fish and water temperature

Question
Knowing that trout may hang out where the water temperature is optimal for oxygen or food, how does water temperature get measured at depth?

Answer
I had a zoology professor whose doctoral study was focused on hibernating procupines.  He indicated that one of the most difficult things to do was get a body temperature becuse it's impossible to get a sleeping porcupine to hold a thermometer in its little mouth.  The moral to this story is that if you want a temperature of anything in particular, the thermometer must go where it needs to go in order to measure the temperature.  In the case of your question it needs to go to the water depth for which you want the temperature. So, the easy answer is that you put a weight on the thermometer and drop it down to the depth you're measuring.  

Having said that, trout will actively seek 64-degrees or the closest temperature to it.  Sixty-four degrees, is the optimal temperature for a trout body to function at its peak.  Below 50-degrees and the trout will start to shut down.  Above 70-degrees and the trout will start to shut down.  So, you can safely guess that trout actively feeding and swimming around will be found where water temperatures are in the 55-65 degree range.

Les Smith
Outdoor Education
Nevada Department of Wildlife