Do all freshwater eels migrate?

The catadromous migration of freshwater eels, in which they migrate from freshwater streams to the sea to spawn1, is widely accepted. The proportion of time spent in freshwater and ocean habitats can be determined by studying the ratio of strontium (Sr) and calcium (Ca) in the otoliths (ear-stones) of the eels. Here we use this technique to show that Atlantic and Pacific eels collected in the ocean have spent their entire lifetime there and have never migrated into fresh water. This finding indicates that freshwater eels need not be catadromous, and that populations from the sea contribute primarily to future recruitment.