta., Canada T6G 2E9 Received February 22, 1991 Accepted October 30, 1991

HARWOOD, L. A., and STIRLING, I. 1992. Distribution of ringed seals in the southeastern Beaufort Sea during late summer. Can. J. Zool. 70: 891 -900. The distribution and relative abundance of ringed seals (Phoca hispida) in the southeastern Beaufort Sea were examined through systematic aerial surveys in August-September of 1982 and 1984- 1986. All data analyzed were collected by the same observer when sea state was 5 2 on the Beaufort Scale and when there was no forward glare. In late summer and early fall of 1982, 1984, and 1986, ringed seals occurred singly and in groups, to an observed maximum of 21 seals. Groups of seals were clumped into large areas of aggregation which appeared to persist for several weeks. Densities in aggregation areas ranged from 121 to 326 seals1100 km2, approximately 6- 13 times greater than regional mean densities. The geographic extent of aggregation areas (350 -2800 km2) and the numbers (1 in 1984, 2 in 1982, 3 in 1986) and locations of aggregations varied among years. Ringed seals tended to aggregate most frequently and in greatest numbers in waters north of the Tuktoyaktuk Peninsula, in the general area where the Cape Bathurst polynya occurs in winter. The relative abundance of ringed seals varied among the years of the study, reaching a maximum in 1982 (42.20 seals1100 km2), declining through 1984 (14.731100 km2) and 1985 (7.921100 km2), and increasing again in 1986 (19.351100 km2). HARWOOD, L. A., et STIRLING, I. 1992. Distribution of ringed seals in the southeastern Beaufort Sea during late'summer. Can. J. Zool. 70 : 891 -900. Des survols akriens systkmatiques effectuks en aoQt-septembre en 1982 et en 1984- 1986 nous ont permis d'ktudier la rkpartition et l'abondance relative des Phoques annelks (Phoca hispida) dans la partie sud-est de la mer de Beaufort. Toutes les donnkes analyskes ont kt6 comeilkes par le meme observateur quand l'ktat de la mar ktait 1 2 a l'kchelle Beaufort et quand il n'y avait pas d'kblouissement. A la fin de l'ktk et au dkbut de l'automne, en 1982, 1984 et 1986, les phoques se tenaient seuls ou en groupes contenant un maximum observk de 21 animaux. Les groupes se tenaient sur de grandes surfaces de rassemblement qui semblaient prkvaloir pour plusieurs semaines. Aux points de rassemblement, il y avait de 121 a 326 animauxl100 km2, ce qui reprksente des densitks de 6 a 13 fois plus klevkes que les densitks rkgionales moyennes. L'ktendue gkographique des surfaces de rassemblement (350-2800 km2), le nombre de points de rassemblements (1 en 1984, 2 en 1982 et 3 en 1986) et leur position variaient d'une annke a l'autre. Les phoques avaient tendance a se regrouper plus frkquemment et en plus grands nombres dans les eaux au nord de la pkninsule Tuktoyaktuk et dans la rkgion oh s'ouvre la polynie au cap Bathurst en hiver. L'abondance relative des phoques a varik au cours des annkes de l'ktude; elle a kt6 maximale en 1982 (42,20 phoquesl100 km2), a diminuk jusqu'en 1984 (14,731100 km2) et 1985 (7,921100 km2) pour augmenter de nouveau en 1986 (19,351100 km2). [Traduit par la rkdaction]

Introduction Oil and gas exploration activities in the early 1970s led to extensive study of the spring distribution of ringed seals (Phoca hispida) in the southeastern Beaufort Sea (Stirling et al. 1982; Kingsley 1986). Distribution of ringed seals during the late summer and fall open-water period is less understood because available information is based on localized field studies and incidental sightings (see reviews by Harwood et al. 1986 and Smith 1987). In contrast to the spring, when ringed seals are dispersed at low densities throughout the southeastern Beaufort Sea (Kingsley 1986), during late summer they often form large, loose aggregations (Renaud and Davis 1981; Harwood and Ford 1983; McLaren and Davis 1985; Harwood and Borstad 1985; Smith 1987). Similar aggregations have been reported in late summer from the eastern High Arctic (Ellis 1957; Finley and Johnston 1977) and the Sea of Ohkotsk (Fedoseev 1965). 'Present address: Joint Secretariat, Box 2120, Inuvik, N.W.T., Canada XOE OTO.

Printed in Canada I lmprime au Canada

Here we describe patterns of ringed seal distribution and relative abundance in the southeastern Beaufort Sea during the open-water period, based on the results of systematic aerial surveys flown in August -September of 1982, 1984, 1985, and 1986.

