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Site fidelity and along-shore range in Hector's dolphin, an endangered marine dolphin from New Zealand
Biological Conservation 108 (2002) 281–287 www.elsevier.com/locate/biocon
Site fidelity and along-shore range in Hector’s dolphin, an endangered marine dolphin from New Zealand Stefan Bra¨gera,1, Stephen M. Dawsona,*, Elisabeth Slootenb, Susan Smithc, Gregory S. Stoned, Austen Yoshinagad a
Department of Marine Science, University of Otago, PO Box 56, Dunedin, New Zealand b Department of Zoology, University of Otago, PO Box 56, Dunedin, New Zealand c 32A Winchester Street, Lyttelton, Christchurch, New Zealand d New England Aquarium, Central Wharf, Boston, Massachusetts, USA
Received 27 July 2001; received in revised form 13 March 2002; accepted 15 March 2002
Abstract To document site fidelity and the alongshore range of individual Hector’s dolphins we analysed sightings of 32 photographically identified dolphins, each seen 510 times at Banks Peninsula, New Zealand, between 1985 and 1997. The furthest two sightings of an individual were 106 km apart. All other individuals ranged over less than 60 km (x=31.0 km, SE=2.43) of coastline. Gender did not significantly influence alongshore range (female x=30.4 km, SE =3.21, n=18; male x=27.4 km, SE=5.68, n=5). Site fidelity was high: for example, on average, individuals were seen in Akaroa Harbour for about two thirds of the years they were known to be alive. These data suggest that impacts on Hector’s dolphins are most appropriately managed on a small spatial scale. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Cephalorhynchus hectori; Hector’s dolphin; Movement; Site fidelity; Management
1. Introduction Of marine cetaceans, Hector’s dolphin Cephalorhynchus hectori has an extraordinarily restricted distribution. The species is found only in New Zealand, and is fragmented into at least three genetically distinct populations (Pichler et al., 1998; Pichler and Baker, 2000). The species’ small size and inshore distribution make it highly vulnerable to coastal gillnetting, especially that for school shark Galeorhinus galeus, rig Mustelus lenticulatus and elephant fish Callorhinchus milii (Dawson, 1991). Gillnet mortality has been high and unsustainable in the Canterbury region from about 1970 until at least 1988 (Slooten and Lad, 1991; Dawson and Slooten, 1993; Slooten et al., 2000), when the Banks Peninsula Marine Mammal Sanctuary was established. This sanctuary is an 1170 km2 area in which * Corresponding author. E-mail addresses: [email protected] (S. Bra¨ger); steve.dawson@ stonebow.otago.ac.nz (S.M. Dawson). 1 Present address: Scharstorfer Weg 12, D-24211 Schellhorn, Federal Republic of Germany.
commercial gillnetting is illegal, and amateur gillnetting is subject to tighter regulation than elsewhere in New Zealand. Bycatch continues in the Canterbury region immediately outside the sanctuary (Baird and Bradford, 2000) and elsewhere in the species’ range (Dawson et al., 2001). Population viability analyses (Martien et al., 1999; Slooten et al., 2000) indicate that, throughout much of its range, gillnetting is causing population decline. For these reasons the conservation status of Hector’s dolphins has recently been revised to ‘‘Endangered’’ (IUCN, 2000). The North Island population is listed as ‘‘Critically Endangered’’ (IUCN, 2000; see Dawson et al., 2001, for review). Whether they focus on establishing gillnet-free sanctuaries (Dawson and Slooten, 1993), employ gear modifications or pingers to reduce bycatch (e.g. Hembree and Harwood, 1987; Kraus et al., 1997; Stone et al., 1997, 2000) or set limits on allowable bycatch (Wade, 1998), efforts to manage bycatch are usually spatially based. Obviously, sanctuaries should be sized so that they contain the movement range of the individual animals for which protection is sought. The issue
0006-3207/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(02)00124-6
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of scale is a two-edged sword, however; there are also penalties if the scale of management is too big. For example, if the spatial scale of management is larger than the scale of the animals’ movement, and the impacts are not uniformly distributed, some population units may be overexploited while others remain virtually unimpacted (Wade and Angliss, 1997). A local population of Hector’s dolphins around Banks Peninsula, on the east coast of the South Island, has been studied continuously since 1984/1985 (e.g. Slooten and Dawson, 1988; Stone and Yoshinaga, 2000). Hector’s dolphins are most frequently seen within the first kilometre from shore. Offshore distribution in summer is well known, thanks to offshore transects undertaken by small boat (Dawson and Slooten, 1988), aircraft (Brown et al., 1992) and most recently by an intensive line-transect survey based from a 15-m catamaran (Dawson et al., 2000). There are data showing diurnal movement of Hector’s dolphins in Akaroa Harbour. Stone et al. (1995) reported that dolphins tended to be seen entering Banks Peninsula’s Akaroa Harbour in the morning, and leaving in the late evening. Winter off-shore distribution is less well known but is presently under investigation. In order to manage bycatch impacts there is a pressing need for information on how far individuals range alongshore. The purpose of this contribution is to add some of this missing information for the Banks Peninsula region by analysing repeated sightings of 32 frequently observed individuals and how far they ranged along shore. These data will provide some guidance on what would be an appropriate spatial scale for the management of Hector’s dolphins in this region.
