Killer whales (Orcinus orca) at Marion Island, Southern Ocean

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1 Killer whales (Orcinus orca) at Marion Island, Southern Ocean M. Keith, M.N. Bester, P.A. Bartlett, & D. Baker Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Pretoria, 0002 South Africa Received 12 May Accepted 24 November 2000 Killer whales (Orcinus orca) were studied using data obtained on an opportunistic basis between 1973 and 1996 at Marion Island (46 54 S, E) in the Southern Indian Ocean. A clear seasonal pattern of occurrence with the main peak between October and December was evident. Most killer whales were observed within 5 m of the shore and adult males typically occurred further offshore than adult females. The distribution of killer whales around the island was not uniform and more than 80 % of all sightings occurred close to the base station. This was probably due to a concentration of search effort in the vicinity of the base station and to an uneven distribution of prey species. Killer whales occurred in groups with an average of 3.56 individuals and a maximum of 28 animals per group; females predominated throughout the study. Dawn-to-dusk surveys of killer whales in the near-shore waters during 1986, 1989 and 1990 showed small, statistically non-significant peaks of sightings in the early morning and late afternoon. Twenty-six killer whales were individually identified from photographs, eleven of which recurred at Marion Island. Modified photogrammetric methods may improve current photogrammetric analysis of cetacean dorsal fins. Key words: population structure; seasonal patterns, photogrammetry. INTRODUCTION The cosmopolitan killer whale is thought to be most abundant in colder waters at higher latitudes of both hemispheres (Heimlich-Boran 1988; Leatherwood et al. 1984), and appears to be widespread in the Indian Ocean (Dalheim 1981). Although killer whales can be observed throughout the year at Marion Island, they are more commonly seen from October to December, coinciding with the seasonal haul out of the southern elephant seals, Mirounga leonina, and certain penguin species (Condy et al. 1978). A similar pattern of occurrence has been reported for killer whales at the Îles Crozet (Guinet 1991a; Ridoux 1986). The movements of killer whale groups are related primarily to movements in their food supply (Dalheim 1981). Individual populations may target a preferred prey species in a given area and may shift prey preferences in response to seasonal variation in prey abundance (Condy et al. 1978; Heimlich-Boran 1988). In the Southwest Indian Ocean, at the Îles Crozet, killer whales feed on elephant seals, penguins, whales and fish (Ridoux 1986). Author for correspondence. mnbester@zoology.up.ac.za Present address: Ichaboe Island, Ministry of Fisheries and Marine Resources, P.O. Box 394, Lüderitz, Namibia. Deceased. The overall aim of this study was to determine the current status of the killer whale population at Marion Island and to establish if its structure had changed in the 22 years since the pioneering ( ) study of Condy et al. (1978). Specific aims were to determine: 1) the temporal and spatial occurrence of killer whales in near-shore waters; 2) the age and sex composition of the population; 3) the characteristics (photo-identification) of the individuals frequenting Marion Island, and 4) if the identified individuals return annually. MATERIALS & METHODS Study area and data collection Marion Island (46 54 S, E) is the larger of the two islands within the Prince Edward Island group and is situated in the Southern Indian Ocean north of the Antarctic Polar Front (Fig. 1a). The island has an area of 296 km 2 (Condy et al. 1978) with a circumference of approximately 90 km (Wilkinson 1992). Data were collected on an opportunistic basis between 1973 and 1996 by researchers operating within the South African National Antarctic Programme (SANAP). The base station at Marion Island is situated on the east coast (Fig. 1b) and most human activity is concen- 36(2): (October 2001)

