What is a marine "alien" (or non-native species)?

Alien species have become established in UK waters, but are not native to the UK (ie. they haven't been part of our natural marine environment for the past 5,000 years). Invasive aliens are those species that have in some way disrupted native UK species or habitats as a result of their population explosion, behaviour or effect on the natural ecosystem. For example, the zebra mussel was first recorded in UK waters in 1824, but wasn't considered "invasive" until around the year 2000.

Unfortunately, as with the zebra mussel and Chinese mitten crab, there are often no native predators or competitors for many alien species, and as a result, their populations can mutliply exponentially in a rapid period when environmental conditions for breeding and settlement become suitable.

Alien species in our seas are generally transported via ballast water in container ships, which is taken up at foreign ports (along with the larvae of animals and plants from those waters) and discharged here. Many of these larvae can stay alive for weeks, to be discharged (along with the ballast water) in UK waters when these container ships take on cargo in UK ports and harbours. Other vectors include the outside of ships hulls (transportation of attached adults), commercial fish and shellfish farming of foreign species, or amateur aquarists spilling species into natural waterways.  

The only short-term cost-effective way of removing aliens is to physically remove them, be it from ships hulls, or water pipes. Researchers based at the University of Cambridge have developed potassium chloride poisoning of the zebra mussels which works at a local scale, but doesn't damage native wildlife, and anti-foulants (click here to see some images) are regularly used on ships hulls. These products are often environmentally problematic in themselves.

MCS is lobbying government to set up a marine non-native species recording and rapid response unit within JNCC (to remove early invaders where practical), and to ratify the international ballast water convention with other NE Atlantic and international states to treat ballast water. MCS also raises awareness on the issue, and carries out our own annual surveys of the brackish waters of the Thames.

Some alien species:

1. The Slipper Limpet Crepidula fornicata

^{The American slipper limpet forms vast mats of individuals clinging on to the seabed,
and any solid object (like the scallop below) (Photos from the Salcombe Estuary, Devon)}

^{Slipper limpet UK distribution (Summer 2007)}

^{Slipper limpets have increased exponentially in some UK sheltered conditions -
the graph above is from the Fal estuary in Cornwall.}

2. The Chinese Mitten Crab Eriochier sinensis

^{Originally from Asia, The Chinese mitten crab burrows into river banks causing erosion, and has caused problems in UK rivers by being a voracious predator, and clogging up water inlets for power stations. Adults annually migrate downstream from freshwater areas to spawn in the salty waters of estuaries. Photo © The Natural History Museum, London.}

^{UK Mitten crab population distribution as of Summer 2007.}

^{The Thames mitten crab exploded in the early 1990's perhaps as a result of reduced rainfall levels, allowing the crabs to migrate further upstream. In the slower-flowing Seine in Northern France, crabs have been established since the early 1900's, probably as a result of its slow flow.}

3. Japanese Knotweed Sargassum muticum

^{Sargassum muticum is a beautiful, but fast-growing and tough algae that colonises sheltered areas (particularly rock pools) in UK waters. It smothers native algae, strips waters of essential nutrients, and can dominate rocky shore communities in rockpools.}

MCS surveys

MCS carries out annual alien species surveys in the upper tidal reaches of the Thames. You can find out more about these surveys here.