2. Methods 2.1. Fieldwork Photo-identification surveys were conducted in the inshore waters of Banks Peninsula (Fig. 1), South Island of New Zealand, between November 1984 and March 1997, following the procedure described by Slooten et al. (1992). For the purposes of this paper, the study area was from Godley Head to Birdlings Flat (Fig. 1). Surveys were conducted at speeds of 10–15 knots in small (3.9–6.6 m) outboard-powered boats. Survey effort was concentrated within 1 km of the shore. On the open coast, we followed a standardised alongshore pattern (described in Dawson and Slooten, 1988). In Akaroa Harbour, we either followed a standardised zigzag route (described in Dawson, 1991) or proceeded opportunistically from one dolphin group to the next. On sighting dolphin groups, the boat was stopped or slowed to idling speed to photograph any distinctive individuals present. A relatively low proportion of
Hector’s dolphins in this area is reliably identifiable (Slooten et al., 1993), and only identifiable individuals were photographed. Details of the photo-ID survey methods, ID categories, and matching procedures are given by Slooten et al. (1992). To reduce the possibility of matching errors, only those individuals with very obvious identifying marks (category 1 or 2 as in Slooten et al., 1992) have been used. Resightings reported here come exclusively from photographs, not from individuals recognised in the field without being photographed. Photographs were taken at close range (usually < 7 m). Since 1990, Nikon F4s, N8008s, and N90s autofocus cameras have been used with lenses ranging from 35 to 300 mm, most frequently a Nikkor AF 80–200 mm f2.8 ED lens. Most photographs were taken on Kodak TMAX 400, Kodachrome 64, or Fujichrome 100 films. Locations of sightings were determined from coastal charts, or, since 1990, using Global Positioning System navigation equipment. 2.2. Sighting analysis For this study, we analysed the distribution of 32 individuals with 510 sightings between 1984/1985 and 1996/1997 (total number of these sightings=506). This dataset includes our sightings of ten individuals compiled by Stone (1992). Survey effort varied considerably within and between years (Table 1). Up to 1990, fieldwork by Dawson and Slooten focused predominantly on Akaroa Harbour. After Smith’s (1992) study in the northern half of the Banks Peninsula study area (1990– 1992), fieldwork by Bra¨ger, Dawson, Slooten, Stone, and Yoshinaga was mainly concentrated in the coastal waters south of the peninsula between Steep Head and Birdlings Flat (Table 1). Most data were collected during summer. There was substantial field effort only in the winters of 1985–1986 and 1994–1996. Alongshore ranges were estimated by calculating the shortest linear distances between the two most extreme sightings without crossing land (i.e. ‘‘observed range length’’; Lehner, 1996). Uneven distribution of survey effort over the years and the five subareas (Table 1) could bias the estimation of movement ranges. To correct for this, we used survey effort data to calculate expected sighting rates for each individual by subarea. The number of sightings in each of the five subareas (northwest, northeast, southeast, Akaroa Harbour, and southwest; see Fig. 1) was divided by the number of complete survey trips (max. one per day) within each area while the individual was known to be alive (i.e. from the season with its first sighting to the one with its last; Table 2). Observed sighting rates were tested against the expected values using a 2*5 Log-likelihood ratio test of goodness-of-fit (Sokal and Rohlf, 1995). The boundaries of subareas
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Fig. 1. Map of the Banks Peninsula study area showing places names and quadrants.
3. Results
from 53 to 106 km. Excluding this sighting, the 32 ranges were on average 31.0 km 2.43 SE long (Fig. 2). All individuals were sighted in Akaroa Harbour several times (max.=37 sightings). For 28 of the 32 individuals the majority of sightings was made in Akaroa Harbour.
3.1. Size of Hector’s dolphin movement ranges
3.2. Potential impact of variation in effort
Distances between the most extreme sightings of the same individual were between 8 km (i.e. all sightings near Akaroa Harbour) and 106 km. The most extreme sighting, however, was of one individual which was observed outside the study area once. This increased the greatest distance between its most extreme sightings
The distribution of effort (Table 1) introduces a geographical and seasonal bias into the distribution of sightings. Most sightings of the 32 individuals occurred between late austral spring and early autumn (November–March). Off the open coasts of Banks Peninsula almost all photo-identification effort was within 800 m
reflect a typical day’s survey effort (i.e. a day’s field effort would typically be confined to one subarea).