2 164 African Zoology Vol. 36, No. 2, October 2001 a b Fig. 1. a, Position of Marion Island within the Prince Edward Island group, situated in the Southern Indian ocean; b, map of Marion Island with some of the beach codes used in the Pinniped Monitoring Programme (solid circles depict the different field huts distributed around the island). trated in the immediate vicinity. Consequently the highest number of opportunistic observations were made along the east coast. In order to control for search effort, dawn-todusk surveys (DDUs) were made in three separate years (1986, 1989 and 1990). DDUs were in addition to the opportunistic sightings. In each of these years, during the anticipated peak in sighting frequencies (Condy et al. 1978), an observation point at the base station was manned for about four consecutive days from dawn to dusk. The opportunistic observations covered the period 03:00 to 19:59 SAST and the DDU observations the period 04:00 to 18:59 SAST. At every sighting (opportunistic observations and DDUs) the date, time, age, sex (when possible), distance from the shore, and group size were recorded. An individual s sex and age were primarily determined by using body size, size and shape of the dorsal fin, following Leatherwood

3 Keith et al.: Killer whales at Marion Island 165 et al. (1976). Individuals were assigned to one of four age classes: adult, subadult, calves and unidentified. Those individuals that could not be observed clearly due to increased distance from shore were classified as unidentified. Repetitive sightings of the same individual were excluded. All individuals sighted within a time frame of approximately 10 minutes were assumed to belong to a group. Four different near-shore zones were established following Condy et al. (1978): zone 1: within 5 m of the shore; zone 2: m from the shore, usually extending to the first kelp belt (Macrocystis sp.); zone 3: m from the shore, usually extending to the second belt of kelp; zone 4: beyond 500 m from the shore. A fifth zone (zone 5; m from the shore), was added during analyses, after it was found that zones 1 and 2 were often not separated by the observers. Data manipulation Temporal and spatial characteristics of killer whale sightings and group sizes and sex ratios were extracted from the data base, and means and standard errors of the means (s.e.m.) calculated. The duration of the study allowed the separation of the opportunistic observations into three time periods ( = the 1970s; = 1980s, and = 1990s) for comparative purposes. The different years data were investigated separately for any remarkable deviation from a common pattern, before they were lumped into different decades to facilitate comparison. Owing to the absence of data from mid-1981 to the beginning of 1983, and the sporadic nature under which the data were obtained, the generation of a seasonal pattern for killer whale occurrences around Marion Island required interpolation of opportunistic observations for the unrecorded time periods. The variation in search effort and the number of observations recorded throughout the study necessitated focus on the three DDUs to establish the daily pattern of occurrence in nearshore waters. The beach codes used in the Pinniped Monitoring Programme on Marion Island (MM001 MM068, clockwise from the Base station, Fig. 1b), were used to plot the spatial distribution of killer whale sightings around the Island. Photogrammetric assessment of relative dorsal fin size We used the photogrammetric methodology of Heimlich-Boran (1986) which provides only relative measurements of dorsal fin size. Measurements taken directly from photographs, were translated into a dorsal fin ratio (DFR) (i.e. fin height/fin width at base) after Heimlich-Boran (1986). The height to width ratio (HWR) used by Olesiuk et al. (1990) proved problematic due to the unreliable placing of the mid-point of curve and was not used for photogrammetric purposes in the present study. Photographs were taken using a Minolta SRT 101 camera, with a mm lens, using 100 to 400 ASA Illford film. All available photographic material (1975, 1986, 1987, and 1989) was examined, and a photo catalogue was developed. Individuals were identified using relative dorsal fin size ratios, injuries, scratches, unique cuts, gouges and any other deformities. The poor quality of many of the photographs limited photogrammetric analysis. Negatives were enlarged by projection to a fin height of 10 cm using a BRAUN AG7 slide projector and the contour of the fin was traced onto a sheet of A4 paper. Traced dorsal fins were usually those that were parallel to the photographic film plane, with the dorsal fin (DF) and part of the back of the animals fully visible. Dorsal fin ratios were determined (mean and standard deviation) from 195 tracings of individuals with distinct nicks