But by the early 1990s the Blackfoot was in such bad shape--its valley scarred by clear cuts, its tributaries poisoned and turned orange with acid mine runoff--that the movie had to be filmed on rivers to the south.

FORBES: Riverkeeper

The acid from the lime, lemon or orange juice changes the structure of the proteins in the fish much in the same way that heat does. (If you make it at home, you can observe the flesh turning from translucent to opaque.) Therein lay my answer.

NPR: Cool and Comforting Ceviche