The Ottawa River may currently be the best place in Canada to observe the Hickorynut, Obovaria olivaria, a freshwater mussel recently listed as an endangered species. I came to this conclusion based on dives I made starting in the late summer, in a specific area of this river.

In August and September of this year, my team and I picked up on research conducted one year back on the distribution and abundance of Hickorynut populations in the Ottawa River.

Two men stand on a river shore, near a motor boat.
André Martel, researcher at the Canadian Museum of Nature, on the right, and Mark Graham, Vice-President of Research at the Museum, scout the Ottawa River not far from “Coulonge Lake”. Dr. Martel is looking for the rare Hickorynut Mussel, Obovaria olivaria, in the Unionidae family. Image: Jacqueline Madill © Canadian Museum of Nature

We knew from the start that the Hickorynut’s preferred habitat differs from that of other freshwater mussels. This mussel prefers settling in deep areas of major rivers (over 3 m in depth), in strong currents far from riverbanks, and on sandy bottoms.

This time round, however, in addition to accounting for the species’ habitat preferences, we decided to focus our efforts on areas where sturgeons, its host fish, were most abundant in the river.

A man who is seated in a boat examines mussels he holds in his hands.
André Martel looks at mussels collected from the shallow waters of the Ottawa River. Image: Jacqueline Madill © Canadian Museum of Nature

Lake sturgeons (Acipenser fulvescens) are known to play a major role in the reproductive cycle of the Hickorynut. Biologists believe that it is on the gills of this fish that Hickorynut embryos or larvae, also called glochidia, attach themselves to complete their development. They are then dispersed throughout the river. Lake sturgeon populations have greatly diminished in the Ottawa River, compared to their historical levels. They are still present, however, especially in wild, free-flowing reaches, far from the influence of dams.

To spot the best sturgeon habitats in the Ottawa River, we consulted researcher Tim Haxton’s doctoral thesis as well as an article he jointly published with Scott Findlay. Then, we matched this information with data on the Hickorynut’s preferred habitat. As you will discover further on, this strategy for determining diving sites paid off well beyond our expectations!

Two male divers seated in a boat study a map.
André Martel (on the right) and Mark Graham study a map of the river. Before choosing a diving site in which to search for the rare mussel, scientists get information on the river depth, current, river bottom, and the presence of yellow sturgeon, the host need for the mussel’s reproduction. Image: Jacqueline Madill © Canadian Museum of Nature

During the month of September, we made several dives in an area of the river that was still relatively free-flowing and wild, far from big cities and hydro-electric power stations. We targeted the Finlay Islands area, near the village of Waltham, slightly upstream from Fort-Coulonge, Quebec. This part of the Ottawa River is called “Coulonge Lake”. The Finlay Islands make up a unique ecological reserve in Quebec.

To conduct this fieldwork, I was assisted by other divers, Nancy, Andy, Mark and Tanya, as well as my research assistant Jacquie. Vince, a local resident familiar with navigating in this sector, also helped us out by scouting the area before the dives.

View of a big expanse of a river in summer.
Spectacular view of the Ottawa River near the Finlay Islands ecological reserve, looking downstream (the village of Waltham is behind the photograph). The population of Hickorynuts, just off the banks, is probably the most important in Canada. Image: André Martel © Canadian Museum of Nature

The Finlay Islands area matched all the criteria needed to find Hickorynuts: a long reach where the river is wide, with a medium depth (3 to 6 m) across nearly the entire width. The current is strong, and there is no dam nearby. The bottom is sandy and, above all, the data confirm the presence of an abundant sturgeon population in the area.

Right from the first dives, we hit the jackpot! Indeed, diving to a depth of about 4 meters, far from the island banks and staying well within the current (5 to 10 cm/sec), we were very surprised to come upon a vast population of Hickorynuts in this wide stretch of the river! We used a 1 m2 (1 m x 1 m) quadrat to estimate the average density of this population at 0.8 individuals per square meter, which is high for a rare species.

