WILMINGTON, Del. — When migratory fish follow their ancestral instinct to swim up Delaware’s Brandywine Creek during this spring’s spawning season, they will find, for the first time in more than 200 years, that their route is not blocked by a dam.

The fish — American shad, hickory shad and striped bass — have been unable to return to their traditional spawning grounds in the Pennsylvania section of the creek about 25 miles to the north since a series of dams was built across the creek by early American settlers, starting in the mid-18th century.

This year, the fish will be able to swim past the site of a dam that was demolished by the city of Wilmington last fall, allowing them to move as far as the next barrier, Dam 2, about three-quarters of a mile upstream, where large numbers are expected to create a sudden bonanza for anglers.

Beginning next month, “there will be thousands of American shad sitting here,” said Jerry Kauffman, a University of Delaware professor. “This area will be full of fishermen because it will be a big fish magnet. It’s going to be like Christmas.”