Sources : Salmon

Pliny the Elder [1st century CE] (Natural History, Book 9, 32): It is also a fact of nature that different fishes hold the first rank in different places ... in Aquitaine the river salmon is preferred to all sea-fish. - [Rackham translation]

Gerald of Wales [c. 1146 – c. 1223] (Topographia Hibernica, Distinction 2, 41-42): [At the top of a river waterfall] is a small cavity, hollowed out in old times by holy men, into which the salmons leap in great numbers from below, the distance of the length of a full-sized spear, in a manner so wonderful that it might be thought miraculous, unless such were the habits of the fish; for this species has the natural instinct to take such leaps. Their peculiar mode of leaping is as follows. Fishes of this sort naturally struggle against the stream; for as birds fly against the wind, so fishes swim up the current. Upon meeting, however, with any very precipitous obstacle, they bend their tails backward towards their mouths, and sometimes, in order to gain more power for their leap, firmly compress their tails in their mouths. Then suddenly releasing themselves from the sort of circle thus formed, with a particular jerk, like the sudden reaction of a bent rod, they spring from the bottom to the top of the leap, to the great astonishment of the beholders. - [Forester translation, 1863]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Fish 7.71): Salmon, as he says [i.e. Liber rerum], are fish which the barbarians call lastas. They grow to a great width and length. They are robust in strength, and all their agility is from the power of their strength rather than from lightness of body. For they are ponderous and heavy. The salmon, as Pliny says, was in ancient times preferred to all river fish, and especially, as he says, in Aquitaine. As for these, they produce offspring and cannot live long in the fresh waters of ponds, unless they have free passage to the sweet waves of rivers; but even where it finds an obstacle in the passage, joining its head to its tail, it flies over with a bent body to where it wants to go. This is exactly what it does when it feels trapped. They do the same thing when they want to cross over to the salty waters of the sea. For they grow most easily and rapidly where sea water grows and shrinks when it meets fresh water. Hence in river waters the smallest salmon are found, about the length of a finger. But salmon of a foot in length are very rarely found in fresh waters, because after a year has passed they are wont to pass over to the salt waters of the sea where they mix with fresh water; having grown to their due strength, they return again to the place of their birth. Its flesh is red, and although it is sweet and much appreciated, it quickly fills those who eat it. - [Badke translation/paraphrase]