Traditionally, in the Catholic church, eating fish is allowed during periods of fasting. See, for instance, this Wikipedia article. So the presence of the golden trout is doubly good for the protagonists: not only is tasty food available, but it is the kind of food that they are allowed to eat even during fast periods.
The figure of the gluttonous monk was a medieval commonplace, as in the Canterbury Tales. (If you google "monk gluttony" you will find many instances.)
A few paragraphs before the trout, the text has this passage:
When I observed this error of the women, and saw how the maidens kept their eyes upon me, I became frightened, and wondered if I could resist should temptation accost me; and often I thought, with fear and trembling, that vows and prayer and penance alone do not make one a saint; one must be so pure in heart that temptation is unknown. Ah me!
That is, the girls looked at the narrator in a way that made him, um, notice. So here we have a hint of two sins, gluttony and lust, in the first few pages of the story. Without reading the rest of the story, it looks to me like the author is setting up the narrator as a man of malleable morals.
