Sea Turtles and Sea Snakes (Class Reptilia)
Hawksbill turtle, Eretmochelys imbricata, at the Kermadec Islands. Photographer Malcolm Francis
Franz Smith
General Description
Marine reptiles represent a group of cold-blooded diving animals of the class Reptilia. Marine reptiles tend to be widely distributed in the world's warmer waters and lay their eggs on tropical islands. Despite a wide distribution, several of these species are under threat as a consequence of human activities. Sea turtles, of the order Testudinata, are believed to have lived on the Earth for 250 million years, before the time of the dinosaurs. These animals are characterised by long, paddle-shaped limbs and a generally reduced shell, making them agile swimmers and divers. Sea snakes, of the order Squamata, are also adapted to life at sea, with a paddle-like tail used for sinusoid swimming.
Seven species of marine turtles and sea snakes occur within the New Zealand region. The frequency of sightings over the years has been correlated with the influx of warmer water associated with the East Australian Current. There are also seasonal trends to the sightings, with peak records occurring between the months of January and May, with fewer records over winter and spring.
Four species of sea turtles of the family Cheloniidae have been registered within the region, including the loggerhead turtle, Caretta caretta, the green turtle, Chelonia mydas, the hawksbill turtle, Eretmochelys imbricata, and the olive ridley turtle, Lepidochelys olivacea. Sea turtles are primarily benthic grazers, where their diet consists primarily of seagrasses, however their diet can also include sponges and jellyfish. Female sea turtles typically reach maturity after 20-25 years and are estimated to reproduce in two- or three-year cycles. Individual clutches of eggs may number between 50 and 150 eggs. The leatherback turtle, the most distinctive species of sea turtle that occurs in the region is included in its own family, Dermochelyidae, and is the largest sea turtle in the world, obtaining a maximum length up to 3 m and weighing over 900 kg. The leatherback is the only sea turtle that is considered to be able to tolerate colder waters, having the ability to regulate its body temperature. Diving depths of up to 1500 m have been documented for adult leatherback turtles.
Two species of sea snake have been recorded in the region, including the yellow-bellied sea snake, Pelamis platurus, family Hydrophiidae) and the banded sea snake, Laticauda colubrina (family Laticauddae). The yellow-bellied sea snake has been recorded most frequently. All of the specimens that have been registered were adult sized, with greater than 50 cm in total length for P. platurus and smaller individuals for L. colubrina, just under 30 cm in total length.
In terms of the frequency of sightings, the most common species that occur within the region are the leatherback turtle and the yellow-bellied sea snake, followed by the green turtle.
Status
Despite a broad distribution pattern for most species, all species of marine turtle are listed as threatened by IUCN classification criteria. Populations of leatherback turtles have declined drastically worldwide, with an estimated 95% decline throughout its range since the 1980s. The Pacific leatherback populations appear to be crashing, where monitored populations from 1985 to 1995 have dropped between 6500 animals to less than 500. The world population of leatherback turtles is believed to be between 30,000 and 40,000 individuals.
Key Locations
Most of the records of marine reptiles, including snakes, in the New Zealand region are concentrated in the northeast of the North Island, with occasional records as far south as Cook Strait. The leatherback turtle occurs most widely throughout New Zealand, with records common on the west coast of the South Island, as well as records near Otago Peninsula, Banks Peninsula and Kaikoura. Two leatherback turtle sightings have also been recorded at the Chatham Islands.
The Kermadec Islands are likely to be frequented by marine reptiles, however limited observations in the area may account for few records. Small groups of green turtles have been historically recorded from Raoul Island, although breeding at the Kermadecs has not been recorded.
Summary of Threats
Within the region, marine turtles are likely to be susceptible to some fishing activities, including set netting, pelagic longlines, and potentially trawling, as well as motorboating activities. As the distribution of most of the sea turtles occurs in the northeast portion of the North Island, the risk of these activities are likely to be delimited to high-usage areas. Forty-one per cent of the turtle records in New Zealand occurred in entangled lines or nets. Worldwide, sea turtles are also threatened by heavy metal contamination, such as mercury. As the diet of some sea turtles, such as the leatherback, include jellyfish, the consumption of plastic bags and other plastic debris has been identified as a hazard to these animals.
Sea snakes within the region appear to be less at risk from fishing activities, where 95% of the recorded specimens were washed ashore dead or moribund on northeast coasts.
Typical Habitats
Marine reptiles occur in open water areas, particularly individuals that occur as rare visitors to the North Island, and their breeding beaches are likely to be located in tropical or subtropical areas outside of the New Zealand region. The leatherback, which is thought to have resident feeding grounds within the New Zealand region, tends to forage over the continental shelf while sightings suggest that they may spend a significant amount of time in near-shore areas adjacent to steep topography (eg, the West Coast of the South Island).
State of Information
The knowledge of the biology and ecology of marine reptiles is relatively well studied outside of the New Zealand region. As the leatherback turtle is thought to spend a more significant time within the New Zealand region, further study of its movements, diet and behaviour would possibly help better conserve the species and/or the resources that it depends on.
Significance for Maori
From the lack of information, it appears that sea turtles and sea snakes held little significance to the Maori people.
Key References
Gill, B J. 1997. Records of turtles and sea snakes in New Zealand, 1837-1996. New Zealand Journal of Marine and Freshwater Research 31: 477-486.
Table 17: Turtles and Sea Snakes (Class Reptilia) in New Zealand
| Taxon | Common name | Breeding species | Endemic species |
|---|---|---|---|
| Order Testudinata (turtles) | |||
| Family Cheloniidae | |||
| Caretta caretta | Loggerhead turtle | 0 | 0 |
| Chelonia mydas | Green turtle | 0 | 0 |
| Eretmochelys imbricata | Hawksbill turtle | 0 | 0 |
| Lepidochelys olivacea | Olive ridley turtle | 0 | 0 |
| Family Dermochelyidae | |||
| Dermochelys coriacea | Leatherback turtle | 0 | 0 |
| Total turtle species | 5 | 0 | 0 |
| Order Squamata (snakes) | |||
| Family Hydrophiidae | |||
| Pelamis platurus | Yellow-bellied sea snake | 0 | 0 |
| Family Laticauddae | |||
| Laticauda colubrina | Banded sea snake | 0 | 0 |
| Total sea snake species | 2 | 0 | 0 |
| Total marine reptile species | 7 | 0 | 0 |