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Table 1 Survey effort over 13 years showing number of days on the water in the four quadrants of the study area (NW–NE–SE–SW) plus Akaroa Harbour NW (Godley Head– Wakaroa Point)
NE (Wakaroa Point– Steep Head)
1984/1985 1985/1986 1986/1987 1987/1988 1988/1989 1989/1990 1990/1991 1991/1992 1992/1993 1993/1994 1994/1995 1995/1996 1996/1997
1 1 0 1 0 1 22 28 2 2 3 5 4
1 1 1 1 0 0 20 22 2 2 1 4 3
1 2 5 3 0 1 4 9 4 7 6 5 3
2 92 79 68 19 24 23 44 19 80 62 52 29
1 3 6 2 0 1 9 15 14 21 16 12 6
Total Median
70 2
58 1
50 4
593 44
106 6
of shore. Offshore transects at Banks Peninsula show that in winter we would expect to find about half (46%) as many dolphins in this 800 m wide strip as in summer (Dawson and Slooten, 1988). Field effort was recorded in detail for surveys by Dawson and Slooten and by Bra¨ger (n=577 days total, of which 61 were in winter). Combining this field effort with dolphin availability within the surveyed strip suggests that, if there was no difference in alongshore ranges between summer and winter, about 5% (4.86%) of the sightings should have been made in winter. Of 461 sightings, 14 were made in winter (May–September). This is not significantly different from expected (Log-likelihood ratio test of goodness-of-fit, P=0.10), hence there is no evidence that the dolphins under study went somewhere else in winter. However, the extent and range of their offshore distribution beyond the 800 m strip in winter is unknown. Seven of the individuals were seen year-round, in a consecutive summer and winter. Six of these were seen in the following summer as well (i.e. summer–winter– summer). Observed sighting rates in the five subareas show which parts of the study area were most frequently used by each individual (highlighted in bold in Table 2). Almost half of all individuals (n=15) had highest sighting rates between Birdlings Flat and Timutimu Head (southwest quadrant) whereas none was ever sighted between Godley Head and Wakaroa Point (in the northwest quadrant). Eight individuals had their ranges ‘‘centred’’ in Akaroa Harbour and another eight individuals between Te Ruahine Point and Steep Head (southeast quadrant). Only one of the 32 individuals had its highest sighting rate between Steep Head and Wakaroa Point (northeast quadrant). For 14 individuals the distributions of sightings were not explained by the distribution of sighting effort (see
SE (Steep Head– Te Ruahine Point)
Akaroa Harbour
SW (Timutimu head– Birdlings Flat)
P values in Table 2), indicating site fidelity to a particular area. However, since the same null hypothesis is being tested in each case, Bonferroni adjustment for multiple testing (Rice, 1989) is probably warranted (it seems to be a matter of opinion; Perneger, 1998) and shows that four of these results are significant at a ‘‘tablewide’’ P40.05 level. Sighting rates, after correcting for effort, also differed significantly among the four subareas (ANOVA of the non-zero sighting rates after squareroot-arcsin transformation: F=4.349, df=3, P=0.007). Post hoc Fisher PLSD tests detected significant differences in average sighting rates between the Akaroa Harbour and southwest as well as between Akaroa Harbour and southeast, and also between southwest and northeast subareas (Fig. 3). 3.3. Factors affecting alongshore range Beyond 10 sightings (the criterion for inclusion in this study), along-shore range appears to be largely independent of the number of sightings (Fig. 4). Furthermore, gender did not influence along shore range significantly for the 23 individuals of known sex. The five males had an average range of 27.4 km 5.68 SE whereas 18 females had an average range of 30.4 km 3.21 SE (t= 0.448, P=0.67, n.s.). 3.4. Site fidelity in Akaroa Harbour All 32 individuals were seen in Akaroa Harbour repeatedly in different years. Fidelity to this subarea can be expressed as the proportion of summers with sightings in Akaroa Harbour out of all years the animal was known to be alive (i.e. between its first and last sighting). On average, each individual was seen in Akaroa
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S. Bra¨ger et al. / Biological Conservation 108 (2002) 281–287 Table 2 Sighting rates of the 32 individuals in the five subareas, correcting for effort Individual
Number of times sighted
Number of seasons from first to last sighting
NW
NE
SE
Akaroa Harbour
SW
Goodness-of-fit P-value (2-tailed)
BB12.020 FL67.050 FS67.840 FSQ67.030 FSV.065 FSV.770 FS67.890 FSV.028 FSV.315 FSV.370 FL67.010 FL67.160 FL67.240 FSL67.015 F67.110 FSQ67.120 FSV.350 F13.010 FL45.020 FSV.340 FL67.100 FSV.265 FL13.050 FL67.210 FSV.310 FSL45.060 FSL45.010 FL13.040 FSV.305 FSL67.017 FL67.020 FSV.010
10 10 10 10 10 10 11 11 11 11 12 12 12 12 13 13 13 14 14 14 15 15 17 17 18 21 22 25 26 28 29 40
2 2 5 12 10 8 10 5 7 11 6 12 10 5 3 11 10 13 4 12 10 10 7 9 5 11 3 11 8 11 13 12
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0.019 0 0 0.021 0.067 0 0 0 0 0 0 0.018 0 0.017 0 0 0.020 0 0 0 0 0 0 0 0 0 0 0
0 0 0.080 0.061 0.023 0 0 0 0.042 0 0 0 0.024 0.059 0.100 0.021 0.024 0.040 0.200 0.043 0 0 0.028 0.029 0.091 0.022 0.300 0 0.030 0.064 0 0
0.115 0.058 0.033 0.010 0.017 0.024 0.016 0.018 0.020 0.012 0.036 0.008 0.022 0.062 0.038 0.014 0.020 0.015 0.047 0.011 0.027 0.009 0.046 0.033 0.057 0.021 0.063 0.032 0.053 0.032 0.032 0.063
0 0 0 0.010 0 0.020 0.034 0.080 0 0.040 0.048 0.067 0 0 0.273 0.039 0.023 0.019 0 0.060 0 0.115 0.023 0.014 0.083 0.081 0.364 0.071 0.088 0.088 0.094 0.029
0.7039 0.5901 0.0866 0.0589 0.2047 0.1749 0.1309 0.0207 0.0952 0.0635 0.2642 0.0012* 0.0791 0.0131 0.0912 0.2169 0.2715 0.3410 0.2695 0.0057 0.0414 <0.0001* 0.0334 0.0902 0.8150 0.0039 0.0138 0.0039 0.0082 0.0019 0.0003* 0.0002*
Goodness-of-fit of the geographical distribution of sightings to the geographical distribution of effort was tested via 2*5 log-likelihood ratio tests (Significant values in bold: *=significant after Bonferroni adjustment).
