Diel Cycle and Effects of Water Flow on Activity and Use of Depth by Common Carp

Common Carp Cyprinus carpio is among the most popular and commercially important fishes globally. For this reason, it has been introduced worldwide and is invasive in many regions, with well-known ecosystems impacts. Like many other freshwater invaders it is thought not to tolerate strong flows well, but knowledge of the effects of flow on their activity, habitat use, and diel cycles are limited, despite this being crucial information for management and control. By means of ultrasonic telemetry we investigated depth use and activity of Common Carp in a small reservoir with a very low water residence time on the main stem of the Ebro River, the largest river by discharge in the Iberian Peninsula, over a 19-month period. The activity of carp and their use of depth displayed low seasonality compared with abiotic factors. However, carp exhibited diel vertical migration patterns, mostly in the warm season, shifting from deep positions near the reservoir bottom during the night (with decreased activity) to shallow waters during the day. This pattern included extensive use of hypoxic waters (<1.1 mg/L dissolved oxygen) and occurred largely at night. Activity and habitat use also varied among individuals and were significantly related to water flow; carp were less active and used shallower water during increased flows. The individual variability, the behavioral adaptation to refuge from high floods flows, and the extraordinary resistance to hypoxic waters might help explain why Common Carp is one of the most successful invasive freshwater fish species.