Effects of Water Level and Substrate Slope on Availability of Suitable Habitat for Young-of-the-Year Muskellunge in Georgian Bay

Abstract:

Georgian Bay supports a self-sustained Muskellunge Esox masquinongy fishery, but recruitment failure is suspected in a southeastern Georgian Bay (SEGB) subpopulation. Absence of young-of-year Muskellunge at historic nursery sites in SEGB has been attributed to sustained low summer water levels; however, presence of young of year in northern Georgian Bay (NGB) wetlands during low-water years casts doubt on the relative importance of water level to Muskellunge recruitment. Instead, habitat for young of year is likely dependent on both water level and nearshore bathymetry; differences in young of year presence between SEGB and NGB in low-water years may be due to differences in site geomorphology. Digital elevation models were used to calculate slope and core nursery area (CNA; area from shore to the 1-m contour) for 20 historic nursery sites in SEGB and 17 confirmed nursery sites in NGB using 1981 (high) and 2012 (low) water-level conditions. Mean slope of NGB sites was significantly higher under high water levels (8.35° versus 6.78°). In contrast, mean slope of SEGB sites was similar between high (0.94°) and low (0.73°) water levels. Mean CNA was significantly higher at low water levels in NGB (0.11 ha versus 0.15 ha) and SEGB (1.7 ha versus 2.5 ha). Between 1981 and 2012, the shoreline migrated lakeward 20 m in SEGB versus only 10 m in NGB while the 1-m-depth contour migrated 50 m in SEGB versus 22 m in NGB. Both regions experienced similar water levels, so steeper nearshore slopes appear to mitigate effects of water-level fluctuations. Absence of young of year during the 2012 survey in SEGB was likely due to the interaction between sustained low water levels and shallow slopes exposed by the low water levels that negatively affected the suitability of young-of-year Muskellunge habitat.