catadromous
adjective
ca·tad·ro·mous kə-ˈta-drə-məs 
: living in fresh water and going to the sea to spawn
catadromous eels
compare anadromous 
Example Sentences
Recent Examples on the Web American eels are North America’s only species of catadromous fish. 
Lela Nargi, Washington Post, 24 May 2022 This was my Holy Grail—a catadromous, trout-like mullet that lives in the clear rivers that tumble from the mountains. T. Edward Nickens, Field & Stream, 27 May 2020 As catadromous fish, European eels spend most of their adult lives in freshwater rivers, streams, and estuaries before returning to the open ocean to spawn and lay eggs. National Geographic, 25 Mar. 2020
Lela Nargi, Washington Post, 24 May 2022 This was my Holy Grail—a catadromous, trout-like mullet that lives in the clear rivers that tumble from the mountains. T. Edward Nickens, Field & Stream, 27 May 2020 As catadromous fish, European eels spend most of their adult lives in freshwater rivers, streams, and estuaries before returning to the open ocean to spawn and lay eggs. National Geographic, 25 Mar. 2020 Word History
Etymology
probably from New Latin catadromus, from cata- + -dromus -dromous
First Known Use
1880, in the meaning defined above