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Why am I finding dead frogs in the garden?

Sent in by Laura Allard, Sunbury-on-Thames

There are several possible causes of death of these frogs. In winter, the most likely cause of death is anoxia (lack of oxygen).

Many frogs choose to overwinter in ponds. They can breath through their skin, so can stay submerged for long periods. However, in very cold winters, ponds can freeze for long periods. As vegetation in the pond decomposes, it uses up oxygen and the frozen condition prevent more oxygen being absorbed into the water. Frogs that have died of anoxia will float to the surface when the ponds thaw.

This is a natural occurrence and does not impact on frog populations. However, it can be distressing to see and there are steps you can take to allow oxygen into the pond. Placing a ball in the pond before it freezes over and then removing it once frozen will leave a hole that allows oxygen and other gases to enter and escape. Placing a bowl of hot water on an already frozen pond will also create a hole.

Disease is another possible cause. There is growing concern worldwide about the effects of disease on amphibians. In the UK, the most common disease is ranavirus. This can cause lethargy, emaciation, sores and bleeding. The bleeding can sometimes occur under the skin making the frogs look red. However, this disease is most commonly reported in the summer in temperatures above 25C.

Predation is also possible. They are most vulnerable to predators in spring when large numbers gather to breed. Herons, rats and cats are most likely to predate them in a garden and bodies could be left behind. However, this is unusual and there would normally be an obvious wound on the frog.

Large number of dead frogs can be reported to the Frog Mortality Project, which is run by Froglife and the Institute of Zoology.

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