and/or gouges following the technique of Defran et al. (1990) as modified by Karczmarski and Cockcroft (1998). The distance between the top of the dorsal fin and the top of the most distinct notch (A), was divided by the distance from the top of the fin to the bottom of the notch (B) to provide a ratio (Fig. 2a). When individuals did not possess any clearly visible markings on their dorsal fin, an alternative dorsal fin ratio was calculated from the fin height to fin width at the base following Heimlich-Boran (1986). Two different methods were used to establish standard points from which the width of the fin base could be measured. In the first method, circles were drawn that fitted into the anterior and posterior curvature of the dorsal fin. The two points at which each circle made contact with the body and dorsal fin were joined, and another line was drawn from the centre of each circle perpendicular to the first line, and extending to touch the body. These two points were linked to give a base length (BL; Fig. 2b). The perpendicular distance to the top of the fin was divided by the BL to obtain a ratio for the modified Heimlich-Boran method (HBR1, Fig. 2b). Secondly, by using the circles with the largest radius that fit in the curvature on either side of the dorsal fin tracing, a single point where each

4 166 African Zoology Vol. 36, No. 2, October 2001 a b and M for an adult male (e.g. MM) and F for a known adult female (MF). A number after the code was given to each identified individual. The unknown sexes were recorded as M and a number. The dorsal fins were divided into the upper third (UP), middle (MID) and lower third (BOT) and characteristic features recorded (notch, a V-shaped jagged indentation; nick, a small groove or notch; gouge, a large piece of DF missing). Scarring (SC) other than on the dorsal fin was noted as miscellaneous scarring (MISC). Statistical analysis Chi-square tests were used to determine if the distribution of killer whale sightings in the nearshore zones (all sightings and males and females separately) were significantly different from random. A non-parametric Kolmogorov-Smirnov test (Conover 1980) was performed on diurnal activity patterns and analysis of variance was used to test for relationships between the two modified Heimlich-Boran methods. Significance was set at P < 0.05 for the ANOVA. RESULTS c Fig. 2. a, An example of the computation of the dorsal fin ratio (DFR) (from Defran et al. 1990); b, example of the adapted Heimlich-Boran (1986) DFR method (HBR1); c, example of the second modified version of the Heimlich-Boran (1986) DFR method (HBR2). circle last touched the body on either side of the dorsal fin was connected, to give a base length (BL). Using this line another perpendicular distance to the highest point of the dorsal fin was obtained, and a ratio (height/bl) calculated (HBR2, Fig. 2c). Photo-identification The Dolphin Biology Research Institute (DBRI) cataloguing system (Urian & Wells 1996) was used to compile a photo catalogue. Each identified whale was coded by the letter M for Marion Island Spatial distribution The distribution of the killer whales in nearshore waters at Marion Island showed a clear pattern with significantly more (P < 0.001) sightings in zone 1 than further offshore (Fig. 3). During , opportunistic observations of killer whales were most common in zone 1 (34.4 %), while the other three zones had a similar range of sighting frequencies ( %). In the other two decades (1980s and 1990s), the highest number of sightings also occurred in zone 1 (59.7 % and 62.6 %, respectively), after which sightings decreased with increasing distance from the shore. When all opportunistic observations were pooled, the frequency of occurrence of sightings decreased with an increase in distance from the island. The numbers of observations for zone 5 were higher for the 1970s (7.0 %), than the 1980s (1.2 %) and 1990s (1.9 %). In the three DDUs, significantly more (P < 0.001) sightings of killer whales were recorded in zone 1 (Fig. 3). During the 1986 DDU zone 4 (26.2 %) had the second-highest percentage frequency of sightings while zone 3 (16.6 %) was second-highest during the 1990 DDU. The combined data from the opportunistic observations ( ) and the DDU surveys showed a similar, statistically

