 Some teleosts are migratory ; certain fresh water species move within river systems on an annual basis ; other species are anadromous , spending their lives at sea and moving inland to spawn , salmon and striped bass being examples . Others , exemplified by the eel , are catadromous , doing the reverse . The fresh water European eel migrates across the Atlantic Ocean as an adult to breed in floating seaweed in the Sargasso Sea . The adults spawn here and then die , but the developing young are swept by the Gulf Stream towards Europe . By the time they arrive , they are small fish and enter estuaries and ascend rivers , overcoming obstacles in their path to reach the streams and ponds where they spend their adult lives . 
