Childrey Village Website

A common vision of a good pond is a relatively large, deep, permanent water body with a few scattered bank side trees providing some shade - but not too much. There are some plants – but plenty of open “weed” free water. To many this perfect pond is also incomplete without a few ducks and/or fish.

This type of pond, commonly seen on many village greens, is just one type of pond. It is much favoured by man as it has historically supplied everything we needed. There is easily accessible plant free water for soaking cartwheels or pumping out water for drinking or other purposes. Ponds fed by or constructed on streams and rivers provided a source of power for mills. Fish and ducks perhaps provided food, and in more modern times an easy opportunity to see large “wildlife”. Thus it has been assumed that this is the best type of pond there is and the idealised target when managing a pond for all reasons including wildlife.

However, this type of pond is usually very artificial in nature and a study of ponds in more natural environments reveals most ponds to be small and shallow. They often do not retain water all year round and in many cases dry out every year. They may be shaded, silted up, choked with plants – completely different from the classic pond described above. In summary, potentially there are an infinite variety of pond types and sizes. The animals and plants that live in them will in part be controlled by the type and quantity of water supply and geological and hydrological factors. The single most important factor for a good wildlife pond is a clean supply of water (i.e. unpolluted not necessarily clear). Other factors will influence the number and variety of species that live in the pond – with the two most critical being the variety of habitat and structure within the pond, quality of the surrounding habitats and the proximity and linkage to other freshwater habitats (sources of potential colonising species).

Fish when present in ponds are one of, if not the top, aquatic predator. Depending on the species concerned they can eat both animals and/or plants. Many natural ponds would not have fish – either they would never colonise but even if they did fish could not survive regular drying out or other seasonal conditions. The fish themselves may be easy prey for other predators such as birds. Ponds in floodplains could be regularly colonised during periods of flood, but the further a pond is away from an easy colonisation route such as flooding the less likely fish would naturally colonise. Many traditional village ponds are located in places where fish would not naturally occur – and are present only because of the way the pond has been modified or managed.

Fish are one of a large variety of organisms in a pond. There is no one mix of species (i.e. algae, mosses and liverworts, “higher” plants, small or large invertebrates, fish, birds or mammals) that is best or better than any other. Each pond will be different. Fish are one of the top predators and can, depending on the species concerned, feed on both animal and plant life. Fish eat the eggs, larvae and adults of invertebrates and amphibians. Some species also browse on plants and can indirectly predate on animals living in or using the plants in the pond. The actual effect fish will have will depend on the number and type of fish and the size and type of pond. In general, the presence of fish in a pond, even small fish, often means either a reduced abundance of other animals (in terms of sheer biomass) and/or reduced number (i.e. diversity) of species. Other potential effects of fish include a reduction in water quality due to either enrichment through their faeces and/or with bottom feeding species (e.g. Carp) the continual disturbance of silts, which causes cloudy and further enriched water. This poor clarity of water and associated raised nutrient levels will mean some species that require clean, clear water will not be able to survive.

Some species groups are more vulnerable to fish predation than others. Most amphibians are particularly vulnerable to predation by even small fish, which eat their eggs and larvae. If the adult amphibians do not breed then even those species with longer-lived adults will eventually decline - unless animals from other nearby ponds continually top up the population. Common Frogs do breed in ponds with fish but only do well when there is plenty of well-vegetated shallows where fish do not do well (places for spawning and tadpoles to hide). Even small fish such as Sticklebacks can devastate populations – in some ways they are more destructive as small fish can live in and gain access to even well vegetated shallow water not used by larger fish. Newts, especially the protected Great Crested Newt are also vulnerable - as their larvae tend to swim in open water. Species with generally more secretive larvae (e.g. the Smooth or Common Newt) can do better. Common Toads though not immune to fish predation as larvae, seem to cope where other species fail because of the toxins they contain, (However, the Common Toad is suffering other problems related to changing weather patterns and appears to be in severe decline across large parts of the UK.)

For the better-studied larger aquatic invertebrates there is a marked hierarchy of vulnerability. When fish are introduced into a pond often the first group of animals to decline and be lost are dragonflies and damselflies. Water Beetles are also lost early on. Other species may do well with fish present, for example there are some Leeches that feed on fish and thus these can do very well in ponds with fish. However in general, the number of individuals and diversity of larger invertebrate species is often reduced in ponds with fish.

Thus it is important to decide what the purpose of a pond is when making decisions about how it is managed and if fish are to be deliberately included as part of the species mix in the pond. The dumping of otherwise unwanted or surplus fish is to be avoided for many reasons. For one the introduction and removal of fish is strongly regulated and requires a licence from the Environment Agency. Fish movements are controlled to ensure the welfare of fish (they should only be put into suitable sites) and the spread of undesirable species (including non-native species such as Goldfish) and disease (fish need to be checked before being moved). Licences may be refused on grounds other than fish health and site unsuitability - for example where a pond contains a protected or rare species that would be affected by fish, e.g. the Great Crested Newt.

