What Could be the Reason for all the Fish Dying?

As they say in CSI, we’re a bit short on forensics; we have the unfortunate victims and a suspect – in this case a Cotoneaster bush – but not much in the way of other evidence!

Cotoneaster berries – and specifically the seeds – are known to be a mild stomach irritant to humans and there are American studies which suggest that some wild birds eating large numbers of them on a long term basis can be poisoned – so your friend may have a point. However, I’ve a niggling suspicion that something else is going on.

Lack of Oxygen?

Assuming that there’s no obvious signs of damage or disease on your mother’s fish – the most common cause of mass fish deaths is lack of oxygen. Now I don’t know how many fish there are in the pond – or what kind they are – but if the unfortunate 11 represents all or nearly all of them, poor oxygenation could be one likely cause. If your water recently went green, or a large amount of plant material has found its way into the pond – and this might include those Cotoneaster berries – the breakdown of the algae or other vegetable matter could have taken the oxygen out of the water. The same might have happened if the water level became unusually low or your pump packed up – but without knowing more of what the cop shows call the “circumstantial evidence”, this is only speculation at best.

Read more: Worried about lack of oxygen in your pond? Check for these signs

Water Quality

It is probably worth testing the water for all the usual things – ammonia, nitrite, nitrate and pH – if you haven’t already. This might give some clue as to what has happened and it may also be a good idea to clean out any settlement residue in the filter and change around a third of the pond’s water to be on the safe side.

If this still leaves you none-the-wiser, the only thing left to try is to cast your mind back over the last few weeks and see if anything out of the ordinary has happened. Have any pesticides or fertiliser been used around the pond, have you changed your brand of fish food or dosed your pond with some treatment or other? It is a bit of an exercise of grabbing at straws, but it might just make things a little clearer.

Fish Factors

The fish themselves might also be a factor in this; if they, like the pond and our prime suspect, the Cotoneaster bush, have also been around for a long while, their advancing years may have played a part.

Now I’m not suggesting that they all died of old age within a couple of days of each other – but like humans, as fish get older, their natural resistance diminishes and their ability to bounce-back drops. Their age could have made them less able to cope to some change in the pond that you might not even have been aware of – but without knowing a bit more about the fish themselves, this is pure speculation again!

The bottom line is, you may never know – but test the water and cast your mind back over events around the pond during the last few weeks and you might find your answer.

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Last Modified: May 13, 2023