Return To Index

Environmental Issues
Going Green

By Alan J Hartley

Invasive Fish And Pond Plants



Releasing imported animals into an environment where they do not belong naturally, can, and usually does, lead to all sorts of problems. We are all familiar with the ecological disasters caused by rats escaping from ships on various islands in our not too distant past and the problem of rabbits in Australia in slightly more recent times. Yet the lesson does not seem to have been learnt from them. We still go on releasing unnatural wildlife even though there are laws against it in this and many other countries. Perhaps part of the problem is that people don’t think that fish are really animals and can’t cause problems or spread as they are forced to live in water. Many are released into the wild simply because they have grown too big for their tank/pond and the owner does not know what to do with them. This is almost certainly the case with Catfish.

The movement and importation of the Wels Catfish is banned in this country and it is illegal to release them into open Waterways such as canals and rivers. They simply grow too big, have no real natural predators and will eat any wildlife that they can get into their prodigious mouths. Having said that there used to be quite a market for some of the slightly smaller species of catfish that would only grow up to some 5 or 6 feet! When I ran an aquatics department many years ago I must have sold thousands over the years and I often wonder how many are swimming round our rivers and canals, dumped, after out growing their tanks.

Grass Carp have also been banned, in the past, from being imported into this country and being released into the wild, although when available for sale in the aquatic trade, they too are popular. They will in theory eat the “Blanketweed” that plagues many pond keepers, but they are not to be recommended as in a well planted pond they will strip it bare of all vegetation in an urge to satiate their massive growth rate and ravenous hunger. In America grass carp are widely grown as a food fish in the same way we grow trout. Farmers who rear them actually mow grass from the fields and scatter the grass cuttings on the water to feed them, although they will take commercial fish food as well. Because Grass Carp are prodigious feeders they have been used experimentally in the Everglades to clear the vegetation that threatens to block the water ways. In many states of America however they are considered a pest stripping rivers of all their vegetation and so they have been banned.

Other aquatic animals can cause dramatic problems beside fish. We are all familiar with the wildlife disaster caused by the importation and subsequent escape/release of the American Signal Crayfish, but most people do not know of the developing problems caused by Bull Frog tadpoles in the South of England.

Every year 1000s of them are sold in the aquatic trade to the public and they quickly develop into giant bull frogs. Growing nearly one foot long, they have no real natural predators when mature, because they are simply too big for birds like Herons to eat. Foxes may take a few, but they are the only animal that we have in the UK which is really capable of killing them. As such they are starting to breed without restraint and stripping parts of the countryside bare of smaller wildlife including frogs, newts and even small birds that stray to near to their cavernous mouths.

Imported plants that are released into the wild can also cause severe problems. When people think of imported plants that have caused ecological troubles they immediately think of the Giant Hogweed and little else. The Hogweed was imported in Victorian times by gardeners as a decorative garden herbaceous plant and conditions were ideal for it with no natural predators, so it spread rampantly with nothing to restrict its spread. The Giant Hogweed is spreading along and choking, many waterway banks throughout the country. What makes this plant even worse is that its juices are poisonous so great care has to be taken when cutting it down. The Hogweed spreads by underground shoots that go very deep making it even more difficult to eradicate or control.

However, there are many other plants that are now being acknowledged as being a potential threat to native species and wildlife. Some are very unassuming in their appearance and on first examination seem completely innocuous. You might imagine that one category of plants that could not possibly pose a threat are water plants, because by their very nature they are trapped in the water source they need to survive.

I don’t know that “Duckweed” or “Fairy Moss” are actually on the banned list, but most gardening experts now advise against putting them in your garden pond. You might think that they are restricted to your pond, immobile, and can’t escape into natural water-courses, but nothing could be further from the truth. Any type of bird, or indeed any wildlife at all including frogs, hedgehogs, or even foxes, are quite capable of getting one or two tiny pieces of the plants on their bodies when they have a drink from your pond. The specks of plant can survive out of water for many hours on a cool day until the animal takes another drink elsewhere in a different pool. It only takes one single speck, a couple of millimeters across, of this type of plant, to start a new colony of plants with its rapid division.

Other pond plants that people are advised not to add to their ponds include some of the rapidly growing “Oxygenating” plants such as Canadian Pond weed. Admittedly, they do not spread as easily, but they can “Escape” and occasionally do so. Some, slow moving natural water courses and many static ones regularly get choked with these plants and have to be routinely “Raked” to clear them.



Gardening Tips
By Mrs FM


Unusual & Old
Fashioned Fruit


Herbs & Other
Edible Plants.


Books By
Alan J Hartley