Although the odd dead frog is something every pond keeper is likely to come across every now and then, four in one day is going it a bit! Unfortunately the fact that they’re bloated doesn’t give much of a clue as to what might be behind things; that’s just something that happens to frogs once they’re dead.
I think you’re absolutely right not to suspect any involvement by your fish on this occasion. While fish and small, edible tadpoles don’t mix very successfully, unless your pond is full of pike or piranha (and I’m assuming yours isn’t, obviously!) adult frogs aren’t much bothered by them.
Obviously, it’s a bit difficult to be absolutely certain, but I think exhaustion seems the most likely explanation for the deaths, especially if where you live, spawning was well underway by the end of March.
It’s principally a problem for ‘early’ females; when they arrive at the pond at the start of the breeding season, they can often be grabbed by a large number of potential suitors, with some staying on-board long after she has spawned, particularly if she is too weak to force them off. The net result of all the exertion involved in wrestling with over-amorous males, the actual spawning itself and then trying to leave can sometimes exhaust a female to the point where she has just used up all her reserves and sadly, dies. Your description of a dead frog with a live one – which I’d guess was smaller and male – clinging on to it would seem to bear this idea out. Females arriving later in the season don’t seem to be mobbed so completely, which makes exhaustion deaths less common towards the end of spawning.
A similar thing can happen to male frogs too – particularly older or weaker ones, for whom the whole hurly-burly of competing for the females simply saps their strength.
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An Ongoing Problem?
If this wasn’t an isolated incident, however and these recent deaths are part of an ongoing problem, then there may be something else at work but just what, is difficult to say. The good news, however, is that it’s unlikely to be either of the two most well-publicised ‘frog-killers’ – chytrid fungus or ranavirus.
To date ‘chytrid fungus’ – Batrachochytrium dendrobatidis – has only been seen in three locations in the UK while ranavirus usually comes with easily spotted symptoms, including emaciation, skin ulcers, rotting limbs and bleeding. Besides, it’s most prevalent when the temperature tops 25 degrees C, making it an unlikely suspect for frog deaths in late-March, no matter where in Britain you live!
If you do find that you’re seeing more dead frogs – especially out of the breeding season, when spawning exhaustion obviously doesn’t apply – it’s well worth getting in touch with your local wildlife trust or the Institute of Zoology (who run the Frog Mortality Project in partnership with Froglife).
Hopefully, however, you’ll have no more dead frogs to worry about – just swarms of tadpoles to watch growing up (well apart from the ones the fish eat, anyway!).