Can a frog drown

info fauna
Coordination office for amphibian and reptile protection in Switzerland (karch)

For several years, reports have been coming in that several to dozen or even hundreds of dead frogs have been observed in ditches and ponds, especially garden ponds, in the winter months. These are mostly common frogs, more rarely water frogs, which immigrated to the pond in question to overwinter in autumn. Normally this works without any problems: the oxygen dissolved in the water can be absorbed directly through the skin; Breathing is guaranteed even under an ice cover, surfacing is not necessary.

Apparently, conditions are now increasingly emerging, especially in isolated water places, which are fatal for the frogs. It is certainly not because the ponds are freezing through. Ice sheets over 20 cm thick are very rare. That is why it is not an insufficient depth of the pond that is responsible for the death of the animals, but rather an unfavorable ratio of water volume to water surface. Shallow but large ponds seem less affected than small, funnel-shaped ponds. However, the course of the weather during a winter is certainly also responsible for the development of the phenomenon; however, the connections have not been fully clarified. It is also not entirely clear when the animals die. The dead frogs are often only noticed after thawing in spring. Interestingly enough, the event also occurs frequently in garden ponds that are seven to ten years old.
 
The degradation processes of organic material (dying aquatic and marsh plants, fallen leaves, algae floss) are certainly partly responsible for the death of the animals. There are three explanations of how this can lead to animal death:
  1. The degradation processes consume oxygen, which can then become very scarce and lead to the animals asphyxiating.
  2. Poisonous degradation products may also form, which are concentrated under the ice cover. The animals cannot escape and are poisoned.
  3. The hypothesis has already been put forward that poisonous substances (e.g. acids) are introduced with the fallen leaves. With increasing over-fertilization of our entire Mittelland landscape and the mostly high autumn and winter temperatures in recent years, the degradation processes are running at full speed and require a correspondingly large amount of oxygen. With completely isolated bodies of water (especially garden ponds on foil), no exchange with the soil and no water renewal can take place, so that the effect is intensified.
What can we do about this frog death?
First of all, it should be noted that the phenomenon rarely hits the same pond several times in a row; Furthermore, the frog population is usually not endangered in its existence, since some of the adults and the young always hibernate on land and are not affected. Therefore, frog spawn is almost always produced in the following spring.
Possible actions:
  • Individual dead animals can be left in the pond; larger amounts should be taken out and buried.
  • Do not allow dying marsh and aquatic plants to sink into the pond in autumn, but cut or rake out and compost.
  • Do not leave fallen leaves in the pond; possibly cover or rake out the pond with grating in autumn.
  • Do not let the mud layer become too thick; "General cleaning" every couple of years: pump out, shovel out the sludge, cut back reed beds vigorously.
  • Try to keep one area free of ice at all times. In severe frost, however, this is hardly possible. Water pumps can do a good job, but are questionable for energy reasons. Perhaps you will find a good idea yourself?
  • If the pond is to be renovated, consider enlarging it (large surface!).
  • With dense barriers around the pond (as in the case of rescuing amphibians from traffic), the frogs are prevented from immigrating into the water in autumn. However, the fence must be in place from August to December.
We would like to point out that these measures should only be taken if the problem has arisen or there is reason to fear that it will occur. Intact ponds should be disturbed as little as possible.