CONSERVATION work on a historic bridge has been halted after a rare and jawless blood-sucking fish was discovered spawning under it.

North Yorkshire County Council said the work on Grade II listed Thornton Bridge, over the River Swale at Helperby, near Thirsk, had ended 13 weeks early after the Environment Agency had issued an alert that lampreys breeding there could be upset.

Lampreys, some of which are classified as threatened species and are protected under the Salmon and Freshwater Fisheries Act 1975, spawn from March to July, but the season can end earlier depending on species and water temperature.

They are considered an important element in river ecosystems, as they are among the few survivors of the jawless stage in vertebrate evolution, for their nutrient processing role, as a food source for other animals and as they act as a buffer for salmon from predators.