Yes, yes, yes, literally it couldn't ....but I know this river, it's on my doorstep
and what is now a scoured channel was a wide shallow river, fordable by foot for great lengths of it, that ran from the Firth right up into the into the heart of southern Scotland, with dozens of burns and rivelets running into it.
Even now, just post industrialisation, we can see the salmon in the river. Just look over the bridge or down at the weir and there they are. Pre industrialisation they ran in their millions.
Look at some of the images of the salmon runs in Canada, and that would have been here too. The Clyde, the Spey, the Dee, the Forth, the Don......every river running down to the sea then still does so now, and there are salmon in all of them.
I learned to guddle wee trout as a child, for my father scorned the man who left a fish with it's mouth ripped apart with a hook; it was said that a man could just lift the fish out of the river with his hands....I've done it. No reason they couldn't have done it back then.
Interesting the turf and surf bit though; we're kind of taught that fishing was a mainstay until something like this research comes up and folks have to reassess the evidences they have.
cheers,
M