A recent survey of the Thames has found huge numbers of non-native species including the highly invasive Zebra mussel, Dreissena polymorpha.
A survey taken in the Thames from Richmond through to Teddington Lock by the Marine Conservation Society (MCS), Zoological Society of London (ZSL) and Thames Landscape Strategy found greatly increased numbers of non-native species compared to the last survey taken in 2002.
The most marked increase was in populations of the Zebra mussel. In 2002 a survey found only 25 individuals, whereas this survey found populations of up to 830 individuals per square metre.
The Zebra mussel is native to the freshwater lakes of Russia. It was thought that this species was originally brought over by cargo ships carrying the mussel larvae in ballast water which off-loaded into the Thames. This mussel, named after its stripy shell, only grows up to 5cm in length but can severely affect waterway ecology.
Zebra mussels come in several colours - hence their species name polymorpha. Picture US Geological Survey, Creative Commons.
Breeding females are extremely prolific and can produce up to 40,000 eggs a year and 1,000,000 eggs in a lifetime. Thus, populations can increase hugely and can number 11,000 individuals per metre square and up to 20cm deep. Each mussel can filter one litre of water and hour meaning that in areas where numbers are high, water becomes clearer allowing greater penetration of light and a huge increase in weeds.
The riverbed topography is altered producing a population skew to bottom feeding fish and a decrease in mid-feeding fish. They also attach to native UK species preventing them from feeding and breathing. There are no natural predators of the Zebra mussel in the UK.
The huge numbers that can congregate in restricted areas also means that this species is extremely costly. In the US, where this species has been present only since the 1980s, it is estimated that the annual cost due to the Zebra mussel is $3-5 million dollars.
This is due to the mussels clogging water inlet valves of water treatment plants, and attaching to the bottom of ships and boats increasing drag and thus fuel consumption.
Other species found to have increased in numbers in the survey included the Asian Clam, responsible for clogging riverbeds and pipes, and the Chinese mitten crab which cause bankside erosion by digging into the riverbed.
Possible solutions to the problem of non-native molluscs include toxic pellets of salts known as bio-bullets, anti-foulant paints, direct removal and hot-flushing.