Cephalorhynchus Hectori) in Porpoise Bay, New Zealand
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NewBejder Zealand & Dawson—Hector’s Journal of Marine dolphins and Freshwater in Porpoise Research, Bay 2001, Vol. 35: 277–287 277 0028–8330/01/3502–0277 $7.00 © The Royal Society of New Zealand 2001 Abundance, residency, and habitat utilisation of Hector’s dolphins (Cephalorhynchus hectori) in Porpoise Bay, New Zealand LARS BEJDER INTRODUCTION Department of Biology Hector’s dolphins (Cephalorhynchus hectori van Dalhousie University Beneden 1881) have been studied on a large scale Halifax, Nova Scotia, B3H 4J1 (c. 1000 km) throughout their geographic distribu- Canada tion (Dawson & Slooten 1988; Bräger 1998). On an STEVE DAWSON† intermediate scale (c. 90 km), one regional popula- Department of Marine Science tion (around Banks Peninsula) has been studied in- University of Otago tensively for more than a decade (Slooten & Dawson P. O. Box 56, Dunedin 1994). Little is known about small populations, how- New Zealand ever, or about habitat utilisation at fine scales (<5km). email: [email protected] Hector’s dolphins are restricted to New Zealand, and have a strictly coastal distribution. Despite wide- ranging survey effort, there is no evidence of alongshore movement of more than a few tens of Abstract Theodolite tracking and boat-based kilometres (Slooten et al. 1993; Bräger 1998). Cur- photo-identification surveys were carried out in the rent distribution is highly localised and fragmented austral summers of 1995/96 and 1996/97 to assess into genetically distinct populations (Pichler et al. abundance, residency, and habitat utilisation of 1998). The species is listed by the International Hector’s dolphins (Cephalorhynchus hectori van Union for the Conservation of Nature as “Endan- Beneden 1881) in Porpoise Bay, on the south-east gered” (2000 IUCN red list of threatened species. corner of the South Island of New Zealand. Data are www.redlist.org. International Union for Conserva- consistent with the model of a small resident popu- tion of Nature and Natural Resources, Species Sur- lation that is visited occasionally by members of vival Commission). From both ecological and neighbouring populations. Mark-recapture analysis conservation perspectives, these observations high- of photographically identified individuals, along light the need for an understanding of small, isolated with data on the proportion of animals bearing iden- populations. tifying marks, indicates a local population of 48 dol- This study is based on two research methods that phins (95% CI = 44–55) in 1996/97. Dolphins spent have each proven powerful in studies of the biology a large proportion of their time in a small area in- and behaviour of cetaceans. Photographic identifi- side the bay. Dolphin sightings were more congre- cation of naturally distinctive individuals facilitates gated in successive time periods from early morning studies of movement patterns, social structure, and to late afternoon. No pattern of diurnal movement population parameters such as abundance, age at first into and out of the bay was observed. reproduction, birthing interval, survival rates (e.g., Hammond 1986; Wells et al. 1990; Rugh et al. 1992; Keywords Hector’s dolphin; Cephalorhynchus Slooten et al. 1992; Best et al. 1995). Theodolite hectori; abundance; residency; habitat utilisation; tracking has proven extremely useful for document- New Zealand; theodolite; photo-identification ing small-scale movements (e.g., Würsig & Würsig 1979, 1980) and habitat utilisation (Smith 1993). Because it offers reliable position fixing without disturbance to the study animals, it has also been †Corresponding author. widely used to document responses to various M00008 stimuli, including acoustic alarm devices (Todd et Received 18 February 2000; accepted 20 October 2000 al. 1992; Goodson & Mayo 1995) and boats (Baker 278 New Zealand Journal of Marine and Freshwater Research, 2001, Vol. 35 & Herman 1989; Polacheck & Thorpe 1990; Acevedo 1991; Kruse 1991; Corkeron 1995; Barr 1997; Bejder et al. 1999). Dawson & Slooten (1988) describe a general in- shore movement of Hector’s dolphins in summer time and an offshore movement in winter time. Stone et al. (1995) described a diurnal pattern in which dolphins tended to be seen entering Banks Peninsu- la’s Akaroa Harbour in the morning, and leaving in the late evening. Limited data on habitat utilisation are available from resightings of distinctive individu- als in the long-term study at Banks Peninsula (Slooten & Dawson unpubl. data), but there is no detailed information on movement patterns over time scales of hours to months. In this contribution we provide data on the number of Hector’s dolphins using Porpoise Bay, information on individual resi- dency of dolphins using the bay, and a fine-scale analysis of their distribution within the bay. MATERIALS AND METHODS Study site Porpoise Bay (46°39¢S, 