Multiple drivers shape winter swarms of the bloody-red shrimp

The bloody-red shrimp Hemimysis anomala, a Ponto-Caspian mysid, has invaded many large lakes in Europe and North America. It exhibits a pronounced seasonal behaviour, forming massive winter swarms (WS) whose dynamics are likely driven by different processes implying reproduction capacities, responses to resource availability and/or predation pressure. We hypothesize that these different processes may not be mutually exclusive but could rather be successively involved in explaining the formation and disappearance of WS. We also expected that water temperature may be a major indirect driver of WS dynamics by indirectly controlling the above processes. To test these hypotheses, we used a combination of high-frequency video monitoring (through acoustic and visible-infrared cameras), direct diving observations, and diet analyses to track the dynamics of a WS of H. anomala in Lake Geneva at different time-scales. Our results reveal adult-juvenile successions during swarm formation while swarm ending was associated to the presence of only juveniles suggesting an effective implication of reproduction for WS formation and possibly its collapse. Temperature had a negative indirect effect on H. anomala abundance during the WS collapse and was only partly mediated by the littoral returns of perch (Perca fluviatilis) for which active predation was clearly identified from videos.