Does anyone on the Forum have any idea of what the lowest pH (Potentiometric Hydrogen Ion Concentration) of water that a native brown trout could survive and maybe thrive in the UK?
For simplicity, pure water is said to be neutral, with a pH close to 7.0. Water with a pH less than 7 are said to be acidic, typical of peaty upland waters and water with a pH greater than 7 are said to be alkaline eg lowland waters.

I think I know the highest and it is higher than the amount when ions are expected to stop exchanging across the gills. Wouldn't the lowest be a sliding scale of productivity from 7?

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Taken from Freshwater fish of the British Isles by Nick Giles.

"When the water is below pH 5.5 and is low in calcium and high in aluminium and heavy metals, trout (and salmon) eggs and fry are killed outright. Fry trying to hatch are unable to to rupture their egg membranes, possibly because the enzyme which helps the to break through is inactivated in very acidic (less than pH4.5) conditions. The embryos are, therefore, entombed in their eggs and quickly die."

"In September 1984, 117 fresh-run salmon and sea trout were killed in the Afon Glaslyn during a spate when the water pH was 5.4-5.9 and aluminium concentrations were high."

Not exactly what your looking for as it didn't give a specific figure.

Ben

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"Water is peculiar stuff. It circulates mysteriously, rising invisibly from the sea as vapour, forming clouds, falling as rain, creating streams and rivers that return again to the sea. You can swallow it in sips, but it can swallow you whole if you fall into a deep place. You can't catch hold of it, but it can catch hold of you, even if you just look at it."

It is not just pH but a balance of bicarbonates and carbonates which can buffer changes from dissolved carbon dioxide or even ammonias but don't change pH.

I'd guess from just over pH 9 to under pH 5.

Salmo trutta is more resistant to metal ion levels than Salmo salar. A reason they have co-existed I suppose.

The acid rain that liberated lethal aluminium salts - classically from Forestry deep machine ploughed hillsides - could have been pH 3 to cause the mortalities mentioned above by Ben.

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Save the softmouth trout

Thanks so far chaps.
I was just wondering whether anyone on the Forum had any real data on peaty upland wild brown trout stillwaters so I could find out what the pHs typically are.

Try having a dig through SEPA reports, they'll probably log pH's of rivers.

There's a fair bit in 'The Trout' by Doctors Winifred Frost and Margaret Brown. But it's over 50 years old.
She records that Gammarus lacustris shrimp introduced to acid lochs survive at levels down to pH 3.0.

I know the EA in Wales experimented with Salvelinus fontinalis introductions in acidic lakes a few years ago - they were presumed slightly more tolerant of peaty water. Something may well be published on-line if you search.

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Save the softmouth trout

I think the typical pH's (spot samples) are slightly misleading as it is 'acid flush' events which can take out trout, trout fry and eggs. ie heavy rainfall off usually peaty catchments, particularly following a dry spell. pH can drop spectacularly during these first flush episodes.

I have to go through my literature but from my memory (not necessarily precise) it is a pH value of 6.4 or somewhere around that, that does not effect survival of the eggs. Below that it starts to show. Will have to go through books if i find enough time.

BlueOne

Ok, after a quick look, in better trout habitat in one of case studies value of 5.5 is given as the one that still enables normal life of brown trout.

If you look at Ephemerella`s post, The biology of streams and rivers, gives the following data:

at pH 5.0 mortality of trout and salmon dramatically increased in presence of aluminium (0.35mg/l)

mortality was low even at pH 4.3 withouth aluminium

As said before that is only a part of the story, pH has a profund effect on hatching of eggs.

I know i have some data somewhere and even diagrams, but am not able to find it at the moment.

BlueOne