Harbour during 65% of all summers. Some individuals (n=6) were resighted there every summer of their known life. This indicates strong site fidelity even to a relatively small part of the known range of movement. However, no dolphin was seen exclusively in the harbour (Table 2).
4. Discussion The results of this study indicate that individual Hector’s dolphins at Banks Peninsula show high site fidelity, and have typical alongshore ranges of less than 60 km. Most ranges were heavily influenced by single ‘outliers’ (distant sightings). Burt (1943, p. 351), who is widely credited with developing the home-range concept, cautioned that connecting ‘‘the outlying points gives a false impression of the actual area covered. . .[and] may indicate a larger range than actually exists.’’ If the 10% farthest sightings are disregarded,
the average alongshore range is reduced by about 40% to 18.8 km (SE=1.66). Some of the larger ranges may be inflated as a result of sightings over more than a decade, with possible long-term shifts in range. There is no evidence of seasonal migration, other than an offshore shift in distribution in winter (Dawson and Slooten, 1988, Bra¨ger, 1998). Two shortcomings of our study are that our results do not reveal the route by which dolphins reached the locations where we saw them, and our relatively small sample size in winter. Of immediate management interest is that even the largest range (106 km) is rather small. This suggests that a relatively small sanctuary, such as that at Banks Peninsula (113 km of open coast, ignoring the harbours and bays) would be expected to contain the normal alongshore range of individual dolphins whose home range includes the central part of the sanctuary. Dolphins normally living near the edges of the sanctuary would continue to be at risk of entanglement immediately outside the sanctuary boundaries.
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Fig. 2. Frequency distribution of individual alongshore ranges of 32 Hector’s dolphins sighted 510 times around Banks Peninsula.
Fig. 3. Differences in mean sighting rates of 32 individual Hector’s dolphins among four subareas of Banks Peninsula (Fisher PLSD test: =significant at P40.05).
To reduce bycatch within the critically endangered population of North Island Hector’s dolphins, in August 2001 the Minister of Fisheries closed most of their habitat to gillnetting. Though this closure is currently being challenged via court action, it seems likely that area closures will remain a important tool in the management of Hector’s dolphin bycatch. Data on alongshore movement can help set the boundaries of such closures. The mean alongshore range observed at Banks Peninsula was just over 30 km. If North Island Hector’s dolphins move on a similar scale to those at Banks Peninsula, it is clear that any protected area should extend alongshore for at least 30 km beyond
Fig. 4. Relationship of alongshore range and the number of sightings among 32 Hector’s dolphins.
where they are normally seen. These estimates of alongshore range also suggest that localised mortality, such as bycatch in fisheries, could profoundly impact the local dolphins while having negligible impact on those 100 km away. Of course this also means that there is limited potential for local depletion to be replenished from nearby populations. Data on movement range and site fidelity are of obvious evolutionary interest. Considering that mating occurs mainly in summer (personal observation), extreme site fidelity over many summer seasons, together with small ranges of movement, could act to restrict gene flow between populations and lead to genetic isolation (Mettler et al., 1988). Indeed, Pichler et al. (1998) found three distinct populations of Hector’s dolphins, on east and west coasts of the South Island and in North Island waters. The few samples available from the south coast of the South Island suggest that these animals are genetically different also (Pichler and Baker, 2000). Very limited alongshore movement is very likely to be the mechanism by which these genetic differences have arisen.
Acknowledgements This study was possible due to financial support from Reckitt Benckiser Ltd., University of Otago (Postgraduate Scholarship and Divisional Grant), Department of Conservation, New England Aquarium, Greenpeace, and Cetacean Society International. We thank several volunteers for their help on the water and in the lab. David Fletcher, Leszek Karczmarski, Franz Pichler, Martin Thiel and Bernd Wu¨rsig provided helpful comments on earlier versions of the manuscript. For
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help in the field GS and AY thank Alistair Hutt and SS thanks Dave Tattle and Rennie Bishop.
