fish will usually lay in diffrerent thermoclines depending on species warm water fish will lay in warmer ones than say kokanee you can find probably online what temps different fish prefer. When trolling for a specific species you can find the correct depth to troll at by looking at the fish on the graph and the ones that are in the thermocline for the species your after, the ones out of that thermocline might be a different species. ussually when you are fishing for kokanee if you are catching trout you want to troll deeper but if you were to look at your graph all the fish that were showing up on it look the same, thats when the thermocline can come in handy. I did see a book from North American Fishing club that listed preferred thermoclines for different type of fish, keep in mind also that when fish are spawning they sometimes move shallower and into warmer waters and move back deeper when they are done.
Very good explanation on therclines fish4life, and by no means trying to start crap... but after some consideration how does a Hummingbird 100 measure different temps at different depths, usually the transducer measure surface temp or the the temp the transducer is at. I run a Lowrance X510c, and all I get is surface temp.
I'm pretty sure it would measure it by water density. The colder water being more dense and the warmer being less dense. The sonar can pick up where the temp change happens but can't actually tell the temperature.
Yep it will measure it by density. Knowing where the thermocline is can be EXTREMELY helpful for finding kokes, for instance, which will be laying below it. Trout as well. If you have ever gone diving in a lake you know that the water will be warm, warm, warm then cold as HELL all the sudden. That's the thermocline. Also, water that seems murky or stained due to algae bloom will often be clear below the thermocline due to the difference in water density. The murk is buoyant and will pack up in the lighter water (surface).
I dont know exactly how they work maybe water temp drops so many degrees at so many feet kinda like altitude but it is just aguidline and warm currents or springs would throw this off. But it is maybe a point to start and you could adjust depth until you are catching what you want. I think a digital thermometer would work also but temp might be a little off as you are bringing it up through the wamer water colums.