The temperature of....the water!
Important when trying to establish feeding
temperature and depth of fish. Also important for rate of retrieve. Cold
water, slow retrieve, also deeper fish. Very cold water forms ICE which can
really spoil a good days fishing although watching a trout try to take a DRY
FLY through ice can be entertaining.
In warmer summer temperatures the fish may also be deeper due to increased
oxygen levels in cooler, deeper water (cold water dissolves more oxygen than
Courtesy of Carl