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Short- and long-term response of fishes to water-level drawdown in the St. Clair National Wildlife Area

Regional Peer Review - Ontario and Prairie Region

March 21-24, 2023
Virtual Meeting

Chairpersons: Sarah Bailey and Dawson Ogilvie


The St. Clair National Wildlife Area (NWA) is a 352 ha dyked wetland complex located on the east shore of Lake St. Clair, managed by the Canadian Wildlife Service (CWS) of Environment and Climate Change Canada (ECCC). Dykes and other water control structures were installed between 1940 and 1980 to maintain wetted area and habitat function following drainage and other landscape modifications to nearshore areas surrounding Lake St. Clair (ECCC 2018). The St. Clair Unit consists of an East cell and West cell, and supports 35 plant and animal species listed under the Species At Risk Act (SARA), including Lake Chubsucker (Erimyzon sucetta; SARA-listed Endangered). The East and West cells support at least 16 other species of fish (Barnucz et al. 2021). This Ramsar Wetland of International Importance is also a globally significant bird area. The St. Clair NWA management plan recommends periodic water-level drawdowns to replicate conditions experienced in natural coastal wetlands, thus allowing for regeneration of native aquatic vegetation through seed bank exposure and other habitat modifications (ECCC 2018). Unlike natural coastal wetlands, the dyke system prevents fishes from accessing deeper refuges outside the cells during water-level reductions. Water-level drawdowns up to 95% volume compared to normal operating level (NOL) have been proposed for the East cell, with reductions occurring for eight weeks starting no earlier than April 1st. Full drawdown conditions will be experienced June 1st through September 15th.

Concerns were raised by Fisheries and Oceans Canada’s (DFO) Species at Risk Program (SARP) and Fish and Fish Habitat Protection Program (FFHPP) over the impact such a drawdown would pose to Lake Chubsucker in the East Cell. In May 2020, in response to a request by DFO FFHPP, DFO Science conducted a regional CSAS Science Response Process to evaluate the ecological impact of water-level drawdown on Lake Chubsucker in the St. Clair NWA. Specifically, the objectives were to better understand:

Drawdown increments of 0.05 m up to 0.75 m were modelled to evaluate changes in wetted area, depth, and patch availability, and how this related to Lake Chubsucker abundance. The changes in habitat availability depended on the drawdown increment, with a drawdown of 0.3 m resulting in a 50% loss of water volume and mean depth, and a drawdown of 0.6 m resulting in a 50% loss of wetted area, a 99.96% reduction in deep water refuge, and a nearly quadrupling of isolated habitat patches. Density-dependent and -independent effects would increase in severity with drawdown increment. Lake Chubsucker, a spring spawner (April through June), has an estimated population size of 234–247 adults in the East cell, which is below the minimum viable population size (600–1,000 adults). Furthermore, the area of the East cell (60.93 ha) is less than the minimum area for population viability (100 ha). Overall, an increased drawdown depth would increase the likelihood of harm to the Lake Chubsucker population. The greatest initial increase in the probability of extinction would occur at drawdown increments between 0.20–0.45 m below NOL, and the probability of extinction would reach 1.0 at drawdown increments between 0.45–0.75 m below NOL. The East cell is depth-limited (mean depth of 0.58 m; 0.23 ha with depth > 1.0 m) under NOL, raising concerns over death of fish beyond Lake Chubsucker should habitat space be reduced further. The creation of deepwater habitat and ensuring connectivity among habitat patches could increase the amount and quality of refuge habitat, and would likely benefit Lake Chubsucker and other fishes during drawdown (DFO 2021).

This previous advice evaluated the short-term consequences for Lake Chubsucker during and immediately following drawdown conditions. However, the long-term effects of the drawdown may provide habitat improvement for Lake Chubsucker and other species by increasing wetland productivity, improving nutrient cycling, and regenerating a diverse macrophyte assemblage. The consequences of inaction are also unclear. Therefore, FFHPP has requested DFO Science provide additional advice to better understand the short- and long-term (i.e., 10 year) response of Lake Chubsucker and other fishes to drawdown in the East cell, including the impacts of proposed mitigation and offsetting measures (e.g., timing and intensity of: deepwater habitat creation or patch connectivity, removal/control of AIS, piscivorous fish removal, salvage and reintroduction of Lake Chubsucker). Although this advice will be developed specifically for the East cell of the St. Clair NWA, it is expected that the general outcomes would apply to other dyked wetlands in the Great Lakes basin.


The objective of the peer-review is to evaluate the short- and long-term response of fishes (including Lake Chubsucker) in the St. Clair National Wildlife Area under three water-level drawdown scenarios:

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Participation to CSAS peer review meetings is by invitation only.

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