Twenty-four hours in the life of Zingel asper

For the first time, radio‐tracking observations were made over 24 h cycles during the growing season to quantify the habitat use and home range of Zingel asper, a critically endangered fish species, endemic to the Rhône River with a high risk of extinction (IUCN). New advances in radio telemetry with regard to the size of tags provided an opportunity to collect behavioural and habitat use information while remaining non‐lethal. Zingel asper was characterized by small ranges of habitat use and, contrary to previous assumptions, the species was diurnal as illustrated by a smaller home range during the night than during daytime.