Dynamics of mother-offspring common dolphins (Delphinus sp.) engaged in foraging activities in the Hauraki Gulf, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Ecology, Massey University, Auckland, New Zealand
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Date
2010
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Massey University
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Abstract
This study investigated the dynamic of mother-offspring common dolphins (Delphinus sp.)
engaged in foraging activities in the Hauraki Gulf, New Zealand. While previous studies have
demonstrated the importance of Hauraki Gulf waters for feeding and nursing common
dolphins, no previous study has systematically assessed the dynamics of mother-offspring
pairs engaged in foraging activities. During 228 hr of survey time, a total of 49 independent
focal groups were recorded between March and July 2009. All observations were conducted
from the surface using the platform of opportunity, Dolphin Explorer, a 20 m tour boat.
Variables examined included group size, composition, offspring presence, predominant
behavioural state, foraging strategies employed, associated species, mother-offspring
relationship, and location of offspring within groups. This study tested the hypothesis that
foraging strategies employed by mother-offspring pairs would be influenced by
environmental and anthropogenic parameters, group size, and associated species. It also
examined the influence of associated species and foraging strategies on the mother-offspring
dynamic within focal groups.
Sighting Per Unit Effort (SPUE) and Offspring Per Unit Effort (OPUE) varied by month, with
SPUE and OPUE being highest in March and July, respectively. Mother-offspring pairs
predominantly engaged in cooperative foraging strategies (97%), and were less likely to
engage in individual strategies. Cooperative feeding groups likely provide better protection to
young and vulnerable dolphins, as well as a more prolific food supply to lactating females and
their offspring. Water depth, sea surface temperature and group size had no influence on
foraging strategies employed by mother-offspring pairs. Groups comprising immature
dolphins typically altered their behaviour more frequently in presence of vessels, with
presumed mothers keeping their offspring away from boats in 80% of encounters. However, a
limited sample size likely impeded the statistical significance of this observation. Behavioural
changes and potential vessel avoidance during foraging activity may occur as a response of
perceived threat. Cooperative foraging strategies employed by focal groups had no significant
influence on the location of the young. Offspring were observed in echelon position during
93.6% of the time dolphins foraged, and were never separated from their assumed mother
during individual foraging strategies. Echelon position may enhance better predatory
protection, as well as learning during foraging activities.
Mother-offspring pairs were observed in association with Australasian gannets (Morus
serrator) and Bryde’s whales (Balaenoptera brydei) during 68.4% and 8.4% of encounters,
respectively. Prevalence of dolphin cooperative feeding is likely to attract and benefit
associated species. While neither species were found to affect the foraging strategies
employed by mother-offspring pairs, diving gannets did affect the position of motheroffspring
pairs within feeding groups, with offspring typically located on the peripheral edge
of the focal group. Conversely, no such affect was detected for associated feeding involving
Bryde’s whales. Since mothers permit offspring in the centre of mono-specific feeding
groups, it seems plausible that the use of peripheral regions maybe associated with the
presence of mixed-species feeding aggregations. This apparent shift in mother-offspring
position is unexplained, although may represent perceived risk by the assumed mother.
Results presented here extend our knowledge of foraging behaviour and mother-offspring
common dolphin dynamics in the Hauraki Gulf, New Zealand.
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Dolphin behaviour, Mother-offspring relationship