The elusive European eel travels 4,000 miles from the Sargasso Sea to the South Downs.
The Western Sussex Rivers Trust is helping by removing barriers and installing fish passes on the River Rother and River Ems.
This will aid their upstream journey.
River Rother in South Downs National Park (Image: Jeff Travis) Aimee Felus, chief executive of the trust, said: “By removing manmade structures which limit how easily fish species, including the extraordinary eel, can move around, we are helping improve migration routes and their ability to reach better feeding spots.”
The eels undergo several metamorphoses throughout their lives, and can grow up to 1m long.
Jan Knowlson, biodiversity officer for the South Downs National Park, said: “The eels we have in the National Park are truly remarkable, and their incredible migration underlines the important connection between our rivers and seas.
“So healthier rivers ultimately mean healthier seas and vice versa.”