Scotland's Marine Atlas: Information for The National Marine Plan

Scotland's Marine Atlas is an assessment of the condition of Scotland's seas, based on scientific evidence from data and analysis and supported by expert judgement.



Non-native species are those species deliberately or unintentionally introduced into the wild by human action outside their natural distribution. Some become invasive where they establish, proliferate and spread in ways that cause damage to native biological diversity and a range of other activities. Invasive non-native species pose one of the most significant threats to marine biodiversity, especially in the light of climate change.

There are a variety of activities that have the potential to bring non-native species into Scottish waters. These include shipping (commercial and recreational), the seafood industry (aquaculture and certain fisheries), scientific research and escapes from public aquaria. Introduction can be either accidental or intentional. Shipping and aquaculture are the most likely sources of their introduction into Scotland, as well as their natural spread from other areas where they are already established.

Vessels of all sizes have the capacity to transport a wide range of species in a number of ways. Ballast water and associated sediments within ballast tanks are important vectors in the global spread of marine non-native species. Non-native species are also transported in other ways including in seawater pipework, in sea chests, attached or entangled on equipment such as anchors and anchor chains, and as fouling on hulls.

Aquaculture has made a significant contribution to the introduction of invasive species. This can include the intentional cultivation of a non-native species within a 'contained' environment that subsequently results in self-sustaining populations in the wild through the escape of adults or the dispersal of larvae. However, more frequently it is the result of unintentional movement of non-native species with fish or shellfish stock. Up to now the extent to which aquaculture has been a factor in the introduction of non-native species in Scotland is relatively small, although the arrival of the Japanese skeleton shrimp Caprella mutica in 2002 illustrates the potential.

Caprella mutica

Caprella mutica
© Liz Cook/ SAMS

Marine non-native species known to be in Scottish territorial waters

A number of non-native species are known to already be present in Scottish waters but a comprehensive list is lacking. Only a few of these species are currently considered invasive (*), but there are a number that have become widespread and well established.



Codium fragile subsp. tomentosoides a green alga

A green alga which is now found throughout UK inshore waters.

Spartina anglica common cordgrass*

Originated from North America and, after hybridising with the native UK species, is now the most common saltmarsh grass in the UK.

Heterosiphonia japonica a red alga

Originating from Japan, first sighted in the Moray Firth, was reported in Loch Sunart on the west coast in 2008 and from Loch Gairloch in 2010.

Asparagopsis armata harpoon weed

Occurs throughout western and northern Scotland as far as Shetland but absent from east coast.

Sargassum muticum wireweed*

Originating from Japan, arrived in Scotland in 2004.

Elminius modestus acorn barnacle*

A native of Australia and New Zealand it is well established in Scottish waters, where they can outcompete the native barnacle species.

Caprella mutica Japanese skeleton shrimp

A predator on several native species, and is now widespread in western Scotland.

Styela clava leathery sea squirt*

Originating in the Far East it is a fouling organism that smothers native species and may affect aquaculture equipment if present in high density.

Didemnum vexillum carpet sea squirt*

Found in the Clyde in late 2009.

Botrylloides violaceus a colonial sea squirt

Native of Japan, first recorded in south-west England in 2004 and recorded in Troon yacht haven in 2009 and Wemyss Bay in 2010.

Corella eumyota a sea squirt

Widespread through much of the southern hemisphere was first report from various locations around Oban in 2009.

Tricellaria inopinata a bryozoan

Originating from the Pacific, first recorded in Troon yacht haven in 2009.

Mytilus trossulus a mussel

(There is some doubt as to whether this species is a native species or not which is as yet unresolved)

Native to Alaska, Canada and the Baltic Sea, first identified in Loch Etive in 2008 and now known from various location on west coast.

Mytilus galloprovincialis a mussel

Native to the Mediterranean occurring in various west coast sea lochs and north-east coast of Scotland.


The potential impact of marine invasive non-native species can be significant, with implications for both biodiversity and the economy.

  • These species have the capacity to affect the ecology of native communities of animals and plants, even becoming a dominant species when well established.
  • They may be a nuisance species for activities such as shipping and aquaculture by fouling equipment and vessels. This may result in lowered production at fish/shellfish farms and increased costs associated with their control or removal.


Action is needed to minimise the risk of non-natives entering the UK and reduce the risks associated with the movement of species outside their natural range. Priorities will include:

  • minimising and managing the negative impact of invasive non-native species in a cost-effective manner;
  • continuing to raise awareness (particularly of preventative measures with relevant stakeholders);
  • developing effective risk assessments for the potential vector activities; and
  • developing best practice in the detection and management of marine non-native species.

Future potential invasions

It is clear that Scotland, in common with other maritime nations, has a growing problem with marine invasive non-native species. This trend seems liable to continue with other species, such as:

  • Chinese mitten crab ( Eriocheir sinensis)
  • Slipper limpet ( Crepidula fornicata)

which are already found in England where they have become well-established and could arrive in Scotland in the near future.

Crepidula fornicata

Crepidula fornicata
© Copyright Keith Hiscock

Sargassum muticum

Sargassum muticum, (wireweed), is an invasive non-native brown alga originally from Japan but now found extensively in Europe and North America. It is a fast-growing species able to form dense stands where conditions suit and by virtue of its life-history traits, is ideally suited to spread rapidly once established in a new region. Fronds of the alga readily become detached and can then disperse via natural drift. Crucially, these fronds can remain reproductively active for several weeks enabling dispersal over a wide area. S. muticum was first recorded in the UK in 1973 but did not reach Scotland until 2004 when it was found in Loch Ryan. It has subsequently spread up the west coast of Scotland.

Sargassum on Cumbrae

Sargassum on Cumbrae
© Lorne Gill/ SNH

Didemnum vexillum

Didemnum vexillum (carpet sea squirt) was discovered in the Firth of Clyde in 2009. This fast-growing colonial sea squirt has the potential to cause significant ecological and commercial impacts, based on experience in North America, New Zealand and several European countries. Carpet sea squirts are fouling organisms with a prodigious capacity to form dense and extensive growths on vessels' hulls, marina pontoons, ropes and aquaculture equipment. Their translocation is principally associated with vessel movements. Work is underway to try to eradicate the species before it is able to spread more widely.


  • Prevention of new species being introduced.
  • Dependence on coordinating action in neighbouring countries.
  • Need to respond rapidly to future colonisations.
  • Establishment of a mechanism/network to ensure early recording of new arrivals.

Carpet sea squirt growing over a creel

Carpet sea squirt growing over a creel
© Copyright SNH/ SAMS


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