While my wording may seem incorrect, I do not think I am totally wrong. I agree that the adult spawners are long range migratory fish from St Clair etc, they migrate up the Thames all the way to London to spawn every spring. These eggs turn into fish and from some studies I've read are likely hatched in the same area as their parents did and lived part of their "fry-stage" to juvenile lives in the upper Thames where by fall they move out. But they are in some way similar to salmon/trout where they will return each year, for why would a lake walleye swim all the way up the Thames if it had not some instinctual spawning need to do so back to their birth area? There are 99% of the fish classed as short spawners that spawn within 3 miles of where they were born in the lakes/reservoirs. This 1% is different.
So, it is these fish I'd be concerned with. Catch and remove the spawners and as a few years go by the whole thing collapses as there are not enough migratory fish left with a memory to make the migration.
Please educate me if this premise is wrong. (Not being sarcastic as I could be totally wrong here and leave it to greater minds to assist)