South America Invaded: Golden Mussel

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The golden mussel is a freshwater bivalve native to China and south-east Asia. During the 1960s it became established in Hong Kong, and later in Japan and Taiwan. In 1991 it was detected in South America in the Rio de la Plata, the river mouth separating Argentina and Uruguay. The mussel probably arrived in the ballast water of ships visiting the ports of Buenos Aires and Montevideo. Aided by human movements and boat traffic, it rapidly spread upstream, advancing at an average rate of 240 km per year. Today it occurs throughout the Parana River system that links Argentina, Uruguay, Paraguay, Brazil and Bolivia.

Foul play

As an invader, the golden mussel has much the same impact as the zebra mussel Dreissena polymorpha in the Great Lakes and adjacent waterways in North America. It is a serious pest at some sites, causing massive fouling problems. It clogs the intakes, pipes and filters of water treatment facilities, industrial plants and power stations, reducing flow velocity, compromising efficiency and representing a hazard to cooling systems. By improving the habitat for micro-organisms, it results in increased surface corrosion. Decaying dead mussels pollute drinking water systems, while empty shells add to the fouling problem. The mussel can also affect aquaculture farms by fouling cages and nets and competing with other filterfeeders for planktonic food. Furthermore, boats plying the local rivers for fishing, tourism and trade typically carry the mussel as an unwelcome passenger. Fouling on boat hulls increases drag – which reduces speed and increases fuel expenses – and may damage the hull surface, while clogging of the cooling water intakes may cause the engine to overheat, with costly consequences.

The effects of fouling by the golden mussel, together with the ongoing remedial actions required, may have a significant economic impact. Of equal concern, though, is the environmental impact of this invader. Its high reproductive capacity means that populations increase rapidly to form dense, homogenous mussel beds, resulting in reduced benthic biodiversity. Indigenous bivalves are smothered and starved as the mussels settle on top of them and compete with them for food, while other invertebrates and aquatic plants are displaced due to habitat modification. At least one fish species is known to have changed its diet by targeting the abundant mussel as its main food item, and other vertebrate predators have no doubt also altered prey selectivity in response to the invasion.

The invader’s ripple effect on the food chain is especially disturbing given that the golden mussel has reached the Pantanal, a high-altitude floodplain that straddles parts of Brazil, Bolivia and Paraguay. With an area of some 150 000 km2 during the wet season, this is the world’s largest wetland, and is recognised as a World Heritage Site because of its rich biodiversity and abundant wildlife. Fortunately, the natural phenomenon known as ‘dequada’ may limit the golden mussel’s establishment in the Pantanal. Concentrations of dissolved oxygen in the water decrease to some extent every year during the annual flood phase, and the mussel is unable to survive extreme low-oxygen conditions.

Regional cooperation and international experience

In response to the threats posed by the golden mussel, Brazil’s Ministry of the Environment established a National Task Force to combat the invader, and launched an Emergency Action Plan to prevent its further spread. A public education and awareness campaign is an important part of the plan, which is being implemented by local coordination committees.

Brazil also joined forces with its neighbours Argentina, Paraguay and Uruguay to share knowledge and experience. The Globallast golden mussel project brought together a multi-national team of researchers, and marked the beginning of regional cooperation in South America on ballast water and aquatic bio-invasion issues.

Researchers in South America are drawing upon North American experience in dealing with the zebra mussel, and several control methods are being evaluated for implementation against the golden mussel. These include mechanical cleaning, chemical and thermal treatment, carbon dioxide pellet blasting, antifouling paint application, freezing and desiccation. Treatment with a hot, chlorine solution will kill fouling mussels in water systems, and can then be used as an ongoing maintenance measure to prevent new infestations. It is important that the wastewater is not released back into the natural environment, as it will be toxic to other organisms.

Various methods of treating ballast water have been proposed, including filtration to remove ballast stowaways, or the application of biocides, ozone, ultra-violet light or electric currents to kill them. However, most of these are not practical or economically viable for application in South America at present.

Special care should be taken when transporting smaller boats, such as skiboats and yachts, from one area to another. All fouling organisms should first be scraped

off the boat and trailer well away from the water, and properly disposed of. Leaving the boat exposed on land for a week during hot, dry weather will kill most mussels, so that they will either fall off or be easier to scrape off. The hull, bilges and trailer can also be rinsed with a dilute solution of chlorine (500 cc in 10 litres of water), providing this will not contaminate any nearby water body. Water should never be transferred from one water body to another, in case it contains the microscopic larvae of the golden mussel.

Population Explosion!

South America’s first record of the golden mussel was at Bagliardi, near Buenos Aires, in 1991. At that stage, the density of the population was only 5 mussels per m2, but a year later this had increased to 36 000 per m2. In 1993, the figure had more than doubled to 80 000 per m2 and by 1998 had reached a staggering 150 000 mussels per m2!

Other marine invasions on the south-east coast of South America are: 

• the NE Pacific barnacle Balanus glandula, which colonised the Rio de la Plata during the 1960s and now occurs as an intertidal belt on rocky shores
• the Australian tubeworm Ficopomatus enigmaticus, which forms extensive reefs that modify estuarine ecosystems, and 
• the Pacific oyster Crassostrea gigas, which is spreading rapidly in shallow bays.

 

Reference: Matthews S. & Brandt K.   South America Invaded: the growing danger of invasive alien species.  Global Invasive Species Programme 2006