Methods Study area Sea ice covers most of the offshore Beaufort Sea during winter. Parallel to the mainland coast and the west coast of Banks Island, there is a system of shore leads which open and close throughout the winter, and a polynya, of variable size, which forms in the vicinity of Cape Bathurst (Smith and Rigby 1981). In spring, the landfast ice disintegrates and the remnant ice moves seaward. From June through September there is a gradual expansion of the coastal open-water zone, with maximum open water usually occurring in September (Hammill 1987). Freeze-up begins in late September or early October and is usually complete by November (Marko 1975). There is a-continental shelf of variable width along the mainland coast. The water depth in most of the study area is less than 100 m (Fig. 1). The physical features of the southeastern Beaufort Sea, such

Can. J. Zool. Downloaded from www.nrcresearchpress.com by Texas A&M University on 06/07/13 For personal use only. 892 CAN. J . ZOOL. VOL. 70, 1992

HARWOOD AND STIRLING


as complicated coastal morphology, steeply sloping bottom topography at the edge of the continental shelf and near Cape Bathurst, the large freshwater discharge of the Mackenzie River, and coastal upwellling driven by prevailing winds, are conducive to the formation of oceanographic fronts, known to concentrate zooplankton (Borstad 1985; Thomson et al. 1986). Data collection and analysis Six systematic aerial surveys were conducted over the southeastern Beaufort Sea during late August and September of 1982, 1984, 1985, and 1986 (Fig. 1). These consisted of 26 north-south transects spaced at intervals of 16 km (1982) or 20 km (1984- 1986). While these surveys were undertaken to examine the distribution of the bowhead whale (Balaena mysticetus; reported in Harwood and Ford 1983; Harwood and Borstad 1985; Norton and Harwood 1986; Ford et al. 1988), a further objective was to record sightings of all marine mammals, including ringed seals, bearded seals (Erigruzthus barbatus), and beluga whales (Delphinapterus leucas). In AugustSeptember 1986, four additional, local systematic surveys and one reconnaissance survey were flown over areas where ringed seals aggregated in late summer of that year (Fig. 1). The primary objective of these surveys was to count seals, although other marine mammals sighted were also recorded. The survey area was divided into three strata based on the basis of geographic and oceanographic features: 'west': Yukon coastal and offshore waters (19 000 km2), which tend to be deeper than elsewhere in the study area and are affected to some extent by the Mackenzie River plume under certain wind regimes; 'mid': the area north of the Mackenzie River delta and adjacent offshore waters (26 000 km2), strongly influenced by the plume of warm, turbid waters; and 'east': the continental shelf waters north of the Tuktoyaktuk Peninsula (35 000 km2), similar hydrographically to the mid stratum, but affected to a lesser extent by the Mackenzie River plume, and having the greatest amount of open water during summer and fall (Fig. 1). Four of the six regional surveys were completed without significant spatial or temporal gaps in survey coverage (August 18-24, 1982; August 18-27, 1984; August 18-24, 1985; August 3 1 - September 10, 1986). Data from these four surveys are used in all analyses and form the basic data set. Even these surveys had unsampled areas, owing to (i) occurrence of localized areas with glare, fog, and (or) sea state >2; (ii) truncation of transects if consolidated sea ice was encountered; and (iii) operation of two aircraft simultaneously on adjacent transects and using data collected from only one observer in one aircraft. While it is not expected that these interruptions in sampling will alter the broad trends we present here, they do require that the data be interpreted accordingly (e.g., the surveys delineate some but not necessarily all areas of seal aggregation). Portions of other regional surveys and the local surveys that were conducted under optimum conditions were used to provide additional inforrnation on aggregation areas. Surveys were conducted from a de Havilland Twin Otter aircraft equipped with either a Global Navigation System (GNS-500) or Collins LRN-70, a radar altimeter, and a bubble window at the second left seat. Target survey speed was 200 kmth (108 kn), but varied _+ 15% due to wind effects. Surveys were attempted when sea state was 5 or less on the Beaufort Scale of Wind Force, and when ceilings were greater than 500 ft (152 m) asl, minimum conditions for observing bowhead whales (Davis et al. 1982; Norton et al. 1987). Survey altitude was 1000 ft (305 m) as1 for 83 % of the basic data set surveys and 500 ft (152 m) as1 for the remainder and for the local surveys. To minimize glare, surveys were conducted when the sun was most directly overhead (1 1:00 - 17:00). Whenever possible, the following information was recorded for each marine mammal sighted: species, number of individuals, number in group, time of sighting, location of sighting (from GNS), number of degrees from horizontal, habitat characteristics, estimated distance between individuals in body lengths, behaviour, relative rate and direction of movement, and the presence of seabirds. Detailed records of survey conditions and ice conditions were kept throughout.

