ok CB, my friend, since you say you are being serious.

Now you are talkin about body density. That is at least believable because different species of fish could have enuff difference in body density to display differently in much the same way as different bottom densities such as sand vs. mud display differently.

But "differences in body temperature" ???

No way.

1) Fishfinders use sonar. They send and receive sound waves at specific frequencies. They don't measure temperatures at depth.

2) Fish are cold-blooded animals. Their body temperature is the same as the water they are swimmin in, regardless of species. Let's say the water is 50 degrees and a bass and a blue are swimmin under your transducer. Both fish are at 50 degrees.

3) Let's say that the fishfinder did measure temperatures at depth with "thermal imaging", and not with sonar. Then those fish would be invisible because they are at the same temperatures as the surrounding water. (When a fishfinder is displaying a thermocline or temperature break, it is able to show water at different temperatures because water at different temperatures has different densities. So does water of different salinities.)

I hope this helps.