Coastwatch has published an awareness pamphlet (downloadable below as a pdf) about the alien invasive species Sargassum muticum or Japanese seaweed.
The pamphlet, which was distributed at Sandycove on Dublin Bay on Monday (July 22), appeals to citizen scientists and coast walkers to report any sightings to the National Biodiversity Data Centre.
Sargassum attaches to rock or stone (sometimes to another seaweed) from midshore down to about four metres sublittoral and can also be found in rock pools.
Plants look like thick brown hair in water, or like an orange-brown mat on water at low tide. Apart from the sturdy perennial tree-like base held firmly by its small round holdfast, the annual stipes look delicate, with branches hanging off like hankies on a washing line when held up, the pamphlet explains.
The Coastwatch team at Sandycove on Dublin Bay
Native to waters off Japan and Korea, the species is known to have been present at the Isle of Wight in 1973.
It was first recorded in Ireland at Strangford Lough in 1995. It has since spread and been prolific from Norway to Spain.
Sargassum muticum is also known to have invaded North America.
The species can occupy hard substrates on sheltered shores where it forms dense monospecific stands and covers rockpools excluding other species.
Wind and waves can raft it in, or tear off the long annual stipes and dump them on bathing beaches during summer, Coastwatch explains.
Dense Sargassum muticum stands can reduce the available light for understory species, dampen water flow, increase sedimentation rates and reduce ambient nutrient concentrations available for native species such as seagrass beds.
“As a general rule, quick early removal of invasive alien biota is the best approach. However, that is not always possible,”the pamphlet says.
“ If you know your area well and see a few stipes appear, nip it off right at the holdfast and take it with you onto land so it can't float away with the tide. Once you see Sargassum- children are good at spotting it too - research viability of removal and permits needed,” it advises.
“It is crucial that this is done before it sheds its germlings. After germlings spread it is too late for removal that year,”it says.
Coastwatch has published an awareness pamphlet (downloadable below as a pdf) about the alien invasive species Sargassum muticum or Japanese seaweed
Coastwatch has developed a checklist and removal protocol that is available to the public. It advises caution – “before removing plants to prevent spread, make sure you identified Sargassum correctly, and not a native seaweed”.
There are four similar native species: Pod weed (Halidrys siliquosa) looks similar from a distance, but has long pods. Two others have bluish iridescence in the water, whereas Sargassum hasn't.
The least common but most similar look-alike is the Bushy Berry Wrack Gongolaria baccata ( Cystoseira baccata).
Check the Irish seaweed site for more information. You can also send Coastwatch a photo of a plant with hold fast and location for assistance.
More information from Coastwatch co-ordinator Karin Dubsky at email [email protected]
