In the book Tuck Everlasting by Natalie Babbitt, Winnie Foster does not like any creatures to die because she hates to see them suffer. This relates to the fish and the mosquito. The trout is beautiful to her, and it appears as a creature that can suffer. On the other hand, the mosquito is an insect that bites and can cause harm, unlike the trout that is helpless.
This relates to an incident later in the story when she rescues the toad from the dog and pours the magic water on it. Her action allows the toad to live forever. Only briefly does Winnie realize that she has used the magic water given to her to rejoin Jesse when she is seventeen and keep her immortal. Winnie learns to respect the value of life and death during her stay with the Tucks.