Coastguard teams were called after a diesel slick was spotted on the water in Beadnell Harbour.
Howick Coastguard Rescue Team and Seahouses Coastguard were called out after a report of a slick in the harbour yesterday afternoon.
The teams found a quantity of what is believed to be diesel floating on the surface of the water. Witnesses believed it had come from a boat that was leaving the harbour as the teams arrived.
A description of the boat was passed to Humber Coastguard, along with photographs of the vessel, and the boat was monitored for a period of time by Coastguard rescue officers off the coast at Beadnell.
Humber Coastguard filed a pollution incident with the relevant agencies and, following advice, the diesel was allowed to disperse naturally.
Howick Coastguard Rescue Team said: "If you ever notice any pollution on the beach or on the water, please inform the Coastguard on 999 immediately. By informing us quickly we can prevent any further spread and quickly mitigate any effects of the pollution. What the caller to this incident did was exactly right.
"Remember, in an emergency at sea or on the coast, or if you think someone may be in difficulty, always call the Coastguard by dialling 999."