Residents in the city have been surprised to see a dolphin swimming in the River Wear.
The Common Dolphin has been spotted making its way up the river towards the Stadium of Light earlier this week.
Staff at Durham Wildlife Trust were initially concerned for its welfare, but according to further sightings and reports, the dolphin appears to be in good health and has not displayed any signs of distress.
Emily Cunningham, Durham Wildlife Trust’s Marine Biologist, said: "It is rare to see a common dolphin so far upstream, they are normally found further offshore, though we've had reports of Bottlenose Dolphins heading up the Wear in the past - probably in search of migrating salmon or sea trout.”
The dolphin was initially spotted at Whitburn on Saturday, then again on Sunday near the Stadium of Light and again on Tuesday near the Glass Centre in Sunderland.
It was brought to the attention of the Trust via social media by local birder Mark Newsome.
Emily said: "Whilst we don't currently believe that this animal is in distress, we are keeping an ear out, so please do let us know of any further sightings.
"It's important not to approach, feed or get in the water with the dolphin - to do so is a prosecutable offence and would cause significant stress to this wild animal.
"If you see any signs of distress (swimming in circles, listing to one side, floating listlessly or stranded on the riverbank), please notify the British Divers Marine Life Rescue immediately on 01825 765546.
"Common dolphins are one of many marine mammal species that can be spotted off our coastline, though it's extremely rare to see one so far upstream.
"It's been a great month for dolphin sightings off our coastline - with bottlenose dolphins spotted feeding off Hendon and again at Seaham harbour in December.
"The best way to spot dolphins is to get yourself up to a good viewpoint, such as a clifftop or harbour wall, on a calm day, scan the waves with your naked eye and keep binoculars to hand in case you spot a splash that needs further investigating.
"Sightings are reported throughout the year so keep an eye on the sea next time you're at the coast. Our seas are full of surprises, few people realise just how rich our seas in the North East really are."
Durham Wildlife Trust, which manages 35 nature reserves from the Tees to the Tyne, is inviting members of the public to share their wildlife sightings via Twitter @durhamwildlife.