Following the results of a recent survey, a national charity has issued advice to help prevent fish and amphibian deaths in garden ponds during hard winters.
The winter of 2009/10 was the hardest for 30 years in the United Kingdom, causing garden ponds to freeze over, the ice persisting for over a month in some places.
Following reports from pond owners that they had found dead frogs and other amphibians following the freeze, the national charity Pond Conservation launched the Big Pond Thaw Survey, in which it asked pond owners to tell them more about their ponds during the freezing weather. The charity wanted to find out if the things people did to protect their pond wildlife, such as making a hole in the ice and running a pump were having an effect, along with whether the ponds had seen any amphibian or fish deaths.
The results of the survey showed that in most cases, those people who reported back had found dead frogs, usually less than ten. But a few people found very large numbers of dead amphibians.
Making a hole in the ice didn’t seem to have made any difference to the likelihood of either fish or amphibian deaths.
Running a pump did seem to reduce the likelihood of deaths, although there were only a small number of ponds with pumps in the survey, so Pond Conservation says that the result should be treated with caution.
Looking at the results of the survey more generally, Pond Conservation says that when it comes to possible causes of the deaths, there is a hint that more amphibians died in deeper ponds, and in ponds where the snow lay for longer. Fewer amphibians were found dead where a pump was run, and/or where there were plants present, suggesting that the deaths might be associated with lack of oxygen in the water, although a build-up of toxic gases can't be ruled out at this stage.
So, with colder weather predicted again before the end of the winter, what can we do to reduce such pond deaths during cold spells?
Pond Conservation offers the following advice to pond owners:
- Wide and shallow ponds are more likely to have higher oxygen levels in the water than deeper, more 'tank-shaped' ponds. The standard advice is that deeper ponds run less risk of 'freezing solid' but Pond Conservation says: "We know that most ponds didn’t have more than more than a few centimeters of ice, even during the very coldest days of the 2009-10 winter – so 'freezing solid' isn’t the problem."
- Don't allow leaves and sediment to build up on the bottom of the pond, as it leads to de-oxygenation of the water.
- Underwater plants and algae will help oxygenate the water, including mosses which don’t die-back in winter.
- If you are worried about low oxygen levels in your pond, it is worth making a surface hole and keeping a pump running so that the water is stirred up – this can move oxygen from the surface to deeper waters.
- Clearing snow from the top of a frozen pond can help, but is only really likely to work if your pond has lots of underwater plants (or algae) which can then oxygenate the water.