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Harrow 50 acre Harefield no2 total fish kill

Mark Swaby

Senior Member
About 2 weeks ago a total fish kill occurred on this 50 acre local lake due to oxygen levels being too low for life to survive.50 plus year old carp,massive numbers of pike,bream, tench,etc and lots of catfish all perished.Some things were seen prior to the kill. 1) About a month ago the sunny/warm days stopped and we had a dreary damp week,all the algae/weed starrted to dye off. 2)The lake water changed from clear to dark 3 weeks before the kill 3) The fry started to shimmer on the surface like it was raining, small fish were jumping around the edges. 4)The bigger fish were gasping on the surface the night the kill occurred shortly afterwards all the fish dyed.When the water oxygen levels were tested afterwards they were around 0.2ppm and you need above 2ppm for any survival.Local lakes were testing at levels like 7. The EA turned up after the kill but it was too late,so you need to do things yourself.Make sure your fishery lake managers have oxygen test kits and test daily if possible towards the end of the summer,the lowest levels are around 3 am but if you see levels around 3 then you need to start doing something.Get your club to buy pump/generator units or approach local clubs to share.Thunder storms and these conditions are very bad for dropping the oxygen levels.I was told all this by a friend who was involved and hope i have got all things correct, he had never seen anything like it and never wants to see it again.
How was dissolved O2 measured? Metering after a crash is marginally informative at best. With the rapid breakdown of organic mater and surge in Ammonia, this is then a spiral that causes serious problems. The toxic form NH3 can kill some fish at concentrations of 0.25ppm.
There are quite a range of water chemistry issues that can cause death in fish. If I had a lake personally I'd be testing at least 5 parameters with regularity and tbh O2 isn't on that list.

Mark Swaby

Senior Member
As i said Stephen i was told this by a person at a committee meeting last week by one of our club bailiffs who tried to help Harrow.I know nothing of how things are tested.I will talk to him and find out how he does it.We are all amateurs trying to understand how to do things for our club better for the future.If you can explain how to do things better/properly we would be very grateful.I put it out there hoping to get some sort of more advanced info and understanding.It did seem a bit bad that days after all this the HS2 lorries turned up to rip everything apart.
Completely understand that and your approach is admirable. I certainly wasn't knocking you.
Unfortunately my background isn't in commercial fishery management. I have however spent about 70% of my life caring for various aquatic organisms both professionally and for pleasure. Currently I work for a manufacturer of water treatmemt technologies, 99% based around commercial swimming pools.
More than happy to discuss things and share what I do know. If any of it applies and benefits that a winner.
Terrible News.
Spent fair bit of my youth on North Harrow Waltonians water and the LAA water opposite after the pike.
With Reg, who you know Mark.

Also the BA Stockers water for the Tench.
Happy days. So sad to hear this news.