893

Rough seas and glare from the sun significantly reduce the detectability of ringed seals (Harwood and Ford 1983) and other marine mammals (Davis et al. 1982; Holt and Cologne 1987), therefore survey data were used only if they were collected when sea states were Beaufort 0 (sea like a mirror), 1 (ripples but without crests), or 2 (small wavelets with glassy crests that do not break), and there was no forward glare. The requisite conditions occurred along 11 500 linear km, or 49% of the total transect distance flown. Under these ideal, calm conditions, seals produce a clearly visible surface disturbance which moves and covers a relatively large area. In the case of open-water surveys, then, the observer relies on a movement cue and not on the detection of a small dark or silvery head in the water. Once a seal or group of seals has been detected in this manner, the observer examines the location and counts the number of seals visible. Movement and the associated surface disturbance are also the usual cues for initial detection of bowhead whales, and thus a similar search image is used for both species. However, because of their small size, reliable sighting of seals within the disturbance area appears limited to the inner 400 m of the strip. In 1982, Harwood and Ford (1983) found 74% (n = 148 groups) on the inner 400 m of their 800-m strip. Consequently, a 400 m wide survey strip was used for calculation of seal densities in all subsequent analyses (e.g., number of seals seen along the inner 400 m of striptarea of 400 m wide strip), regardless of the width of the transect searched. Using this approach, survey coverage for seals was 2.1 % of the total study area. For consistency, all data analyzed and reported herein were collected by the same observer (L. Harwood), except for one comparison described below, which required data collected by two observers. The strip-transect method was used (Caughley 1977), with designated search widths, per side, of 800 m in 1982, 1000 m in 1984 and 1985, and on most (82%) regional surveys in 1986. A narrower search width of 400 m was used for a small (18%) portion of the regional surveys in 1986, and on the 1986 local surveys. Survey procedures were as described in Norton and Harwood (1985). For the purpose of the surveys, a group of seals was defined as two or more individuals within an estimated distance of five body lengths of each other. Where possible, hand-held Suunto PM5t360S inclinometers were used to determine the lateral distance of sightings from the flight path, and in 1986, to mark the transect search area on the bubble window. The angle of depression from horizontal was measured when the animal was abeam of the aircraft, and the lateral distance from the aircraft calculated on the basis of this angle and survey altitude. The strip was not offset from the transect center line, as the observer was able to consistently view this area using the bubble window. While the available search time was shorter nearer to the transect line than at outer edges of the transect, these differences are considered inconsequential in terms of seal counting under the ideal survey conditions encountered in this study. The region was subdivided into 228 essentially square subareas or 'grid cells' (Robertson and Robertson 1987). Each had an approximate surface area of 353 km2 and at least one 18.5 km transect segment, within which was the sampling unit. Using results from all surveys, two tests were done to see if changes in survey altitude and differences in width of the search area had a significant effect on seal detectability. First, for each stratum, a three-way Kruskal -Wallis test (Zar 1984; SAS Institute Inc. 1985) was done to determine the effect of the width of the search area, survey altitude, and year on ringed seal densities along the transect segments. Ringed seal densities pertaining only to the inner 400 m of the search areas were used. Second, to further examine differences in detectability of seals, using search widths of 1000 m and 400 m, mean densities of ringed seals recorded by the primary observer and an additional, experienced observer were compared. Both observers used bubble windows. Seal counts along the inner 400-m strip of the left side 1000-m transect were compared with counts from the 400-m transect on the right side. The effect that surveying the outer 600 m had on seal counts along the inner 400 m of the left side transect was assessed with a MannWhitney U-test, at constant altitude (1000 ft) for 68 transect segments under optimum survey conditions (26 August 1986). Assuming that

CAN. J. ZOOL. VOL. 70, 1992

894


seals were equally likely to occur on either side of the flight path over the duration of the survey, and that both observers were of sufficiently equal skill in this context, the left side 400-m counts were tested against the right side 400-m counts to determine if they were consistently different and thus determine the effect of the extra search area. We believe these assumptions to be valid, given the results of other seal surveys in the Beaufort region which suggest that differences in the number of ringed seals counted by experienced observers are not statistically significant (Stirling et al. 1977; Frost et al. 1988). Relative abundance Using the transect as the sampling unit, mean regional density (R) was calculated for the four complete regional surveys (eq. I), and variance (si using eq. 2, developed by Kingsley and Smith (1981) specifically for systematic aerial surveys:

where Yi is the total number of o_n-transect sightings, Xi is the total area surveyed, and d, = Y, - R(Xi). Confidenc~intervals with a Bonferroni correction were calculated about each R value to evaluate density differences among years (Neter and Wasserman 1974). Ringed seal density for each stratum was calculated (eq. I), and each value multiplied by the total area of the stratum to obtain an index of stratum abundance. These were summed over all strata for each year to provide an index of annual regional abundance. Since seals at the surface that were missed by observers, and seals that were under water during the survey pass, could not be accounted for, the estimates are indices of relative abundance, not total population size. Regional distribution The distribution of ringed seals was evaluated using clump factor ratios derived by Kingsley et al. (1985) to describe ringed seal distribution during spring in the eastern High Arctic (1980 - 198I), and in the southeastern Beaufop Sea in 1974- 1979 (Stirling et al. 1982). Mean regional density (R) for the four complete regional surveys was calculated using eq. 1. Variance (si) for R was calculated two ways, using eqs. 2 and 3 (Cochrane 1963, cited in Kingsley and Smith 1981)):

where d, = Y, - R(x~). An estimate of survey precision is given by the error coefficient of variation (Ek = SkIR, where k = 1,2). Clump factors (C,), measures of dispersion, are given by Ck = (Et) (CY,), where k = 1,2. C2 is a measure of clumping between adjacent transects, C, is a measure of clumping as it occurs over all transects, and the ratio C,/C2 provides an indication of the homogeneity of distribution across all transects (e.g., by longitude). Distribution is depicted with contour density maps (e.g., Bonnell and Ford 1987). The densities of seals along transect segments were calculated using SAS Institute Inc. (1985), and assumed to apply to the grid cell in which they were located. Linear interpolation of density values at grid cell center points was used to produce and smooth contour lines depicting annual distribution of ringed seals using the Surface I1 Graphics System (Sampson 1978) and a fixed contour interval of 15 seals/100 km2. We defined an aggregation as a peak exceeding 6 times the contour interval value ( > 9 0 seals/100 km2).