5 Keith et al.: Killer whales at Marion Island 167 Fig. 3. Offshore distribution patterns of killer whales in the demarcated zones during the opportunistic surveys in the periods , and , the three dawn-to-dusk (DDU) surveys carried out in 1986, 1989 and 1990, and the combined opportunistic ( ) and dawn-to-dusk surveys (1986, 1989 and 1990), respectively. significant pattern with more than 50 % of observations in zone 1 and 20 % or fewer in the other zones (Fig. 3). Spatial distribution of males and females Opportunistic observations and the DDUs revealed that both male and female killer whales were more abundant than expected in zone 1 (P < 0.001; Fig. 4a,b). For females, the opportunistic observations revealed a general trend of decreasing sightings with increasing distance from the shore. However, the DDU surveys of 1986 and 1990 showed a different pattern, with the zones further out to sea (zones 3 and 4) returning slightly higher sighting frequencies than zone 2 (Fig. 4a). While the data for males fit the same general pattern, the frequency of sightings in zone 1 was lower than for females, and in the zones further out to sea, was higher than for the females (Fig. 4a,b). The DDU for 1986 produced a reversed trend, with the greatest number of sightings in zone 4 (Fig. 4b). Distribution around the island Most sightings were made in the vicinity of the base station (beach codes MM001, MM002, MM ) with fewer sightings at other sites around the island (Fig. 5a). This apparent trend reflects a concentration in both opportunistic observations and the DDUs in the vicinity of the base station. If the sightings from the immediate vicinity of the base station are omitted, and the data re-plotted (Fig. 5b) it is apparent that sightings of killer whales were more frequent on the east coast of Marion Island (beach codes MM and MM ; Fig. 1b). Seasonal occurrence The occurrence of killer whales off Marion Island changed seasonally in a pattern that was repeated throughout the study (Fig. 6). An increase in the number of sightings occurred at the onset of October and lasted to the end of December after which there was a decline in sightings during January. A small increase in sightings occurred in late April to early May after which the killer whale sightings, during most years, decreased to almost zero during the winter months. Group size and sex ratios Mean group size for the whole study was 2.9 ± 0.03 individuals (based on opportunistic observations) and 2.8 ± 0.08 (based on the DDUs) of which adult females comprised the largest class (38 % and 28 % for the opportunistic observations and DDUs, respectively; Table 1). Based on opportunistic observations, adult males ranked second (24 %) while the DDUs indicated that subadults formed the second largest component of the groups (21 %; Table 1). The largest group comprised 28 individuals and was seen at Marion Island in December 1991.

6 168 African Zoology Vol. 36, No. 2, October 2001 a b Fig. 4. a, Observed frequencies of occurrence of adult female killer whales in the different offshore zones during the different opportunistic and dawn-to-dusk surveys, including the combined data sets; b, observed frequencies of occurrence of adult male killer whales in the different offshore zones during the different opportunistic and dawn-todusk surveys, including the combined data sets. The sex ratio of killer whales differed significantly between decades (opportunistic observations) and between the three DDUs ( 2 > , d.f.=8,p < 0.001) and also between the DDU and the opportunistic observation data ( 2 = , d.f. = 4, P < 0.001). Diurnal frequency of sightings The number of killer whales sighted each hour (expressed as a percentage) during the DDUs did not vary significantly (P > 0.05) nor were there significant differences between the three DDUs (Fig. 7). There was, however, a general trend with a greater frequency of sightings in the morning and late afternoon (Fig. 7). Photo-identification and photogrammetry Approximately 200 tracings of killer whales dorsal fins were analysed and 42 % of these did not possess any identifiable markings. Twenty-six killer whales were identified and 11 were re-sighted in more than one season (Table 2). Relative dorsal fin size was calculated, using photogrammetric methods, for 18 killer whales (Table 3). There were significant differences in dorsal fin ratios generated by the two Heimlich-Boran (1986)

