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Zebra Mussels

Zebra Mussel. Photo credit: Gina at http://www.scientificillustrator.comZebra mussels originated in the Balkans, Poland, and the former Soviet Union.

Zebra mussels get their name from the striped pattern of their shells. However, the pattern has been seen to vary greatly to where there are no stripes, only dark or light coloured shells. Zebra mussels can grow to a maximum length of about 50 mm (5-10 mm in the first year) and live four to five years. They inhabit fresh water, usually at depths of two to seven meters.

They were first discovered in North America in 1988. The first account of an established population came from Canadian waters of Lake St. Clair, a small water body connecting Lake Huron and Lake Erie. By 1990, zebra mussels had been found in all the Great Lakes.

It is highly likely that the presence of zebra mussels in the Great Lakes was a result of a ballast water introduction. Its rapid dispersal throughout the Great Lakes and major river systems was due to its ability to attach to boats navigating these lakes and rivers.

Optimal temperature for spawning is 14 to 16C. Over 40,000 eggs can be laid in a reproductive cycle and up to one million in a spawning season. Spawning may last longer in waters that are warm throughout the year.

Zebra mussels are notorious for their ability to reproduce. In one location researchers noted that in one square metre, Zebra Mussel populations jumped from 1000 to 700 000 in six months. Each mature Zebra Mussel can filter one litre of water a day. There are now enough Zebra Mussels in Lake Erie to filter the entire volume of the lake once a week.

Navigational and recreational boating can be affected by increased drag due to attached mussels. Small mussels can get into engine cooling systems causing overheating and damage. Navigational buoys have been sunk under the weight of attached zebra mussels. Fishing gear can be fouled if left in the water for long periods. Deterioration of dock pilings has increased when they are encrusted with zebra mussels. Continued attachment of zebra mussel can cause corrosion of steel and concrete affecting its structural integrity.

Zebra Mussel Distribution—1988 to 2004

Sightings distribution - 2004

Most of the biological impacts of zebra mussels in North America are not yet known. However, information from Europe tells us that zebra mussels have the potential to severely impact unionids (native mussels) by interfering with their feeding, growth, locomotion, respiration, and reproduction. Researchers are observing some of these effects as they study interactions between zebra mussels and native unionids in the Great Lakes. (USGS, 2002).

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. Overall, the impact is a reduction in the amount of food available to native species. In fact, Zebra Mussels have caused drastic declines in the native Great Lakes mussels commonly called clams. They infest the exposed clamshell to the extent the clam cannot get enough food to survive.

Zebra Mussels can significantly change the nature of the lake bottom, affecting fish habitat and spawning. In zones near the shore where mussel populations dominate, they appear to be changing the natural process along the shoreline by trapping nutrients and disrupting the normal flow of these nutrients into deeper waters. The mussels also excrete nutrients creating an environment that may be linked to water quality problems, such as algal fouling on rocky shorelines, off-tastes in drinking water and lethal outbreaks of botulism in wildlife, especially during warm water periods.

Industries that use river water for cooling, like Bruce Power near Tiverton, millions of dollars per year to remove the encrusted mussels clogging intake or outflow pipes and structures. The sharp shells can also be a danger to swimmers. Decay odour along beaches and historical sites like shipwrecks encrusted with mussels are having a negative impact on tourism in many areas around the Great Lakes. (NWRI, 2004)

The feeding activity of zebra mussels results in changes in the normal energy cycle within a water column. While each mussel can filter about one litre of lake water per day, 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 outbreaks 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 die­offs. The outbreaks 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. (OFAH, 2004)

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