Wireweed
What action is going on for this species?
The following will give you information on what work has been taking place through the Species Action Framework:
Species background
Wireweed (Sargassum muticum) is a large olive brown seaweed that can grow to more than a metre long.
Why is this on the Species Action List?
Wireweed is an invasive, non-native species which meets criterion 2 of the Species Action Framework, as posing a significant risk to marine biodiversity, primarily through competition with indigenous marine plant life.
Although only very recently found in Scotland, it has spread rapidly elsewhere in the UK since it first appeared in 1973, and now seems to be doing so in south-west Scotland.
Habitat, distribution and abundance
Wireweed is normally found on the lower shore or in shallow sub-tidal waters and in mid-shore rock pools. It generally attaches to rocky substrates, but it can also be found on sandy or muddy shores where stones are present. Wireweed prefers sheltered to slightly exposed habitats as it can be damaged by wave action in more exposed sites.
In Scotland it is currently known to occur at various sites in the south-west, from Loch Ryan to Loch Fyne.
General ecology
The plants have long main fronds with lateral branches that hang like washing from a line when held horizontally out of the water. The lateral branches support distinctive grape-like air bladders. In autumn, the branches break off from the basal structure and they can be found floating individually or in large masses. These detached fronds remain reproductively capable and contribute to this species' ability to expand its distribution rapidly. The basal structure remains attached to the rock and will regenerate new branches in the spring time.
History of invasion and expansion
It is native to the western Pacific but has become established in many other regions as an invasive, non-native species. It was first recorded in the UK in 1973 on the Isle of Wight, and has since spread along the south coast of England and into Wales and Ireland.
The first reports of wireweed in Scotland were in Loch Ryan in 2004 and, since then, populations have also been found at Great Cumbrae Island and on the North Ayrshire coast. In July 2006, wireweed was reported in Loch Fyne, and it may already be present, but as yet unidentified, in other locations in the Firth of Clyde.
Impacts on other biodiversity and conservation interests
Wireweed is a fast-growing species that can compete with native seaweeds and seagrasses through over-growing, shading and abrasion. Germlings are well-developed when released, and so may have an advantage in competition for attachment sites with other macroalgal species. However the competitive ability of wireweed may be reduced at lower salinities and this may limit their ecological impacts, for example in Scottish sea lochs with high freshwater input.
It is considered a nuisance in harbours and shallow waters where large floating masses may become a hazard to commercial and recreational boating through entangling in propellers or blocking engine cooling systems. The floating mats can also affect watersports such as swimming, wind surfing and sailing. Wireweed can foul fishing nets and lines and can seriously impact on oyster beds and other aquaculture structures.
What should I do if I find wireweed?
If you think you have found wireweed, please report your sighting.
The 'Species Lead' at Scottish Natural Heritage
Fiona.Manson@snh.gov.uk Tel 01738 444177