7 Keith et al.: Killer whales at Marion Island 169 a b Fig. 5. a, The spatial distribution of killer whale sightings off Marion Island from 1973 to Data are presented as percentage frequencies; b, the spatial distribution of killer whale sightings between Macaroni Bay (MM003) and Duiker s Point (MM067) during Data are presented as percentage frequencies. methods for one individual, and between individuals (ANOVA, P < 0.001). DISCUSSION Spatial distribution: near-shore distance Condy et al. (1978) found that killer whales sighted off Marion Island, had a near-shore distribution of 34 % in zone 1, 27.6 % in zone 2, 24.5 % in zone 3, and 13.9 % in zone 4. Our study shows the same trend although a greater percentage of sightings (>50 %) were in zone 1. As southern elephant seal pups (Mirounga leonina) remain in near-shore waters during play and local post-weaning dispersion (Panagis 1981; Lenglart & Bester 1982; Wilkinson & Bester 1990), and elephant seal females and penguins enter and exit the water here at the land-sea boundary, it is likely to be the most rewarding hunting area for killer whales. Guinet et al. (1992) found a high predation rate of killer whales on weaned elephant seal pups at the Îles Crozet, with 25 % of pups weaned on a particular beach being taken. The results from the opportunistic observations and the DDUs are slightly different, particularly in terms of the numbers of sightings in the zones further

8 170 African Zoology Vol. 36, No. 2, October 2001 Fig. 6. Percentage frequencies of observations of killer whales for the study period. Data are represented on a monthly basis and were interpolated for months when no recordings were made. No data were available for the period October 1980 to November off-shore and this probably is due to differences in observational techniques. In the present study, the frequency of observations of female killer whales in zone 1 was slightly greater than of males and similar results have been reported for the Îles Crozet (Guinet 1991b). There, adult males rarely participated in social beaching and were never observed to strand themselves completely on the beach (Guinet 1991b). This difference in the behaviour of the sexes may be related to the difference in body size between adult males and females. However, individually variable-hunting techniques may contribute to this segregation as Lopez & Lopez (1985) found a solitary adult male that did on some occasions intentionally strand to capture prey, while Guinet (1991b) found only females performing this type of hunting technique. The beach physiognomy on Marion Island (a paucity of gently sloping sandy beaches) apparently does not readily allow this type of stranding behaviour. Distribution around the island More than 80 % of the sightings of killer whales were recorded in the vicinity of the base station and this was probably due to the high number of Table 1. Mean number of killer whales (± standard error of the mean) recorded for each sex and age class for the opportunistic and dawn-to-dusk surveys during The percentage frequency composition ( %) of the different age and sex classes are included in brackets Adult males 0.84 ± 0.03 (25.7) 0.61 ± 0.02 (20.2) 0.74 ± 0.02 (26.2) 0.70 ± 0.01 (23.9) Adult females 1.16 ± 0.05 (35.9) 1.12 ± 0.03 (38.7) 1.10 ± 0.03 (38.7) 1.12 ± 0.02 (38.0) Subadults 0.48 ± 0.04 (14.6) 0.25 ± 0.02 (8.8) 0.29 ± 0.02 (10.1) 0.31 ± 0.01 (10.5) Calves 0.25 ± 0.02 (7.8) 0.35 ± 0.02 (12.1) 0.32 ± 0.02 (11.1) 0.32 ± 0.01 (10.8) Unidentified 0.52 ± 0.06 (16.0) 0.55 ± 0.03 (19.5) 0.37 ± 0.03 (14.3) 0.47 ± 0.02 (16.9) All animals 3.24 ± ± ± ± 0.03 DDU-1986 DDU-1989 DDU-1990 Combined DDU Adult males 0.41 ± 0.04 (16.2) 0.46 ± 0.04 (16.7) 0.53 ± 0.05 (16.2) 0.46 ± 0.02 (16.4) Adult females 0.60 ± 0.04 (22.5) 0.86 ± 0.05 (31.5) 1.01 ± 0.05 (30.9) 0.79 ± 0.03 (28.3) Subadults 0.26 ± 0.03 (10.1) 0.71 ± 0.05 (25.9) 0.96 ± 0.08 (28.9) 0.60 ± 0.03 (21.5) Calves 0.28 ± 0.04 (10.5) 0.42 ± 0.04 (15.5) 0.58 ± 0.06 (17.9) 0.40 ± 0.03 (14.5) Unidentified 1.09 ± 0.12 (40.7) 0.28 ± 0.07 (10.3) 0.07 ± 0.03 (6.1) 0.53 ± 0.06 (19.4) All animals 2.64 ± ± ± ± 0.08

