How very annoying for you! Without knowing a lot more about the layout of your garden and its surroundings it’s a bit difficult to be too sure, but there are a few possible culprits which spring to mind.
One obvious possibility is that some budding local young naturalist is doing a spot of spawn “re-location”; having been just such an annoying little nuisance myself as a child where my beloved amphibians were concerned, I know you should never underestimate the determination of frog-loving youngsters! However, the activities of such well-intentioned, if deeply irritating, invaders normally take place during daylight hours – and can usually be curtailed by good fences and a few strategically-planted spiky bushes.
Ducks would probably be the second suspect for spawn theft; mallards in particular seem to be quite partial to frogspawn and they usually turn up at night – or at least around dusk. If the water surface seems a bit oily (from their feathers) and the pond edges muddier than normal, you’ve probably found the guilty parties.
However, none of this really explains why the spawn should hang about for a week and then disappear; it all seems a little too regular for small children or hungry ducks! If the jelly changes in appearance at all during the week it lies there – especially if it goes milky or starts to lose its structure – it sounds as if the eggs themselves are not viable, which would suggest that something else could be going on.
I wonder if you have checked your pond water quality recently. Factors such as a shift in pH, changes in nitrates and so on might be causing problems for the spawn. If you do have water quality records going back to any of the years before this started happening, it might give a clue – and if you haven’t been testing previously, now’s a good time to start.
The only other possibility which springs to mind is that the local frog population isn’t quite as healthy as it was fifteen years ago. Many amphibians are in decline – principally due to the effect of the highly infectious chytrid fungus; an ill or ageing population of frogs might simply not be producing as good quality eggs as before. Your local Wildlife Trust should be able to tell you about the general status of frogs in your area.
What to do About It
One thing you might like to try in the spring is to take some of the spawn as soon as you notice it and leave it somewhere safe – perhaps a garage or shed – in a bucket of pond water and keep a careful eye on it. Nothing’s likely to get at it, so if your problem really is of a feathered or short-trousered kind, you should get a horde of wriggling tadpoles in the normal run of things. On the other hand, if the spawn simply starts to disintegrate, you’ll be able to watch what’s happening, which might give a better clue as to the cause.
The only other suggestion that might be worth trying is to suspend a suitable mesh cover over the pond itself, to prevent anything getting at your spawn; it won’t tell you what has been treating your garden as a snack bar, but it might make it a little less appealing in the future.
Good luck, come February.