Thursday, 16 September 2010

Night of the killer shrimps.

Last week the Environment Agency sent its biologists down to Paxton Pits to look for an invasive alien. The "Killer Shrimp" - Dikerogammarus villosus was discovered in Grafham Water by anglers who noticed that it was three times the size of our native freshwater shrimp (it can be up to 30 mm long).

It is native to the Black Sea region, and has already entered the Rhine system where it is upsetting the ecology by eating everything it can. This shrimp is a predator, while our native one is basically a detritus feeder. It could have come here in sailing boats that have competed abroad, or in the kit of travelling anglers. If so, it could easily spread to other sites such as Rutland Water. It might also be moved by birds.

Having identified the beastie, Anglian Water and the Environment Agency were keen to make sure that it was only in the reservoir and not in nearby water bodies such as Paxton Pits, or the River Great Ouse. So far Grafham seems to be the only site in the UK. It is a big worry because I have no idea how the shrimp could be removed without killing off everything else in the lake.

The shrimp was not found at Paxton, but the biologists did report that they found blue-green algae and a highly invasive weed from New Zealand called Crassula. We know about both of these, but there is nothing we can do to remove them.

We also know that we have Zebra Mussels in our lakes. These come from the Black Sea too. They were carried in ships' bilges to the Great Lakes in the USA, then back to the Thames. They are an economic pest because they block pipes at power stations, pumps and even on engines.

On the land, we made an effort to eradicate some more aliens this week. We found some quite large clumps of Himalayan Balsam on the river bank and pulled it all up. We also tried to eliminate two colonies of Japanese Knotweed (see photo, left) by pulling and burning. I'm sure we haven't seen the last of it though.

Some of our neighbours in the village have been using the reserve as a dump for their garden waste. I hope the examples above illustrate the dangers of introducing non-native species which can become invaders.