Since the ice and snow thawed quite a few people have been reporting dead fish and dead amphibians.
What’s been killing these animals?
One obvious answer is lack of oxygen, though it may not be the only one.
The goldfish (Carassius auratus), koi carp (which are a form of the common carp, Cyrpinus carpio) and frogs people have been finding dead are all quite resistant to low oxygen levels, the goldfish especially so.
Goldfish can survive for days to several weeks at 0 degrees C in the complete absence of oxygen – and pretty much indefinitely in cold weather with low oxygen levels.
Even more remarkable is the close relative of the goldfish, the crucian carp (Carassius carassius – so the same genus as the goldfish).
The crucian carp is the fish world champion at staying under the ice in winter and can tolerate literally months without oxygen.
Crucian carp are happy in silty ponds, like this pond in Norfolk, with low oxygen levels. In winter they can survive in cold water under the ice for months without oxygen
In carefully controlled laboratory studies crucian carp have survived four and a half months without oxygen. So resistant are these fish that the first people to report their resiliance – who studied the animals in Finland where most ponds are frozen in winter – were not initially believed.
Both goldfish and crucian carp survive by shutting down their metabolisms, and respiring without oxygen. To do this they need a big energy store: the crucian carp’s is huge, the biggest known in any vertebrate in proportion to its size. The goldfish isn’t so good at this but can still survive for a good time.
Our common frogs (Rana temporaria) are also well known for hibernating underwater. They do it in much tougher climates than the UK as well – ranging north of the Arctic Circle – and can hibernate for 8-9 months of the year.
But to do this they need oxygen and, although frogs can tolerate short periods with no oxygen at all, they’re nothing like as good at it as zero-oxygen tolerant fish. Probably 4-7 days is as much as they can manage, not as long as a goldfish, and nowhere near as long as crucian carp.
So frog deaths due to lack of oxygen are likely to come first, before the kind of fish we commonly keep in ponds.
And a strong candidate for the cause of death is lack of oxygen.
But we can’t yet rule out some of the other possibilities completely: poisoning by hydrogen sulphide, carbon dioxide, ammonia or methane, all of which could be present in de-oxygenated ponds, is still a possibility.
But, although toxic gases are often suggested as a cause of amphibian mortality, the jury is still out on on this: an experienced amphibian ecologist I was speaking to this week was unaware of any data or published reports to back up this idea. This doesn’t mean it doesn’t happen, just the evidence isn’t there yet to decide one way or another.
