Environment A to Z of Canal Wildlife
A to Z of Canal Wildlife: D - F

Dragonflies & Damselflies

DragonflyThe Dragonflies and Damselflies, the most colourful and easily recognised of the canalside insects, are among the fastest flying insects in the world. Estimates of their speed vary from 35 to 60 mph and fossilised remains indicate that they have existed for some 300 million years.
These ferocious insects are all carnivores preying upon other flying insects, sometimes even their own kind. There are 27 species of Dragonflies, so called because they are as fierce as dragons, and 17 species of Damselflies in Britain.

They are easily distinguishable, both have four large wings but they hold them differently when at rest, the damselfly having two pairs of similar wings which are held vertically over their slender bodies. The Dragonflies have two pairs of dissimilar wings, up to 4" across, which are held out horizontally at right angles to their stouter bodies when at rest.

The Dragonflies are the much stronger flyers and some, including the Brown Aeshna, may travel some distance from water in the search for prey, even coming into gardens. The weaker flying Damselflies remain among waterside vegetation. The so called 'hawker' Dragonflies, such as the Common Aeshna and the Emperor, spend a lot of time patrolling the canal whilst the 'darker' Dragonflies, like the Common Sympetrum have a favourite perch from which to pounce and dart after insects.

It is common to see these insects flying around in tandem, the male and the female are joined together as part of the mating procedure. Before he searches for a female, the male transfers his sperm to a special pairing organ on the underside of his body. When he finds a mate he grasps her behind her head using claspers at his tail end. The female twists her body forwards below him to receive the sperm, then they fly together to find a suitable egg laying site. For some this may be on vegetation at or near the water surface. Others lay in the mud, some just drop the egg into the water. The Banded Agrain take more care, both sexes crawl together below the waters surface to plant their egg. The adults live only a month or so.

Once hatched the immature stage of these insects is entirely aquatic, the Damselfly nymphs mature in a year but some Dragonfly nymphs may take 2 or 3 years. The nymphs of the two groups are quite different, the slender Damselfly nymph lives among the underwater vegetation, the slower stouter Dragonfly nymphs live in the mud at the bottom.

Like the adults they are entirely carnivorous, eating anything from tiny insect larvae to large tadpoles depending on their species and size. They are not very active creatures and may full prey themselves, but some can move quickly if threatened, propelled by a jet of water from the rectal cavity. Generally they live quietly, undergoing about 12 moults of skin as they grow. There is no pupal stage. When fully grown they emerge from the water, from May to August depending on the species and the weather conditions.

They climb up a plant stem into the air where their nymphal skin splits. The insect emerges, its wings expand and it flies away as a perfect adult, the canal environments most colourful and well known insect.

Duckweed

Duckweed

Duckweed floats on the surface of the water. Under favourable conditions countless billions will form green carpets completely covering the entire canal for miles. Many people find it unsightly, but it is a sign of clean and healthy still water.

The Duckweeds seem to grow best where the water contains a high concentration of organic matter. Partially silted canals with a lot of dense bankside vegetation, but with clear, still open water suit them admirably.

The common lesser Duckweed, Lemma Minor, eaten by ducks and fish, consists of a tiny flat round leaflike structure called a thallus with a single root hanging beneath. It multiplies by each thallus producing buds which break off, increase in size and in turn produce their own buds. It rarely flowers and then only in shallow margins or where drying mud is exposed to the sun. The flowers are minute and borne at the edge of the thallus in a pocket.

Ecotone

The shores of lakes and ponds and the banks of rivers are good examples of ecotone. Ecotones are habitats which form the boundaries between two other habitats. Simply put the canal bank is an ecotone between the surrounding dry land (generally farmland) and the open water of the canal. This ecotone is an extremely important 'man made' wildlife habitat which supports a great diversity of plants and insects.

In the still water of the canal extensive areas of bankside vegetation develop, forming a gradation from the true aquatic plants to those on dry land. The most conspicuous are the reeds and sedges, especially the common reed, Phragmites communis, which will grow in water varying from three feet deep through to almost dry land. The Common Reed is an important plant of the canal margins, dying back in the winter and throwing up fresh green shoots the next spring. It provides protection to a great many insects, and birds like the reed warbler who build their nests on the stems. The hollow stems provide shelter for overwintering insects and then convenient perches for dragonflies in the summer.


