The Eradication of Topmouth gudgeon from a Llanelli lake

A non-native invasive species of fish - Topmouth gudgeon (or Stone moroko)- has been identified at Sandy Water Park, Millennium Coastal Park, Llanelli, a lake at the Millennium Costal Path.

Topmouth gudgeon are classified as a ‘Category 5 Species’ being a - Non-native species of ‘highest’ risk under the Import of Live Fish Act 1980 (ILFA) and considered one of the most ‘potentially’ damaging non-native fish species to invade Western Europe. For this reason, it is banned from sale in the UK.

Its presence within the lake not only threatens the local wildlife but if it were to spread it could have a serious impact on our native wildlife and habitats on a national basis.

This is one of only 34 known Topmouth gudgeon populations in the wild in the UK.

What are Topmouth gudgeon and where do they come from?

Topmouth gudgeon are a small freshwater fish belonging to the (cyprinid) carp family. They originally came from Asia and have spread rapidly throughout Eastern and Western Europe. They are silver in colour, with iridescent purple sides. They have a characteristic upturned lower jaw, and mature males can have a prominent hump behind the head. Adults are approximately 8 to 10 cm in length and will live for approximately four years.

Why are Topmouth gudgeon a problem?

If they escape, or are intentionally introduced into the wild, they have the potential to harm our native wildlife and habitats.

Topmouth gudgeon are invasive because:

  • They are very small and easily transferred accidentally with native fish.
  • They reproduce rapidly, spawning up to four times per year
  • They eat the eggs and larvae of native fish
  • The male guards the eggs from other predators
  • They out compete native fish for food and habitat
  • They also carry the threat of new (novel) diseases and parasites that our native species are at risk from
  • They are able to survive in poor, low oxygen conditions which might otherwise be inhospitable to our native fish
  • In large numbers, they could also become a pest species to anglers, impacting on recreation

How did they get into the lake?

It is not currently known how Topmouth gudgeon first entered the lake. However, introduction of Topmouth gudgeon to other lakes can be traced back to illegal fish movements and restocking.

What are we doing?

Natural Resources Wales and Environment Agency plan to eradicate the invasive non-native Topmouth gudegeon from a popular lake in Llanelli.

In the first week of January, 2023, the two agencies will start work to remove the fish from Sandy Water Park to protect the future of the lake, as well as waterways throughout Wales and England.

The work has been funded by Welsh Government and is supported by landowners, Carmarthenshire County council.

Why are we doing it?

Topmouth gudgeon is a highly invasive non-native fish species. Its presence represents a significant threat to the ecology and wildlife in our rivers and lakes throughout the country. 

The Environment Agency, as Agents of DEFRA, are the enforcing authority for the Salmon & Freshwater Fisheries Act 1975 (SAFFA) and the Import of Live Fish Act 1980 (ILFA) which regulate the movement and keeping of fish in the wild.

Alongside the Agency, we are required to take appropriate action to ensure the licensing, containment, management and where appropriate eradication of invasive non-native species like Topmouth gudgeon.

How are we going to remove Topmouth gudgeon from the lake and stream?

Firstly, as many of the larger healthy fish species, such as carp and tench, will be removed from the lake, health-checked, and relocated.

After that, a piscicide containing Rotenone will be applied to the water to kill the Topmouth gudgeon population.

What is a ‘piscicide’?

A piscicide is a chemical like a pesticide or herbicide specifically designed to kill fish.

What is Rotenone?

Rotenone is a naturally occurring organic substance found in the roots of tropical plants in the bean family (Derris spp & Lonchocarpus spp). As a piscicide Rotenone is applied either as a powder from ground up plant roots or extracted from the roots and formulated as a liquid. In this case we will be using a liquid formulation.

How does it work?

When applied to water, it is absorbed through the gills of fish. Rotenone prevents a biochemical process at the cellular level making it impossible for fish to use the oxygen absorbed in the blood.

Is it a humane method?

Yes – the dose applied will ensure that the fish are removed rapidly with minimum stress.

Will it kill other animals?

Rotenone is selective to fish, which is why it is used. Mammals, birds and shellfish are highly resistant to the chemical and are not affected. Animals that eat it either directly or through eating animals that have been exposed, will not be affected because all animals have natural enzymes in the digestive system that break down Rotenone.

Insects, amphibians and crustaceans are also far more resistant than fish. Although they will be affected, the levels we will use will minimise this and recovery will be rapid.

Are there any risks to human health?

Extensive studies indicate that Rotenone does not pose a hazard to human health either through direct or indirect oral, dermal (skin) or respiratory intake.

As a precaution, highly trained staff will carry out the operation with all necessary protective equipment and risk assessments.

What will happen to the Rotenone?

Rotenone is a naturally occurring organic substance. It breaks down when exposed to light, heat and oxygen. When applied to water it will break down to carbon dioxide and water in a few weeks depending on conditions.

How will you check this?

