Stay cool: Habitat selection of a cyclopoid copepod in a north temperate oligotrophic lake

1. We studied the vertical distributions of Cyclops cf. sibiricus in Lake Toya, a north temperate oligotrophic lake. During the winter circulation period, their distribution was vertically homogeneous both day and night. During the summer stratification period, C. cf. sibiricus stayed below the thermocline. Diel vertical migration (DVM) was pronounced in advanced developmental stages, although the upper limit of the migration became deeper as the thermocline gradually descended. This seasonal change was observed throughout the 4-year study period, implying that thermal structure is the primary determinant of C. cf. sibiricus distribution. 2. In a field experiment, C. cf. sibiricus incubated in the summer epilimnion, which most of the population never experience, developed faster and grew better than in their original habitat. We consider that trans-thermocline DVM would not have evolved because of possible disadvantages such as the cost of migration offset the benefit observed in the field experiment. The rapid temperature change at the thermocline may act as a swimming-cost estimator for the copepod. 3. Low food availability in deep water during the summer stratification period seemed to determine the lower limit of C. cf. sibiricus distribution, and the copepod minimised the risks of predation by fish via DVM. These results suggest that C. cf. sibiricus modified their distribution seasonally to obtain maximum benefit in terms of individual fitness.