Multiple comparisons say serpent not salmon⚕️

In Irish mythology, several primordial beings that personify old age and ancient knowledge are described as taking the shape of a salmon. Most notably, this includes Fintan mac Bóchra and Tuan mac Cairill.

The Welsh Hanes Taliesin (16th c.) has a similar story of how the poet Taliesin received his wisdom, that also involves shape-shifting into the form of a fish.

Heinrich Zimmer suggested that the episode may have been transferred from Scandinavia as part of the heritage of the Norse-Gaels. This is supported by further circumstantial evidence regarding Norse motifs in the Fenian cycle, including his suggestion that the name of the Fianna can be traced back to an Irish rendering of Old Norse fiandr "enemies (pl.)" > "brave enemies" > "brave (free) warriors" (Zimmer 1891, p. 15ff),.

In the Icelandic Völsunga saga (late 13th century), these motifs also recur: Odin, Loki, and Hœnir slew an otter that they later found to be Ótr, the son of the dwarf Hreiðmarr. The treasure Hreiðmarr was receiving as ransom was then protected by his son Fáfnir, who took the shape of a worm or dragon. 

On behalf of his brother Regin, Fáfnir was later slain by Sigurð. Regin asked Sigurð to cook Fáfnir's heart for him to eat, but, like Fionn, Sigurð tasted it and gained knowledge of the speech of birds. Thus he learnt about Regin's treachery and confronted him. 

Similarly, Saxo Grammaticus (Gesta Danorum, V.2.6-V.2.8, 12th c.) describes how Eric acquired eloquence and wisdom by eating the snake-infested stew his step-mother Kraka had prepared for his half-brother Roller.

The motif is well-known in European folktales and corresponds to Aarne–Thompson type 673 (KHM 17, see The White Serpent's Flesh), frequently found in Central and Eastern Europe, but also in Scotland, Ireland, Scandinavia, in the Baltic countries and occasionally also outside Europe. 

As both the majority of fairy tales and the Germanic versions point to a serpent being digested, but not a salmon, it seems likely that the salmon is a substitute for an original serpent.