New Zealand’s Freshwater Crayfish Ecology and Conservation
The koura, or freshwater crayfish, is endemic to New Zealand. It can be difficult to spot as it is often the same colour as the rocks at the bottom of its stream. It also stays hidden during the day, preferring to move around at night. Although genetic research is ongoing, there are two currently recognised species of koura found in New Zealand. The larger species (80 millimetres long) has very hairy pincers and is found in the east and south of the South Island and on Stewart Island. Koura in the North Island and in Marlborough, Nelson and the West Coast of the South Island are slightly smaller (about 70 millimetres long) and have less hairy pincers.
The koura lives in freshwater streams, lakes, ponds, and swamps. Koura typically shelter between stones but they also can burrow into mud. Koura living in swamps will sometimes burrow deep into the mud when the swamps dry out over summer, waiting until the water returns to re-emerge. Some koura can live on the bottom of very deep, clear lakes in the South Island at depths of up to 60 metres.
The koura belongs to a group of animals called ‘crustaceans’, which all have a hard, shell-like covering. This covering eventually gets too small for the koura, and the animal must then shed the old layer in a process called moulting. The old shell-like skin splits and is left behind, while the new skin underneath hardens. During moulting, crayfish are very vulnerable to predators.
Koura, also known as kewai,use their four pairs of walking legs to get around quite quickly. They do not crawl as their common name ‘crawly’ suggests! When alarmed, the koura will switch into reverse gear, flicking its tail forwards violently to shoot backwards into shelter. The koura’s first pair of legs are pincers, which are used mainly for catching food and fighting orwarning off invaders. Instead of hunting for their food, koura are scavengers that feed on old leaves and small insects that float by in the water or settle onto the
lake or river bottom.
between 20 to 200 berry like eggs under the side flaps of her abdomen, when she is said to be ‘in
berry’. Small koura hatch about 3 to 4 months later looking exactly like their parents in miniature. They cling to their mothers with their pincers until they are nearly 4 millimetres long. By their fourth year they are 20 millimetres long and become adults.
Koura also act as an indicator species, signalling to scientists when conditions have become unhealthy for a range of organisms in that habitat. Scientists can therefore monitor koura populations in order to determine the overall health of a lake or river system. If freshwater crayfish are present in a stream, that is a good sign that the stream is clean.
New Zealand freshwater animals such as the koura are in danger because their habitats are disappearing. Less than 9% of New Zealand’s wetlands remain; most have been drained for farming. Many streams, rivers and lakes are now highly polluted, and the loss of riverbank vegetation can lead to erosion, making the water muddy and killing the food source for aquatic insects. Other threats include:
- Illegal harvesting: koura may legally be gathered for personal consumption up to a limit of 50 crayfish per day. However, the selling, trading or possession of koura for the purposes of sale or trade is illegal.
- Introduced mammals: Although not a lot is known about the effects of land mammals on freshwater ecosystems, research has detected crayfish in theguts of trapped stoats, which suggests that land mammals may pose a threat to koura.