Tall fishy tales!! Long dead 7th C monks not knowing one fish from another for goodness sake.
How can small brown river trout be compared to the mighty salmon who battles up large rivers, overcoming rocks and weirs to return to its home to mate. Of course the celts thought the salmon was sacred and deserved a place of honour......
A quote from Anne Ross by the way, re Lydney Temple, which of course overlooks the Severn
"the sacred salmon of Llyn Lyw in the Mabingoen is associated with the Severn. O'Rahilly suggests that the fisherman hooking the salmon on Lydney Park bronze, and the salmon itself, are both manifestations of the God Nodon. These he would compare to the Finn's salmon of wisdom, whereby he gained access to magical powers"......
still, if it was a small well you would need a small fish living in it.