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Invasive species are species that have made their way into the UK that are having a harmful effect upon native UK species or environments. In this new environment, free from their predators, herbivores - in the case of plants - or parasites, there is often little to prevent them from thriving. They tend to out-compete our own species which suffer massively as a result.
Invasive Non-Native Species
Why are invasive species a problem?

Invasive species can do all sorts of damage to an existing ecosystem. This includes changing habitats and starving native species because they begin to monopolise the food supply.

They may eat native species, which sometimes have no defences against them, or even parasitise them. They take over food supplies, light and even nesting sites. In many cases, they even bring new diseases with them, which is what happened when grey squirrels were introduced into this country. They brought with them squirrel pox, which affects them fairly mildly, but against which red squirrels had no immunity or natural defence.

Left alone, invasive species can breed very quickly and begin to dominate habitats, which leads to the death or massive reduction in the numbers of our own native wildlife.

All in all, invasive species are a massive threat to our own native species. They are one of the biggest causes of ecosystem damage and there are so many of them that some ecosystems are facing a huge struggle to survive. They can cause chaos in a finely-balanced habitat and are considered by many scientists to be an even bigger threat to biodiversity than climate change. In many cases, they can also have a large economic impact on the areas they take over.

How do invasive species spread?

Very often humans are the reason behind invasive species.

We move animals or plants around the world for our own ends and often it doesn't turn out quite the way we expect. A local fish farm, for example, decided to farm signal crayfish (Pacifastacus leniusculus) which appeared to be a highly lucrative market. The owner dug a deep rounded pit, lined it with concrete to stop them escaping, and stocked five thousand crayfish. Much later, when he came back to check how they were doing, he found only one. The rest had made their way into a small stream that ran beside the property, presumably finding entrance to the main watercourse from there.

With each female crayfish capable of laying hundreds of eggs and the adults capable of killing and eating many of our own river lifeforms, the damage from this will be considerable.

Problems often start from the most innocent of intentions; a foreign plant might look nice in the garden but, from there, it begins to expand far beyond its owner's expectations. Trimmed back to keep the plant looking nice, the fragments - some far smaller than you might think capable of growing again - survive, take root and proliferate across and beyond the areas to where they were transported for disposal. Creatures intended as pets also escape into the environment, begin to breed and, with no natural predators, begin to massively expand their populations to the detriment of our own native wildlife. Ornamental ponds, stocked with non-native species, are often flooded and their inhabitants thereby released into our ecosystems, which has happened with almost all small, invasive fish species, while some are even thought to have been deliberately and illegally released.

Not that every non-native species is going to pose a great danger to our ecosystems. Some species, like the grass carp, which are very unlikely to breed in our waters because they are too cold, may be a non-native species but they are not an invasive species. There is no danger of them taking over the environment in the way that they have done in warmer American waterways. Similarly while pumpkinseed are thriving in a few limited lentic (standing water) ponds, there is no evidence of them representing a threat to lotic (running water) waterways. There is also considerable debate about species such as zander. This top predator was surrounded by fears that it would decimate natural populations of fish such as roach. It does not appear to have done so but, if caught in a canal, must currently be killed rather than returned. In other UK areas, where a licence is held by the water concerned, this is not the case.
Sometimes invasive species are not deliberately spread but hitch a ride on human transport. In the 1800's, for example, a shipment of American - believed to be Pacific oysters - contained many slipper limpets, which were disposed of. Unfortunately that disposal process saw them introduced into our seas, from where they spread to become a destructive nuisance to our native mussel and oyster beds often hundreds of miles from the original port.

