Sadly, adult amphibian deaths can be caused by many things. In the spring, lots will be eaten or killed by predators like herons, rats, foxes, grass snakes and domestic cats, which make the most of lots of frogs gathering in one place to breed. Female frogs may die of exhaustion after spawning, particularly in very cold or very hot weather, and some may be drowned by over-amorous males!
Some frogs overwinter at the bottom of ponds, staying alive by breathing through their skin. In severe winters when a pond is completely frozen for a long time, vegetation will start to decompose and reduce the oxygen levels in the water. This can suffocate the frogs and other animals under the water. Once the ice melts the dead frogs will float to the surface.
If there is no evidence that predators, breeding or harsh environmental conditions are the cause of your frog deaths, then it could be an infectious disease. Like all animals, frogs can become ill as part of their natural lifecycle and the odd death isn't of concern. The worry is when there are repeated mass mortalities which threaten a population's long term survival.
The most common amphibian disease in Sussex is ranavirus. This is often referred to as 'red leg disease' but this is misleading as infected frogs don't necessarily have red legs and some healthy frogs naturally have red skin. The disease is most noticeable between June and August when adult frogs and toads can be found dead or dying in or around the pond. Symptoms of ranavirus include:
- Redness of the skin
- Breakdown of the limbs
- Drowsiness
- Abnormal wasting
- Skin ulcers
- Bleeding
But not all infected individuals display symptoms.
It is very unlikely that you have done anything to cause these deaths, so don't clear out the pond or remove it. Just remove the dead animals and bury or burn the bodies. Frogs don't need to breed every year to be successful and there is no reason to think that the deaths will happen again next year.
You can help us understand diseases better by reporting any mass amphibian deaths to the Garden Wildlife Health initiative.