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Site fidelity and along-shore range in Hector’s dolphin, an endangered marine dolphin from New Zealand Stefan Bra¨gera,1, Stephen M. Dawsona,*, Elisabeth Slootenb, Susan Smithc, Gregory S. Stoned, Austen Yoshinagad a
Department of Marine Science, University of Otago, PO Box 56, Dunedin, New Zealand b Department of Zoology, University of Otago, PO Box 56, Dunedin, New Zealand c 32A Winchester Street, Lyttelton, Christchurch, New Zealand d New England Aquarium, Central Wharf, Boston, Massachusetts, USA
Received 27 July 2001; received in revised form 13 March 2002; accepted 15 March 2002
Abstract To document site fidelity and the alongshore range of individual Hector’s dolphins we analysed sightings of 32 photographically identified dolphins, each seen 510 times at Banks Peninsula, New Zealand, between 1985 and 1997. The furthest two sightings of an individual were 106 km apart. All other individuals ranged over less than 60 km (x=31.0 km, SE=2.43) of coastline. Gender did not significantly influence alongshore range (female x=30.4 km, SE =3.21, n=18; male x=27.4 km, SE=5.68, n=5). Site fidelity was high: for example, on average, individuals were seen in Akaroa Harbour for about two thirds of the years they were known to be alive. These data suggest that impacts on Hector’s dolphins are most appropriately managed on a small spatial scale. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Cephalorhynchus hectori; Hector’s dolphin; Movement; Site fidelity; Management
1. Introduction Of marine cetaceans, Hector’s dolphin Cephalorhynchus hectori has an extraordinarily restricted distribution. The species is found only in New Zealand, and is fragmented into at least three genetically distinct populations (Pichler et al., 1998; Pichler and Baker, 2000). The species’ small size and inshore distribution make it highly vulnerable to coastal gillnetting, especially that for school shark Galeorhinus galeus, rig Mustelus lenticulatus and elephant fish Callorhinchus milii (Dawson, 1991). Gillnet mortality has been high and unsustainable in the Canterbury region from about 1970 until at least 1988 (Slooten and Lad, 1991; Dawson and Slooten, 1993; Slooten et al., 2000), when the Banks Peninsula Marine Mammal Sanctuary was established. This sanctuary is an 1170 km2 area in which * Corresponding author. E-mail addresses: [email protected] (S. Bra¨ger); steve.dawson@ stonebow.otago.ac.nz (S.M. Dawson). 1 Present address: Scharstorfer Weg 12, D-24211 Schellhorn, Federal Republic of Germany.
commercial gillnetting is illegal, and amateur gillnetting is subject to tighter regulation than elsewhere in New Zealand. Bycatch continues in the Canterbury region immediately outside the sanctuary (Baird and Bradford, 2000) and elsewhere in the species’ range (Dawson et al., 2001). Population viability analyses (Martien et al., 1999; Slooten et al., 2000) indicate that, throughout much of its range, gillnetting is causing population decline. For these reasons the conservation status of Hector’s dolphins has recently been revised to ‘‘Endangered’’ (IUCN, 2000). The North Island population is listed as ‘‘Critically Endangered’’ (IUCN, 2000; see Dawson et al., 2001, for review). Whether they focus on establishing gillnet-free sanctuaries (Dawson and Slooten, 1993), employ gear modifications or pingers to reduce bycatch (e.g. Hembree and Harwood, 1987; Kraus et al., 1997; Stone et al., 1997, 2000) or set limits on allowable bycatch (Wade, 1998), efforts to manage bycatch are usually spatially based. Obviously, sanctuaries should be sized so that they contain the movement range of the individual animals for which protection is sought. The issue
0006-3207/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(02)00124-6
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of scale is a two-edged sword, however; there are also penalties if the scale of management is too big. For example, if the spatial scale of management is larger than the scale of the animals’ movement, and the impacts are not uniformly distributed, some population units may be overexploited while others remain virtually unimpacted (Wade and Angliss, 1997). A local population of Hector’s dolphins around Banks Peninsula, on the east coast of the South Island, has been studied continuously since 1984/1985 (e.g. Slooten and Dawson, 1988; Stone and Yoshinaga, 2000). Hector’s dolphins are most frequently seen within the first kilometre from shore. Offshore distribution in summer is well known, thanks to offshore transects undertaken by small boat (Dawson and Slooten, 1988), aircraft (Brown et al., 1992) and most recently by an intensive line-transect survey based from a 15-m catamaran (Dawson et al., 2000). There are data showing diurnal movement of Hector’s dolphins in Akaroa Harbour. Stone et al. (1995) reported that dolphins tended to be seen entering Banks Peninsula’s Akaroa Harbour in the morning, and leaving in the late evening. Winter off-shore distribution is less well known but is presently under investigation. In order to manage bycatch impacts there is a pressing need for information on how far individuals range alongshore. The purpose of this contribution is to add some of this missing information for the Banks Peninsula region by analysing repeated sightings of 32 frequently observed individuals and how far they ranged along shore. These data will provide some guidance on what would be an appropriate spatial scale for the management of Hector’s dolphins in this region.
