Alien Invasive Species


Introduction of alien species


The introduction and spread of alien species in Canada poses a serious threat to our environment, economy and human health and safety. The cause and effect of alien invasive species is often unequally distributed between the sectors that are pathways of invasion, and the sectors that are directly affected by them. In the case of ballast water, species unintentionally introduced through this vector have had wide-ranging impacts on the aquaculture and freshwater fisheries, but not on the shipping industry per se.


Ecological impact


The introduction and spread of alien species into any ecosystem is highly undesirable because of the negative and irreversible changes that can result. Alien species with no natural predators can wipe out native species by altering their habitat, feeding on them excessively, or by using up a particular food source on which native species depend.

Some species can also alter an ecosystem's dynamics. The zebra mussel, for example, filters large amounts of phytoplankton from water, which can improve water clarity but the increased light penetration has caused considerable problems for walleye (Stizostedion vitreum) populations and other organisms that are adapted to survive in cloudy water.

Quagga mussel (Dreissena bugensis) (bottom photo) a native species threatened by the zebra mussel (Dreissena polymorpha)

The increased light penetration also results in the increased growth of algae such as Cladophora to much greater depths. Zebra mussels are also a major threat to endangered North American freshwater unionids, such as the quagga mussel (Dreissena bugensis) by competing for food and encrusting their shells, resulting in mortality.


Economic impact


Until the introduction of sea lamprey (Petromyzon marinus) to the Great Lakes in 1955, there was no general understanding that alien species could be both an environmental and an economic problem. Sea lampreys have decimated lake trout, whitefish and chub populations in the Great Lakes resulting in severe losses to the Great Lakes recreational and commercial fisheries. The Great Lakes Fishery Commission reported that sea lamprey control, assessment and research cost $13.5 million in 2001.

Once introduced to Lake St. Clair in the early 1980s, zebra mussels quickly expanded their range throughout the Great Lakes. The zebra mussel is notorious for firmly attaching itself to any solid surface including hulls of boats, rigging and motors. As a collective result, zebra mussels are now present throughout the Eastern Seaboard to the mouth of the Mississippi River at the Gulf of Mexico.

Zebra mussels have caused tremendous problems for industrial raw water users by clogging water intake pipes resulting in significant increases in annual operating costs to the automotive industry, water purification plants, and electric power utilities.


Sea lamprey Petromyzon marinus (shown attached to a lake trout) led to the collapse of lake trout (Salvelinus namaycush) populations in the Great Lakes during the mid-20th century. (Photo by U.S Fish & Wildlife Service)


A cluster of zebra mussels on a small rock. (Photo by J. Ellen Marsden, Lake Michigan Biological Station)

The cost to keep the water intake pipes at Ontario Power Generation facilities clear of zebra mussels from 1990 to 2000 was estimated at close to $44 million. The total estimated impact on industries, businesses and communities is over $5 billion.

Alien species can cause severe economic hardship for communities that depend on aquaculture for their livelihood in both marine and freshwater environments. This is especially significant because aquaculture largely depends on local and rural-based communities, so that the impacts from loss of income due to invasive species can directly affect employees, and also their neighbours.

Ballast water is thought to have been the source for the introduction of an oyster parasite called MSX to Cape Breton, which has spread to Prince Edward Island, where the oyster industry is more firmly established. The economic impact to the oyster industry from damages, and the potential future loss of revenues to the industry could extend far into the future and escalate to millions of dollars.

For example, toxic phytoplankton can infect local mussel populations, negatively impacting mussel farmers. Invasive tunicates or sea squirts, pose a serious threat to the marine ecosystem in the Atlantic Provinces as well as the shellfish harvesting and aquaculture industries. Since the introduction of the clubbed tunicate (Styela clava) into the waters of Prince Edward Island in 1998, the issue of controlling invasive species has become a priority with the Island's aquaculture industry, as mussel aquaculture employs about 1,500 people and has an annual harvest value of over $23 million.

The European green crab (Carcinus maenas) has invaded many parts of the world, where its appetite for commercially valuable clams and crabs has threatened important fisheries. Within Atlantic Canada the green crab was first observed in the early 1950s after they had been present in New England for over 100 years. It has recently entered the Gulf of St. Lawrence through the Canso Strait and is expanding its range much more rapidly than it did along the outer coast of Nova Scotia.

Environment and human health impact

Alien species can also destroy the environment and threaten human health and safety. For example, the Chinese mitten crab (Eriocheir sinensis), which is able to live in both fresh and salt water, was found in the lower St. Lawrence River in 2004. They can travel hundreds of miles through inland waterways and reproduce in enormous numbers, burrowing into riverbanks and causing soil erosion and destruction of levees.

Chinese mitten crab
(Eriocheir sinensis)

The crabs are a danger to human health because they are a host of the oriental lung fluke (Paragonimus westermani), a parasite that can penetrate the skin or be ingested and can cause severe illness in humans and other mammals. Through their filtering activity, zebra mussels take in hazardous compounds such as polychlorinated biphenyls (PCBs). Fish and waterfowl that eat the mussels carry those poisons into the food chain.