There are few definitive ecological studies of ponds looking at the effects of fish on other life. However, using some local examples of sites it is easy to illustrate the negative effects they can have on other species.

There is a fine looking pond in the village of Crays Pond (South Oxfordshire), which like Childrey Pond has been stocked with Goldfish and/or similar species. The pond has a good diversity of habitats and should be a rich site for all types of wildlife. However, a survey of the pond reveals all of the invertebrate life to be present in the outer parts of the pond made up of densely vegetated shallow water. The central open water section of the pond is more or less plant free through the continual browsing by the fish and other animal life more or less absent. The effects of small fish, which can live in shallow areas and relatively densely vegetated pond habitats, are seen in several local ponds. In Oxford there is a well-known pond in Risinghurst, part of the land once owned by the author C S Lewis. Historical records suggest the pond was stocked with larger course fish, the fate of which is not clear. However, in more recent times the pond has been stocked with Sticklebacks (there are no historical records for them) and a comparison of the invertebrate fauna of the period with and without Sticklebacks shows a marked decrease in species, and a very marked decrease in the overall abundance of invertebrates. There are few places to hide as the size of Sticklebacks allows them free and easy access to more or less all parts of the pond. At a site in Sutton Courtenay there are two well established ponds, one of which remained fish (Stickleback) free for many years and one of which they were introduced many years ago. Two species of newt can be found in both ponds, but the population of newts was much higher in the much smaller pond, which until 2006 or 2007 had no Sticklebacks at all. Adult newts are still seen in the smaller pond, but in 2008 there was no larvae recorded at all – almost certainly because of the predation of the eggs and larvae by the many thousands of small Stickleback fry that are now present. The longer-term future of the newts on this site (Smooth and Great Crested) depends on adults being replaced – and plans are being devised to control or remove fish from the small pond at least.

Another well-known pair of ponds is in the village of Woodcote (the Greenmoor Ponds). These consist of large, deeper open water Lower Pond and a smaller shallower very well vegetated Upper Pond. The Lower Pond has been used in the past as a fishing pond and historically stocked with fish (Carp only). The Upper Pond has until 2008 never had any fish at all as it was deemed not to worthwhile as a fishing site. In 2008 persons unknown stocked the Lower Pond (illegally) with a mix of at least six species of fish and it is suspected that Carp (Crucian or Crucian hybrids) were also put in the Upper Pond. Few ponds have been surveyed as well as these two and there is a good series of surveys over the last twenty years or so. Invertebrate surveys of the Upper Pond just before and soon after the fish introduction shows the loss of most of the dragonfly and damselfly species and many of the water beetles. (The impact of the fish in the Lower Pond is harder to assess as in 2000 its water supply dried up and only in the last three years has it been restored. Such major changes mask and make it difficult to tease out individual factors such as the introduction of fish.) Ongoing fish control measures have been introduced to both ponds.

Goldfish are very effective predators, and where they have been introduced they often result in ponds that are very poor for smaller animal species. One example of this is the pond on the Millennium Green in Drayton, with the open water part of the pond being reduced to an invertebrate free zone by the fish – with a much reduced fauna from what would otherwise be expected given the type of pond elsewhere in the shallower parts of the pond. The fish also stop the growth of all plants in their end of the pond, and the water is very cloudy in part as a result of their continual bottom feeding.

In summary Fish as top predators in ponds have a significant impact on other life, both animal and plants, in ponds. Depending on the size and type of pond and number and type of fish the impacts will vary. Large numbers of fish in small ponds can cause the demise of most other animal species and reduce the quantity of plants – a key habitat for animal species. Small fish are just as efficient predators as large fish – in many cases for invertebrates and other small animals (including amphibian eggs or larvae) perhaps more efficient. Goldfish are known to be very effective predators. The impact of fish may not be restricted to direct predation but also through indirect impacts on water quality by enrichment through their faeces and/or by their feeding habits (especially bottom feeding species). In small simple ponds, i.e. those with little variation in habitat especially those dominated by open water habitats, even relatively small populations of fish can dramatically reduce the diversity and abundance of invertebrates in particular. Thus it is important to have a clear idea of what a pond is for when devising a management or restoration project. If “wildlife” in its broadest sense (i.e. plants, invertebrates, amphibians and not just easy to see obvious species such as fish) is important, serious consideration should be given to the possible effects fish may have, and a definite policy as to their desirability decided upon. Similar consideration is needed for maintaining and/or encouraging resident ducks - which can have similar impacts as fish on smaller ponds at least.

Rod d’Ayala, Ecological Consultant, Pond Adviser for Pond Conservation - March 2009