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Posted by Ilse Steyl Posted on 2015-07-06

Fish mortality on the Twin Rivers

There has been an extensive fish mortality along both the Longford and the Duke of Northumberland’s Rivers (DNR). This is part of a wider problem affecting various catchments across London. Heathrow Airport has swiftly responded to the problem

Firstly it is well known that rivers can suffer from low oxygen events when you have a storm event following a period of warm, dry weather. The telemetry on both the DNR and the Longford River shows a low oxygen event entering the Twin Rivers on the afternoon of Saturday 4th July and extending into the evening. Oxygen levels then recover shortly afterwards. This seems to be consistent with a low oxygen event impacting the River Colne catchment following the storms in the early hours of the morning, which then enters the Twin Rivers later that day. It is also significant that the water temperature remains high at circa 21C. As fish are cold blooded creatures, their respiration rate increases as the temperature rises. There is also a physical effect that means oxygen is less soluble in water as temperature increases. This creates a vicious circle in such summer storm events as the fish need to breath faster as less oxygen is available.

Although Heathrow moved the Twin rivers as part of the T5 development, there are no discharges from their catchments into either the Longford or the DNR. Staff from Heathrow Ltd. walked the river together with the Environment Agency on Sunday 5 July and found dead fish along the section they manage.  Whilst there is no suggestion that these fish were killed by anything other than a natural event, they arranged for their contractors to remove the bodies for safe disposal. On a more positive note, it was noticed that some fish had survived this event, mainly in the area adjacent to the south west corner of the airport. They have been using a tanker to suck water out of the river and spray it into the air to create a small refuge area by re-aerating the water. Readings taken on Monday showed that this approach has succeeded at a local level.

Heathrow Airport continues to liaise with the Environment Agency and offer assistance where possible.

Aeration Equipment

Re-aerating water at the Twin Rivers, Heathrow Airport