Koi go dormant in
the deepest, densest part of the pond where the water temperature
stays 39°F. DO NOT DISTURB THIS WATER! It is
absolutely imperative that this naturally warm, dense water stays at
the bottom of the pond. DO NOT put a
circulating pump at the bottom of the pond to keep the surface from
freezing over. DO NOT put an airstone or
bubbler at the bottom of the pond. DO NOT
keep the waterfalls flowing.
All of these things could create a condition known as
SUPERCHILLED WATER. In temperatures below 32° ice starts
to form on still water. Moving water does not freeze, but is still
at or below the freezing point of 32°. This superchilled water
has individual molecules that have frozen into microscopic ice crystals.
As soon as this superchilled water stops moving the microscopic
ice crystals allow the water to freeze solid almost instantly.
If the naturally insulated dense water at the bottom of the pond
is disrupted and becomes superchilled the same microscopic
ice crystals begin to form in the koi's gill tissues. Gill cells
begin to rupture one by one, slowly suffocating the fish. The fish
may never 'freeze solid' in the conventional sense, but the fish
IS freezing to death on a cellular level. Ruptured
cells may heal over but are replaced with scar tissue that either
does not transfer oxygen or transfers oxygen poorly. The fish may
survive the winter but in a poorly oxygenated pond the fish may
show stress in the heat of July and August when dissolved oxygen
in the water is at its lowest. The pond owner may think the over-wintering
process was successful only to find fish going belly up in mid-summer.
Large koi seem to be harmed by superchilled water more so than small
koi. One explanation is that young koi are still in
the midst of their growth stage at one and two years old. This is when they
are actively putting on body mass the fastest. They may also have
a stronger regenerative ability at this stage and can thus regenerate
gill cells more reliably than mature fish. Another explanation may
be that small koi have smaller gill tissues which can access
smaller air bubbles in the water and therefore they have a higher concentration
of oxygen in their gill cells which in turn helps to deter the
formation of ice crystals within the tissues. More dissolved oxygen = less ice crystal
formation = less damage. This is all speculation on my part, but
the end result is the same. Big koi die easier than small koi in a
poorly wintered pond.
PROTECT THE WARM DENSE LAYER
OF WATER IN THE BOTTOM OF THE POND!
