There are an increasing number of cases where otters are taking goldfish and koi from garden ponds. Otter numbers are on the up, and they are on the look-out for food. They might already have taken some of your fish, when you blamed a heron.
What is the real level of risk to your pond and can you do anything about it?
The otter increase
Otter numbers in the UK had dropped very low by the 1970s, partly due to the effects of certain pesticides. Since then, due to safer use of agro-chemicals and deliberate protection measures for otters, their numbers have steadily increased.
In our area (Severn Vale and Cotswolds/Upper Thames) surveys between 1977-86 found no sign of otters across all 114 sites checked. Yet by the survey of 2009-10 otters were found in over 60% of those sites, a remarkable recovery. Numbers have continued to increase since then, and there are reports of otters in virtually all English river catchments now. There are healthy populations of otters in Wales and Scotland too.
Garden pond raids
From my own experience in our locality, I had only been personally aware of two possible cases of fish being taken 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.
However, I now know of at least seventeen cases since 2014 where otters have been confirmed or suspected. These occurrences have been in three distinct areas, two in Gloucestershire (Minchinhampton, near a small tributary of the River Frome; and Cheltenham, near both the River Chelt, and Noverton Brook), and the other in North Wiltshire (one near a feeder stream for the Cotswold Water Park, and the other on a tributary of the River Ray near Swindon). It is noteworthy that ten of these cases were in town centre gardens, some up to 500m from the nearest stream. There have also been reports in newspapers of otter attacks on garden pondfish in a number of other parts of the UK.
How would I know if otters had been in my pond?
Otters will often visit between dusk and dawn (though feed during the day too) and most customers affected have been aware that something had happened by the following morning. 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 on one 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 are substantial creatures with brownish coats, and can be a metre long. 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… 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.
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, and 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. If otters get a taste for garden pondfish, then perceivably they may, like foxes, look 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, or maybe they can sense the traces of fish aroma coming from the 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, and 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 types)
Ordinary pond cover-nets and heron deterrents (see the post 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, whilst others are taking a chance and restocking with a few low value fish. There is no doubt that otters will impact pondkeeping in future, with at-risk ponds less likely to be stocked with ornamental fish.
UPDATE December 2017: I am continuing to receive further reports of fish kills, mostly considered to be by otters. The issue has also received more coverage in The Times and on BBC Radio2’s Jeremy Vine Show, reflecting the increasing prevalence of otters. My trade association OATA has collaborated with the UK Wild Otter Trust to produce a leaflet on protecting ponds from otters . It notes that attacks are much more common during the winter months. The Trust is also collecting details of incidents to find out which parts of the country are most affected (see the leaflet).
 Environment Agency Survey (pdf)
 Fencing to keep out otters (pdf)
 Garden pond protection from otter predation (pdf)
Telling the difference between otter and mink
Conference Report on Otters and Fisheries (2012)
Otter fact sheet