Griff,

What you say here does make some sense. As l as posted, the majority of trout l see trailing lamprey are below the confluence, and for sure any fish running in to the NF will have that attached to it and it would remain so unless something caused it to let go.

But as l also said l have seen them now as far up stream as White hole, but only a few cases.

After l read Toms post it got me thinking. The entry of lamprey into the great lakes was due to the lakes being open to other water sheds.
Species that habit the oceans and around estuaries will survive in freshwater.

What took place in the great lakes is that they nearly wiped out the lake trout.

That has me interested as l have as yet not seen a lamprey stuck to any other species other than salmonids, trout or salmon.
I have seen very large lamprey nearly 24 ins long in UK waters in the past, also attached to Atlantic salmon.
I am not suggesting that they do not attach to other species.

But l would hazard a guess here, that the skin of a salmonid is not as such a scaled fish like members of the carp family black bass, suckers etc. I have as yet not seen one with a lamprey attached.

It may well be that the lamprey cannot due to the hard scale formation find a hold on the fish.
Trout also will rest very still at the bed of a river and provide the perfect opportunity for a lamprey to attach to them.
That is how they attack Salmon, at night.

Interesting also that Toms' post intimated that they were not at all common in these rivers at one time.
Was it the introduction of Salmonids that changed that. Did they find, those that were here the perfect host and then multyply.
That interests me.
So far as the dilution of water quality is concerned l am not convinced that it would make any great difference at least to the lamprey.

Some of the estuaries they are known to habit in the UK, are subject to at times great amounts of toxins, that is in turn washed out by the tide flows and flood water over a period of time as tides levels fluctuate and floods clean the rivers. The lower end of the river mouth tends to be more stable so far as levels of toxic build up in and around the level of water rise from the ocean. As water is twice a day pushed up stream in the river mouth and falls back. That causes contaminant to move back and forth so to speak.
That factor also being one of the reasons why the two species of shad that migrate into some of the UK rivers has declined, as they spawn in the lower sections of the river and also spawn in the surface and not substrate as such. The eggs then track down stream to ward the ocean.

I believe the same is also true for some species of fish found around the coasts here in the US.

Davy.