The New Zealand common dolphin (Delphinus sp.) : identity, ecology and conservation : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Zoology, Massey University, Auckland, New Zealand
Common dolphins (genus Delphinus) are poorly understood within New Zealand
waters. Prior to this study, most information relating to the taxonomy, population
structure, diet and pollutant loads of this genus relied upon untested assumptions.
Furthermore, factors affecting the occurrence, demographics and habitat use of common
dolphins in the Hauraki Gulf remained unknown. This lack of empirical data has
resulted in the inadequate recognition and management of New Zealand Delphinus.
Inappropriately classified by the New Zealand Threat Classification System, the
anthropogenic impacts that affect this genus have clearly been overlooked. The present
study examines behaviour of common dolphins in the Hauraki Gulf and details analyses
undertaken on tissue samples collected from around New Zealand. Results detailed
here challenge many of the untested assumptions about this genus within New Zealand
The taxonomy of New Zealand common dolphins was assessed using 92 samples
analysed for 577 base pairs (bps) of the mtDNA control region (D-loop). New Zealand
samples were compared with 177 published sequences from eight other populations
from around the world. New Zealand Delphinus exhibited a high genetic variability,
sharing haplotypes with both short- (D. delphis) and long-beaked (D. capensis)
populations. Indeed, the New Zealand population showed significant genetic
differentiation when compared with most other populations world-wide. Furthermore,
intrapopulation analyses revealed significant genetic differentiation between Hauraki
Gulf individuals and other common dolphins sampled within New Zealand waters.
Results suggest habitat choice and site fidelity may play a role in shaping the
fragmented population structure of New Zealand Delphinus.
Data relating to the occurrence and demographics of common dolphins in the Hauraki
Gulf region were collected during boat-based surveys between February 2002 and
January 2005. In total, 719 independent encounters, involving one to > 300 common
dolphins were recorded. Dolphin presence was significantly affected by month, latitude
and depth. Group size varied significantly by month, season, depth, sea surface
temperature (SST) and latitude, and was highly skewed towards smaller groups
comprising fewer than 50 animals. Calves were observed throughout the year but were
most prevalent in the austral summer months of December and January. Group
composition was significantly affected by month, season, depth and SST. The yearround
occurrence and social organisation of Delphinus in Hauraki Gulf waters suggest
this region is an important nursery and potential calving area.
The effects of diel, season, depth, sea surface temperature, and group size and
composition on dolphin behaviour were investigated using activity budgets. Foraging
and social were the most and least frequently observed behaviours, respectively. A
correlation between group size and behaviour was evident, although behaviour did not
vary with the composition of dolphin groups. Resting, milling and socialising animals
were more frequently observed in smaller groups. Foraging behaviour was prevalent in
both small and large groups, suggesting foraging plasticity exists within this population.
Behaviour differed between single- and multi-species groups, with foraging more
frequent in mixed-species aggregations, indicating the primary mechanism for
association is likely prey-related.
Stomach contents analysed for forty-two stranded and eleven commercially by-caught
individuals collected from around North Island, New Zealand between 1997 and 2006,
revealed arrow squid (Nototodarus spp.), jack mackerel (Trachurus spp.) and anchovy
(Engraulis australis) as the most prevalent prey. Stranded individuals and dolphins bycaught
within neritic waters fed on both neritic and oceanic prey. Moreover, a mixed
prey composition was evident in the diet of common dolphins by-caught in oceanic
waters, suggesting inshore/offshore movements of New Zealand Delphinus on a diel
basis. Additionally, prey differences were also evident in the stomach contents of
common dolphins sampled from within the Hauraki Gulf.
Trace elements, polychlorinated biphenyls (PCBs) and organochlorine (OC) pesticide
levels were determined in five stranded and fourteen by-caught Delphinus sampled from
around New Zealand between 1999 and 2005. Generally, levels of trace elements were
low. However, concentrations of OC pesticides were similar in range to those
previously reported for Hector’s (Cephalorhyncus hectori) and common bottlenose
dolphins (Tursiops truncatus). Organochlorine pesticides dieldrin, hexachlorobenzene
(HCB), o,p’-DDT and p,p’-DDE were present at the highest concentrations.
Markov chain models were used to assess the impact of tourism activities on Delphinus
within the Hauraki Gulf. Foraging and resting bouts were significantly disrupted by
boat interactions. Both the duration of bouts and the time spent in these two
behavioural states decreased during boat interactions. Additionally, foraging dolphins
took significantly longer to return to their initial behavioural state in the presence of a
tour boat. Impacts identified are similar to those previously reported for the common
bottlenose dolphin, a coastal species typically considered to be more susceptible to
cumulative anthropogenic impacts.
Data presented here reveal the nature and apparent susceptibility of New Zealand
common dolphins to human-induced impacts, namely fisheries by-catch, pollution and
tourism. This in conjunction with taxonomic uncertainty, lack of abundance estimates
and the year-round use of inshore waters for feeding, clearly warrants immediate
attention from managers. Furthermore, the current threat classification of New Zealand
Delphinus should be reconsidered in light of population uncertainties, and in view of the
susceptibly to human-induced impacts revealed by the present study.