For a discussion of the Celtic and Indo-European folk motif "The Oldest Animal(s)" see Bromwich (* notes for Triad 92).
Wise salmon are a common Irish myth. Hazel nuts containing knowledge were said to drop into streams and be eaten by Salmon (for a summary see Muhr*). In addition the "Salmon of Knowledge" is a feature of the Fionn mac Cumhail tales (information). This might seem a long way from our wonder, but note this reference in the Life of Cadog (English Translation), written in the 12th C, but set in the 'Dark Ages' and probably based on older materials...
At another time it happened that blessed Cadog one day sailed with his two disciples...from the island of Echni, which is now called Holm, to another island, named Barren, Barry (Map). When he had prosperously touched port, he asked his aforesaid disciples for his enchiridion, that is, manual book. But they confessed that through forgetfulness they had left it behind in the aforesaid island...Burning with anger he uttered a reproof of this sort against them, ‘Go ye, never to return.’...the aforesaid volume being recovered, they soon returning on their watery course even to the midst of the sea...the boat being unexpectedly overturned, they were drowned... Cadog, desiring to refresh with food his body wasted with fastings, bade his followers to get fish for him for supper. As they were going to sea for the purpose of fishing, they found on the sand a salmon of wondrous size, and rejoicing they bring it back to their preceptor. When they had disembowelled it, they found the aforesaid book in its entrails, unhurt by any injury of waters, and white.
While the latin Lives of St.David tell that his father was sent hunting in south-western Wales by an angel for a stag, a salmon, and a swarm of bees, and the lives interpret this as indicating David's future power, abstinence, and wisdom* (though it is not clear whether they are in this order). Both references show the importance of Salmon in the South of Wales, and the Cadog story is specifically set in the Severn.
The Severn area has had a long tradition of Salmon fishing. Gerald of Wales* notes:
River-fish are plentiful, supplied by the Usk on one side, and by the Wye on the other; each of them produces salmon and trout; but the Wye abounds most with the former, the Usk with the latter. The salmon of the Wye are in season during the winter, those of the Usk in summer...
While Thomas Churchyard* has:
A thing to note, when Sammon failes in Wye,
(And season there: goes out as order is)
Then still of course, in Oske doth Sammons lye,
And of goodfish, in Oske you shall not mis.
And this seemes strange, as doth through Wales appeare,
In some one place, are Sammons all the yeers:
So fresh, so sweete, so red so crimp withall,
As man might say, loe, Sammon here at call.
The Severn estuary itself has a long history of core(/a)dau ("fish weirs") and fish traps, indeed, the Gwent Levels have the highest concentration in Wales*. On the Severn the weirs were largely wicker fences directing fish on the ebb of the tide into basket traps, or supporting the traps themselves*. In particular the area was known for its putchers - open weave baskets for catching salmon, though these appear to have been an 18th C. invention, prior to which historically known baskets seem to have been designed to catch a range of fish*. The traps at Goldcliff appear to have been worked right up until the end of the 20th C*. A Saxon charter of 956 CE notes 104 weirs at Tidenham.
Of particular note are the traps and weirs found at the outfall of the River Troggy near Caldicot. These were both considerable and old, dating in some cases to prior to the Norman invasion, and, indeed, within the Pill, to the early Bronze Age. Ironically, Hazel seems to have been used in constructing some baskets*. The link between Salmon and the area continues on the other side of the river; when the Severn Tunnel was being dug, the roof fell in and drained a shallow pool on the English side opposite the Troggy, called Salmon Pool*.
However, there is one problem with the Salmon, and this is that the Atlantic Salmon which we have in Britain doesn't tend to grow big enough to be our fish. There are, of course, some large salmon. The largest officially recorded Atlantic Salmon appears to be that caught in Restigouche River (New Brunswick, Canada) in 1990. It weighed 72lb (32.66kg), and measured 68.5" (174cm). This appears to be around the size of the largest Salmon reported from Norway and Russia, where the largest Atlantic salmon are supposed to live. In Britain the largest official freshwater Salmon appears to be that caught in the Tay by G.W.Ballantine in 1922 which weighed 64lb (29.03kg). However, a Scottish Fisheries Board expert apparently reported in the 1920s that "an illicitly caught salmon had been taken in the Forth that weighed 103 pounds" (46.72kg)*. The largest caught in the Severn area appears to be the Salmon caught by Robert Crawshay at the turn of the last century in the Usk of 44lbs (19.96kg)*. However, most Salmon are an order of magnitude smaller than these. A more likely candidate for a fish large enough to pull a man, and reach advanced years, would be a Sturgeon. There is a 'Royal' Sturgeon in Hereford Museum, caught in the Wye by James Postans of 8"6' (2.6m) length and 182lbs (82.55kg) (Photo). A European Sea Sturgeon (Acipenser sturio) caught off the Orkneys in 1956 was 3.18m and 700lbs (317kg)*.
While Sturgeon are now very rare, the European Sea Sturgeon was once much more common. A late-Saxon list of fish caught at Tidenham on the Severn near the Wye lists Sturgeon, porpoises, and herrings - the latter apparently being more important than the Salmon catch of later years (* BookX). Most notably, there was a large influx of the North American sturgeon (Acipenser oxyrinchus) between 800 and 1200 CE into northern European waters*. While the evidence for this is strongest in the Baltic, one might wonder if the sudden appearance of this fish in the Severn at this time didn't contribute to the story - though it would be a pretty poor fisherman who couldn't tell the difference.
Finally, it is worth noting that the disturbance of the bore has been used to fish for elvers (young eels) on the Severn for at least 450 years. The removal of elvers was banned in 1553, with a repeat in 1558 and from 1677 to 1778*. The discription by Lynch* on the Severn Science page is from a traditional "Good Friday" elver fishing trip. You can read more about this and the other uses of bore around the world in Donnelly and Chanson*.