P.O. Box 2065

57 Foster Street

Perth, ON

K7H 3M9








The largest invertebrate animals in our local lakes and rivers, native freshwater mussels and crayfish, are all threatened by invading alien species. Native crayfish are displaced or genetically swamped by the Rusty and Obscure crayfish (Orconectes obscurus and O. rusticus), while native mussels are suffocated by Zebra Mussels (Dreissena polymorpha), which have eliminated them from many lakes and rivers.


Chris Stone dives for mussels.
Move curor over image for larger view.

In 1995 the Bishops Mills Natural History Centre (BMNHC) began sporadic surveys of these species in the Tay River, finding a modest diversity of native mussels and crayfish that couldn't be certainly identified as any of the native or invasive species.


Observations of clusters of Zebra Mussels attached to empty shells in Christie Lake in the fall of 2008, provided the stimulus for a detailed study.  The Friends of the Tay Watershed (FoTW) successfully applied for a grant from the Shell Environmental Fund to conduct a study of  the freshwater mussels and native crayfish which inhabit the Tay River and  Christie Lake from Bolingbroke to Port Elmsley.


Freshwater mussels are a useful biological indicator of water quality and long term aquatic health due to their long life spans.  Chronic water pollution problems can be associated with the disappearance of freshwater mussels. With the loss of mussel species from the invasion of Zebra Mussels, there will be a subsequent change in the ecology of streams and lakes, since native mussels (Unionidae) aerate the bottom by their burrowing, while Zebra Mussels smother the bottom with their matted shells and droppings.


Since June 23rd, 40 sites (13 in Christie Lake and 27 along the Tay River) have been sampled for existing species (living & dead- shells) of clams, snails and crayfish.  Water quality was also monitored by chemical analysis for seven parameters.  This study was guided by the expertise of Dr. Fred Schueler and Aleta Karstad from the Bishops Mills Natural History Centre, and the shoulder-wrenching dexterity of Orion Clark of FoTW..  The valuable assistance of the Lanark Stewardship Council (detailed maps), The Canadian Museum of Nature (loan of equipment) and 15 volunteers  made possible this detailed assessment.


While many observations of other macroinvertebrates, amphibians, and reptiles were recorded and photographed, there were three findings that stand out:


1) Zebra Mussels have been present for a few years, and it's not clear how much they will increase. High definition video recording done by SCUBA divers on the shoals of Christie Lake found Zebra Mussels of a size that suggests they were at least four years old.  The Zebra Mussels were found at five other sites in the Tay River in the course of the survey though in as-yet very low numbers. Analysis of the water chemistry data collected in the survey will suggest whether they have the potential to increase explosively, or will remain at low abundances.  It is vitally important to identify the native mussel species before their destruction by Zebra Mussels, if this is to occur.


2) Elktoes are fairly common in a specific habitat. The Elktoe (Alasmidonta marginata), is a fairly  common species of mussel in southwestern Ontario, but in eastern Ontario it is common only in the Mississippi River below Almonte. A single shell was found in 1995 in the Tay River in Perth, and another in 2007, but the survey found them living in a particular habitat: single layers of broken rock over flat bedrock. The biggest of these populations was on the flats above the OMYA water intake, where Elktoes were at a density of about 1 in every 2 square metres. The vast majority of the mussels found at all sites were the Eastern Elliptio (Elliptio complanata); small numbers of six other species were found.


3) Most crayfish seem to be hybrids among a native species and two invaders. Uniformly tannish Crayfish found all along the Tay show a mixture of the characters of the native Orconectes  propinquus,  and the introduced O. rusticus, and O. obscurus, which are known from nearby lakes. Specimens will need to be studied before definite conclusions are drawn about their status.  The native Blue-claw Crayfish, Orconectes virilis, was also found in smaller numbers.

For further details, look for postings on the Friends of the Tay Watershed website, and the Bishops Mills Natural History Centre webpage at