No sign of Tadpoles in the Pond

Spring has finally arrived and we have been enjoying a period of settled, warm weather with temperatures up to 20 degrees Celsius (not bad for our part of the world, at this time of year).

Staff have been keeping a close watch on the pond, after the virtual disappearance of the tadpoles just a few days after hatching.  The last sighting was on Tuesday morning (April 29th) when one tadpole was seen swimming close to the large clump of Elodea towards the centre of the pond.  The pond looks very healthy, the water is clear, well oxygenated and there is certainly plenty of invertebrate life in the pond, but no tadpoles.

We suspect that the tadpoles may have been eaten, it is certainly true that we would expect only a very few to reach the adult stage and leave the pond as little froglets.  According to informed literature mortality rates for these animals can be as high as 99.95% but it is slightly shocking to have to contend with the thought that hundreds of tadpoles met an untimely death just a few days after hatching and finally freeing themselves from their protective jelly.

Some team members had hoped that the tadpoles had simply concealed themselves amongst the pond weed and were remaining inconspicuous, but as the water has warmed up due to the higher temperatures we would have expected to find more signs of tadpole activity.  No tadpoles have been observed for over a week.

As for what might have caused the demise of our tadpole population, we are still debating a number of theories.  Frustratingly we have no evidence to work with, the tadpoles were around in profusion a couple of weeks ago and now there are virtually no signs of them.  Many could be hiding in the weed or silt at the bottom of the pond but if this is not the case then we have to consider what might have wiped them out.

Tadpoles have many predators and succumb to fungal and viral infections.  The absence of any bodies to be seen, coupled with the relatively healthy state of the tadpole population and the healthy state of the pond itself leads us to discount these theories.

We think they have been eaten, but by what?  Certainly, Blackbirds and Robins eat tadpoles and catch them by wading into the shallows.  We have seen Blackbirds in particular around the pond, we are away of a nest site nearby and we have observed these birds taking a bath in the pond but no one has seen them catching and eating the tadpoles.

The majority of us suspect that the other pond life has decimated the tadpole population.  Once out of their protective jelly and swimming freely they would have been easy prey for the newly emerged water-boatmen.  However, the main culprits may be the numerous damsel-fly larvae that can be seen in the pond.  There are a large number and these are known to be voracious predators.  We suspect that many tadpoles may have ended up being eaten by these creatures.  We have a number of damsel-fly nymphs in the pond, they can be distinguished from other larvae by the three appendages that stick out like fans from the back of the thorax.  These are not part of the tail but the caudal lamellae, a set of gills.  There are a number of different sized larvae in the pond, this is not surprising considering that they can spend up to 5 years at this nymph stage

These larvae are ambush predators and hunt mainly at night and at the moment they are the number one suspects as we set out to investigate our tadpole disappearance.

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