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Presence of the zebra mussel in Magog Bay, Lake Memphremagog

Presence of the zebra mussel in Magog Bay, Lake Memphremagog

To consult the report (In French only)

Invasive zebra mussels were inventoried in 2018 to determine their potential proliferation in Lake Memphremagog. A biologist and the MCI patrollers found low levels of the empty shells of these bivalves, evidence that their arrival is recent, likely 2 or 3 years ago. However, the Lake's physico-chemical profile makes the 10 first metres around the whole lake a likely site for their expansion. Further surveillance needs to be undertaken in future years.

Evaluation of the Presence of Zebra Mussels in Lake Memphremagog /Magog Bay in July 2018

Members of Memphremagog Conservation Inc. (MCI) lake patrol with biologist Isabelle Picard have reported on invasive zebra mussels (Dreissena polymorpha) in areas of Lake Memphremagog near Magog, which were first observed in 2017.
Biologists have determined that these small mussels disrupt natural ecosystems by attaching to the larger native bivalves (of the family Unionidés), sometimes preventing the larger mussels from opening their valves, affecting their breathing. They also advantageously compete for oxygen and food sources such as plankton, filtering up to a litre of water a day. They reproduce rapidly and can thus eliminate the natural mussels from their territories as well as reduce food sources for young fish and other invertebrates. Their filtering action increases the clarity of water and encourages the growth of aquatic plants at greater depths, further disturbing the eco-equilibrium. Native species can become maladapted to this transformed environment: Fish and other animals can be affected by the proliferation of harmful algae caused by its presence. The zebra mussel is also a vector for avian botulism which has killed thousands of aquatic birds in Lake Erie.
First observed in Lake St-Clair in 1988, the zebra mussels rapidly spread in the St. Lawrence River corridor by 1990, including areas on the south shore, Lake Champlain and the Richelieu River. Early preventative measures such as boat washing stations seem to have slowed its progress compared to the U.S., where its expansion has been very rapid. About 30% of the bodies of water that have been evaluated south of the St. Lawrence corridor possess physical and chemical conditions that allow zebra mussel survival.
Using snorkelling gear, the MCI patrollers scanned transect areas of Magog Bay likely to have the mussels and found relatively low quantities compared to 100 or 1000/m2 found in other locations. The life cycle of these bivalves is 2 to 3 years, and few empty shells were found. None having been found in a 2016 inventory also suggests that the invasion is recent. The absence of small specimens indicates that no mollusc larvae were able to attach to the substrate again this year, and that the present population is a result of reproduction from last year or the previous one.
Zebra mussels require water temperatures above 12°C to reproduce, and in Europe require low acidity with calcium levels in the water of 25,4 to 28,3 mg/l. But in North America, the calcium threshold seems to be lower - around 12 mg/l. They do not tolerate acidic conditions, and are usually found in waters of pH more than 7,2 to 7,4 with calcium concentrations above 10-12 mg/l. They remain in a state of stress when calcium levels are below 20mg/l.
Lake Memphremagog offers a potential niche for zebra mussels in the first 10 metres around most of the lake (calcium levels between 11,4 and 20,9 mg/L) while the Magog and Saint-Francis Rivers also have similar profiles of pH and calcium with intermediate potential for their establishment. Each female can liberate up to 1 million larvae. Presently the zebra mussel appears to have sufficient numbers to stress the native mussels, since the small mussel species have been found attached to the larger species.
We must redouble our vigilance in Lake Memphremagog and to follow the expansion of their downstream populations and in other water bodies in the region, especially Lake Massawippi which has optimal water conditions (calcium levels of 20 mg/l) for the zebra mussels to thrive.



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