The water is also cooler.

The essential reason for these long perilous journeys to these headwaters are that the water is:

• pristine
• cool
• clear
• fast
• running shallow
• full of gravel
  
  These conditions are absolutely essential to maintain the very high oxygen levels the fish and their eggs need to survive and produce offspring.
  
  Air is absorbed by natural water-courses at the interface (the place where air and water meet - ie the surface). This air is then mixed by convection throughout the water mass. A tiny part of the oxygen in the air (air contains about 21% oxygen)
  is transferred into the water to be used by fish, plants and bacteria.
  
  On a calm summer's day in a lake the amount of oxygen in the lake would be lowered. The reasons are twofold:
  
  The interface between the air and the water is at a minimum because there is no surface movement.
  The higher the temperature of water the less oxygen (or air) the water can absorb and hold.
  
  On the other hand during a gale in winter the oxygen levels in a lake would be at their highest. The reason for this is the direct opposite to those above namely ........
  
  The interface for air transfer into the water is at a maximum - waves rise, crash, break along the bank and continuously present a new air/water interface across which air can be absorbed by
  the water.And because the water is colder it can hold more oxygen.
  
  So what has this got to do with pond keeping?
  
  Notice I said oxygen in water is used by plants fish AND bacteria.
  
  Biofilters are absolutely essential in almost all ornamental garden garden ponds to purify the waste products excreted from the fish.
  In this way the fish are prevented from poisoning themselves to death.
  
  A biofilter can only work if it gets oxygen to feed its own bacteria 24 hours per day 7 days per week. Just like you and me biofilter bacteria die if they do not have air (oxygen) to breathe. This oxygen is contained in the water being pumped through the biofilter.
  
  Stop the pump, then you stop the water flow and after about 5 or 6 hours all the bacteria in the biofilter are DEAD.
  
  The bacteria survive a while because there is normally a bit of water left in the filter. Bacteria use this as their emergency oxygen supply.
  This is why we say after about 5/6 hours the bacteria die and not immediately the pump is stopped. Once the bacteria die and you continue feeding the fish toxic chemical build up in the pond water occurs. Do this when you have a highly stocked pond, the water is warm, possibly full of algae and you have a recipe for potential
  fish mortalities.
  
  The conclusion:
  
  Water must be pumped through a biofilter 24 hours per day, every day without fail except for very short maintenance periods. You cannot go to bed, shut off the pump until the next morning and
  expect your fish to like it.