Dr. Passovoy,
I
have an urgent problem with my one of my koi ponds. As the ice is thawing today
there are a number (7 so far) of dead koi. This pond was put in in 2000 and has
been well established. There are are approximately 30 fish from 2 years old to
over 15 years. The pond is over 3 feet deep at the center and we have probably
only lost 4 or five fish since it was installed. Normally we run the falls which
connect to another pond until January but had to shut it down and start the heaters
the first week of December this year. The fish I looked at this morning don't
have any outward signs of fungus or damage and only one seemed to have been dead
for an extended time. The others seem to be recent. There are live fish in the
other pond and I have seen some of the smaller ones and a large frog in the pond
in question but it still has a lot of ice on it so I don't know the total fish
count.
Could there be a contaminate leaf debris that is fatal to fish that would have
blown in? I can't think of any, or that would have not blown in previously.
Hi Bob,
The three most common causes of winter-kill are thermal shock, icing over and
hydrogen sulfide accumulation. The last two are related. Thermal shock occurs
when the temperature of the pond changes suddenly, usually as a result of the
sudden introduction of snow. Heavy snowfall, or more commonly, the failure of
a protective structure and the sudden introduction of large quantities of snow
will do it. Weaker fish will tolerate this poorly and will die as a result. The
sudden introduction of what is essentially pure water without dissolved minerals
can also catastrophically lower alkalinity in the pond and destabilize the pH.
Sudden changes of any kind to the water quality in a closed system at low temperatures
are dangerous, though those same low temperatures are actually protective for
ammonia!
Icing over locks in the pond's surface and leads
not only to eventual oxygen depletion but also to accumulation of hydrogen sulfide,
the major breakdown product of anaerobic bacterial breakdown of organic debris
trapped between and behind your rocks and gravel. Experienced koi keepers go to
great lengths to keep their ponds unfrozen, employing high-capacity pond heaters
(not the gadgets purchasable at the Farm and Fleet, but big coil heaters capable
of maintaining 15,000 gallons at above-freezing temps all winter) and greenhouse
structures to protect the pond from snow, ice and debris. If your pond does ice
over, attempts to break up the ice with a blunt instrument damages the fish's
balance mechanism (sort of like us getting trapped in a belfry during the All-England
No-Holds-Barred Smackdown Bell-Ringing New Years Marathon [see The Nine Tailors
by Dorothy Sayers for details]) and they'll stress out and die as well.
Bacterial, viral and parasitic causes are much less likely in very cold water,
since water temps below 40 degrees inhibit the activity of all of these microorganisms.
Water quality and thermal issues are much more likely.
Necropsies won't
tell you much in this case, as the cold water already guarantees that no bacteria
or parasites are active.
Bob
