why does thoreau say that one cannot fish without losing "self respect"

In this chapter, Thoreau has a loss of self-respect when fishing. It's a base instinct which he has lost but which would be reawakened if he were to go back to live in the woods. Mostly this is because fish make an unclean diet, with all the cleaning they need. There is an instinct against animal food, and any man who wants to preserve his poetic faculties should give up eating it. Larvae eat gluttonously; butterflies can survive on drops of honey.