Every year between January and May the East Australian Current delivers an influx of unusual marine visitors to New Zealand waters. Among them are several species of marine turtle, sightings of which are becoming increasingly common.
Some 250 species of turtle are known, most occurring in tropical zones. Large land-dwelling species are often referred to as tortoises, while those that are hard-shelled and aquatic are called terrapins or turtles.
Structurally, turtles are unique in the animal kingdom. The upper shell, under which the head, limbs and tail can be more or less completely withdrawn, is called the carapace. The lower, flatter shell is termed the plastron. The two parts of the shell are connected to the vertebrae and a series of expanded ribs. These anatomical features, along with the unique placement of limb girdles inside the rib cage and the possession of horny beaks instead of teeth set turtles apart from other reptiles.
Marine turtles have long flippers which enable them to migrate over great distances, often averaging speeds of between 20-30 kilometres per hour. One juvenile leatherback turtle was clocked at 40 kph.
Seaweed and jellyfish form a large part of the diet of marine turtles. Perhaps to handle such gelatinous fare, the mouth and esophagus are lined with rubbery protuberances resembling the open arms of sea anemones. However, like other reptiles, turtles have dry, scaly skin and a body temperature that is much more dependent on ambient conditions than our own. The leatherback is able to withstand much colder sea temperatures than can other turtles, due to its ability to maintain a body temperature which is up to 5°C warmer than sea temperature.
Like almost all reptiles, turtles lay eggs. Typically, the female comes ashore on a sandy beach, where she may lay several clutches of a hundred or more eggs in holes she excavates. She then refills the holes and departs. Depending on the species, incubation time ranges from six weeks to more than seven months. Hatched by the heat of the sun, young turtles dig their way to the surface and then trek to the ocean.
Only a small percentage of each clutch reach maturity. Predators and turtle hunters take their toll, and others are drowned in fishing nets. Nesting grounds are also diminishing as a result of coastal development. The trade in live turtles and turtle products doesn’t assist the creature’s survival. Stuffed specimens, shells, shell carvings, skins, leather articles, meat, eggs and oil form the basis of a thriving industry.
This combination of factors has led to all species of marine turtles becoming endangered. In recognition of their plight, they are protected under the International Convention for the Trade in Endangered Species (CITES). However, many countries do not recognise CITES, and marine turtle numbers continue to decline. For instance, the number of loggerheads nesting fell by up to 80 per cent in the 10 years prior to 1993. In the Solomon Islands, 18,650 kilograms of tortoiseshell was exported between 1983 and 1990, before the enforcement of a trade ban. That quantity represented 20,000 turtles.
In an attempt to prevent the extinction of these creatures, the South Pacific Regional Environment Program (SPREP) is coordinating a study of the migration and life cycle of marine turtles in the Indo‑Pacific Region.
In New Zealand, records of turtle sightings are held on the Biosite database maintained by the Department of Conservation. There are some 160 records dating back to 1885, when a loggerhead turtle was observed at the entrance to the Manukau Harbour.
Five species of turtle have been sighted in New Zealand waters, and 60 per cent of sightings occur in Northland. By far the most common species reported is the leatherback. Generally only seen at sea, this species—the largest in the world—can weigh up to 970 kg and measure three metres in length. As its name suggests, this species lacks a top shell, having instead a rubbery skin.
Four species of much smaller hard-shelled turtles (rarely exceeding 1.5 metres in length) account for the remainder of the sightings, and are the species likely to he encountered on beaches. They are difficult to tell apart. The best keys to use are colour pattern and the number of costal shields, or plates, on the upper shell and head. The green turtle (named for the colour it imparts to soup, not for its body colour) has four pairs of costal shields and one pair on its head. The loggerhead—brown to dark brown in colour—has five pairs of costals and two pairs on the relatively small head. The olive ridley, of which there have only been a handful of sightings, is olive-green and has the same shield configuration as the loggerhead. The hawksbill possesses distinct overlapping shields, except in old animals where they are side by side. It has four pairs of costals and two pairs of prefrontal shields on its head. Its shell is a wonderful mix of strongly marbled shades of brown.
Equal numbers of turtle sightings around the New Zealand coast have been at sea and on beaches. A number have been found entangled in lines and nets, and many of these have drowned. Those still alive are generally in an emaciated state, having been too long in waters that are colder than they can readily endure. Typically, they feel very cold to the touch, with baggy skin and eyes that are recessed far into the sockets. Often they will also be covered in seaweed and algal deposits as a result of not being able to swim fast enough to deter growths.
Kelly Tarlton’s Underwater World in Auckland has been rehabilitating stranded or trapped turtles since 1985. Turtles from as far away as Christchurch have been delivered to the aquarium, where they are gently rewarmed and rehydrated. Many are diagnosed with pneumonia and are often unable to float. They are placed on a foam mat in a special tank that has warm water underneath it and warm water above, dripping on to the shell.
An artificial jelly-like mixture consisting of vitamins and mussels is fed to the animal until it is restored to normal health. It is then released into one of the main tanks, where it dines on greens, herrings and mussels.
Recuperating in the aquarium at present are a green and a loggerhead. The average stay is six months. The turtles are then tagged and transported back to their native waters courtesy of naval or cruising ships sailing to Raoul Island and beyond. The most recent release was in May 1997, when a young hawksbill found at Glinks Gully in Northland was sent on his way.
Marine turtles need all the help they can get. If you see a turtle, alive or dead, look for a tag on its flipper, record its location, try and identify it or take a photograph and contact the nearest Department of Conservation office. If you are lucky enough to find a live turtle ashore, contact or take the turtle to the Department or Kelly Tarlton’s. In most cases, the turtle will be able to be nursed back to health and, in time, returned to the sea.