Paranephrops

Genus of crayfishes

Paranephrops is a genus of freshwater crayfish found only in New Zealand. They are known by the English common names freshwater crayfish the latter from their Māori name of kōura. The two species are the northern koura, Paranephrops planifrons, found mainly in the North Island, but also in Marlborough, Nelson, and the West Coast of the South Island, and the southern koura, Paranephrops zealandicus, found only in the eastern and southern of the South Island and on Stewart Island/Rakiura. Both species are a traditional food for Māori, and a small koura aquaculture industry supplies the restaurant market.

The northern koura (P. planifrons) reaches lengths of about , whereas the southern koura (P. zealandicus) is slightly larger – – with relatively shorter antennae. Their first pair of legs (chelipeds) are pincers used for scavenging food and warding off predators or other koura. The chelipeds in P. zealandicus are much hairier at their tips than those of P. planifrons. The four pairs of well developed walking legs are used for most movement, but the pleopods are small and no use for swimming; when alarmed, koura can flick their tails forward violently to propel themselves backwards at speed. Fossil evidence of Paranephrops in Pleistocene sediment demonstrate occurrence in or near marginal marine habitat, unfortunately there is little published literature about occurrences of fossilized Paranephrops.

Eels, perch, catfish, and trout are the major aquatic predators of koura. Terrestrial predators include rats, kingfishers, shags, scaup, stoats, and kiwi. Shag populations in the Rotorua lakes district in the North Island of New Zealand have been shown to feed on koura as the bulk of their diet. Predation on koura by trout is thought to be restricted to larger adult trout. Streams and lakes with established populations of trout have been shown to affect koura abundance. Cannibalism in koura is most likely to occur when koura are sick or moulting. Cannibalism can be a greater problem in high density situations where competition for shelter and territory is greatest. Juvenile koura can be consumed whole by larger koura, and this presents problems for aquaculture in ensuring continuity of intergenerational growth. Koura use their chelae for both attack and defence, and when one limb is lost, the koura will divert energy for overall growth to restoring the lost limb. The only disease known to seriously affect koura is "white tail disease" caused by the microsporidian parasite Thelohania contejeani. This parasite causes degeneration of striated muscle in the tail area, which turns the tail a pale white colour and correspondingly leads to death soon after. Water temperature and calcium concentrations are thought to be the key variables determining koura growth rates. P. zealandicus has high survivability (>80%) rates below 16 °C (61 °F), but temperatures above this correlate with lower rates of survivability. Higher death rates are thought to be associated with increased activity of koura at higher temperatures. Greater activity by koura increases cannibalistic behavior, and increased activity may also affect water quality with the greater production of ammonia as a waste product. Survival of koura also increases with higher calcium concentrations in the water, and this is thought to be in part due to a lower incidence of moulting related deaths and decreased risk from predation. A calcium concentration value of 20–30 mg/L in water is thought to be ideal for maintaining koura growth and survivability in aquaculture setups. and up to 60 weeks for P. zealandicus in Otago streams.

Farming of koura is currently undertaken by a small number of companies within New Zealand. Sweet Koura Enterprises Ltd and New Zealand Clearwater Crayfish Ltd are two such operators. Koura is sold solely to the high end restaurant trade, where they are commonly eaten as an entrée dish. Koura are harvestable once larger than in total length, which can take 2–5 years in P. planifrons. The suggested depth for ponds used to farm P. zealandicus is
A number of environmental challenges exist for koura farmers in ensuring optimum growth and survivability of stock. Environmental contamination of fresh water supplied to ponds from other land use activities, such as livestock farming, can affect survival. Other risks come from the introduction into the ponds of predators such as carp, eels, and birds; however, these can be controlled by steps such as placing netting across pond surfaces. Algal blooms creating anoxic conditions and cannibalism caused by high density stocking of ponds are also challenges to koura farmers.

The kōura was formerly abundant in New Zealand, and was a major traditional food source for Māori. The two main ways that kōura could be caught are ruku kōura, free diving to collect crayfish, or tau kōura. Tau kōura are bundles of Pteridium esculentum (bracken fern) that were constructed and placed into the water, where the crayfish would begin to inhabit and feed on the fern spores. The bundles would be carefully lifted out with a net. Kōura was commonly eaten during the early colonial era in New Zealand, but became less popular due to habitat destruction and predation by introduced trout species.