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Quick answer
Dead amphibians in ponds are likely to have suffered during icy
weather. Any found away from water could have been caught out by
a sudden drop in temperature or attacked by a predator.
Further
information
Some amphibians, primarily frogs, will choose to overwinter at
the bottom of ponds because the water temperature there stays
fairly constant (whilst doing so they breathe through their
skin). In severe winters, when a pond is completely frozen over
for a prolonged period of time, various noxious gases which are
released from decomposing vegetation/debris within the pond
become trapped under the ice. This can 'suffocate' not only the
frogs but many other organisms which are down there. This is
known as winterkill. Lack of oxygen is not normally a problem as
plants in the pond will still be producing it, providing enough
light can get through the ice - try to clear snow from icy ponds
if it's lying for any length of time. It's also a good idea to
prepare your pond for winter by giving it a bit of a clear out
in the autumn; if necessary, re-stock with native oxygenating
plants too. After the ice has melted, the dead frogs will float
to the surface of the pond and will often appear bloated. It can
take some time for the bodies to rise.
Death of a few individuals through winterkill is a natural
process and not a problem for the population, but some garden
pond owners decide to reduce this cause of mortality. The best
way to prevent winterkill is to place a ball in a pond before it
freezes over and then remove it after the ice layer has formed.
The hole that remains should allow gases to exit the pond. If
the pond has already frozen over, place a bowl / pan of hot
water on top of the ice until a hole has formed. In the longer
term, removing excess decaying material from the base of your
pond may help (but leave some material behind as it is a great
habitat for many pond animals).
Other winter deaths can be due to sudden changes in temperature
- as frogs and toads may take advantage of milder patches of
weather to come out and forage they can be caught out by
unexpected frosts. They are also more susceptible to predation
at this time as the cooler weather makes them more slow-moving.
Preparing your pond for winter.
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