The Use of Winter Water Temperature and Food Composition by the Copepod Cyclops vicinus (Uljanin, 1875) to Provide a Temporal Refuge from Fish Predation

SIMPLE SUMMARY: Predator avoidance mechanisms play a critical role in the survival and stable population growth of prey. Here, we describe a new defense strategy for Cyclops vicinus, which is vulnerable to fish predation. Long-term data (January 2014 to February 2019) showed that C. vicinus was abundant in winter when the foraging activity of fish was lower. This pattern was reversed in spring, summer, and autumn. C. vicinus is consumed frequently by fish because it has a body size larger than that of other cyclopoid copepods (Mescyclops leuckarti and Thermocyclop sp.). In this respect, winter formed a seasonal refuge when C. vicinus populations could grow efficiently. In addition, there was an abundant phytoplankton presence (Cyclotella sp. and Rhodomonas sp.) in winter. These species formed a food source that supported the population growth of C. vicinus. The evolution of the predator avoidance mechanisms of prey contributes significantly to the security of local biodiversity and the stability of the freshwater food web. ABSTRACT: Frequent predation induces various defense strategies in prey, including morphological changes or migration patterns in zooplankton. We hypothesized that the winter dominance of Cyclops vicinus in the Upo Wetlands, South Korea, is an evolved temporal defense mechanism to avoid fish predation. Long-term data (2014–2019) showed that fish consumed the most cyclopoid copepods from spring to autumn. Lepomis macrochirus preferentially consumed C. vicinus; thus, C. vicinus density was lower from spring to autumn. However, C. vicinus was abundant in winter when fish consumed fewer copepods. Nauplii density began to increase in late autumn (October–November), and their population growth was fueled through consumption of Cyclotella sp. and Rhodomonas sp. Culture experiments showed that Cyclotella sp. contributed more to the growth stage (copepodite or subadult) after nauplii than Rhodomonas sp. C. vicinus density was lower in the winters of 2013 and 2016 when the densities of these phytoplankton prey species were lower. In summary, although winter conditions were suitable for copepod survival and population growth, C. vicinus relied heavily on the diversity and species composition of its food sources. The winter dominance of C. vicinus could increase regional biodiversity and contribute significantly to the stability of the freshwater food web.