2. Methods 2.1. Fieldwork Photo-identification surveys were conducted in the inshore waters of Banks Peninsula (Fig. 1), South Island of New Zealand, between November 1984 and March 1997, following the procedure described by Slooten et al. (1992). For the purposes of this paper, the study area was from Godley Head to Birdlings Flat (Fig. 1). Surveys were conducted at speeds of 10–15 knots in small (3.9–6.6 m) outboard-powered boats. Survey effort was concentrated within 1 km of the shore. On the open coast, we followed a standardised alongshore pattern (described in Dawson and Slooten, 1988). In Akaroa Harbour, we either followed a standardised zigzag route (described in Dawson, 1991) or proceeded opportunistically from one dolphin group to the next. On sighting dolphin groups, the boat was stopped or slowed to idling speed to photograph any distinctive individuals present. A relatively low proportion of
Hector’s dolphins in this area is reliably identifiable (Slooten et al., 1993), and only identifiable individuals were photographed. Details of the photo-ID survey methods, ID categories, and matching procedures are given by Slooten et al. (1992). To reduce the possibility of matching errors, only those individuals with very obvious identifying marks (category 1 or 2 as in Slooten et al., 1992) have been used. Resightings reported here come exclusively from photographs, not from individuals recognised in the field without being photographed. Photographs were taken at close range (usually < 7 m). Since 1990, Nikon F4s, N8008s, and N90s autofocus cameras have been used with lenses ranging from 35 to 300 mm, most frequently a Nikkor AF 80–200 mm f2.8 ED lens. Most photographs were taken on Kodak TMAX 400, Kodachrome 64, or Fujichrome 100 films. Locations of sightings were determined from coastal charts, or, since 1990, using Global Positioning System navigation equipment. 2.2. Sighting analysis For this study, we analysed the distribution of 32 individuals with 510 sightings between 1984/1985 and 1996/1997 (total number of these sightings=506). This dataset includes our sightings of ten individuals compiled by Stone (1992). Survey effort varied considerably within and between years (Table 1). Up to 1990, fieldwork by Dawson and Slooten focused predominantly on Akaroa Harbour. After Smith’s (1992) study in the northern half of the Banks Peninsula study area (1990– 1992), fieldwork by Bra¨ger, Dawson, Slooten, Stone, and Yoshinaga was mainly concentrated in the coastal waters south of the peninsula between Steep Head and Birdlings Flat (Table 1). Most data were collected during summer. There was substantial field effort only in the winters of 1985–1986 and 1994–1996. Alongshore ranges were estimated by calculating the shortest linear distances between the two most extreme sightings without crossing land (i.e. ‘‘observed range length’’; Lehner, 1996). Uneven distribution of survey effort over the years and the five subareas (Table 1) could bias the estimation of movement ranges. To correct for this, we used survey effort data to calculate expected sighting rates for each individual by subarea. The number of sightings in each of the five subareas (northwest, northeast, southeast, Akaroa Harbour, and southwest; see Fig. 1) was divided by the number of complete survey trips (max. one per day) within each area while the individual was known to be alive (i.e. from the season with its first sighting to the one with its last; Table 2). Observed sighting rates were tested against the expected values using a 2*5 Log-likelihood ratio test of goodness-of-fit (Sokal and Rohlf, 1995). The boundaries of subareas
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Fig. 1. Map of the Banks Peninsula study area showing places names and quadrants.
3. Results
from 53 to 106 km. Excluding this sighting, the 32 ranges were on average 31.0 km 2.43 SE long (Fig. 2). All individuals were sighted in Akaroa Harbour several times (max.=37 sightings). For 28 of the 32 individuals the majority of sightings was made in Akaroa Harbour.
3.1. Size of Hector’s dolphin movement ranges
3.2. Potential impact of variation in effort
Distances between the most extreme sightings of the same individual were between 8 km (i.e. all sightings near Akaroa Harbour) and 106 km. The most extreme sighting, however, was of one individual which was observed outside the study area once. This increased the greatest distance between its most extreme sightings
The distribution of effort (Table 1) introduces a geographical and seasonal bias into the distribution of sightings. Most sightings of the 32 individuals occurred between late austral spring and early autumn (November–March). Off the open coasts of Banks Peninsula almost all photo-identification effort was within 800 m
reflect a typical day’s survey effort (i.e. a day’s field effort would typically be confined to one subarea).
