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  6. Two-decade study shows dramatic colonization of invasive zebra mussels in Eastern Ontario’s Rideau River

Two-decade study shows dramatic colonization of invasive zebra mussels in Eastern Ontario’s Rideau River

André Martel © Canadian Museum of Nature.

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Zebra mussels attached to the shell of a native mussel called the Giant Floater. The zebra mussels prevent it from opening, from normal filter feeding, and from reproducing normally. Image taken below the lower locks at Clowes, just north of Manotick on the Rideau River. 

Ottawa, October 3, 2018 – Invasive zebra mussels have been known in Canadian waterways since they first arrived from Europe into Lake Erie in 1986. Now, scientists with the Canadian Museum of Nature report the rapid colonization of the species over 26 years in Eastern Ontario’s Rideau River, which includes the Rideau Canal system, a UNESCO World Heritage Site.

The results, published today in the Canadian Journal of Zoology, represent the longest-known monitoring study of the invasive species in a small-river system (defined as a few hundred kilometres or less). The first zebra mussels were identified in the river in 1990, and two decades of monitoring the 100-km waterway reveals some areas where there are hundreds of thousands of individuals within a square metre.

“The increase in their numbers in the early years was just stunning,” says lead author Dr. Andre Martel, senior research scientist and malacologist with the Canadian Museum of Nature. “This long-term study shows that the species was quick to establish itself and is here to stay, even reaching significant densities upstream where we thought conditions might not be ideal for their growth.”

The zebra mussel (Dreissena polymorpha) originated in the Caspian Sea region of Europe. The tiny invertebrates (adults are about 15-30 mm long) are highly-efficient filter feeders, and the females can produce up to 1 million eggs per year. The average life span is about three to five years; they often enter new waterways attached to boats. Zebra mussels outcompete native mussels for food—consuming vast quantities of nutrient-rich plankton that would otherwise be available for other species, including native mussels.

© Canadian Museum of Nature

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In the fall of 2010, two volunteers count zebra mussels in quadrats (each 0.5 by 0.5m) on the floor of lock 31, at Smiths Falls, Ontario. Image: © Canadian Museum of Nature.

It was in fall 1990 that Martel first identified zebra mussels in Ottawa’s Mooney’s Bay. Knowing the species had taken over waterways in Europe and was rapidly expanding in the Great Lakes, Martel and senior research assistant Jacqueline Madill set out to document its presence throughout the Rideau.

To do so, they took advantage of the zebra mussel’s ability to grow and attach to all hard surfaces, including boulders and bedrock as well as filtration pipe, sometimes resulting in thick layers as the individuals attach to each other. Starting in 1993, they began monitoring 13 sites along the Rideau River (at most locations every year) and mainly at canal locks operated by Parks Canada.

The river is a popular waterway for boating, fishing and other recreational uses, as it flows downstream from the community of Smiths Fall, Ontario, through rural areas and farmland into the suburban and urban regions of Ottawa.

Each October and November, Parks Canada would draw down the water levels at the locks. With the Agency’s cooperation, Martel and Madill, joined by students and volunteers, would climb down using specially constructed ladders to count and sometimes collect the zebra mussels attached to the exposed walls and bases of the locks. Samples were also taken to the lab, where more accurate numbers could be counted using microscopes.

Numbers from the downstream third of the river close to the National Capital Region showed rapid growth, becoming pervasive by 1996. The growth was exponential, sometimes as high as a 1,000 to 10,000-fold increase over time. The highest densities measured were comparable to the worst-case scenarios reported in the Great Lakes in the 1990s.

By the early 2000s, the team counted hundredfold increases in the middle 40-km stretch known as the “Long Reach” (from Burritt’s Rapids to Manotick), which was not surprising to Martel. “We had hypothesized that this part of the river would provide a favourable environment for growth, given that it acts like a narrow lake, with a slow, sluggish current and no locks. These conditions are favourable to the larvae, allowing them to grow and develop and settle on their way down the river,” he explains.

© Canadian Museum of Nature

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In fall 1996, André Martel and two museum employees prepare to count zebra mussels at a lock along the Rideau River. The researchers built the ladders to access the base of the locks.

By the late 2000s, the team was picking up significant, though less numerous, counts for zebra mussels in the farthest upstream reaches of the river near Smiths Falls. At the same time, over the past 20 years, Martel and Madill have also documented a significant loss of native mussels (at some sites observing close to 100% mortality), in areas where the zebra mussels have taken over.

As colonies of the tiny zebra mussel grow, they effectively “choke” out and smother the larger native mussels. The zebras attach to the native mussels by secreting hundreds of byssal threads, which prevent the native species from feeding or reproducing, killing them over a few months or years. 

The data compiled by Martel and Madill is being incorporated into a worldwide review of river systems colonized by zebra mussels in order to understand the trends and the impacts of their presence. These will include a 10-year study of the Hudson River in the United States, as well as those for waterways in Germany, Ireland, Scotland, Poland and other European countries.

“No doubt the zebra mussel has proven to be amazingly successful in establishing itself  over a relatively short period of time, as we have shown through this exhaustive study of the Rideau River over more than 20 years. At the same time, we have observed significant changes where native mussels have died off and the ecology of the river may be impacted further. All this will require further research,” says Martel.

About the Canadian Museum of Nature
The Canadian Museum of Nature is Canada's national museum of natural history and natural sciences. The museum provides evidence-based insights, inspiring experiences and meaningful engagement with nature's past, present and future. It achieves this through scientific research, a 14.6 million specimen collection, education programs, signature and travelling exhibitions, and a dynamic web site, nature.ca.

Information for media, including photos and video:
Dan Smythe
Media Relations
Canadian Museum of Nature
613.566.4781; 613.698.9253 (cell)
dsmythe@nature.ca