9 Keith et al.: Killer whales at Marion Island 171 Fig. 7. Diurnal distribution of observations of killer whales recorded during the dawn-to-dusk (DDU) surveys. Data are presented as percentage frequencies. potential observers present there. However, prey distribution and availability may also play a role in the frequency of occurrence of observations around the island. The areas away from the base station with high percentage frequencies of sightings support colonies of penguins and elephant seals. Most breeding (and moulting) elephant seals occupy the eastern coasts (Condy 1977; Wilkinson & Bester 1990) and this could explain the predominance of killer whales on the east coast. Despite the large number of fur seals (Arctocephalus tropicalis) residing on the west and north coasts of Marion Island (Hofmeyr et al. 1997), relatively few sightings of killer whales were recorded there. This might be related to the behaviour, distribution and difference in the seasonal haulout cycle of these two (of three) seal species. Seasonal occurrence of killer whales The seasonal occurrence of killer whales at Marion Island reported here supports the findings of Condy et al. (1978) and Guinet (1991a) who found a clear pattern of seasonal occurrence of killer whales, with the maximum frequency of observations in October, November and December, on Marion Island and the Îles Crozet respectively. Voisin (1972) also recorded a peak in killer whale occurrence between September and November, with a smaller peak from March to May, at the Îles Crozet. Condy et al. (1978) found a steep decline in the number of sightings during January and February, with another, less pronounced, increase in sightings occurring from March to May, a pattern which is similar to that reported here. Roux (1986) reported a clear seasonal cycle of occurrence at the temperate, low latitude Amsterdam Island with killer whales being rare from July to August, and more common in February and March, three to five months later than the peak in abundance around the sub- Antarctic islands. The observed difference in the timing of peak abundance of killer whales might be related to dispersing killer whales passing through different latitudes at different times (Mikhalev et al. 1981). The high frequency of sightings in the 1980s may be the result of the increased human activity on the island, owing to the presence of increased number of staff involved with the feral house cat (Felis catus) extermination programme (Bloomer & Bester 1992). The low numbers of opportunistic sightings in October and December 1986 can possibly be related to the DDU survey being conducted at that time. As at the beginning of October 1989, and in October and December 1990, the DDU data were excluded from analysis of the seasonal occurrence of killer whales which was based on opportunistic sightings. Group size The overall mean group size for killer whales at Marion Island was approximately four individuals (Table 1) which is similar to the peak in group size

10 172 African Zoology Vol. 36, No. 2, October 2001 Table 2. Photo catalogue for the identified killer whales with their assigned code (MM = adult males, MF = adult females and M = unknown sex). The number of available photographs (n) for each year group as well as number of tracings for each of the identified individuals and basic descriptions as used in the Dolphin Biology Research Institute (DBRI) cataloguing system by Urian & Wells (1996) are included. Code DBRI description (n = 16) (n = 62) (n = 53) (n =28) MM1 3 UP MM2 2 Typical MM3 1 2 UP, SC MM4 2 4 UP & MID NICK MM5 2 ENT, UP, MID 3 MM6 1 1 Typical MM7 2 1 UP MM8 1 1 ENT, UP, MID 2, BOT MM9 1 1 Typical, MID MF MID NOTCH 2 MF MID, MISC MF 2 1 Typical M2 1 MID UP M3 1 MISC M4 1 UP M5 5 1 UP & MID NICKS 2 M6 1 BOT, MISC M7 1 MISC. M8 1 MID M9 1 UP M10 1 BOT M11 1 FOLD, MID M12 1 BENT M UP M14 1 MID M15 1 BOT (3.5 ± 1.99 individuals) reported by Condy et al. (1978) for October. A number of studies throughout the world have reported mean group sizes for killer whales which vary from six to 30 individuals (Mikhalev et al. 1981; Lopez & Lopez 1985; Baird & Dill 1995), although groups may rarely include several hundred animals (Mikhalev et al. 1981). Sex and age composition In the present study, groups of killer whales comprised predominantly adult females whereas Condy et al. (1978) calculated that groups of killer whales had a sex/age composition of 28.7 % adult males, 21.4 % adult females, 24.7 % subadults, 7.8 % calves and an unidentified class of 17.4 %. By contrast, at the Îles Crozet, adult males represented 71.1 % of all the killer whales sighted (Voisin 1976). While these results may indicate real differences in group composition, comparison between studies is complicated due to the difficulty in distinguishing between a subadult male and an adult female. Diurnal frequencies of sightings The results from the DDUs indicate that killer whales were equally common throughout the day. The apparent increased near-shore activity in the mornings and afternoons could be related to the movement of elephant seal under-yearlings which occurs at dawn and dusk (Lenglart & Bester 1982). Photo-identification and photogrammetry Twenty-six killer whales were individually identified from photographs and eleven of these recurred at Marion Island. Only three individuals could be positively identified as females, based on the continual presence of a calf during a particular

