The ecology and conservation of green turtles (Chelonia mydas) in New Zealand : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Marine Ecology at Massey University, Albany, New Zealand
Historically, little consideration has been given to the occurrence, ecology or
conservation of the green turtle (Chelonia mydas) in temperate New Zealand.
Located geographically at the southern boundary of the distributional range of
green turtles in the southwestern Pacific, reports of this species in New Zealand
are often overlooked as occasional visitors or stragglers incidentally carried by
ocean currents. This convention may be reasonable when considering the
temperature constrained distribution of this poikilothermic marine reptile. Despite
this, green turtles have been reported in New Zealand waters for more than 100
years, yet no study has undertaken any in depth investigation as to their
occurrence in this region. Therefore, this thesis investigated the presence of green
turtles in New Zealand waters to test the hypothesis that their occurrence is
ephemeral and incidental.
Opportunistic data and samples collected between 1895 and 2013 was collated,
reviewed and analysed to investigate several lines of empirical enquiry, including
spatio-temporal distribution, population structure, genetic origin, diet composition
and anthropogenic effects. Sighting, stranding, and incidental capture revealed a
year round presence of post-pelagic immature juveniles to large sub-adult green
turtles across northern New Zealand (ca. 34°-38° S). Such occurrence exists
despite sea surface temperatures averaging only 14 C during austral winters.
The aggregation exhibited a female:male sex ratio of 1.7:1 which is similar to that
reported from proximate warm temperate foraging grounds in eastern Australia.
Size frequency data indicated that green turtles recruit to neritic habitats of the
North Island at ca. 40.8 cm curved carapace length. This reflects a natural postoceanic
settlement pattern rather than oceanic-phase stragglers incidentally blown
ashore by storm and other stochastic events. Supporting this rationale for natural
recruitment, diet component data demonstrates that once green turtles settle into
New Zealand’s nearshore coastal habitats, they transition to a benthic foraging
strategy. Notably, green turtles in New Zealand do not ontogenetically transition
from omnivory to obligate herbivory with age, but instead consume a variable diet
of primarily macroalgae and benthic macro invertebrates. Overall, the confirmation
of feeding in New Zealand substantially extends the southern foraging limit for green turtles in the Pacific Ocean.
Genetic analyses of ~770 base pair sequences of mitochondrial (mt) DNA was conducted on 42 stranded green turtles to characterize the genetic structure of this aggregation. Results identified 15 haplotypes including one orphan haplotype from widely dispersed green turtle stocks across the western, central, and eastern Pacific Ocean. When compared to other regional nesting rookeries and foraging grounds, the New Zealand aggregation exemplified its unique composition, predominantly due to the large proportion of haplotypes from the endemic eastern Pacific clade. These results provide a genetic link to east Pacific stocks in the southwestern Pacific; identifying previously undefined regional connectivity and trans-oceanic dispersal for eastern Pacific green turtles.
In order to assess potential human impacts, gross necropsies were conducted on green turtles found stranded in northern New Zealand between 2007 and 2013. Anthropogenic effects predominantly associated with the ingestion of plastic marine debris were identified as the likely cause for the majority of strandings in the North Island. Propeller strike and incidental capture in recreational fisheries were further shown to impact green turtles, particularly for turtles inhabiting neritic habitats adjacent to densely populated urban centres of northeastern New Zealand.
Overall, data presented here supports the hypothesis that New Zealand northern neritic habitats provide a transitional developmental habitat for immature green turtles at the edge of their range in the southwestern Pacific Ocean. Genetic analysis reveals this aggregation is unique when compared to other regional foraging grounds, exhibiting links to discrete genetic stocks from across the Pacific Ocean. In addition, the cause of the exponential increase in records observed over time remains unclear, therefore warrants further research and monitoring of this endangered marine reptile; particularly in light of climate-mediated environmental change presently experienced in the region.