Zebra Mussel
( Dreissena polymorpha )
Zebra mussels are what is known as a freshwater mollusk, these mollusks are native to the Black and Caspian Sea region of Asia. It is believed that the mussel was introduced to the Great Lakes via water ballast dumping from foreign ships in the early to mid 1980s. Since that time they have spread throughout all of the Great Lakes passively carried by water currents or intra-lake ship ballast discharge. Zebra mussels have been spread unintentionally by people into waterways such as the Trent-Severn and Rideau Canal and some inland lakes in southern and central Ontario.
Characteristics
Zebra mussels filter plankton from the surrounding water at a rate of up to one litre per day. The mussels are relatively small (3 cm in length) and live approximately 2-3 years. Their shell is brown, cream and yellow striped and triangular in shape with one edge flattened. Small threads called byssal threads attach the mussel to hard surfaces.
Zebra mussels have spread very quickly in Ontario waters. They normally begin to reproduce when water temperatures warm to around 12° C. Female zebra mussels can produce between 40,000 and 1 million eggs per season. These develop into microscopic, free-floating larvae called veligers. At about three weeks the sand grain-sized larvae begin to form shells, start to settle and attach to any firm surface using their byssal threads. Zebra mussels prefer warm, shallow water, and slow currents. The quagga mussel has a similar life cycle and affect on the ecosystem but prefer colder, deeper waters.
Tolerant of a wide range of environmental conditions, zebra mussels flourish with few natural predators, a lack of disease and parasites, favourable temperatures and food supply as well as a high reproductive rate. Diving ducks, the freshwater drum, and a few other fish species such as the introduced round goby eat zebra mussels, but do not significantly reduce their population levels.
Distribution
Zebra mussels were discovered in Lake St. Clair near Detroit in 1988. Zebra and quagga mussels have now spread to all the Great Lakes and are showing up in inland waterways and lakes throughout North America. The spread of zebra mussels has been more rapid than initially predicted by the experts.
Download the Zebra Mussel distribution map ( PDF File, 950 kb )
Impact
Significant changes to aquatic ecosystems have been documented as a result of the introduction of zebra mussels. Zebra mussels filter out large amounts of phytoplankton, and compete with many species of zooplankton which are an important food source for young fish. One such species is Diporeia which is a tiny shrimp-like organism that lives in the bottom mud and who's source of food is settling algae from the water column. Since zebra mussels invaded in the 1980s, there has been a decline in the numbers of Diporeia which normally make up to 70 percent of the living biomass in a healthy lake bottom. Species such as whitefish and other prey fish including alewife, bloater, smelt and sculpin directly depend on Diporeia as a food source. The decline in Diporeia may be linked to declines in numbers and the condition of species such as whitefish, sculpin, smelt and young lake trout from various Great Lakes. This may further have an impact on sport fish such as adult salmon, trout and walleye which feed on prey fish.
The feeding activity of zebra mussels results in changes in the normal energy cycle within a water column. Each mussel can filter about one litre of lake water per day, however, not all of what they consume is digested. What they don't eat is combined with mucus as "pseudofeces" and is discharged onto the lake bottom where it accumulates. Organisms that benefit most from these changes are those that live on the lake bottom such as invertebrates (which include aquatic insects, worms, snails, etc.) and aquatic plants. This filtering causes the water to become clearer allowing more sunlight to penetrate the water column. Changes in weed growth patterns occur and forces some fish, such as walleye that are light sensitive, to find new habitat.
When zebra mussels filter the water, they also remove contaminants which become concentrated in their tissues. Although this may sound like a positive thing, organisms that feed on zebra mussels may accumulate these contaminants in their own tissues. An example are some duck species such as Lesser and Greater Scaup, which now feed on zebra mussels, have elevated levels of contaminants in their tissues which may influence their survival and/or reproduction success. Another invader, the round goby, which predominantly feeds on zebra mussels, may accumulate contaminants in their tissues and may pass those contaminants on to sport fish species which are now consuming them.
Scientists are also finding a link with zebra mussels and the occurrence of toxic blue-green algal blooms or microcystis. Zebra mussels will spit out microcystis into the water and at the same time eat other algae that may be competitors with or help control microcystis. The mussels also produce nutrients that further fertilize microcystis.
Botulism Type-E (Clostridium botulinum) has also been found in the tissue of zebra mussels when out-breaks occur. Species that feed on zebra mussels such as round gobies, freshwater drum and ducks such as scaup may be impacted by eating infected zebra mussels which could result in die-offs. The out-breaks may go further up the food chain still when ducks, loons, grebes, gulls and/or other fish eat the infected species and other animals such as racoons scavenge the contaminated victims.
Zebra mussels will cover any hard surfaces including: rock, metal, rubber, wood, boat hulls, native aquatic animals such as clams and crayfish and even aquatic plants. Some fish prefer rock, and boulder substrates to spawn, and can be deterred from spawning in the areas that are covered in zebra mussel colonies. In some areas, populations of native clams have significantly decreased or completely disappeared as a result of infestation. Zebra mussels will colonize on top of native clams preventing them from opening and closing and also exposing them to predators, parasites, disease, and noxious water quality. Respiration and feeding for native clams is difficult as a result of zebra mussel colonization on their shells.
Navigational markers and fishing buoys used for directing boat traffic can accumulate large numbers of zebra mussels, causing them to sink. Commercial fishing gear such as trap nets and gill nets can also collect enough zebra mussels to render the equipment useless and difficult to retrieve. The hulls of boats and ships can become so infested that sailing efficiency can be impaired. Zebra mussels colonize industrial, boat and domestic water intake pipes, reducing flow rates, and even preventing water flow. It is estimated that facilities on the Great lakes spend millions of dollars each year to combat zebra mussels.
People are directly affected by zebra mussels in a number of different ways. Colonies of zebra mussels on the lake bottom can cut the feet of swimmers. Zebra mussels wash up on beaches creating a foul odor, and an unpleasant sight. They also cover ship wrecks and other articles or sights of interest for scuba divers.
Prevention
Presently, the best defence we have against the spread of zebra mussels is to restrict its movement between waterbodies. Once they become established in a lake, there is no known way to eradicate them. Please take the following precautions to help prevent the spread of zebra mussels and other exotic species:
Inspect your boat, trailer, boating equipment, fishing tackle and nets and remove any visible plants or animals before leaving any waterbody.
Drain water from motor, live well, bilge and transom wells while on land before leaving the waterbody.
Empty your bait bucket on land before leaving the waterbody. Never release live bait into a waterbody, or release animals from one waterbody into another.
Wash/Dry your fishing tackle, nets, boat and equipment to kill harmful species that were not seen at the boat launch. Some species can survive for several days out of water, so it is important to:
- rinse your boat and equipment with hot tap water (> 50° C); or
- spray your boat and equipment with high pressure water (250 psi); or
- dry your boat and equipment for at least five days, before transporting to another waterbody.
Call the Invading Species Hotline at 1-800-563-7711 if you find zebra mussels in an area not shown on the distribution map.
Zebra mussels and dozens of other non-native species, were introduced to the Great Lakes in the ballast water of ocean-going ships. Canadian and U.S. agencies have worked together to initiate guidelines and regulations for shippers to reduce the risk of organisms being introduced in ballast.
Further research and lobbying efforts will improve our ability to prevent ballast introductions.
