I would be willing to bet our forefathers thought the huge catches of salmon they were making would last forever. History has certainly proved that assumption wrong.
"Those were the days my friend, we thought they'd never end"
Mary Hopkin may not have been a salmon angler but that line in her song certainly resonants with me every time I step onto the bank.
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Having followed the salmon runs closely from the South, to Scotland to Scandinavia for a few years now, I don't think the conduit of the previous generations of fishers made much difference. It was probably not possible to stop the decline by C&R and, unfortunately, it's likely not possible for us to steward and leave it in even the poor state in which it is now, with all the care in the world, wether we fish or not. Every fisher and ranger in Europe finds faults which are particular to their local river on which they blame the decline: seals, dolphins, pollution, goosanders, cormorants, too little water, too much water. Yet, despite continuous improvements to riparian habitats, they all go in the same direction.
There will be singular success stories along the way, of which, the only notable ones I know of involve different forms of hatcheries. Considering the low level of income generated by Avon fishers and also the discussion around the subspecies of chalk stream salmon, paired with the concern regarding the genetic degradation caused by hatcheries, I don't see this ever becoming an acceptable option.
Long story short, I think unless the imbalances in marine ecosystems go in an unexpected direction where suddenly abundant food sources and new, more protected feeding grounds become available for salmon, the decline will continue slowly until it reaches unsustainable levels, fishing stops, 100 or so fish continue to run the Avon for a couple of decades until they are wiped out for good likely by successive years of extreme weather events.
This doesn't put me off though, you can't be frustrated over what you can't do a thing about. We are direct, on-the-ground witnesses of the major changes our planet is going through. My grandkids will probably not believe we ever had them in the South of England until I show them a photograph of one of the last, carefully handled salmon to ever run the Avon.