Wednesday 21 February 2018

Wireweed a common sight on our coasts

Japanese Wireweed growing in a rock pool on the seashore.
Japanese Wireweed growing in a rock pool on the seashore.

A recent trip to the seaside renewed my acquaintance with Japanese Wireweed a brown seaweed that has become very common around our coasts in recent years.

Several brown seaweeds may be seen on rocky seashores and on sandy beaches where there are hard surfaces such as coastal protection works, breakwaters or harbours. Japanese Wireweed is distinguished from the others by its unusual yellowish-brown colour, its wiry stems and its very numerous and very small gas-filled bladders that give the seaweed buoyancy.

The bladders cause the metre-long seaweed to float just below the surface of the sea. It grows low on the shore so since it has a restricted range it is best searched for near the water's edge when the tide has fully ebbed.

Wireweed is an invasive alien from the Far East that has managed to colonise our shores. It grows vigorously, reproduces rapidly, can self-fertilise and can spread vegetatively from bits broken off a parent plant. Its bladders keep it afloat ensuring that floating fragments are dispersed widely by wind and currents. It is also known to travel internationally via imported stocks of Pacific Oysters for shellfish aquaculture and by hitchhiking attached to the hulls of cruising yachts.

There are many reports of Wireweed colonising new areas and displacing native local species of seaweed and eelgrasses. Its habit of floating just below the surface causes shading resulting in the Wireweed out-competing other plants for light and sometimes becoming the dominant species as a result.

When Wireweed becomes abundant it causes problems for commercial fishermen fouling propellers, fishing nets and lines. It becomes a serious pest of oyster beds and large floating mats become a nuisance in harbours and marinas.

The plant's botanical name in Latin is Sargassum and enormous floating mats of it and its relatives are so extensive in the North Atlantic Gyre that the area is known as the Sargasso Sea. The gyre is a 3,200km-long stretch of ocean in which floating Sargassum seaweeds are trapped by the combined impacts of three surrounding ocean currents.

Japanese Wireweed was first reported from Strangford Lough, County Down, in 1995 and colonised the rest of the island of Ireland shortly afterwards.

Attempts to date to control or eradicate the alien have met with such limited and temporary success that they have been largely abandoned in the realisation that the invasive and alien Japanese Wireweed is, unfortunately, here to stay.

Wexford People

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