Research Document - 2015/018

Impacts of dredging on fish species at risk in Lake St. Clair, Ontario

By J. Barnucz, N.E. Mandrak, L.D. Bouvier, R. Gaspardy and D.A. Price

Abstract

This research document aims to provide recommendations and to inform management decisions related to the permitting of dredging activities in the waters of Lake St. Clair, Ontario. In addition, it aims to evaluate the potential impacts of maintenance dredging activities as it relates to fish species at risk. To evaluate the impacts of maintenance dredging Fisheries and Oceans Canada (DFO) Science sampled impact sites (dredged, dredgeate) and reference sites. Impact sites were locations experiencing periodic maintenance dredging and dredgeate disposal. Reference sites were nearby locations of similar depth and substrate with no known prior disturbance from maintenance dredging activities. DFO Science conducted a repeated trawling survey to compare the fish community and fish abundance between impacted and reference sites. The impact sites were paired with nearby reference sites with similar depths and substrate type. No fish species at risk were captured at any impact sites (dredged or dredgeate locations). One Eastern Sand Darter (Ammocrypta pellucida) was captured at one reference location. Catch per unit effort (CPUE) was compared between impact and reference treatments for both dredged and dredgeate locations. No significant difference in CPUE between the dredged and reference sites (p=0.6414), and between dredgeate and reference sites was observed (p=0.9156). Seasonal comparisons found a significant difference in CPUE between spring and fall (p=0.0026) as well as spring and summer (p=0.0102). Further, no significant difference in CPUE was observed between trawl repeated trawls (three passes) across all treatments (p=0.4831). These results suggest that the abundance of fish species at risk in Lake St. Clair is likely very low. As such, the direct impact of maintenance dredging on these fishes is expected to be low. Fish abundance within sites was lowest during the spring and fall sampling periods. This result supports the continued use of fisheries timing windows to mitigate impacts of maintenance dredging on the local fish community. Depletion of fishes by repeated trawling within each site was not significant across all treatments. This result does not support the use of repeated trawling as a fish removal technique. Future research on maintenance dredging activities using a before/after design to better understand the impacts on fishes and their associated habitats is recommended. It is also recommended that maintenance dredging activities including timing of activities, disposal locations and monitoring be better communicated between stakeholders, as this communication will aid in future assessment of maintenance dredging activities.

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