Swarms of the Ponto-Caspian mysid Hemimysis anomala in Montreal Harbour, St. Lawrence River, Canada

► Dense swarms of the invasive Hemimysis anomala were found in St.Lawrence River. ► Mysid is established in Montreal Harbour basins but absent outside the harbor area. ► Species distribution was not related to specific water mass properties. ► Species introduction was presumably due to local discharge of ships' ballast waters.