Bluetooth Sonar Fish Finder for Smartphones

P Smith said:

You must see the fishes against your position, not the sonar.

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You put it on your line and cast it out. That's usually going to be within sight of where you are. So it's pretty much like, "OK, so there are some fish right over there".
If you need GPS to get to a spot that's already within sight of where you are - well you've got problems.