Thousands of fish are being moved from a Lancashire aqueduct so a 240m section can be drained for repairs.
A £1.5m project to replace the waterproof lining in part of the Lancaster Canal means several species of fish must be relocated.
The Canal and River Trust is sending a team to move the fish - including pike, bream and eels - to another section.
The work would ensure the aqueduct stays watertight for another 200 years, the trust said.
The aqueduct section of the Lancaster Canal canal, which was built between 1794 and 1797 by engineer John Rennie, crosses the River Lune.
When the canal opened it was made watertight with clay, which has gradually eroded causing "minor seepage", project manager Graham Ramsden said.
The trust wants to preserve "the historic fabric of the canal" and ensure "people can continue to spend time by the water", Mr Ramsden added.
The work is scheduled to be completed by March 2020.