of

Characteristics seal aggregations The density (R,) of ringed seals in a given aggregation was calculated by dividing the number of seals counted within an aggregation (summed over all transect segments within the given aggregation with densities > 9 0 seals/100 km2) by the area (km2) surveyed in that aggregation. For the four complete regional surveys, density was extrapolated to unsurveyed areas of that aggregation to estimate I,,

an index of relative abundance for that aggregation: I, = (R,) x number of transect segments x 353 km2 (the area of one grid cell). For each aggregation, the number of seals per group was tallied and plotted in a frequency histogram. Mean group size, number of groups per aggregation, and distance between groups were tabulated. The geographic extent of each aggregation was estimated by multiplying the number of transect segments in the aggregation by 353 km2 (the area of one grid cell). Dates of the first and last surveys of a given aggregation were used as a minimum estimate of the length of time that an aggregation persisted, although nothing is known about the distribution of seals before, after, or during the interval between the surveys.

Results The location of the 7-9+/10 pack ice edge varied among the survey years. In late summer 1984 and 1986, it was north of the Tuktoyaktuk Peninsula at approximately 71 ON latitude (100- 150 km offshore). Most of the survey area was ice free, as was the case in August 1982, when the edge of the pack was at 72O23'N latitude (300 m offshore). The pack ice edge in August 1985 was 50-70 km from shore (between 70" and 70°20'N latitude) off the Tuktoyaktuk Peninsula, and about 100 km from shore off the Yukon coast. During the four complete regional surveys, weather and survey objectives necessitated some modifications to the transectwidth search area and the survey altitude. Considering all surveys in each of the three strata (west: n = 233; mid: n = 193; east: n = 263 transect segments), densities of ringed seals along the 400-m inner transect did not interact or differ significantly ( p > 0.05) when they were based on counts made using different search widths and altitudes. Similarly, mean densities of ringed seals recorded in 1986 by two experienced observers searching simultaneously (n = 68 transect segments), using search widths of 1000 m (left side) and 400 m (right side), were not significantly different when the left side density was calculated using only seal counts and survey area pertaining to the inner 400 m of the left wide transect (right observer, x = 0.107 seals/km2, SD = 0.258; left observer, x = 0.069 seals/km2, SD = 0.208; Mann -Whitney U-test ,p > 0.05). On the basis of these tests, we pooled data collected using different transect widths and survey altitudes. In total, 884 ringed seals were counted on the surveys, 681 of them during the four complete regional surveys (Table I). The density of ringed seals varied among the 4 years of the study (Table 2), with a maximum in 1982 (42.20 seals1 100 km2), declining through 1984 (14.731100 km2) and 1985 (7.921100 km2), and increasing again in 1986 (19.351 100 km2). However, the densities were significantly different in 1982 and 1985 only (Bonferroni confidence intervals: 1982, +20.34; 1985, f 12.59; p < 0.05). Abundance indices followed a similar pattern (Table 2), varying from a maximum of 42 200 (1982) to a minimum of 6400 (1985). Distribution The density contour maps illustrate patterns of distribution both along and between transects (e.g., by longitude and latitude), and clearly show that from a regional perspective, ringed seals aggregated in 1982, 1984, and 1986, but not in 1985 (Fig. 2). Six areas of aggregation (A-F) were documented during the four complete regional surveys (Fig. 2). Clump factor ratios (Table 2) for 1982, 1984, and 1986, all greater than unity, confirm that there was clumping between transects (e.g., by longitude) and a nonhomogeneous distribu-


TABLE1. Summary of survey effort and on-transect ringed seal sightings in the southeastern Beaufort Sea during late summer of 1982, 1984, 1985 and 1986 No. of transectsa

Survey date


Aug. Aug. Aug. Aug.

No. of transect segments

Size of area (km2)

Survey area (km')

No. of ringed seals on transect

Systematic regional surveysh 16 105 34 837 14 111 34882 25 120 36114 22 160 51 718

18-24, 1982 18-27, 1984 18-24, 1985 31 - Sept. 10, 1986

Other surveysc 14 71 9 34 2 13 4 19 10 54 2 2 Reconnaissance 5

Sept. 5- 12, 1982 Sept. 6-17, 1984 Aug. 21, 1986 Sept. 5, 1986 Sept. 14, 1986 Sept. 23, 1986 Oct. 3, 1986

21 899 9 340 4 242 6 359 16 842 706 lo00

'Whole or part of transect. b ~ o u regional r surveys with no significant spatial or temporal gaps in survey coverage. 'Remaining regional surveys, local surveys, and a reconnaissance survey.