11 Keith et al.: Killer whales at Marion Island 173 Table 3. Photogrammetric means (± standard deviation) of the three methods used to identify individual killer whales (Defran et al. (1990), and adapted versions of Heimlich-Boran (1986)), with the sighting dates and the assigned codes of each identified individual. The individuals are represented with a capital M and a numeral with MF being an adult female and MM representing adult males. The other individuals (M) are of unknown sex. Date Code Defran et al. (1990) Heimlich-Boran Ratio 1 Heimlich-Boran Ratio , 1986, 1987 MF ± ± ± , 1987, 1989 MF ± ± ± , 1987 MF 0.94 ± ± MM ± ± ± MM ± ± , 1989 MM ± (n = 1) 1.17 (n = 1) 1986, 1989 MM ± ± ± MM ± ± , 1987 MM ± ± , 1986 MM ± ± ± , 1989 MM ± (n = 1) 1.19 (n = 1) 1989 M (n = 1) 0.92 ± ± , 1987 M ± ± ± , 1989 M ± ± ± a 1.16 ± ± b 0.85 (n = 1) 0.72 (n = 1) 1989 c 1.49 (n = 1) 1.17 (n = 1) 1989 d 0.86 ± 0.04 year. One female (MF1), with distinct markings on the dorsal fin was first identified in 1973 and was still present around the island in 1993 (G.J.G. Hofmeyr pers. comm.), giving an age in excess of 20 years. Furthermore, this supports the suggestion that marks on the dorsal fins remain essentially unchanged over time (Bigg 1982). The identification of individual killer whales from photographs was complicated by numerous factors. Many of the photographs were taken at great distance and 42 % of the killer whales lacked any distinguishing features. Light reflection and shadows caused false markings on a number of photographs, and in some cases it was necessary to re-evaluate and discard photographs. On occasion, different individuals were identified as the same animal because they each had four distinct scratch marks on the saddle patch. It is thought that these were scars made by the teeth of other killer whales (Scheffer 1969), which may become a permanent and useful mark (Bigg 1982). Furthermore, as shapes can vary considerably in appearance between different orientations (e.g. turn or roll) of an animal to the camera (Bigg et al. 1986), individual dorsal fin shapes were not constant in appearance and could therefore not be used for identification. Finally, since most photographs concentrated on individual killer whales, they yielded no useful information on group structure. The calculation of dorsal fin ratios produced varied results. The Defran et al. (1990) method for photogrammetry could only be used on a few individuals that possessed unique markings on the dorsal fin. The modified Heimlich-Boran (1986) methods used in the present study provided a quantitative way to position the BL. However, it cannot be stated with any certainty that they resulted in a stable position for placing the BL. The arching of the killer whale s body in particular has an effect on the curve of the fin in relation to the body and will affect the position of the BL. The two modified Heimlich-Boran methods gave significantly different results but it is not possible to determine which of the two methods is the more reliable and this may only become evident after obtaining a greater number of good quality photographs. Adult male killer whales should have a dorsal fin ratio (DFR) greater than 1 (Heimlich-Boran 1986) and the modified Heimlich-Boran method (HBR2) gave the same results. On all occasions in the present study killer whales identified as adult males had a HBR2 value larger than one. In the case of MM4, identified as a subadult male, the HBR2

