to completely harden the exoskeleton is achieved by absorption from the water. A lower limit of 5 mg/L of calcium in water for temperate species of koura has been suggested as sufficient to support exoskeleton hardening. Water temperature and calcium concentrations are thought to be the key variables determining koura growth rates. P. zealandicus has high survivability (>80%) rates below , 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. Reproduction The female carries 20–200 eggs under the side flaps of her abdomen, where they take 3–4 months to hatch. Over this time, male sperm production corresponds with females' reproductivity. Once hatched, juvenile koura cling to their mother's abdomen using their pincers to attach until they have reached a length of . At this stage, they resemble adult koura in appearance, having undergone two moults. In Lake Rotoiti in the central North Island of New Zealand, the main breeding period occurs between April and July (autumn–winter), with a second breeding period occurring from October–January (spring–summer). The total breeding length time from peak egg laying to the release of juveniles is estimated to be 28 weeks for the autumn–winter period and 19–20 weeks in spring–summer breeding groups. This difference is attributed to warmer temperatures speeding up the egg development process. In stream populations, this growth period has been shown to take around 25–26 weeks in P. planifrons, and up to 60 weeks for P. zealandicus in Otago streams. Aquaculture 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.