TABLE2. Error coefficients of variation (E), clump factors (C), clump factor ratios, mean density (no.1100 km2), 95% confidence intervals (CI), and standard errors (SE) for ringed seal densities in the southeastern Beaufort Sea during late summer of 1982 and 1984 - 86 Survey

Mean density

CI

SE

El

E2

Cl

C2

C,IC2

Index of abundance

Aug. 1982 Aug. 1984 Aug. 1985 Aug.-Sept. 1986

42.20 14.73 7.92 19.35

27.09-57.31 1.15-21.66 4.75-11.09 13.53-25.17

7.71 6.93 1.59 2.97

0.2441 0.5121 0.2026 0.2218

0.1835 0.4706 0.2007 0.1532

18.30 25.70 2.46 10.33

10.34 21.70 2.42 4.93

1.77 1.18 1.02 2.09

41200 13200 6400 14300

tion across the region in those years. The clump factor ratio for 1985 (1.02) suggests a homogeneous or only slightly clumped distribution, also consistent with depictions on the contour map for that year (Fig. 2). Within the aggregations, seals occurred as individuals and in groups of up to 21 (Fig. 3). In 1982, there was a tendency for larger groups rather than singles or pairs, while in 1984, singles, pairs and groups larger than six were prevalent. In 1985, all sightings were of individuals (n = 56) or pairs (n = 2) seals. In 1986, most seals seen were alone, but there were also five groups of 7 - 16 seals. The distances between groups seen along the same or adjoining transect segments were relatively consistent among areas where the groups were clumped, most groups being separated by 2 -5 km (Table 3). Along 23 of the 30 transect segments within aggregation areas, ringed seal groups were seen in circular or linear formations, but not elsewhere. Seals in circular groups were usually oriented toward the center of the circle. Gulls (Larus spp.) were seen in close association (usually circling directly overhead in groups of up to approximately 50 birds) with ringed seals in four of the six aggregations. Bowhead whales occurred with ringed seals in three of the six aggregations. Although the numbers and specific locations of areas where ringed seals aggregated were not identical among years, aggregations were detected most often in the general area north of Cape Dalhousie (Fig. 2). The aggregation areas

varied in size from approximately 350 km2 in nearshore waters off the Yukon coast (August 1986) to 2800 km2 north of Cape Dalhousie (August 1982; Table 4). More seals were seen in aggregations in August 1982 (89% of seals counted) than in either August 1984 (58 %) or August - September 1986 (46%). Abundance indices for ringed seals in the six aggregations varied from 400 (Yukon coast, 1986) to 8700 (north of Cape Dalhousie, 1982). Densities in aggregations ranged from 121 to 326 seals1100 km2, whereas over the region as a whole, observed densities ranged from 7.92 to 42.20 seals1 100 krn2. Portions of four of the six aggregations were surveyed again 19 -26 days after their first observation (Table 4), and in each case, observed seal densities indicated that an aggregation was still present in the same general area. Although it is not known if the same seals occurred in aggregation areas between the surveys, it appears that the aggregation areas were attractive to ringed seals for a period lasting up to several weeks (as opposed to only a few days).

Discussion In late summer and early fall of 1982, 1984, and 1986, ringed seals occurred singly and in groups up to an observed maximum of 21 seals. The groups themselves were clumped into large aggregation areas, where the densities of seals were

FIG.2. Patterns of distribution of ringed seals per group observed during aerial surveys in the southeastern Beaufort Sea in August 1982, 1984, and 1985 and August - September 1986.




August 1982 (n = 6 4 groups)

V

1

2

3

4

6

6-10

August 1984 (n = 44 groups)

11-15 16-21

Group Size

Group Size

- August 1985

August-September 1986 (n = 126 groups)

(n = 58 groups)

Group Size

Group Size

FIG.3. Distribution of ringed seals observed in the southeastern Beaufort Sea during systematic aerial surveys in August 1982, 1984, and 1985 and August-September 1986. TABLE3. Group sizes and distances between groups in ringed seal aggregation areas in the southeastern Beaufort Sea, late summer of 1982, 1984, and 1986 Aggregation areaa

Group sizes observed

Mean group size

No. of groups

Distance between groupsb (km)

Mean distance between groups (km)

'For locations of areas see Fig. 2. b~alculatedamong nearest-neighbour groups sighted along same transects in a given aggregation.

