Thames (Richmond) surveys of alien species
The Marine Conservation Society, the Zoological Society of London and the Thames Landscape Strategy have joined forces to survey the Thames at Richmond for alien species and threatened UK marine wildlife. The partnership is taking the opportunity to study the animals that colonise the river bed during the annual 'draw-off' - where the gates are closed at Richmond leaving the river to reach natural river levels without the influence of the tide. The annual event is used to clean and maintain docks, and to remove the river bed of rubbish and silt. This provides a unique opportunity to survey the species on the river bed, right out to the middle of river. You can download a short film of the survey.
Study Site
The Thames at Glovers Island, Richmond just before the survey (November 2007)
Species recorded
This survey recorded the Asian Clam Corbicula fluminea - The Asian Clam is a known biofouler in power plant and industrial water systems and has also caused problems in irrigation canals and pipes. Ecologically, this species can alter benthic substrates and complete with native mussel species for food and space. In addition, Asian clams appear to be capable of tolerating polluted environments better than many native bivalves.
The Asian clam - Corbicula fluminea
The native depressed river mussel - Pseudanodonta complanata - The threats to this species are not fully known, but are likely to include water pollution, physical disturbance of river banks and channels, drought, and the collection of individuals for garden ponds and aquaria. The Zebra mussel can attach itself to the depressed mussel, thus threatening it's ability to feed and burrow.
The depressed river mussel - Pseudanodonta complanata
The Duck mussel - (along with the swan mussel with the genus Anodonta spp.) are very vulnerable to the expansion of the zebra mussel as the zebra mussel not only colonises the riverbed, but can attach to the outer shell of UK threatened native bivalves.
The Painters mussel - Another UK freshwater clam.
Eels Anguila anguila - The eel is a major commercial species in the river Thames. It is very important as a keystone predator in our river systems. It's numbers in NW Europe have declined considerably over the past few decades.
A common eel 'caught' by MCS Biodiversity Policy Officer's son, Yannick Solandt in 2006.
They tend to shelter under large stones at low water
The Zebra mussel Dreissena polymorpha - a species which invaded the Thames in the early 18th century. It was recorded in the Richmond area in a University of Cambridge survey between 200 and 2002 with only 25 counted in a 5 minute search. This species has been responsible for millions of dollars worth of damage in the Great Lakes of the US, clogging water inlet valves, attaching to solid infrastructure, ships hulls, and sinking mooring chains - they can reach densities of 10,000 or more per square metre, forming dense reefs. They individually filter feed about 1 litre of water per hour, and are extremely fecund broadcast spawners (up to 1,000,000 eggs can be released per female). Their abundance can be a detriment to native wildlife, using up space, and colonising the shells of native clams (see below).
Zebra mussels attached to native duck mussell
The Chinese mitten crab Eriocheir sinensis - This species has become ever more widespread in UK rivers in the past 20 years. MCS described its spread in the Summer 07 issue of Marine Conservation. It threatens our habitats and species by being a voracious predator, and adults dig into river banks causing erosion, and have clogged up the inflow pipes of power-station cooling pipes.
chinese mitten crab © Natural History Museum
Survey results (2007)
The surveys only counted a single depressed river mussel, and 21 duck and painters mussels from 37 0.25m2 quadrats. This low count was in contrast the large quantity of zebra mussels counted on the survey - one single quadrat alone (at low water on the Petersham Meadow side of Glovers Island) recorded 208 individuals - over 830 individuals per m2 (Fig 1). No mitten crabs were recorded at either the Glovers Island or Orleans Gardens sites.
Figure 1. Distribution and densities of native and non-native molluscs at the 3 study sites.
The three study sites all had similar substrate habitats:
West Glovers (Marble Hill Park)
East Glovers (Petersham Meadow)
Orleans Gardens
Conclusions
The most apparent patterns were that of the high density of zebra mussels attached to the river bed itself (generally in gravel areas and on rocks). However, what was also apparent was that the mussels also attached to the native clams (see picture above of duck mussel with attached zebra mussels), which inhibits the ability of the clams to burrow and feed.
Duck mussel extracting its foot prior to burial in the mud at Petersham Meadow.
The survey has revealed densities which are much higher than the anecdotal report of 25 individuals recorded on a 5 minute walk in Spring 2002 (Article published in 2004). At Petersham Meadows, the quadrat placed at lowest water recorded 208 individuals - 832 per m2! The next survey will be carried out in November 2008.
What can be done about the spread of zebra mussels and other alien species?
The same researchers who published the article in 2004 have trialled a toxic pellet which is absorbed by the filter feeding mechanism of the mussel, and doesn't harm other native wildlife (as it quickly dissolves in time). This method works for a localised area, but not for the whole Thames area. An alternative is either direct removal, use of hot water flushing, or potentially the use of anti-foulant materials. Eco-Sea is a consultant engineering firm which is developing a non-toxic anti-foulant to treat ships hulls, and water utilities equipment specifically against Zebra mussel infestation. They are currently trying to sell the product to the US market for use in the Great Lakes where millions of dollars a year are spent to control infestations - we are now witnessing the start of this problem in UK lakes and rivers.
In summary, the mussels and mitten crabs are here to stay - we are only able with current technology to provide short-term or small-scale eradication of populations (principally of fouling organisms). The ecosystems effects of these species can be great, and the results of our surveys will be reported to the Environment Agency and Defra.
