Invasive Alien Species in Northern Ireland

Sargassum muticum, Japanese weed

Sargassum muticum
© Julia Nunn
Sargassum muticum (Yendo) Fensholt
Sargassum muticum is a brown seaweed now found on the shores of the British Isles, locally abundant especially on the southern shores of England. Attached plants were first recorded in 1973 from the Isle of Wight in the British Isles. It was discovered in Strangford Lough, County Down in 1995 and reported to the DoE (N.I.) on 16 March 1995. It can grow up to 12m long with a lifespan of 3-4 years, producing dense floating mats. It reproduces both sexually and asexually by floating branches which break off and continue to grow and reproduce.
Sargassum muticum is a large brown seaweed of the Division Phaeophyta, attached to the substrate by a perennial conical holdfast up to 5cm in diameter. From this the perennial main axis grows to a maximum of 5cm high. The leaf-like laminae and primary lateral branches grow from this stump. In warm southern waters and over one season it can grow to 12m. However, in the British Isles, it generally grows to no more than 1-4m long. In British waters, the holdfast gives rise to a single terete main axis; from this grow secondary and tertiary branches that are shed annually in the late summer leaving the short stalk and holdfast.
The numerous small, 2-6mm, air vesicles are stalked and provide buoyancy. The reproductive receptacles are also stalked; they develop in the axils of leafy laminae and contain both male and female gametes within separate conceptacles, thus they are self-fertile.
Country of origin
The natural range of this species is from south-east Asia to southern Russia, including Japan.
Europe: Scandinavia, Norway, Baltic Sea, Helgoland, Netherlands, Great Britain, Ireland, north France, the Iberian Peninsula, Italy, Adriatic. East Asia: Japan and China. Western North and Central America: Alaska, British Columbia to California.
The first recorded extension of this species was along the Eastern Pacific coasts of British Columbia, Washington and Oregon. It colonised the entire length of the Pacific coast of North America.
It is considered to have entered European waters in the late 1960s or early 1970s, imported with the oyster Crassostrea gigas from America. It was first recorded in the British Isles from the Isle of Wight in 1973 and later from France and the Netherlands.
Location in Ireland
Strangford Lough, County Down discovered on 15 March 1995; Cashel Bay, County Galway, also Counties Kerry, Sligo, Wexford and Cork.
Life cycle
A period of two to three years is usually required for the development of a breeding stock. As the plants become older, the number of fronds increases and the reproductive output increases. Within three months the plants may become fertile. The plants are monoecious and 50-70 conceptacles can mature on a single receptacle. In the first season, a small 15g plant can release 500,000 eggs. One S. muticum plant can potentially give rise to a whole population. Fronds can also break off and drift away; these cannot reattach, but can, however, remain alive for up to three months and drift some distance away and enable long distance dispersal. A further method of dispersal follows the attachment to small stones and, as the plants grow, the buoyancy results in the stone being lifted and carried by currents to another site (stone-walking). Human action can also disperse the alga by introducing the Pacific oyster (C. gigas) to further areas.
Wildlife and habitat impacts
It was originally feared that S. muticum would outcompete and displace Zostera marina and other species of brown algae; this, however, does not appear to be so. Indeed, S. muticum has been found to support a diverse range of epibionts as indigenous algae. Further, it does grow in some areas not used by indigenous algae and may even increase productivity and provide shelter for fish and other marine creatures.
Human impacts
Because of its prolific growth it has become a great nuisance, forming large detached mats, drifting and clogging marinas, recreational areas and other sports facilities. It can foul fishing lines, clog the intake pipes of boats, trap debris and drift ashore.
Key vectors
Believed to have gained distribution through transportation with Japanese oysters and the fertile fronds being carried by currents or boats to new areas.
Management measures
None recommended. Efforts to remove the plants often did not remove the perennial base from which the algae could grow again next year. In fact S. muticum responded to physical damage by regenerating quickly and rapidly recolonised cleared areas. This resulted in even denser populations.
Further information
Davison, D.M. (1999). Sargassum muticum in Strangford Lough, 1995 - 1998; A review of the introduction and colonisation of Strangford Lough MNR and cSAC by the invasive brown algae Sargassum muticum. Environment and Heritage Service Research and Development Series.No. 99/27.

Farnham, W.F., Fletcher, R.L. & Irvine, L.A. (1973). Attached Sargassum, Found in Britain. Nature 243 No. 5404 pp.231 - 232.

Farnham, W.F. (1980). Studies on Aliens in the Marine Flora of Southern England. In Price, J.H., Irvine, D.E.G. & Farnham, W.F (Eds) The shore environment 2: ecosystems pp. 875 - 914. Systematics Association Special Volume 17(b). Academic Press, London and New York.
See the Invasive Species Ireland web site for further information -
Text written by:
Osborne Morton, Curator, Botany, Ulster Museum