The occurrence and habitat use of common dolphins (Delphinus sp.) in the central Bay of Plenty, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Conservation Biology, Massey University, Auckland, New Zealand
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Date
2012
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Massey University
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Abstract
This thesis investigated the occurrence and habitat use of common dolphins (Delphinus sp.)
inhabiting the central Bay of Plenty (BOP), North Island, New Zealand. Although common
dolphins are the most prevalent species of dolphin found off the east coast of the North Island
of New Zealand, there has been no long term empirical investigation of the species in the
central BOP area. Behaviour of common dolphins in the presence of the observation platform
was also assessed to investigate habitat use. Additionally, data describing other cetacean
species occurring within this region are presented in order to place into context the
importance of these waters for Delphinus. Sightings data were collected between March 1998
and May 2011 during 2364 boat-based surveys on board platform of opportunity, tourism
vessel Gemini Galaxsea (a 60 ft ketch sailboat). Variables examined included location, group
size, composition, water depth, time of day, seasonality, behaviour and the presence of
associated species.
Common dolphins were encountered during 54% (n = 1265) of surveys, in water depths
ranging from 5.0 to 197.0 m. Delphinus sightings primarily occurred in the area between
Motiti Island, Mayor Island and Waihi on the mainland. Group size ranged from one to 500+
individuals and was significantly affected by the time of day, month and depth of sightings.
The most frequently recorded group size involved 50 to 100 animals, with larger aggregations
more frequent during the warmer austral months when nutrient upwelling leads to increased
prey availability in coastal waters off the BOP. Groups containing immature animals
accounted for 16% of total sightings and occurred throughout the year, although neonate
calves were only reported during the warmer austral summer months, supporting the concept
of reproductive seasonality in this population. Common dolphin groups sighted within the
central BOP were reported in association with five marine mammal species and 14 avian
species, most frequently with various species of petrel (Procellariiformes) and the
Australasian gannet (Morus serrator). The year round occurrence of common dolphins within
central BOP waters indicates that this region maybe important for Delphinus.
Behavioural data were collected from 162 independent dolphin groups. Overall, forage, social
and travel accounted for the majority of recorded behavioural states, while mill and rest were
less frequent. Behaviour was influenced by water depth, with foraging dolphins encountered
in the deepest waters. Behaviour also varied significantly according to group size, with
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foraging occurring more often than expected in large groups and resting, socialising and
milling occurring more often in smaller groups. The presence of immature animals also had a
significant influence on common dolphin behaviour, with foraging occurring more often than
expected in groups containing immature animals. The presence of associated species varied
according to behaviour, with the majority of foraging groups occurring in the presence of
Australasian gannets. Seasonal and diurnal peaks in behaviour were not evident. Social
behaviours primarily occurred during summer when large nursery groups were also reported
in the central BOP.
In order to place into context the use of central BOP waters for Delphinus, an investigation of
other cetacean species using these waters was undertaken; eleven cetacean species (6
Mysticeti and 5 Delphinidae) were identified as utilising central BOP waters. Sightings
primarily occurred in the area between Karewa Island, Mayor Island (Tuhua) and Motiti
Island in the central BOP. The majority of delphinid sightings occurred in spring and
involved the bottlenose dolphin, Tursiops truncatus (50.5%) and killer whale, Orcinus orca
(42.9%). Pilot, Globicephala spp. (3.8%) and false killer whale, Pseudorca crassidens (2.9%)
sightings were less frequent. Group sizes ranged from solitary to 200+ individuals. Calf
presence was recorded for bottlenose dolphins during summer and autumn and killer whales
during spring and autumn. Pilot and false killer whale calves were not recorded during the
study period. The majority of baleen whale sightings occurred during winter and spring and
involved minke, Balaenoptera acutorostrata/bonaerensis (44.3%), blue, B. musculus (19.0%)
and Bryde’s whales, B. edeni (16.5%). Humpback, Megaptera novaeangliae (10.1%), sei, B.
borealis (8.9%) and southern right whale, Eubalaena australis (1.3%) sightings were less
frequent. Group sizes ranged from solitary to four individuals, with results highly skewed
towards solitary animals (79%). Cow-calf pairs were observed during spring for all baleen
whales except sei and humpback. Cetaceans were primarily observed in association with the
Australasian gannet as well as various species of petrel and shearwater (Puffinus spp.).
Bottlenose dolphins and false killer whales were sighted together in mixed species groups.
Bryde’s whales were also sighted in association with common dolphins.
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Keywords
Dolphins, Bay of Plenty, Dolphin behaviour, New Zealand, Cetacea, New Zealand, Whales, New Zealand