Spawn has died or is not developing properly.
Common causes of spawn failure are late frosts and a lack of light.
Spawn requires warmth and light to develop properly. It can be extremely sensitive to frosts, particularly during long spells of icy weather; though some of the spawn may survive a light frost.
If your pond is too shaded (by overhanging vegetation or overgrown aquatic plants) you might notice that spawn fails to develop as there is not enough light or warmth reaching the water.
Occasionally pond-owners notice a white filamentous fungi covering dead spawn. This is not responsible for killing the spawn, it is a natural decomposer of dead pond matter and has grown after the spawn has died.
In some situations spawn continues to fail year after year – the jelly coating breaks down before the tadpoles are fully developed and ready to hatch. This may just be a case of having been unlucky, with different things affecting the spawn each year, but sometimes it’s unclear what the cause is and how best to help. Moving the spawn to hatch in a bucket of rainwater can sometimes combat this and will help confirm whether it is an issue with the pond or a problem with the frogs laying the spawn.
Spawn/tadpoles have disappeared.
Common causes of disappearing spawn/ tadpoles are predators and cold weather.
Spawn and tadpoles require warmth and light to develop properly. If they have disappeared it may be because they’ve died. Late frosts or a shady pond could be responsible for this. Sometimes spawn sinks out of sight but still develops normally.
The most common cause of disappearing spawn/tadpoles in the numerous predators in and out of the pond. These include fish, newts, water boatmen, dragonfly larvae, birds, rats, foxes and hedgehogs.
A disappearance could also just mean they have just moved on to the next stage – if there is no spawn, it may have hatched, if there are no tadpoles, they may have metamorphosed into froglets.
I’ve found dead tadpoles, what might be the cause?
The death of tadpoles is often related to a lack of oxygen in the water, usually caused by a sudden algal bloom.
If there has been some warm weather and the water has turned green, this indicates that there is a lot of algae growing in the water. Algae removes oxygen, leaving the water ‘anoxic’, and the tadpoles suffocate. To avoid this happening again, make sure the pond is not completely in sunlight and that there are plenty of aquatic plants present, which use up excess nutrients (stopping too much algae growing) and produce more oxygen.
If the water is not green then it could be that some sort of chemical/product has been washed off surrounding land by rain and run into the pond, or even the bark or leaves of a toxic plant have landed in the water. If you suspect this to be the case contact the Environment Agency or Pond Conservation for further information.
Spawn/tadpoles are being preyed on, should I do anything?
No! They form an important part of the foodchain and it’s best to let nature take its course.
Amphibian eggs and their tadpoles make up a vital part of the foodchain; the reason amphibians produce so many eggs is that so few of them survive. You should expect that over 90% of the eggs, tadpoles or young amphibians in your pond will be eaten throughout spring by numerous predators, including: dragonfly larvae, water boatmen, grass snakes, birds and hedgehogs. Larger tadpoles may also prey on smaller, weaker tadpoles.
Tadpoles are an important food source for wildlife, including other amphibians – newts are a predator of tadpoles, especially in the weeks after frog spawning when adult newts are in the pond laying their eggs on pond plants. Garden ponds are often home to more than one species of amphibian – this is a healthy situation and indicates that the pond is functioning well. Normally a cyclical predator-prey relationship will establish – while numbers are high for one they are low for the other until a critical point and the situation becomes reversed.
There is no need to remove newts or other predators though we would advise against deliberately introducing fish to a wildlife pond.
Adding places within the pond for tadpoles to hide could help increase their chances of survival. Potential hiding places include rocks/pebbles, plants and aquatic planters. We don’t advise that pond-owners use pond-netting to keep potential predators out – sometimes the animals you’re trying to attract (like grass snakes or hedgehogs) can become caught and die.
Some people opt to remove some tadpoles from the pond and raise them in captivity, to give the local frog population a helping hand. It is entirely up to you if you wish to do this.