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

Lampreys 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.

The council said contractors had worked seven days a week to paint the Victorian bridge in an attempt to limit the disruption to motorists who had faced 18-mile diversions, completing the initial work three weeks ahead of schedule.

They had intended to start ten weeks of work to re-paint the bridge structure when it received the alert.

It added the work, part of a £197,000 scheme to bolster the bridge which will see it closed to vehicles weighing more than three tonnes, had been put back until June 15.

David Bowe, the authority's environmental services director, said: “No doubt the reopening of the bridge will be a relief to commuters and farmers who we appreciate have been very patient throughout the first phase of the works.

“Thankfully, the spawning fish site was not disturbed during the first phase, but we will now need to delay the re-painting work until later in the summer after the spawning season.”

Residents in nearby villages have expressed frustration over the timing of the work, saying it will diversions for school buses will impact on pupils during exams, while farmers said it would affect agricultural vehicle movements at harvest time.

A Helperby resident, who asked not to be named, said: "I am all for the protection of the rare fish, but the council appears to have gone ahead with scheduling the works again without any consultation with those who rely on the bridge."