Advanced fishing adds a slew of new fish to all different biomes. Meaning depending on what biome you start fishing in will determine what type of fish you can catch. Some fish overlap like cod and squid but in deserts you may catch sandy bass:
In swamps you may catch mud tuna:
And in snowy biomes you can catch frost minnow:
You can even go fishing in lava and catch things like Nether Salmon, Obsidifish and Magma Jellyfish!
But these fish are more than just clever names and in varying locations. These fish can be crafted into their respective blocks and items!