Invasive Species in the Thames
Every year in November the weir at Richmond Lock is lifted to allow a portion of the river to drain naturally at low tide. Here at ZSL we use this opportunity to scout the river bed for any species that are not native to this country's waterways.
What is an invasive species?
‘Invasive’ describes a species from a foreign country that has established a breeding population in UK waters and disrupts our native species and habitats.
Invasive species arrive in British waters via several means. When a ship leaves its port of origin, it takes up ballast water in order to increase stability when crossing open seas. Contained in this ballast water, however, are the larvae of local plant and animal species.
When the ship reaches its destination in a foreign country, these plant and animal larvae are expelled from the ships tanks along with the ballast water. Alternatively, adult species attach onto the hulls of ships in a process known as ‘biofouling’. Trade in the aquaculture industry has also resulted in the importation of foreign species.
With suitable environmental conditions for larval settlement and breeding these species can colonise new waters. In the absence of predators or competition, populations of invasive species can rapidly multiply, exhausting resources used by native species or severely degenerating their habitats.
Surveying invasive species at Richmond
As of November 2007, the Zoological Society of London has teamed up with the Marine Conservation Society and the Thames Landscape Strategy to research both invasive and threatened native species of the Thames river bed. The aim of the project is to monitor how populations of these species change over many years.
The study site at Richmond. The photo looks east towards Petersham road and Abbotts Hall Farm. Photo ©ZSL
The survey takes place every year during November, when the weirs at Richmond Lock are lifted to allow the river between Richmond Lock and Teddington Lock to drain naturally at low tide.
The survey uses 0.25m2 quadrats to record the number of each species found on the lower shore and record the type of substrate present. The annual event is also used to clean and maintain docks, and to remove the river bed of rubbish and silt.
Members of the ZSL survey team at Richmond Lock.©ZSL
What we’ve found so far
The survey has identified several invasive species in the Thames.
Chinese mitten crab (Eriocheir sinensis)
©ZSL This species originates from Asia, but over the past 20 years has become increasingly prevalent in British river systems.
British wildlife is affected by the Chinese mitten crab because it is a voracious predator. It also poses a threat to habitats through the burrowing activity of adults, which leads to the erosion of river banks.
These crabs also cause problems by clogging up water intake pipes of power stations.
Zebra mussel (Dreissena polymorpha)
Invasive zebra mussels attached to a native swan mussel. ©ZSL The Thames was invaded by this species during the early 18th century.
The major threat to British wildlife is from their sheer abundance. Each female can release up to 1, 000, 000 eggs, and adults are capable of reaching densities of 10, 000 or more per square metre.
Our survey found 208 individual zebra mussels in just one 0.25m2 quadrat! These high abundances affect other wildlife by using up space for attachment needed by native species, and even colonizing the shells of other species (pictured) which affects their ability to feed and burrow.
Asiatic clam (Corbicula fluminea)
©ZSL This species was first discovered in Western Europe during the 1980’s. Known as the ‘good luck clam’ in Southeast Asia, they are capable of self-fertilisation and release 2000 juveniles each per day!
This clam competes with our native mussel species for food and space, but is also responsible for altering benthic substrates upon which other species rely.
Furthermore, the Asian clam has a greater resilience against pollution, increasing its potential to outcompete our more sensitive species. It also causes problems to people by fouling water intake pipes of power plants and other industrial water systems.
What can be done?
The high fecundity (fertility) of many of these invasive species plus a greater resilience to pollution over many of our native species makes completely removing them from the Thames an impossibility. The physical removal of invasive animals does nothing to decrease their population as it only takes one or two individuals to release several thousand young.
Dr. David Aldridge from the University of Cambridge has developed an alternative method of removal for zebra mussels, particularly useful for clogged industrial water systems.
His pellet treatment, called the ‘Biobullet’, consists of a toxin enclosed in an edible coating which lures Zebra mussels into eating them. Once filtered out of the water by a mussel, the edible coating breaks down and allows the toxin to take effect.
You may be thinking that this also poses a threat to some of our native species. To prevent this, Dr. Aldridge engineered the pellets to break down after a couple of hours – a short enough time that the toxin would be dissolved before the water flushed out of the pipes into the river system.
Unfortunately, the application for this method is limited to local areas such as water pipes, not for whole river systems like the Thames. To learn more, please visit the links below.
Several companies have tried to reduce the trafficking of invasive species from continent to continent by developing ‘antifouling’ paints. These paints are used on ship hulls to create a surface which is either too slippery for fouling species to attach to, or too inhospitable (biocides).
Nowadays a greater emphasis is placed on the development of anti-stick paints, as it was found that biocidal paints, particularly those containing TBT compounds (organotin tributylin) were leaching toxins into the water and affecting non-target species. Current legislation has globally prohibited the use of organotin compounds in antifouling paints since 1st January 2008.
In a further effort to reduce the spread of invasive species, several methods of treating ballast water used by ships have been proposed. The International Maritime Organisation(IMO) has stated that by 2016, all ships will be required to cleanse their ballast water of unwanted organisms.
Methods include exchanging ballast water in the open ocean, chemical treatment using biocides, heating ballast water, filtration and using UV light. Unfortunately none of these methods are 100% effective and can be very expensive. Certain methods, such of the use of biocides can also have harmful effects on the external environment. For more information of the techniques used to treat ballast water, visit the website below.
For an overview of current and proposed legislation regarding invasive species in the UK, take a look at the UK Parliamentary Office of Science and Technology below.
In short, the invasive species already found in our waters are here to stay. Although there has been investigation into methods for small-scale eradication and control of these species, the technology does not yet exist for projects of a larger scale.