During and after the operation we will take water samples and monitor the levels of Rotenone to ensure the treatment has been effective. Sentinel fish will also be used to check that the treatment has been neutralised and that the stream is suitable for restocking.

We will also be monitoring the status of the other wildlife in the lake, including invertebrates and fish.

Will it be dangerous for the public?

No, the Rotenone and potassium permanganate will be used at exceptionally low doses, which are not toxic to humans. However, to ensure that there are no risks during the operation we will restrict public access (including pets) from the lake and stream. The area will be clearly signposted, the whole area fenced off, and officers will be available to ensure it is kept clear and safe. Walkers should avoid contact with the water until notified that it is safe to do so. 

Are we working with the owners?

Yes – Carmarthenshire County Council agree to remove the Topmouth gudgeon from the lake. NRW is working closely with the lake owners and other regulatory Agencies including The Environment Agency, to ensure successful removal.

They’ve been there a long time, why are you only doing something now?

The risks from this invasive species are so great that Welsh Government have instructed NRW with the aid of the Environment Agency’s to fulfil a 5-year eradication programme to remove Topmouth gudgeon from the UK. Of the 34 known sites, to date the Agency has eradicated 12. NRW is committed to leading the eradications in Wales.

Are we the only ones concerned about Topmouth gudgeon?

No - many government organisations are concerned including DEFRA, The Environment Agency and CEFAS.

Is it legal to use a piscicide?

Yes but only if it is properly licensed.

The use of Rotenone as a piscicide is currently permitted under the Biocidal Products Directive (98/8/EC) and Biocidal Product Regulations.

The Environment Agency also license the use of any noxious chemicals in the aquatic environment (including rotenone and potassium permanganate) under Section 5(2) of the Salmon and Freshwater Fisheries Act (1975).

You haven’t done this before at other sites, why are you doing it at this lake now?

Yes, the Environment Agency has used this method before to eradicate other populations of invasive fish, including Topmouth gudgeon, sunbleak, wels catfish, pumpkinseed and fathead minnow. All operations have been successful, and the sites have now been restored to their former condition as productive fisheries.

Both NRW and The Agency licences and manages sites that contain non-native ILFA species on a site-by-site basis. The level of management is based on variety of factors including: the species, the type of site, the use of the site, the risk of escape, the level of threat posed to the environment, the surrounding habitats/species and so on. Based on these factors management action can range from simply screening the site to total fish removal. The action taken is also strongly linked to the ILFA category the fish are listed under, which indicates their likely risk to the environment.

In the case of the Topmouth gudgeon population in this lake, they are classified as a ‘Category 5’ ILFA species (being of Highest Risk). The threat they pose to the environment is potentially very high and so they are dealt with in a different way to many other non-native species of lower risk categories. 

Can’t you use another method?

No – We have examined and tested all other options for management of Topmouth gudgeon including: no action, screening outlets, rod and line removal, netting / electrofishing removal, egg removal using spawning mats, biological control (introducing other predatory species), drain down and liming. All of these options will only achieve ‘management’ of the population and not ‘eradication’. Due to the threat posed by Topmouth gudgeon it is essential that 100% eradication is achieved – piscicide application represents the only viable option to achieve this in this case.

Public fishing at Sandy Water Park in the future

Going forward, there will be no public fishing at Sandy Water Park.

When are we doing it?

The operation will be carried out in January 2023. During this period, all public access around Sandy Water Park will restricted and fenced off.

Why are we doing it at that time of the year?

  • To ensure success the piscicide application must be carried out in early winter or early spring to allow maximum growth of last year’s young fish but prior to breeding (this will ensure the fish are susceptible to the piscicide).
  • This timing will provide suitable water temperatures to ensure a successful piscicide application and enable effective management of water levels.
  • To allow fish to be returned to the water as soon as possible to reduce any impact on the fishery business.

What else are we doing to prevent this happening again?

  • Regulating every fish introduction and removal in the freshwater environment through fish movement consents and ILFA licenses.
  • Educating anglers, clubs, landowners and fish farm owners about the risks posed by non-native species and importance of managing their activities carefully.
  • Making clear that there is a buyer beware responsibility on the buyer of any fish.
  • Working with CEFAS and the fish farming industry to monitor fish movements.

CEFAS regulate the movement of fish into fish farms and NRW and the Agency regulates the introduction and removal of fish in the freshwater environment. The introduction of non-native species requires further licences under ILFA and the Wildlife & Countryside Act 1981 (WCA).

While CEFAS, NRW and the Environment Agency takes every measure to ensure that stocking is carried out in accordance with our strict guidance every fish supplier and purchaser must be aware that any fish stocking carries with it risks, including the risk of disease, parasites and accidental introduction of non-native species. It is the responsibility and in the best interests of any fish supplier or purchaser to minimise those risks. If suppliers and purchasers don’t minimise risks, their entire farm or fishery could be at stake. We discuss these issues with people who want to stock fish to their waters and provided leaflets to customers with this information.

We also work closely with CEFAS to monitor the import of live fish from Europe to reduce the risk of new ILFA species getting into the UK.

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