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Table 1 Survey effort over 13 years showing number of days on the water in the four quadrants of the study area (NW–NE–SE–SW) plus Akaroa Harbour NW (Godley Head– Wakaroa Point)
NE (Wakaroa Point– Steep Head)
1984/1985 1985/1986 1986/1987 1987/1988 1988/1989 1989/1990 1990/1991 1991/1992 1992/1993 1993/1994 1994/1995 1995/1996 1996/1997
1 1 0 1 0 1 22 28 2 2 3 5 4
1 1 1 1 0 0 20 22 2 2 1 4 3
1 2 5 3 0 1 4 9 4 7 6 5 3
2 92 79 68 19 24 23 44 19 80 62 52 29
1 3 6 2 0 1 9 15 14 21 16 12 6
Total Median
70 2
58 1
50 4
593 44
106 6
of shore. Offshore transects at Banks Peninsula show that in winter we would expect to find about half (46%) as many dolphins in this 800 m wide strip as in summer (Dawson and Slooten, 1988). Field effort was recorded in detail for surveys by Dawson and Slooten and by Bra¨ger (n=577 days total, of which 61 were in winter). Combining this field effort with dolphin availability within the surveyed strip suggests that, if there was no difference in alongshore ranges between summer and winter, about 5% (4.86%) of the sightings should have been made in winter. Of 461 sightings, 14 were made in winter (May–September). This is not significantly different from expected (Log-likelihood ratio test of goodness-of-fit, P=0.10), hence there is no evidence that the dolphins under study went somewhere else in winter. However, the extent and range of their offshore distribution beyond the 800 m strip in winter is unknown. Seven of the individuals were seen year-round, in a consecutive summer and winter. Six of these were seen in the following summer as well (i.e. summer–winter– summer). Observed sighting rates in the five subareas show which parts of the study area were most frequently used by each individual (highlighted in bold in Table 2). Almost half of all individuals (n=15) had highest sighting rates between Birdlings Flat and Timutimu Head (southwest quadrant) whereas none was ever sighted between Godley Head and Wakaroa Point (in the northwest quadrant). Eight individuals had their ranges ‘‘centred’’ in Akaroa Harbour and another eight individuals between Te Ruahine Point and Steep Head (southeast quadrant). Only one of the 32 individuals had its highest sighting rate between Steep Head and Wakaroa Point (northeast quadrant). For 14 individuals the distributions of sightings were not explained by the distribution of sighting effort (see
SE (Steep Head– Te Ruahine Point)
Akaroa Harbour
SW (Timutimu head– Birdlings Flat)
P values in Table 2), indicating site fidelity to a particular area. However, since the same null hypothesis is being tested in each case, Bonferroni adjustment for multiple testing (Rice, 1989) is probably warranted (it seems to be a matter of opinion; Perneger, 1998) and shows that four of these results are significant at a ‘‘tablewide’’ P40.05 level. Sighting rates, after correcting for effort, also differed significantly among the four subareas (ANOVA of the non-zero sighting rates after squareroot-arcsin transformation: F=4.349, df=3, P=0.007). Post hoc Fisher PLSD tests detected significant differences in average sighting rates between the Akaroa Harbour and southwest as well as between Akaroa Harbour and southeast, and also between southwest and northeast subareas (Fig. 3). 3.3. Factors affecting alongshore range Beyond 10 sightings (the criterion for inclusion in this study), along-shore range appears to be largely independent of the number of sightings (Fig. 4). Furthermore, gender did not influence along shore range significantly for the 23 individuals of known sex. The five males had an average range of 27.4 km 5.68 SE whereas 18 females had an average range of 30.4 km 3.21 SE (t= 0.448, P=0.67, n.s.). 3.4. Site fidelity in Akaroa Harbour All 32 individuals were seen in Akaroa Harbour repeatedly in different years. Fidelity to this subarea can be expressed as the proportion of summers with sightings in Akaroa Harbour out of all years the animal was known to be alive (i.e. between its first and last sighting). On average, each individual was seen in Akaroa
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S. Bra¨ger et al. / Biological Conservation 108 (2002) 281–287 Table 2 Sighting rates of the 32 individuals in the five subareas, correcting for effort Individual
Number of times sighted
Number of seasons from first to last sighting
NW
NE
SE
Akaroa Harbour
SW
Goodness-of-fit P-value (2-tailed)
BB12.020 FL67.050 FS67.840 FSQ67.030 FSV.065 FSV.770 FS67.890 FSV.028 FSV.315 FSV.370 FL67.010 FL67.160 FL67.240 FSL67.015 F67.110 FSQ67.120 FSV.350 F13.010 FL45.020 FSV.340 FL67.100 FSV.265 FL13.050 FL67.210 FSV.310 FSL45.060 FSL45.010 FL13.040 FSV.305 FSL67.017 FL67.020 FSV.010
10 10 10 10 10 10 11 11 11 11 12 12 12 12 13 13 13 14 14 14 15 15 17 17 18 21 22 25 26 28 29 40
2 2 5 12 10 8 10 5 7 11 6 12 10 5 3 11 10 13 4 12 10 10 7 9 5 11 3 11 8 11 13 12
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0.019 0 0 0.021 0.067 0 0 0 0 0 0 0.018 0 0.017 0 0 0.020 0 0 0 0 0 0 0 0 0 0 0
0 0 0.080 0.061 0.023 0 0 0 0.042 0 0 0 0.024 0.059 0.100 0.021 0.024 0.040 0.200 0.043 0 0 0.028 0.029 0.091 0.022 0.300 0 0.030 0.064 0 0
0.115 0.058 0.033 0.010 0.017 0.024 0.016 0.018 0.020 0.012 0.036 0.008 0.022 0.062 0.038 0.014 0.020 0.015 0.047 0.011 0.027 0.009 0.046 0.033 0.057 0.021 0.063 0.032 0.053 0.032 0.032 0.063
0 0 0 0.010 0 0.020 0.034 0.080 0 0.040 0.048 0.067 0 0 0.273 0.039 0.023 0.019 0 0.060 0 0.115 0.023 0.014 0.083 0.081 0.364 0.071 0.088 0.088 0.094 0.029
0.7039 0.5901 0.0866 0.0589 0.2047 0.1749 0.1309 0.0207 0.0952 0.0635 0.2642 0.0012* 0.0791 0.0131 0.0912 0.2169 0.2715 0.3410 0.2695 0.0057 0.0414 <0.0001* 0.0334 0.0902 0.8150 0.0039 0.0138 0.0039 0.0082 0.0019 0.0003* 0.0002*
Goodness-of-fit of the geographical distribution of sightings to the geographical distribution of effort was tested via 2*5 log-likelihood ratio tests (Significant values in bold: *=significant after Bonferroni adjustment).
