Dictionary of Ichthyology  

gill = 1) a paired respiratory organ in fishes consisting of gill filaments on the gill arch in the posterior portion of the head and usually providing the primary exchange of gases between the blood and the surrounding water.

Tuna gills from Wikimedia Commons: http://en.wikipedia.org/wiki/File:Tuna_Gills_in_Situ_01.jpg

Canadian Museum of Nature