6- 13 times greater than the regional means. In late summer of 1985, ringed seals were seen singly and in pairs, and no aggregations were observed. Aggregations appeared to persist for several weeks. The specific locations of aggregations varied among years, but occurred most regularly in the general area north of Cape Dalhousie and in western portions of the study area north of the Yukon. Aggregations were also reported in the area north of the Tuktoyaktuk Peninsula in 1980 (corresponds to aggregation E in 1986; Renaud and Davis 1981) and north of Cape Dalhousie in 1983 (corresponds to aggregations B, C, and D in 1982, 1984, and 1986; McLaren and Davis 1985). From these

observations it is clear that aggregations are a regular phenomenon during late summer and early fall in the southeastern Beaufort Sea. The large aggregations of ringed seals seen in the openwater season are different from the small and widely dispersed groups that commonly occur during spring, when seals haul out around holes in the sea ice (Stirling et al. 1982; Kingsley 1986). This may be explained by seasonal changes in behaviour from primarily feeding in summer and fall to reproduction and moulting in spring, changes in habitat from open water to complete ice cover, and differences in the threat of predation from polar bears after freeze-up. During late

CAN. J. ZOOL. VOL. 70, 1992

898


zo zo*g*gzo *g *g zo zo*g*g

summer and fall, feeding is a particularly important activity for ringed seals as they deposit fat reserves with which to survive the winter, and in the case of pregnant females, to support growing offspring (Lowry et al. 1980; Smith 1987). Areas where ringed seals aggregated are known to have oceanographic characteristics favourable for production of zooplankton (LGL Ltd. 1988). Some were examined using in situ data on zooplankton collected concurrently with the 1986 aerial surveys (LGL Ltd. 1988), and it was found that mean densities of euphausiids and copepods were significantly greater in seal aggregation areas than in nonaggregation areas (Harwood 1989). In addition, stomachs and intestines of four seals collected from aggregation area F along the Yukon coast were full and contained the same prey type (Mysis littoralis; Harwood 1989). Smith (1987) found that seals collected from aggregation areas during fall in the Prince Albert Sound also had full stomachs. There the predominant prey items were crustaceans (e.g ., amphipods, euphausiids, and mysids), with fish being slightly more prevalent in the diet of subadults. We also observed seabirds feeding concurrently in several of the seal aggregation areas, as noted by Smith (1987) in Prince Albert Sound. Together, these results suggest that ringed seals aggregate in late summer and fall to feed on prey concentrated in the aggregation areas. Although ringed seals aggregated in the open-water season in most years, their relative abundance varied among the years of this study. In particular, their densities declined markedly between 1982 and 1985, and were still low in 1986 compared with those recorded in 1982. Hunters from Sachs Harbour, a small community on the west coast of Banks Island to the northeast of the study area, reported a reduction in ringed seal pups in their harvests in fall 1984 and continuing low productivity in the 3 years following (Kingsley and Byers 1990). Data on age structure and reproductive status from 1987 to 1989 confirmed a substantial failure of recruitment from 1984 to 1987 but, as in the 1970s (Stirling et al. 1982), this was temporary and reproduction had returned to normal levels by 1988 (Kingsley and Byers 1990). Confirmation that the decline in ringed seal productivity from 1985 to 1987 was real and widespread in the eastern Beaufort Sea was provided by a corresponding decline, with a 1-year time lag, in the productivity of polar bears, which prey primarily on ringed seals (Stirling et al. 1988). If, as we suggest, ringed seals aggregate in open-water areas in order to feed where their prey are more abundant, there should be a relationship between the density of seals in aggregations and the abundance of their prey. Bowhead whales are filter feeders, and like other baleen whales, probably aggregate to feed in areas where prey species are more concentrated (Gaskin 1982). Our observation of bowhead whales feeding in areas where ringed seals aggregated constitutes strong circumstantial evidence for a greater than average biomass of invertebrate prey species there. Although we have no direct measure of prey abundance other than for localized areas in 1986 (LGL Ltd. 1988), we suggest that the positive correlation between density of ringed seals and their productivity reflects this aspect and provides further support for the hypothesis that ringed seals aggregate in the open-water period to feed. The reason for the decline in ringed seal density, and the reduction in the numbers of ringed seal pups born, particularly in 1985, is uncertain, but the decline coincided with heavy ice conditions that prevailed during the summer of 1985. The


similar decline in the numbers and productivity of ringed seals in the study area between 1974 and 1975 also coincided with heavy ice conditions (Stirling et al. 1977, 1982). It is likely that heavy ice conditions have a negative influence on primary or secondary productivity and reduce the numbers of o r disperse the prey species of ringed seals.

Acknowledgements We thank P. Norton, J. Ford, and L. Turney for assistance


in the field, the pilots of Dome Petroleum Ltd. and Kenn Borek Air Ltd. for safe and successful survey flights, and staff of the Atmospheric Environment Service and the oil and gas industry aviation offices for weather information. We also gratefully acknowledge the contributions of the following scientific advisors from funding agencies: R. Hurst, M. Kingsley, J. Montague, E. Pessah, B. Smiley, and J. Ward. We appreciated the opportunity to use data on ringed seals collected during surveys flown under contract to ESL Environmental Sciences Ltd., Vancouver, B .C . We are particularly grateful to J. Holmes, University of Alberta, S. Barry, Canadian Wildlife Service, and M. George-Mascimento, Pontificia Universidad Cat6lica de Chile, for assistance and guidance with analysis and approach. Financial support was provided by Dome Petroleum Ltd., Gulf Canada Resources Inc., Fisheries and Oceans Canada (DFO, Science Subvention Program and Ocean Information Division), Indian and Northern Affairs Canada (Northern Oil and Gas Action Program and Scientific Resource Centre), the Polar Continental Shelf Project, the U. S. Minerals Management Service, the Boreal Institute for Northern Studies, and the Department of Zoology at the University of Alberta. We thank Sam Barry, Canadian Wildlife Service, Michael Kingsley, DFO, Lloyd Lowry, Alaska Department of Fish and Game, Dr. Tom Smith, DFO, and two anonymous reviewers for their thorough and helpful comments on the manuscript.

