The Paranephrops planifrons population in mesotrophic L. Rotoiti consists of 2 bathymetrically and temporally separate breeding groups. Late autumn breeders comprise about 80% of the population, occupy depths mainly above 30 m and at night feed in the littoral zone, where food is 80% more concentrated than elsewhere. Early summer breeders exist mainly below 30 m depth. Utilisation of the whole lake bottom and seasonal changes in food available to the early summer breeders are tentatively given as explanations of this pattern. Crayfish show increased sensitivity to light intensities greater than 150-205 lux and consequently occupy shelters above around 12 m depth. Shelters include any recesses subjected to intensities below this range. This zone accomodates almost the entire juvenile population and ca 10-20% of the adult population. This response to light intensity is believed to be an adaptation promoting avoidance of their main predator, the shag. Nocturnalism is another adaptation reducing losses to predators, including trout. Crayfish at greater depths lie unprotected and inactive by day and ca 70-80% of these adults form a high density band around the lake at a mean annual depth of 19.2±2.8 m. The band has a mean annual vertical depth range of 11.4±2.1 m and densities up to 50 crayfish m⁻². At dusk activity begins in the deepest waters first and crayfish forming the band migrate shorewards, while those above emerge from shelters. Within the hour feeding on detritus mainly, commences amongst the weed beds which extend to 6 m depth. At dawn a downward migration preceeds reformation of the high density band. These dual diel migrations apply almost solely to late autumn breeders and are part of a circadian rhythm, the timing of which is modified by light. Locomotion associated with migratory activity appears to be effected mainly through leg proprioceptors responsive to gravity. Directional orientation at this time and also generally, is inversely related to bottom slope angle. The diel distribution pattern varies seasonally. In summer and autumn the high density band is displaced shorewards by a vertical distance of 3.3 m, to a mean depth of 18.3 m and there is a 4 fold increase in crayfish inhabiting shallow water shelters. This closer association with the main feeding ground is probably an adaptation to meet coincident increases in energy needs. It applies especially to late autumn breeding females, whose period of ovarian development occurs between December and April. Energy requirements appear to be met through food consumed rather than from food stored. Female feeding activity is directly related to temperature but not in males, and prewinter storage is apparently of little importance in both sexes. Hypolimnetic deoxygenation affects early summer breeders mainly and induces a mass migration from the deeper regions to above the 30 m contour by February. Complete recolonisation of these parts follows lake turnover in May. Crayfish occur on all substrate types in approximately equal numbers except very soft muds, which support densities of <0.001 crayfish m⁻².