Eels of the genus Anguilla are described as a catadromous fish, meaning that they spawn in salt water and metamorphose into glass eels when they reach the continental shelf. After the glass eel stage, they become pigmented and colonize freshwater systems. In the fall, they begin their sexual maturation and begin an epic migration during which they may swim 6,000 km back to the Sargasso Sea, where they will spawn.
Eels have a complex life cycle during which they migrate between freshwater and brackish (semi-catadromous behavior). Analyses of otoliths from the European eel (Anguilla anguilla), and also from other eel species in the world, reveal that some eels skip the freshwater phase. In Norway, eels appear to live predominantly in salt and brackish water.
Other individuals are able to alternate between waters that have completely different salinity, temperature, substrate, depth, and other environmental conditions. It is unclear what determines the eel’s life strategy, but the decision to migrate does not appear to have anything to do with gender, since both males and females display this migration flexibility. One hypothesis is that differences in productivity between river and saltwater areas influence the eel’s decision to move between salt and fresh water habitats (facultative diadromous behavior). Primary production in freshwater is often higher at lower latitudes than at higher latitudes. Therefore, eels tend to remain in brackish and salt water environments at higher latitudes, since in these areas migration into freshwater habitats is not as advantageous.