Science Advisory Report 2015/050
Assessment of the impacts of dredging on fish species at risk in Lake St Clair, Ontario
- Newly proposed and ongoing annual dredging activities in harbour mouths in the lower Great Lakes, and specifically along the Lake St. Clair shoreline, may have direct and indirect negative effects on fish currently listed under the federal Species at Risk Act (SARA).
- Near shore bathymetric surveys revealed dredgeate disposal locations showed no drastic changes in depth. Therefore, the dredgeate disperses quickly after disposal or the survey locations were not conducted at the dredgeate disposal sites as proposed. Additional research is needed to determine the speed at which dredgeate disperses after disposal.
- Accurate reporting on the location, timing, and depth of dredging and dredgeate disposal locations, as well as an estimate of the volume of substrate dredged, are important to determine the impact that dredging activities have on fish species at risk.
- The dominant substrate within all sampling sites in Lake St. Clair was sand; therefore, the results of this report apply exclusively to areas where the dredgeate is placed on like substrate – in this case sand-on-sand in Lake St. Clair.
- This report did not review the impact of side casting during dredging.
- As no significance difference was detected between impact (dredged and dredgeate) and reference sites, it was concluded that maintenance dredging and dredgeate disposal may not have a significant impact on fishes within the areas along the south shore of Lake St. Clair.
- Considering the apparent low abundance of fish species at risk (a single fish species at risk, Eastern Sand Darter (Ammocrypta pellucida) was detected in this study) in the surveyed areas of Lake St. Clair, it is suggested that if sufficient mitigation steps are taken, the direct and indirect impacts to fish species at risk could be considered minimal.
- As no significant difference in fish abundance was noted across repeated trawls, indicating no depletion in fish abundance, trawling for fish salvage purposes prior to maintenance dredging is not a recommended mitigation strategy.
- Considering the apparent low abundance of fish species at risk in the surveyed areas of Lake St. Clair, it is suggested that the current timing window of March 15th to July 15th be maintained and remain flexible.
- This survey was conducted using a Control/Impact design utilizing local reference sites as control sites. Sampling sites were surveyed with no prior knowledge of the last maintenance activity for any location. It is recommended that future research on maintenance dredging impacts be conducted using a before/after/control/impact (BACI) approach. Sampling sites should be sampled immediately before and after maintenance dredging activities. Subsequent surveys of sampling sites should follow a time line of weeks and months after maintenance dredging activities. Reference sites would require sampling during this time to account for local variation in fishes and habitat during sampling.
This Science Advisory Report is from the November 28, 2014 regional peer review of the Impacts of dredging on fish species at risk in the lower Great lakes basin. Additional publications from this meeting will be posted on the DFO Science Advisory Schedule as they become available.
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