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
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).
FAQ: protecting pond life during icy weather.
Clearing out your pond.