Isidore of Seville [7th century CE] (Etymologies, Book 12, 6.6): [Fish have names] based on color, as the... varii from variegation; these are commonly called ‘trout’ (tructa). - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Fish 7.86): Trout [truite] are fish, as Isidore says. They live in fresh water rivers, and only in rivers which run with great force from the mountains. It has scales like salmon and meat very similar, but much more delicate, and this only in the summer from July to November. But in winter they have white meat like the rest of the fish and less flavor. They have spots on their backs that are crimson and bloody. - [Badke translation/paraphrase]

Vincent de Beauvais [circa 1190-1264 CE] (Speculum naturale, Book 17.97): The trout is indeed variegated on the outside, like the perch, but red on the inside. And, as it is said, as long as it exists of a lesser age and body, it is nourished only in fresh waters, and is called a trout. But when it grows up and descends to the borders of the sea and drinks from its saltiness, it becomes a salmon; which, however, is by no means asserted to be true in all accounts. Both fishes are delicious and tasty. - [Badke translation/paraphrase]