Partially buried in the sandy river floor, the Hickorynut Mussel is actively filtering water.
A photo of something very few people get a chance to see: a Hickorynut in action in its natural setting. The mussel is filtering the water, partially buried in its favourite substrate: sand. I took this close-up right in the middle of the Ottawa River at a depth of 4 m. You can clearly see the fine papillae along the mussel’s siphon opening. Note the incurrent siphon, with its more elongated shape, in the upper right corner of the mussel. It is through this opening that bacteria and microscopic algae contained in the water enter the mussel as food. Once filtered, the water flows out through the excurrent siphon, further to the left, with a less elongated shape. These freshwater mussels do us a great service by filtering the river water. Image: André Martel © Canadian Museum of Nature

In fact, our data suggest that in the area south of the Finlay Islands, Hickorynuts often made up over 25% of all mussels found in our quadrats. I had never seen this before! These are very encouraging results for this rare species.

Thanks to other dives around the Finlay Islands, we were able to determine that this Hickorynut population is widely distributed, possibly covering the equivalent of many square kilometres of sandy habitat on the river bottom. We estimate that there are tens (if not hundreds) of thousands of individuals in this area!

Of course, the entire area includes other much more common species of freshwater mussels that are not endangered and can be found in great numbers on the river bottom.

Around sixty mussels of different colours and sizes are arranged on a table.
This shows five different indigenous species of freshwater mussels from the Unionidae family, collected in the Ottawa River. The group to the left, all dark brown in colour and rather round in shape, are Hickorynuts, Obovaria olivaria. In the centre, the larger mussels, yellowish in colour with stripes, are Plain Pocketbooks, Lampsilis cardium. The only yellowish specimen without stripes is a Fatmucket, Lampsilis siliquoidea. The six smaller specimens in a group in front are Triangle Floaters, Alasmidonta undulata. The mussels furthest to the right are Eastern Elliptios, Elliptio complanata, a common species. For conservation reasons, most of these specimens were released exactly where they had been collected. Only a few were kept to add to the mollusc collection at the Canadian Museum of Nature. To give you an idea of the size, you should know that the biggest mussel, in the centre to the right, is 11 cm long. Image: André Martel © Canadian Museum of Nature

I don’t think it’s a coincidence if “Coulonge Lake” has such great numbers of Hickorynuts—on the contrary. Let’s look at the facts: the reach just further upstream, called “Allumettes Lake” (going towards Pembroke and Petawawa), as well as the reach further downstream, northwest of Grand Calumet Island, are part of a long reach of the Ottawa River that is still relatively wild.

All along this reach, which spans over 70 km, the river flows freely without encountering any dams. It includes several great rapids that are ideal spawning grounds for several fish species, including the sturgeon. Indeed, it is in this large area that sturgeon populations are at their highest numbers in the Ottawa River—in short, an area hardly impacted by humans that includes a rich aquatic biodiversity. This is truly a haven for the Hickorynut and a natural heritage that must be preserved!

Stay tuned for other findings from our fieldwork on freshwater mussels in Canada, including those in the Ottawa River.

Freshwater mussels in a plastic bag.
Oyster, clam or mussel?
Anyone who has explored Canada’s riverbanks and lakeshores, from Newfoundland to Vancouver Island, has probably already seen and handled freshwater mussels, living or dead, in the form of empty shells. These rather sedentary molluscs live on the bottom of rivers and lakes. They filter bacteria, microscopic algae and detritus contained in the water, lending a helping hand to the aquatic ecosystem and contributing to water quality. They may sometimes be called “freshwater oysters” or “freshwater clams”, but freshwater mussels is the most appropriate term for these indigenous species. They are not to be confused with sea mussels you buy at the fish market, or zebra mussels, an invasive species from Europe!
Image: André Martel © Canadian Museum of Nature

Translated from French.