Harbour during 65% of all summers. Some individuals (n=6) were resighted there every summer of their known life. This indicates strong site fidelity even to a relatively small part of the known range of movement. However, no dolphin was seen exclusively in the harbour (Table 2).
4. Discussion The results of this study indicate that individual Hector’s dolphins at Banks Peninsula show high site fidelity, and have typical alongshore ranges of less than 60 km. Most ranges were heavily influenced by single ‘outliers’ (distant sightings). Burt (1943, p. 351), who is widely credited with developing the home-range concept, cautioned that connecting ‘‘the outlying points gives a false impression of the actual area covered. . .[and] may indicate a larger range than actually exists.’’ If the 10% farthest sightings are disregarded,
the average alongshore range is reduced by about 40% to 18.8 km (SE=1.66). Some of the larger ranges may be inflated as a result of sightings over more than a decade, with possible long-term shifts in range. There is no evidence of seasonal migration, other than an offshore shift in distribution in winter (Dawson and Slooten, 1988, Bra¨ger, 1998). Two shortcomings of our study are that our results do not reveal the route by which dolphins reached the locations where we saw them, and our relatively small sample size in winter. Of immediate management interest is that even the largest range (106 km) is rather small. This suggests that a relatively small sanctuary, such as that at Banks Peninsula (113 km of open coast, ignoring the harbours and bays) would be expected to contain the normal alongshore range of individual dolphins whose home range includes the central part of the sanctuary. Dolphins normally living near the edges of the sanctuary would continue to be at risk of entanglement immediately outside the sanctuary boundaries.
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Fig. 2. Frequency distribution of individual alongshore ranges of 32 Hector’s dolphins sighted 510 times around Banks Peninsula.
Fig. 3. Differences in mean sighting rates of 32 individual Hector’s dolphins among four subareas of Banks Peninsula (Fisher PLSD test: =significant at P40.05).
To reduce bycatch within the critically endangered population of North Island Hector’s dolphins, in August 2001 the Minister of Fisheries closed most of their habitat to gillnetting. Though this closure is currently being challenged via court action, it seems likely that area closures will remain a important tool in the management of Hector’s dolphin bycatch. Data on alongshore movement can help set the boundaries of such closures. The mean alongshore range observed at Banks Peninsula was just over 30 km. If North Island Hector’s dolphins move on a similar scale to those at Banks Peninsula, it is clear that any protected area should extend alongshore for at least 30 km beyond
Fig. 4. Relationship of alongshore range and the number of sightings among 32 Hector’s dolphins.
where they are normally seen. These estimates of alongshore range also suggest that localised mortality, such as bycatch in fisheries, could profoundly impact the local dolphins while having negligible impact on those 100 km away. Of course this also means that there is limited potential for local depletion to be replenished from nearby populations. Data on movement range and site fidelity are of obvious evolutionary interest. Considering that mating occurs mainly in summer (personal observation), extreme site fidelity over many summer seasons, together with small ranges of movement, could act to restrict gene flow between populations and lead to genetic isolation (Mettler et al., 1988). Indeed, Pichler et al. (1998) found three distinct populations of Hector’s dolphins, on east and west coasts of the South Island and in North Island waters. The few samples available from the south coast of the South Island suggest that these animals are genetically different also (Pichler and Baker, 2000). Very limited alongshore movement is very likely to be the mechanism by which these genetic differences have arisen.
Acknowledgements This study was possible due to financial support from Reckitt Benckiser Ltd., University of Otago (Postgraduate Scholarship and Divisional Grant), Department of Conservation, New England Aquarium, Greenpeace, and Cetacean Society International. We thank several volunteers for their help on the water and in the lab. David Fletcher, Leszek Karczmarski, Franz Pichler, Martin Thiel and Bernd Wu¨rsig provided helpful comments on earlier versions of the manuscript. For
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help in the field GS and AY thank Alistair Hutt and SS thanks Dave Tattle and Rennie Bishop.
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