 Which Color Depthfinder Distinguishes Fish Species?
Wild Bill,
Yes, of course the electronics get credit. That's why Furuno's are so loved, they do this better than anyone else.
Here's the deal, deep in the gizmos that make up the fishfinder there is some sort of logic circuit and it works something like this. The fish finder is putting out a pulse of energy with 10X power. Ping! it goes off into the water. The sound hits something and comes back but the return echo is only 1x in strength. There are a number of things that make that return echo weaker than the outgoing one was and they are almost all related to how far it traveled through water of some salinity and some temperature. The other big component of the strength of the return echo is how reflective of sound the target was. A jelly fish does not give good echos. A fish with large hard scales, let's say a Tarpon, gives very very good echos in deed.
Now at the fish finder 10x is going out and the very strongest return that can possibly come back is 1x, so the fish finder will paint a dot on the screen in the appropriate place every time it gets a return echo with a strength between 0.8x and 1.0x. That dot will be bright red If the return echo strength is 0.6x to 0.79x it might paint the dot yellow, and so on through the colors from very harsh red to very gentel green at the soft end of the scale. But you have to remember the hardness or softness of the painted dot is a function of more than the absolute strength of the return, its is also related to the depth from which it came - which is a function of the time of the echo return, not its strength.
So, with that special knowledge, normally reserved for the grand master wizards, you can now see why a Tarpon swimming under the boat will show up as a nice Red target while some small baitfish down below it might only rate a light blue or green. Me, I can never tell anything. Sometimes I yell out, Wow! Shit! Look, there's something down there. Then we continue to fish ......
Thom | 
