European eel life history is complex. Eels are catadromous, meaning that they spawn in the sea and return to freshwater rivers and streams to grow. The European eel breeds in the mid-Atlantic in the south-western part of the Sargasso Sea. Larvae are carried by the Gulf Stream to the continental shelf of Europe in about 1 year, where they metamorphose into colourless elvers or glass eels and enter continental waters. Elvers then start to move up rivers, in UK, in February and April. When about 2-3 they start to develop tiny scales and after 4 years are completely scaled. The freshwater stage is a feeding and growing phase. At this stage they are known as yellow eels. As they mature and grow they change into silver eels. Eels that grow up in freshwater generally become females, while the ones in brackish water become males or females. Males change into silver eels when 6-12 years old (30-48cm), the females when 10-30 years old (50-130cm). As they mature sexually, they descend the river or migrate to the sea. If silver eels are prevented from returning to the sea, they start to feed again and can live for over 80 years. Growth and age at maturity are linked to regional temperature (mature later at colder temperatures). The average length of adults is around 60-80 cm, when they weigh around 1-2 kg. It is thought that they use the earth's magnetic field to find their way to the Sargasso Sea to spawn. Eels die after spawning.