I always used a photo thermometer and would slide it down the fishes thought and take the fishes temp. I will then use my Color-C-Lector probe down till it reads the same temp as the fish. That way I know what depth of water that fish came from. Being a cold blooded critter it will be the temp of its surroundings just before it hit my jig. Courtesy of Dr. Loren Hill my teacher in college.