You can catch fish with just using your fish finder, but your catch ratio will at least double using a speed/temp. I started with just using my Fish Finder looking for bait and fish and did ok, but now I will run to an area drop my probe and if I do not see the water temp I am looking for I dont set up and keep going looking for the right temp. Most of the time you will get fish that did not show up on you FF by just fishing the correct Temp and going the correct downspeed. What you will notice for example is say heading North you will need to go 3.2 SOG on your GPS to put decent action in your lures and catch fish and for the same depth heading South you may only need to go 1.8 SOG for tha same action. This is all caused by the currents below. In my opinion a decent Fish Finder with a Speed/Temp probe is the most deadly cobination you can have.