Kōura or freshwater crayfish (Paranephrops planifrons) are an important component of lake food webs and are a traditionally important species to Māori in the Rotorua Te Arawa lakes district. Prior to European settlement Te Arawa iwi collected large quantities of lake-dwelling kōura for their consumption and for trading with iwi in outlying districts. Today, kōura are considered a ‘taonga’ or treasured species and support important customary fisheries in some North Island lakes where large populations are still present.
Long-term monitoring of the kōura populations have been undertaken at three sites in Lake Rotoiti since 2005 using a traditional Māori harvesting method known as the tau kōura. This method comprises bundles of bracken fern fronds (Pteridium esculentum) laid on the lake bed which kōura take refuge in. There has been a marked and progressive decline in kōura abundance and biomass and an increase in kōura mean size (OCL) in Lake Rotoiti since 2009. Length frequency analysis of kōura data shows that there has been a drastic reduction in the numbers of small and medium sized kōura (i.e., <24 mm OCL) recorded.
These changes in kōura population parameters and size structure have coincided with the establishment of invasive brown bullhead catfish (Ameiurus nebulosus), which are known to directly and indirectly affect kōura through predation and competition for shared food resources. A number of measures are now in place to mitigate the effects of catfish predation on kōura populations in lakes Rotoiti and Rotorua and to prevent their spread to the remaining 14 lakes in the Rotorua region.