What structure in fish performs the same function as lungs mammals?

The structure in fish that performs the same function as lungs in mammals is the gills.

Here's why:

* Lungs: In mammals, lungs are responsible for taking in oxygen from the air and releasing carbon dioxide.

* Gills: In fish, gills extract oxygen dissolved in water and release carbon dioxide.

While the medium they work in is different (air vs. water), the fundamental function of both lungs and gills is gas exchange.