In the Celtic world virtually all fish symbolism was concentrated upon the salmon, which was once a common and abundant food-source among North European peoples. Other fishes - with the exception of the whale for which they used a word borrowed from the Germanic - were practically speaking non-existent and in literature, unless the word ‘fish’ is qualified, it is nearly always synonymous with ‘salmon’.
The salmon is of the same essential nature as the boar, in that both are creatures of sacred wisdom.
Wells of knowledge recur in Irish literature overhung by hazel- or rowan-bushes and in them live the salmon of knowledge who feed on the scarlet berries or the nuts dropping into the water. Whoever eats the flesh of these fish acquires second-sight and knowledge of all things. This is what happened to Finn as a boy. He was the pupil of a bard or file and was busy one day grilling a salmon for his master. As he turned the fish on a spit he burned his finger and sucked it. He instantly became omniscient and was given a prophetic tooth. Thereafter he had only to put his thumb on his wisdom tooth and chew it to become gifted with second-sight.
Salmon, again, was the food of Eithne, the allegorical figure of Ireland, after her conversion to Christianity. With the boar and the wren, the salmon was a particularly druidic creature and one of the symbols of wisdom and spiritual nourishment. It recurs as a primordial creature in the Arthurian tale of Culhwch and Olwen and in the apocryphal account of the Sages of the World in Wales, and in the adventures of Tuan mac Cairill. The body of the salmon is the last stage in his metempsychosis, and after he has lived for a hundred years in this shape, Tuan is caught and taken to the Queen of Ireland. She eats the fish and becomes pregnant by it.