Bonnell, M. L., and Ford, R. G. 1987. California sea lion distribution: a statistical analysis of aerial transect data. J. Wildl. Manage. 51: 13-20. Borstad, G. A. 1985. Water colour and temperature in the southern Beaufort Sea: remote sensing in support of ecological studies of the bowhead whale. Can. Tech. Rep. Fish. Aquat. Sci. No. 1350. Caughley, G. 1977. Sampling in aerial survey. J. Wildl. Manage. 41 : 605 -615. Cochran, W. G. 1963. Sampling techniques. 2nd ed. John Wiley and Sons, New York. Davis, R. A., Koski, W. R., Richardson, W. J., Evans, C. R., and Alliston, W. G. 1982. Distribution, numbers and productivity of the western Arctic stock of bowhead whales in the eastern Beaufort Sea and Amundsen Gulf, summer 1981. Prepared for Sohio Alaska Petroleum Co., Dome Petroleum, and others. Available from LGL Ltd., Box 457, King City, Ont., Canada LOG IKO. Ellis, D. V. 1957. Some observations on mammals in the area between Coppermine and Pond Inlet, NWT, during 1954 and 1955. Can. Field-Nat. 71: 1-6. Fedoseev, G. A. 1965. Feeding of the ringed seal. Izv. Tikhookean. Nauchno-Issled. Inst. Ribn. Khoz. Okeanogr. 59: 2 16-223. (Translated by M. L. Poppino, Davis, Calif., 1977.) Finley, K. J., and Johnston, W. G. 1977. An investigation of the distribution of marine mammals in the vicinity of Somerset Island with emphasis on Bellot Strait, August - September 1976. Prepared by LGL Ltd. for Polar Gas Project, Toronto, Ont. Available from the Arctic Institute of North America, University of Calgary, 2500 University Drive NW, Calgary, Alta., Canada T2N 1N4. Ford, J. K. B., Cubbage, J. C., and Norton, P. 1988. Distribution,

899

abundance and age segregation of bowhead whales in the southeast Beaufort Sea, August-September 1986. Environmental Studies Revolving Funds Rep. No. 089, Indian and Northern Affairs Canada, Ottawa. Frost, K. J., Lowry, L. F., Gilbert, J. R., and Burns, J. J. 1988. Ringed seal monitoring: relationships of distribution and abundance to habitat attributes and industrial activities. Alaska Outer Continental Shelf Environmental Assessment Program, U.S. Minerals Management Service, Department of the Interior, National Oceanic and Atmospheric Administration Project RU No. 667. Available from the Alaska Department of Fish and Game, 1300 College Road, Fairbanks, AL 9970 1- 1599, U. S.A. Gaskin, D. E. 1982. The ecology of whales and dolphins. William Heinemann Ltd., London. Hammill, M. 0 . 1987. The effects of weather on ice conditions in the Amundsen Gulf, NWT. Can. MS. Rep. Fish. Aquat. Sci. No. 1900. Harwood, L. A. 1989. Distribution of ringed seals in the southeast Beaufort Sea during late summer. M.Sc. thesis, Department of Zoology, University of Alberta, Edmonton, Alta. Harwood, L. A., and Borstad, G. A. 1985. Bowhead whale monitoring study in the southeast Beaufort Sea, July-September 1984. Environmental Studies Revolving Funds Rep. No. 009, Indian and Northern Affairs Canada, Ottawa. Harwood, L. A., and Ford, J. K. B. 1983. Systematic aerial surveys of bowhead whales and other marine mammals of the southeastern Beaufort Sea August -September 1982. Prepared for Dome Petroleum Ltd. and Gulf Canada Resources Inc. Available from The Arctic Institute of North America, University of Calgary, 2500 University Drive NW, Calgary, Alta., Canada T2N 1N4. Harwood, L. A., Turney, L. A., de March, L., Smiley, B. D., and Norton, P. 1986. Arctic data compilation and appraisal. Beaufort Sea: biological oceanography-seals, 1826- 1985. Can. Data Rep. Hydrogr. Ocean Sci. No. 5. Vol. 8. Holt, R. S., and Cologne, J. 1987. Factors affecting line transect estimates of dolphin school density. J. Wildl. Manage. 51: 836-843. Kingsley, M. C. S. 1986. Distribution and abundance of seals in the Beaufort Sea, Amundsen Gulf, and Prince Albert Sound, 1984. Environmental Studies Revolving Funds, Rep. No. 025, Indian and Northern Affairs Canada, Ottawa. Kingsley, M. C. S., and Byers, T. 1990. Status of the ringed seal population in Thesiger Bay, NWT, 1987-89. Prepared for Fisheries Joint Management Committee, established pursuant to the 1984 Inuvialuit Final Agreement. Available from Joint Secretariat, Box 2120, Inuvik, N. W.T., Canada XOE OTO. Kingsley, M .C.S., and Smith, G. E. J. 1981. Analysis of data arising from systematic transect surveys. In Proceedings of a Symposium on Census and Inventory Methods for Populations and Habitats, April 1980. Edited by F. L. Miller and A. Gunn. Northwest Section, The Wildlife Society, Banff, Alta. pp. 40-48. Kingsley, M. C. S., Stirling, I., and Calvert, W. 1985. The distribution and abundance of seals in the Canadian high Arctic, 1980-82. Can. J. Fish. Aquat. Sci. 42: 1189-1210. LGL Ltd. 1988. Bowhead whale food availability characteristics in the southern Beaufort Sea: 1985 and 1986. Environmental Studies Program Rep. No. 50, Indian and Northern Affairs Canada, Ottawa. Lowry, L. F., Frost, K. J., and Burns, J. J. 1980. Variability in the diet of ringed seals, Phoca hispida, in Alaska. Can. J. Fish. Aquat. Sci. 37: 2254 -2261. Marko, J. 1975. Satellite observations of the Beaufort Sea ice cover. Beaufort Sea Project Tech. Rep. No. 34, Department of the Environment, Victoria, B. C. McLaren, P. A., and Davis, R. A. 1985. Distribution of bowhead whales and other marine mammals in the southeast Beaufort Sea, August - September 1983. Environmental Studies Revolving Funds Rep. No. 00 1, Indian and Northern Affairs Canada, Ottawa. Neter, J., and Wasserman, W. 1974. Applied linear statistical models. Richard D. Irwin, Inc., Homewood, Ill.


