Effects of benthic substrate, nutrient enrichment and predatory fish on freshwater crayfish (kōura, Paranephrops planifrons) population characteristics in seven Te Arawa (Rotorua) lakes, North Island, New Zealand

Abstract

Populations of kōura, or freshwater crayfish (Paranephrops planifrons White, 1842), were quantified along a eutrophication gradient in seven Te Arawa lakes using the tau kōura, a traditional Māori harvesting method, in April, July and November 2009. Six of the seven lakes contained kōura. No kōura were captured in hypertrophic Lake Ōkaro. Kōura catches were highly variable and mean catch per unit effort (CPUE) varied from 1.5 to 99.3kōura per whakaweku (fern bundle) in the six lakes with kōura present. The highest abundance of kōura was recorded in eutrophic Lake Rotorua and the highest biomass in oligotrophic Lake Rotomā. Kōura orbit carapace length (OCL) ranged from 6 to 52mm, with the largest kōura recorded in lakes Rotoiti and Rotomā. Our results suggest that benthic substrate is more important in determining kōura population abundance than nutrient enrichment (using chlorophyll-a as a proxy) or predatory fish (rainbow trout abundance). Nevertheless, lake trophic status in conjunction with lake morphology appeared to indirectly affect kōura distribution in the sheltered, steep-sided lakes, through hypolimnetic deoxygenation.