Modeling the migration of the American eel in the Gulf of St. Lawrence

Telemetry experiments allow documentation of aquatic animal movements in the ocean, and numerical models can contribute to our understanding of the mechanisms underlying the observed movement patterns. In this study, we used mature (silver-stage) American eel Anguilla rostrata migration out of the St. Lawrence Estuary (SLE)–Gulf of St. Lawrence (GSL) system to illustrate how an individual-based model coupled with a 3-dimensional ocean circulation model can help us understand fish behaviour, and complement ongoing and future telemetry experiments. A suite of 23 numerical simulations that combined various vertical and horizontal swimming behaviours of virtual eels (‘v-eels’) was conducted to evaluate which behaviours most closely matched the observed migratory patterns of silver eels in the GSL. Results indicated that v-eels migrating through the SLE–GSL must employ active swimming and complex orientation strategies in order to migrate in the time suggested by telemetry experiments. The use of selective tidal stream transport is sufficient for v-eels to escape entrapment in the Estuary; however, this behaviour requires active swimming rather than simply drifting in the direction of the appropriate currents in the Estuary in order to eventually exit the GSL within the time frame observed in the field. Orientation towards higher salinity and/or greater water depths in the Gulf was not sufficient for v-eels to successfully escape the GSL, suggesting the use of other variables such as the geomagnetic field gradient is required. The model also allows exploration of the potential effects of climate change on the duration of migration, through changes in hydrological conditions.