Ship's ballast water, changed or renewed at many ports to maintain the ship's stability, also sees the transport of lifeforms from their original environments to new ones, like the Chinese mitten crab (Eriocheir sinensis) which has infested South-East reaches of the Thames and is similarly established in the Dee, Medway, Ouse and Tyne. This invader is a real headache for River Trusts, marine engineers and waterfront architects since it digs itself a burrow to live in. Not a problem perhaps with just one or two individual crabs but this species proliferates very quickly and makes the problem much worse by congregating in huge numbers, leading to severe bank erosion. Similarly planes, especially cargo planes, may inadvertently spread lifeforms to new ranges. With our huge transportation network, it is almost inevitable that lifeforms spread to new ecosystems, where they may become an invasive species and the start of a major problem.

So what has this to do with angling?

Our rivers, lakes and canals are much more fragile than you might think. With the amount of time that we, as anglers, spend on them, we can keep both our eyes and ears open for threats whilst simultaneously ensuring that we do not accidentally make the problem worse.

In some cases, we can report to the Environment Agency species that should not be here, like topmouth gudgeon and the sunbleak or motherless minnow. These are small species but incredibly destructive, outcompeting species such as our native gudgeon, roach and rudd for food while proliferating at an alarming rate. To make matters worse, they also predate on the larvae of other species. Topmouth gudgeons, which have the unsavoury habit of biting the scales of larger fish so that they can nibble on the flesh underneath, also expose those same fish to a much greater risk of infection and disease.

In addition to the Environment Agency, any sightings of invasive species can be reported to the UK Centre for Ecology & Hydrology via the email address
Sunbleak or motherless minnow
Topmouth gudgeon
We also need to make sure that we are not contributing to the problem. Unless we take precautions, our equipment can help to spread the invaders by transporting, unbeknownst to ourselves, larvae and eggs from one habitat to another. When you are talking about species such as Dikerogammarus villosus, commonly referred to as the killer shrimp, this is all too easily done while the resulting damage can be devastating. It is therefore essential that we thoroughly clean off any equipment that has come into any body of fresh water where invasive species are established. This includes boats, float tubes, flippers, waders, landing nets, carp mats, weigh slings, carp cradles, keep nets, wellington boots and any other equipment that routinely comes into contact with water, including both lures and the legs of seat boxes.

It is important to remember that both diseases and invasive species can be spread by damp angling equipment. Chemical dips - as long as they are using approved chemicals correctly - can help but one of the most effective controls is to make sure that all landing nets etc are thoroughly dried before being used again. Direct and prolonged exposure to sunlight is highly recommended with several sources quoting five days between use an ideal length of time to ensure harmful organisms are killed.

In regard to chemical dips, if they are not maintained correctly, they can become part of the problem. Excess dilution, e.g. from rainwater, reduces their effectiveness and, left untended, they can become breeding grounds for the very organisms that they are intended to eradicate.

A guide to invasive species that anglers are likely to come into contact with can be found at Invasive Non-Native Species - Angling Trust . Several are mentioned here together with links to some of the organisations trying to do something about the various problems. Reading through some of this information can be highly enlightening and is much to be recommended.
With freshwater plants, there are a variety of control methods available but you need to combine the most effective treatments with the appropriate time of the year. Chemical treatment, mechanical dredging, manual hand-excavation, plant suppression, mechanical cutting, manual hand-pulling, manual digging and water level management or a combination of the above all have a part to play in invasive plant control but what works for one plant does not necessarily work for another. This is why it is important to let the experts know as soon as possible. They can decide on the most appropriate course of action and put it into practice. The Angling Trust page has links to a lot of information but for an angler the best thing to do is to report anything that we see and, by ensuring that we care for our equipment properly, make sure that we do not add to the problem.

If you see something that you don't recognise, take a photograph on your camera or mobile and look it up when you get home. It may take a few moments but you could be helping to stop a problem before it gets out of hand.
Invasive Species Likely to be Encountered by Sea Anglers
A sample of the information to be found on the Angling Trust website.

Invasive Species Likely to be Encountered by Coarse and Game Anglers
Module 1: Fish Biology
Module 2: Invasive Species
Module 3: Fish Diseases
Module 4: Topography - Lakes
The Sea
Module 5: Angling and Mental Health