Thanks for your answers and link. the preamble to my question would be that a year ago I inadvertently brought some juvenile newts to my pond with some aquatic plants that I had bought. It was nice to see them grow - they ate all the mosquito larvae. Later, I introduced some tadpoles to the pond as I wanted to propagate frogs as a natural predator against the slugs in my vegetable garden. Unfortunately, the tadpoles disappeared and the newts got fatter. Then the newts disappeared. This could have been for many reasons but I suspect that the newts, having eaten everything in the pond, starved to death. I admit my ignorance of the habits of newts.