Tracking the real-time behavior of Hemimysis anomala’s winter swarms using acoustic camera

Tracking the behavior of invasive alien species (IAS) is challenging due to logistical constraints that hinder high frequency surveys. Real-time automated recorders offer a promising solution, but their feasibility needs testing and validation. The acoustic camera is an efficient method to obtain real-time records of the behavior of Hemimysis anomala, a mysid that has invaded a variety of aquatic ecosystems. This approach is also a valuable tool for high-quality video recordings of mysid swarms in shallow habitats. The feasibility of this device to record the continuous behavior, including exit and entry (hereafter referred to as emergence and homing) from a shelter over a diel cycle was validated. Emergence consistently correlated with the onset of civil twilight, while homing displayed more intricate temporal trends. Homing events were longer than emergence events, but both were significantly influenced by light conditions. The termination of emergence was concomitant to the onset of the light preference as already shown by past studies. However, homing showed an earlier onset before light conditions exceeded the preference values, possibly due to inter-individual variability and/or information sharing in swarm behavior. The survey also captured the collapse in the targets number detected by the end of March, underlining the termination of the winter swarm. Furthermore, we demonstrated the importance of high-frequency temporal resolution, with a sampling frequency of at least 10-min, to accurately capture the swarm behavior. Overall, we provide original new insights characterizing H. anomala ecology and encourage the development of real-time automated recorders to track the behavior of IAS.