In the still water of the canal this dying back of the bankside vegetation gradually leads to the shallowing of the water. Due to the build-up of plant remains the edges become drier and this offers an opportunity for other plants to encroach and become established, each in turn supporting their associated wildlife. The alder and the willow are trees that thrive in these conditions. They, in turn , drop their leaves in the autumn and so contribute to the build-up of humus and thus conditions for yet more plants to invade. This general process is called succession and over a long period the canal banks will gradually extend outwards from the dry land until they meet.

Eventually the canal would revert to dry land, strangled out of existence by the very diversity of life it supported! Long stretches of abandoned canals bear witness to this process. However, all is not lost, if the canal is dredged and rewatered the wildlife, in all its diversity, soon returns to start the process all over again.
Hence regular maintenance of the canal is a must if the ecotone with its great diversity of life is to be protected permanently.

Fish

Fish and water automatically go together, so fish and canals equally go together. Forget about trout, salmon and grayling. They all need lots of oxygen so they love fast rivers and tumbling streams, although Rainbow Trout are often used to stock reservoirs where wind gives plenty of waves and hence oxygen. A canal is really a long narrow lake and generally has no current or just a very slow one for most of its length. This means fish found in canals will almost certainly be the same as those found in lakes and ponds.

Perch

Perch are the small boy's favourite fish and are often the first fish caught by a young angler. They are bold and brash (and fearless) with dark green vertical stripes, two red dorsal fins, the front one with very sharp spines which can prick if the fish is not handled carefully. Perch will feed on anything that moves including worms, insects, and (especially) small fish and fry. If you see an explosion of small fish erupting from the water surface you can be pretty sure that a shoal of perch, often five or six inches long, will be hunting below.

Roach

A much gentler fish, but an equally dogged fighter for the angler, is the Roach. Seen from the side it is a medium depth fish with bright silver scales, gradually merging to a deepish green on the back and characteristic red fins. Roach are catholic feeders eating a wide range of small aquatic insects, grubs, even it is said, water weed itself although possibly for the small creatures which live in the weed. Roach do not grow particularly big so a fish weighing one pound is a good fish to the angler and a two pounder is a 'red letter' fish.

Bream

Much deeper in the body and growing much larger is the Bream which can often weigh two, three or four pounds, although they do grow larger still. Bronze in colour, fading on the sides, they are slow moving fish which often patrol up and down the canal in a shoal, feeding as they go. They have a prehensile mouth which can extend a couple of inches or so in order to suck food from the bottom whilst tilting their body only a little downwards.

Carp

Growing much larger still is the Carp which commonly grow to ten or fifteen pounds in weight. The UK record is over fifty pounds! Carp are extremely powerful fish with a thick cylindrical body and large fins, especially the tail, which gives them the ability to swim very strongly and give the angler a battle to remember! The Carp is thought to have been introduced to Britain by the Romans as a food fish. In the Middle Ages the monks used to fish for Carp in their 'stew ponds' on Thursday ready to eat the Carp on Fridays. Whilst the Common Carp is fully scaled there are two sub species, one with no scales at all just a leathery skin, and one with the leathery skin and a few large scales near the head and tail, along the lateral line and along the back.

Pike

Finally, the fish most commonly found in the canals is the Pike........ the fish people love to hate! A Pike is a predator in the water just as a fox or kestrel is a predator on land or in the air and as such helps to keep stocks of other fish in check and culls the weak and injured fish. Keeping stocks in check is an important function, otherwise we would finish up with waters full of stunted fish because of excess fish and limited food supplies. The Pike is superbly designed. It has large jaws with several rows of needle-sharp teeth, pointing slightly backwards, and a toothed tongue for drawing its prey back into its mouth. A small fish caught in that mouth rarely lives to tell the tale! It is streamlined and cylindrical in shape with tremendous acceleration provided by having its dorsal, anal and tail fins very close together at the end of its body. One flick of these from its powerful muscles sends it darting very quickly out of the weeds to strike. There have been many stories over the years about how much a Pike can eat and how they constantly eat. Most of these stories are grossly exaggerated. It is a fascinating sight to see small fish contentedly swimming round the head of a Pike if it is not hungry. is that the behaviour of an out-and-out killer?

Some canals contain fish such as Dace and Chub which generally prefer much faster water, Tench which thrive in gravel pits and muddy pools and occasionally the beautiful Rudd which is like a Roach but with a golden tinge to its body. The children's favourites, Sticklebacks and Minnows also occur. Although Minnows are much more common in fast shallow stretches of rivers, they seem to adapt well to still or slow moving water.

In some ways a canal without fish is like a canal without boats.... it is incomplete!