High-speed footage of elvers (Anguilla anguilla) during their migration upriver in the spring. A high spring tide allows the elvers to climb to reach freshwater ponds, in which they spawn. Elvers are the young of the European eel, which has a complex life cycle. The eel spends most of its life in freshwater ponds and rivers in northern Europe, feeding on insects and other small animals. In the summer, some mature to a reproductive phase and travel to rivers to reach the sea. They then transform into an ocean-going reproductive stage, reaching the Sargasso Sea after several months, where they breed. The resulting eel larvae are planktonic, and are carried by the Gulf Stream into the North Atlantic, feeding and growing as they travel. They turn into a clear, thin form known as a glass eel, and swim towards estuaries. As they enter fresher water they develop pigmentation and become elvers, which swim upstream and climb over wet rocks and ground to reach pools and headwaters, where they mature. The European eel is now critically endangered due to overfishing, human activity and introduced parasites. These elvers were filmed at Stackpole National Nature Reserve in Pembrokeshire, Wales, UK, in April 2018.