900

CAN. J. ZOOL. VOL. 70. 1992

Norton, P., and Harwood, L. A. 1985. White whale use of the southeastern Beaufort Sea, July -September 1984. Can. Tech. Rep. Fish. Aquat. Sci. No. 1401. Norton, P., and Harwood, L. A. 1986. Systematic aerial survey programs and bowhead whales sighted by industry personnel. In Distribution, abundance, and age segregation of bowhead whales relative to industry activities and oceanographic features in the southest Beaufort Sea, August -September 1985. Edited b y W. S. Duval. Environmental Studies Revolving Funds Rep. No. 057, Indian and Northern Affairs Canada, Ottawa. pp. 4-27. Norton, P., Smiley, B. D., and de March, L. 1987. Arctic data compilation and appraisal. Beaufort Sea: biological oceanographywhales, 1848- 1983. Can. Data Rep. Hydrogr. Ocean Sci. No. 5. Vol. 10. Renaud, W. E., and Davis, R. A. 1981. Aerial surveys of bowhead whales and other marine mammals off the Tuktoyaktuk Peninsula, NWT, August-September 1980. Prepared by LGL. Ltd. for Dome Petroleum Ltd. Available from The Arctic Institute of North America, University of Calgary, 2500 University Drive NW, Calgary, Alta. Robertson, E. O., and Robertson, I. 1987. Assessment of the value of stratified sampling for aerial surveys: a case study of bowhead whales in the Beaufort Sea. Can. Tech. Rep. Fish. Aquat. Sci. No. 1500. Sampson, R. J. 1978. Surface I1 graphics system. Kansas Geological Survey, Lawrence.

SAS Institute Inc. 1985. SAS user's guide: statistics, version 5 ed. SAS Institute Inc., Cary, N.C. Smith, M., and Rigby, B. 1981. Distribution of polynyas in the Canadian Arctic. In Polynyas in the Canadian Arctic. Edited b y I. Stirling and H. Cleator. Can. Wildl. Serv. Occas. Pap. No. 45. pp. 7 -27. Smith, T. G. 1987. The ringed seal, Phoca hispida, of the Canadian western Arctic. Can. Bull. Fish. Aquat. Sci. No. 216. Stirling, I., Archibald, W. R., and DeMaster, D. 1977. Disribution and abundance of seals in the eastern Beaufort Sea. J. Fish. Res. Board Can. 34: 976-988. Stirling, I., Kingsley, M. C. S., and Calvert, W. 1982. The distribution and abundance of seals in the eastern Beaufort Sea, 1974-79. Can. Wildl. Serv. Occas. Rep. No. 46. Stirling, I., Andriashek, D., Spencer, C., and Derocher, A. 1988. Assessment of the polar bear population in the eastern Beaufort Sea. Final Rep. to Northern Oil and Gas Action Program by Canadian Wildlife Service, Edmonton. Available from The Canadian Wildlife Service, Edmonton, Alta., Canada T5K 255. Thomson, D. H., Fissel, D. B., Marko, J. R., Davis, R. A., and Borstad, G. A. 1986. Distribution of bowhead whales in relation to hydrometeorological events in the Beaufort Sea. Environmental Studies Revolving Funds Rep. No. 028, Indian and Northern Affairs Canada, Ottawa. Zar, J. H. 1984. Biosatistical analysis. Prentice-Hall, Inc., Englewood Cliffs, N.J.