There are 129 freshwater fish species native to Ontario with the Sydenham River being home to at least 80 species. Many of these species are sensitive to environmental change, 10 species in the Sydenham River have been listed as species at risk and there are several other rare species that live in the river.
In the Sydenham watershed, fish are primarily sampled using seine nets or a technique called electrofishing. But first the survey area needs to be set up.
To set up a site, a 40m portion of the watercourse is excluded using blocknets at the upstream and downstream ends of the survey area – this prevents fish from swimming in or out of the sampling area while the survey is taking place.
Biologists pull a seine net through the sampling site to collect fish. Sampling of the survey area is usually repeated three times or until there are no new species detected. After each sampling run, biologists will see “depletion” meaning the most fish are collected in the first run with fewer and fewer in each subsequent run.
Diagram of a Seine Net
A seine net is a large net with floats along the top and a lead-line along the bottom, it is held by poles on either side. The net has an enlarged area in the middle, referred to as the “bag” that fish are funneled into. After passing the net through the survey area, biologists are able to collect the fish from the bag portion of the net.
Electrofishing involves temporarily stunning the fish with an electric current sent through the water. Biologists scoop up the stunned fish using hand-held nets and collect them in a bucket. As with seine netting, sampling of the study area is typically repeated three times.
Once collected, the fish are quickly identified, measured, and counted before being released back to the river.
The purpose of fish community surveys is to gain important information on the status of fish populations and watershed health. Changes to the fish community over time can tell us about changing water quality and habitat conditions as well as the effectiveness of remediation efforts. This information is also used to guide decisions on potential restoration project sites and watershed planning.
Biological Indicators
The brightly coloured and surprisingly tropical looking Rainbow Darter is an example of a fish species that is sensitive to pollution and habitat disturbance that can be found in the Sydenham watershed. Their presence can help to indicate a healthy aquatic system.
© 2026 · St. Clair Region Conservation Authority - The Sydenham River Watershed