Canadian Forest Service Publications

Measuring 30 years of improvements to aquatic connectivity in the Greater Toronto area. 2018. Choy, M.; Lawrie, D.; and Edge, C.B. Aquatic Ecosystem Health & Management 21(3): 342–351.

Year: 2018

Issued by: Atlantic Forestry Centre

Catalog ID: 39424

Language: English

Availability: PDF (download)

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Instream barriers (e.g. dams, weirs and road crossings) fragment aquatic habitat and prevent the upstream movement of fish, impairing the ability of fishes to complete critical life stages, access critical habitat and for dispersal among local populations. Mitigation efforts have improved aquatic connectivity to some degree, but it has been challenging to quantify the overall improvement in connectivity without long-term and costly field assessments. The development of spatially explicit habitat connectivity indices make it possible to evaluate current stream connectivity, and quantify the improvement prior mitigation projects have had on connectivity. We combined a list of instream barrier mitigation projects completed in five watersheds in the Toronto (Ontario, Canada) area from 1987–2016 (mitigated barriers) and a previously established inventory of all known instream barriers in 2016 (current barriers). The cumulative improvement to connectivity was measured for potadromous (remain in tributaries) and diadromous (move between tributaries and lake) fish species using the dendritic connectivity index. Aquatic connectivity improved for diadromous species between 0 and 14.5% and for potadromous species between 0.1 and 4.4% in the five studied watersheds. Some variation in improvement among the watersheds can likely be attributed to differences in mitigation strategies among the watersheds and a historical emphasis on mitigating instream barriers to benefit migratory salmonid species.

Plain Language Summary

Dams, road crossings, weirs, and other anthropogenic structures have extensively fragmented aquatic ecosystems, preventing the upstream and downstream movement of fish. Restoration projects have focussed on restoring connectivity in aquatic systems by removing barriers or mitigating barriers by installing structures that allow for the passage of fish. Evaluating the success of these projects for restoring connectivity to the entire stream network is difficult because it requires detailed knowledge about the cumulative impact of all barriers in the system. Here we apply a spatially explicit connectivity index that evaluates the cumulative impact of all barriers to five streams in Toronto at two different times (1987 and 2016) to determine how much improvement to connectivity has resulted from barrier mitigation projects over the past 30 years. We find that improvement varied among the five streams and attribute much of this variation to mitigation targeted at restoring connectivity for migratory salmonids or for conserving the few remaining high quality aquatic habitat in Toronto. The connectivity index provides a quantitative metric to evaluate the success of habitat restoration activities and results from its application are being used to evaluate the delisting of Toronto and Region as a Great Lakes Area of Concern. The article was an invited submission for a special issue on the status of the Toronto and Region Area of Concern.