2-2 Reading 1: Zebra Mussels

Part A: The Infestation of Zebra Mussels in Canada’s Waterways

Zebra Mussels have become a serious problem in Canadian waterways. The infestation of this pest has become severe over the last decade mostly due to the fact that, as Zebra Mussels are not native to Canada, they have no natural predators that feed on them, and therefore there are no natural controls within the environment. Before examining possible methods the Canadian government might use to control this pest, the following information from the Ontario Ministry of Natural Resources will help to explain the problem more fully.

Zebra mussels are freshwater mollusks commonly called clams. These small, brown, yellow and white striped clams grow to a maximum length of five centimetres and live up to five years. They were first discovered in North America in Lake St. Clair in 1988. Scientists believe they arrived in the ballast water of a ship travelling from a European port.

Zebra mussels have spread quickly in Ontario. By the end of 1990, they were found in large numbers in Lake Erie and Lake Ontario and have been reported in the remaining Great Lakes and in the St. Lawrence, St. Clair, Detroit and Niagara rivers. A female zebra mussel can, over her lifetime, produce up to 150,000 eggs. Upon hatching, the larvae (known as veligers) are microscopic, and cannot be seen by the naked eye. The veligers are free swimming for about eight to fifteen days during which time they are widely spread by currents.

When zebra mussels begin to form their shell (known as settlers) they become heavier and settle out of the water column. At this time they secrete small fibers known as byssal threads which they use to attach to any suitable hard surface. Upon settling, they become visible.

Zebra mussels attach themselves to any available solid underwater surface, e.g. boat hulls, motors, buoys, water pipes, docks, rocks, reefs, and even other animals such as clams and crayfish. They quickly build up on solid surface in large colonies, up to several hundred thousand per square meter in Lake Erie.

Large concentrations of zebra mussels have been found attached to intake pipes of municipal water treatment plants, power plants and industries. Water flow is greatly reduced, in some cases by more than 50 per cent. To remove the mussels and to prevent further build-up is difficult and expensive.

Zebra mussels may harm our valuable fisheries resources. They consume large quantities of plankton, possibly reducing the amount of food available for some young fish. They also colonize spawning shoals, potentially decreasing the survival rate of fish eggs.

Zebra mussels can be spread unknowingly from the Great Lakes to inland waters by boaters. Invisible in their early life-stages, zebra mussels are easily transported in water, in baitfish buckets and live wells; or as adults attached to boat hulls, engines, trailers, float plane pontoons, fishing gear, scuba diving equipment, or a number of other items. While waterfowl and other wildlife may also disperse zebra mussels by carrying veligers in wet fur or feathers, boaters have the greatest potential for moving zebra mussels from one lake to another. Any boat that travels from the Great Lakes to inland waters, such as the Trent-Severn or Rideau Canal system, or is trailered from the Great Lakes, has the potential to spread zebra mussels to new waters. Adult zebra mussels can live from several days to a couple of weeks in moist, shaded areas, (e.g. in bilges, live wells, inside trailer frames). As mentioned at the beginning of this article, there are no naturally occurring ecological controls for this pest. As the Zebra Mussel was introduced into the Canadian waterway system from Europe, it does not fit into the naturally existing food chain. No other species uses it for food and it consumes huge quantities of food that would normally be available to other species. As a result the Zebra Mussel population is growing at a tremendous rate, creating a great ecological and economic impact and, consequently, the Canadian government must attempt to introduce an environmentally safe method of controlling this pest.

Part B: The Ecological Impacts of the Zebra Mussels (Dreissena polymorpha) in the former European USSR and in North America

The mussel Dreissena polymorpha or Zebra Mussels formed as a species millions of years ago in the huge saline water basin which includes the present Aral, Caspian, Azov, and Black Seas. It was in the mouths of the rivers that flowed into these seas, such as the Volga, Don, Dnepr, and Danube, that there was a stable relationship between Zebra Mussels and other organisms in the area. About 200 years ago, evidently in connection with the construction of canals and pollution of the water basins, Zebra Mussels began to increase their range into North European rivers that emptied into the Baltic Sea. Similarly, about 40-50 years ago, this mollusk began moving northward along the Volga, Don, and Dnepr Rivers when large regions were flooded as a result of the construction of a series of reservoirs for hydroelectric stations.

Long-term Changes in the Zebra Mussel Population

Based on observations and data from a number of reservoirs in the former European USSR, a mathematical model was developed to typify long-term changes in numbers of Dreissena/ Zebra Mussels and other ecosystem components after the introduction of this species. After first being introduced into a system, the number of Zebra Mussels increases relatively slowly at first, then increases rapidly to a plateau about 10-15 years after becoming established, and finally decreases to a stable lower population level (Figure 1).

The decline in numbers is a result of both a decrease in food resources (micro plankton), since this species is a very active filter feeder, and also a result of being eaten by various species of fish. Typically, when the biomass of Zebra Mussels increases, the biomass of the fish population also increases (Figure 1). The dominant fish in the rivers and reservoirs of the former European USSR is the river roach. This fish begins to feed on Zebra Mussels when it reaches a length of about 13-15 cm. In addition to the river roach, other abundant roach species also feed heavily on Zebra Mussels including: Rutilus caspicus, Rutilus heckeli, and Rutilus frissi cutum. Nonroach species that also feed on mollusks are the gustera (Blicka bioerkna), the Aral barbel (Barbus brachycephalus) and sturgeons (Acipensetidae).

The increase in numbers of Zebra Mussels in Lake St. Clair and Lake Erie in the first few years after its introduction far exceeds those increases noted in water bodies of the former European USSR. When the mathematical model developed for predicting ecosystem changes in the former European USSR was applied to changes in the North American Great Lakes, the results indicated that the relatively rapid, explosive increase in Zebra Mussels could only have occurred if there were no (almost no) fish in the Great Lakes able to feed on the mussels. This indeed appears to be the case. In Europe, most fish species that feed on Zebra Mussels are of the variety that have several rows of teeth and are able to crush the mussel shells. Few fish species with this characteristic are found in North America. Fish biomass in the water bodies of North America should decline rather than increase after Zebra Mussel becomes established. With few predators, large populations of Zebra Mussels will develop and proceed to filter the water of plankton, thereby limiting the food resources of larval fish.

Long Term Prospects Bad for Fish

Over the long term, the Zebra Mussel population in North American water bodies will either be limited by the amount of available food, or will be limited by parasites, bacteria, disease, etc. In the former case, a stable population will become evident about 10-15 years after the species is first introduced; in the latter case, the population will fluctuate broadly over periodic intervals (Figure 2). Either way, the fish population will probably decline and remain at low levels.