169°6¢E) is situated on the south-east corner of the South Island of New Zea- land and covers an area of c. 4 km2 (Fig. 1). The bay Fig. 1 Map of study area. is confined by volcanic rock headlands on both its north-eastern and south-western boundaries and has a gently sloping sandy beach. An estuary flows into fin and from body blotches and scars (for more de- the north-eastern end of the bay. A semi-submerged tail on our Photo-ID methods see Slooten et al. reef is situated off the south-western headland. Av- 1992). Only surveys with complete coverage of the erage water depth in the bay is c. 12 m and the maxi- study area were included in the following analyses. mum depth is c. 18 m. Fieldwork was conducted in A “group” was defined as dolphins close (<20 m) the Austral summers of 1995/96 (December–April) to each other. Hector’s dolphins are most often ob- and 1996/97 (November–April). served in groups of two to eight, which often fuse together and split up (Slooten & Dawson 1988). Field methods Dolphins were considered part of the same group if they merged in the time span when photographs were Boat-based fieldwork being taken during an encounter. An encounter was Photographic-identification surveys of Hector’s defined as the period spent with the same group. dolphins were conducted from 3.5–6 m boats at slow Different photographic strategies were adopted in planing speeds (10–15 knots), with the vessel slow- the two field seasons. In 1995/96, photographic ef- ing to <3 knots for photography once dolphins were fort was biased towards the more distinctive dol- sighted. Photographs were taken with 400 ASA phins. In 1996/97, photographs were randomly taken black and white film, using autofocus 35 mm cam- of all dolphins that surfaced in the close vicinity of eras with zoom lenses (28–200 mm), at ranges of the boat. At least four exposures were taken per <10 m. Upon encountering dolphins, group size, dolphin, i.e., if group size was 10 dolphins a mini- number of calves, number of juveniles, location, and mum of 40 exposures were taken at random. On time were recorded. Dolphins were individually average, this assures that all individuals in a group identified from photographs of naturally occurring had a better than 95% chance of being photographed marks such as nicks in the trailing edge of the dorsal (Würsig 1978; Ballance 1990). Bejder & Dawson—Hector’s dolphins in Porpoise Bay 279 Theodolite observations The positions of Hector’s dolphins in the study area were determined using a theodolite on a 27.4 m vantage point on the southern headland overlooking Porpoise Bay (Fig. 1). The height of our observation point was measured by leveling (3 times) from a surveyed trig station. Resulting observation height measurements were within 20 cm of each other. The precision of theodolite fixes is proportional to the instrument’s elevation above sea level and inversely proportional to the distance of the acquired fix. At our site a 20 cm error in theodolite height would cause a position error of c. 7 m at a range of 1000 m (Würsig et al. 1991). Fig. 2 Discovery curve of photographically identified Custom-written software (running on a Hewlett- Hector’s dolphins (Cephalorhyncus hectori) in Porpoise Packard 95LX palmtop computer) allowed auto- Bay, New Zealand, in summer 1996/97. mated storage of theodolite fixes and their timing, and allowed keyboard entry of associated observa- tions. Each position was later converted into (x,y) identifying feature as these do not persist over time co-ordinates using the computer program “T-Trak” (Lockyer & Morris 1990). (Cipriano 1990). Population estimation Land-based observations were made over the same two summers as the boat observations, between The first summer’s photo-ID surveys were used pri- 0600 and 1630 h. The study area was scanned at the marily to establish a catalog of individuals. For the beginning of each tracking day with 10-power bin- second season, we estimated abundance via a oculars. In most cases only one group was present; Chapman mark-recapture estimate of the number of if not, the largest of the groups was tracked via the- marked animals (Seber 1982), scaled up using data odolite. This group was then tracked throughout the on the proportion of animals that bore identifying entire observation period using focal group sampling marks. We considered the first half of this field sea- (Martin & Bateson 1993). If the focal group split, son as the “marking” period, and the second half as the larger of the resulting groups was tracked. The- the “recapture” period. odolite fixes were taken from the centre of the dol- This proportion of animals that bore identifying phin group. Fixes more than 60 s apart (<2% of the marks was estimated via randomly photographing total) were discarded from analyses. This was done whichever dolphins came into range, whether or not to ensure that each track was a continuous observa- they were marked (see Williams et al.