12 174 African Zoology Vol. 36, No. 2, October 2001 value was 0.95 ± However, the DF ratio only reaches a value greater than one after adolescence (Heimlich-Boran 1986). The HBR1 method was less accurate and some females had a ratio larger than one. Human impact on killer whales around Marion Island Marion Island has recently been proposed as an eco-tourism destination (Heydenrych & Jackson 2000) and killer whale watching may become a tourist attraction. Based on the seasonal and diurnal patterns of occurrence, killer whale watching from shore-based vantage points could be viable in the peak season from October to December. While shore-based whale watching does not have a direct effect on the species being watched, there may be an indirect environmental cost on the vegetation and surrounding areas (Findlay 1997). Should shore-based whale watching ever be allowed, it would require strict regulation and be restricted to the base station. With boat-based whale watching, the predictable occurrence of whales, which is of course essential for whale watching, ultimately depends on ecosystem functions that define the habitat relationships of the killer whale (Duffus & Dearden 1993). Alterations to the surrounding ocean may influence the species habitat relationship and could affect the whales attachment for given sites. If boat-based whale watching is ever implemented, it is suggested that care be taken regarding contact with the killer whales, as such an activity could have immediate behavioural effects as well as long-term distribution adjustment effects. Such behavioural changes have been shown by humpback whales (Megaptera novaeangliae) due to an increased traffic of whale watching vessels (Corkeron 1995). ACKNOWLEDGEMENTS We thank the various researchers and relief team members who contributed to this project, and for making their photographs available. The seal researchers were mainly responsible for data collection, and operated with funding provided by the Department of Transport, and latterly by the Department of Environmental Affairs and Tourism, under the advice of the South African Scientific Committee for Antarctic Research (SASCAR) and SACAR, respectively. Analysis of the data and preparation of the manuscript were done under the auspices of the Mammal Research Institute, Department of Zoology and Entomology of the University of Pretoria, and the contributions of Prof. Willem Ferguson, Dr Sampie Ferreira and Greg Hofmeyr are gratefully acknowledged. We also thank Dr Vic Peddemors and another reviewer for helpful comments on the manuscript. REFERENCES BAIRD, R.W. & DILL, L.M Occurrence and behaviour of transient killer whales: seasonal and podspecific variability, foraging behaviour, and prey handling. Canadian Journal of Zoology 73: BIGG, M An assessment of killer whale (Orcinus orca) stocks off Vancouver Island, British Columbia. Report of the International Whaling Commission 32: BIGG, M.A., ELLIS, G.M. & BALCOMB, K.C The graphic identification of individual cetaceans. Whalewatcher 20: BLOOMER, J.P. & BESTER, M.N Control of feral cats on sub-antarctic Marion Island, Indian Ocean. Biological Conservation 60: CONDY, P.R The ecology of the southern elephant seal Mirounga leonina (Linnaeus, 1758) at Marion Island. D.Sc. thesis, University of Pretoria, Pretoria. CONDY, P.R., VAN AARDE, R.J. & BESTER, M.N The seasonal occurrence and behaviour of killer whales Orcinus orca, at Marion Island. Journal of Zoology, London 184: CONOVER, W.J Practical Nonparametric Statistics, 2nd edn. John Wiley & Sons, New York. CORKERON, P.J Humpback whales (Megaptera novaeangliae) in Hervey Bay, Queensland: behaviour and responses to whale-watching vessels. Canadian Journal of Zoology 73: DALHEIM M.E A review of the biology and the exploitation of the killer whale, Orcinus orca, with comments on recent sightings from Antarctica. Report of the International Whaling Commission 32: DEFRAN, R.H., SHULTZ, G.M. & WELLER, D.W A technique for the photographic identification and cataloguing of dorsal fins of the bottlenose dolphin (Tursiops truncatus). Report of the International Whaling Commission (Special issue 12): DUFFUS, D.A. & DEARDEN, P Recreational use, valuation, and management, of killer whales (Orcinus orca) on Canada s Pacific coast. Environmental Conservation 20: FINDLAY, K.P Attitudes and expenditures of whale watchers in Hermanus, South Africa. South African Journal of Wildlife Research 27: GUINET, C. 1991a. L orque (Orcinus orca) authour de l archiphel Crozet comparaison avec d autres localités. Revue de Ecologie (Terre et la Vie) 46: GUINET, C. 1991b. Intentional stranding apprenticeship and social play in killer whales (Orcinus orca). Canadian Journal of Zoology 69: GUINET, C., JOUVENTIN, P. & WEIMERSKIRCH, H Population changes, movements of southern elephant seals in Crozet and Kerguelen Archipelagos in the last decades. Polar Biology 12:

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