A foreign invader has reached the Irish coast, a tough, wiry seaweed that has no commercial value, but could damage the shellfish industry and native species, writes Dick Ahlstrom.
A nasty seaweed has established a toehold on the Irish coastline and looks set to take over the place. It is long on nuisance value, but also threatens the shellfish industry, is expected to clog marinas and could even cause deaths.
NUI Galway's Martin Ryan Institute has been monitoring the spread of Sargassum muticum, a native of Japan which is also known as wireweed. It has a tendency to take things over wherever it migrates and had evolved a particularly successful method of growth and reproduction, explains the Institute's director, Prof Michael Guiry.
Wireweed first appeared here in Strangford Lough in 1995, but it was not until last July that it was spotted in Kenmare Bay. Plants were discovered growing happily in Bertaghboy Bay in Connemara - an important shellfish growing area - last September, and appeared to be at least two or three years old, Prof Guiry said.
It is not so easily spotted given our 7,500 kilometres of coast and the fact that few people work along the shoreline, he said. For this reason he believes it could now probably be growing in many other locations.
"It is a problem because the plant grows very large," he says. "It will shade things. And while it is not like a rhododendron in being able to shade everything out, it will still affect our biodiversity."
There are several ways it could affect us, he says. "One of the down- sides of sargassum is it will clog marinas. It is not called wireweed for nothing." It produces tough fronds that can reach 12 metres long.
It was discovered in Britain in the 1970s and immediately raised fears that it might cause accidental drownings, either entangling swimmers or leading to deaths as boaters attempted to clear ensnared propellers. However, no deaths attributable to wireweed have been noted so far, Prof Guiry says.
It spreads like wildfire wherever it goes. It reached British Columbia in the mid-1940s, most likely attached to shellfish imported from Japan. It colonised more than 6,500 kilometres of coastline, down to Mexico, in just 40 years. It got into the Mediterranean in 1992 and has spread from Perpignan to Venice.
It seems particularly fond of the waters off Brittany, which it reached in the 1960s, again most likely aboard imported Pacific shellfish. There it grows to 12 metres long, whereas it reaches only 1.2 metres off its native Japan.
Although a newcomer here, the longest fronds are already 1.5 metres, Prof Guiry says, warning that with the right growing conditions and nutrients it could easily reach 12 metres here too.
Its success is based on how it grows and reproduces. Sargassums grow from a basal stalk that lives for several seasons on the seabed. Fronds begin appearing in the autumn and grow on into the following summer.
In July and August "sexual receptacles" form on the fronds that provide both sperm and eggs, allowing small plantlets to develop. Wave motion dislodges these, allowing ready-made plants to begin fresh colonies. The baby plants also sail away on the back of the fronds when these detach in the autumn, allowing the plant to extend its reach.
The plantlets happily attach anywhere and if near shellfish cultivation will readily cover the shells. This could cause problems for producers who would either have to clear them off by hand or sell their product with unsightly, wiry stalks attached.
The fronds are full of polyphenols and tannins, making them unpalatable to species that live on seaweeds, so there are no convenient predators to keep them in check.
Nor are they of any commercial value, Prof Guiry points out. The familiar carrageen harvested along our coast contains 70-80 per cent alginate, a thickener prized by food processors. Seaweeds must have 35-40 per cent alginate to be commercially attractive, but wireweed has only between 18-23 per cent alginate.
Researchers at the Ryan Institute have also discovered that other imported species are piggybacking into our waters on board the wireweed. They found a form of polychaete worm, spirorbis, attached to the fronds, adding to the number of newcomers linked to the sargassum.
The Institute has set up an expert group to study wireweed and to advise Government on what should be done about it. A brochure will help the public identify the plants and the Institute has joined Dúchas and Údarás na Gaeltachta to encourage workers along the coasts to watch for wireweed's presence.
The Institute has also opened a wireweed Internet site with a questionnaire prepared by Dr Dagmar Stengel.
The address is www.seaweed.ie/sargassum