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Otter attacks on pondfish
Otters raiding garden ponds
There are an increasing number of cases where otters are taking goldfish and koi from garden ponds. Otter numbers are increasing, and they are on the look-out for food. They might already have taken your fish, when you blamed a heron.
What is the Risk and What can you do?
In the UK, Otter numbers had dropped very low in the 1970s, partly due to the effects of certain pesticides. Since then, due to more restricted use of agro-chemicals and deliberate protection measures for otters, their numbers have steadily increased.
In the local area (Severn Vale and Cotswolds/Upper Thames) surveys between 1977-86 found no sign of otters at any of the 100+ sites checked. By the survey of 2009-10 otters were found in over 60% of those sites, a remarkable recovery [1]. Numbers have continued to rise since then, and now there are reports of otters in every English river catchment. There are healthy populations of otters in Wales and Scotland too. So, in a period of 40 years, the likelihood of otters in Severn/Cotswolds’ rivers has risen from zero, to almost certain. copyright www.watergardensolutions.co.ukcopyright www.watergardensolutions.co.uk
Garden pond raids
I had only been personally aware of two possible cases of attacks on fish in the local area from the late 1980s up to 2013. One of those was from a large pool close to the River Coln, where otters were suspected. The other was from a fish stockholder, where mink were the most likely culprit.
Since 2013 I now know of at least twenty customers’ ponds where otter attacks have been confirmed or suspected. These occurrences are widespread. In Gloucestershire – affected ponds include those in Minchinhampton, Cheltenham, Prestbury, Charlton Kings, Coombe Hill, Gretton, and Swindon Village. In North Wiltshire, ponds near the Cotswold Water Park, Lechlade, and others on a tributary of the River Ray near Swindon. It is noteworthy that ten of these cases were in town centre gardens, some almost 500m from the nearest stream. There have also been newspaper and radio reports of otter attacks on pondfish in a number of other parts of the UK. Others were noted in comments when an early version of this article was first published on our blog (archive).
How would I know if otters had been in my pond?
Most customers affected have been aware that something had happened in the morning after the attack. In some cases substantial fish remains have been left partially eaten at the pondside, with large chunks removed from koi. Even big koi (60cm plus) have been taken. In many cases virtually all fish have been killed or eaten in a single visit. In other cases some large koi have been left in the pond with severe wounds to the body or fins. This is unlike heron attacks, where fish over 50cm are less likely to be taken; wounds tend to be spear shaped or in parallel markings on each side of the body; and where many fish escape, frightened into the depths.
Otters often visit between dusk and dawn, though they feed during the day too. They can attack at any time of year, though the winter months seem to be higher risk. They attack singly, or in family groups (wildlife-cameras catching three or four at a time). They are substantial creatures with brownish coats, and can be a metre long. At this size they are capable of dislodging stones at the pool edge, and knocking planting baskets off the pool shelves – one customer was convinced that the damage to the pool edge could only have been done by a human vandal… until they saw the otter swimming in the pond.
If otters have been in the area, they can leave patches of their distinctive tarry waste (spraints) on nearby rocks/paving. This has a strong, slightly floral odour and may have scales and bones in it. This is unlike mink, which are smaller (up to 60cm long), with much darker coloured coats, and whose waste (scat) is tubular, just over an inch long, and smells unpleasant [2]. Both mink and polecats can take fish too, but otters seem to be the bigger issue.
Is my pond at risk?
If neighbours have had otter problems then you are most definitely at risk. Otters can travel many miles within a territory. They may not return to an area for some months, but they are likely to revisit. From the reports I know of, if you are within 500m of a stream or river, then your pond is at greater risk. Otters obviously have a taste for garden pondfish and they seem, like foxes, to be looking further afield in towns and cities for food.
What can I do to keep otters out?
It is uncertain what alerts the otters to the presence of garden ponds, but they are known to have very good senses of smell and hearing. Perhaps they can hear garden waterfalls and fountains, and they can sense the traces of fish aroma coming from a pond surface. It certainly won’t help to leave fish food (pellets or sticks) outside near the pond, as these have a strong smell.
Angling fisheries have had increasing problems with otters, notably on carp fisheries. Anglers suspect that reduced numbers of wild fish in some streams (possibly due to predation by cormorants or signal crayfish), and increasing densities of otters, might also be driving otters to look further afield. Fisheries have had to take substantial steps to keep them out. Certain designs of heavy-duty fencing have worked, usually in conjunction with a powerful electric line (the thick otter pelt gives them some protection from lower powered devices) [3,4]
Ordinary pond cover-nets and heron deterrents (see the item on Herons) are unlikely to have any impact. Wire mesh covers may help, but are of course very unsightly.
A few customers have been so upset by an otter attack that they have decided not to restock with fish. Others are taking a chance and restocking with a few smaller fish, and others have fitted heavy duty grid covers. There is no doubt that otters will impact pondkeeping in future, with at-risk ponds less likely to be stocked with ornamental fish.
Useful links
[1] Environment Agency Surveys
[2] Telling the difference between otter and mink
[3] Fencing to keep out otters
[4] Garden pond protection from otter predation (pdf)
- Otter fact sheet
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