Pre-Habitat Enhancement Construction Monitoring (2016)
In 2016, the pre-habitat enhancement construction monitoring efforts focused on characterizing the spawning habitat and the use of different substrate types for egg incubation by adfluvial lake trout, lake whitefish, and cisco in the study area. Seven sampling events took place from September 15 to November 2, 2016, to coincide with the spawning phenology of the three species (Supplementary Information Photo 1). This timeframe corresponds to the period when water temperatures for spawning ranged from approximately 1°C to 14° C. Egg deposition areas were identified by conducting surveys while wading or floating across various hydraulic conditions and areas, including depths and velocities. The presence and relative abundance of eggs were determined through underwater snorkel observations of the substrate. The snorkeler gently agitated the substrate with their hand or foot to facilitate the detection of eggs. However, to avoid disturbing the egg incubation areas, some eggs embedded in the substrate may have been missed without thorough agitation. While lake trout eggs are noticeably larger in size, allowing them to be differentiated from coregonid eggs (cisco and lake whitefish), cisco and lake whitefish eggs could not be distinguished based on size alone. Therefore, the results of this study collectively refer to cisco and lake whitefish eggs as coregonid eggs.
A detailed assessment of habitat characteristics was conducted at specific locations where eggs were observed (14 egg plots) and where no eggs were observed (15 non-egg plots) in Reach 1 and the Tailrace Area (Figure 1). Within each egg and non-egg plot, habitat characteristics were evaluated within a 1 m2 frame. To ensure representative sampling, survey plot locations were selected within the approximate spatial extent of the egg deposition area. First, locations where eggs were present were identified, and then additional plots were randomly positioned nearby without eggs. These non-egg plots were typically within a short throwing distance of the plot frame, landing within 2 to 8 m of the plots with eggs. During each survey plot, the following information was recorded: GPS coordinates to accurately locate the plot, total depth of the water column, midwater column velocity, and the proportion of different substrate types. Substrate type proportions were determined based on particle size diameter, with clay/silt defined as < 0.06 mm, sand as 0.06-2 mm, gravel as 2-63 mm, cobble as 64-256 mm, and boulder as > 256 mm (Figure 2A).
Initial snorkel surveys revealed significant mortality of eggs deposited in clumps on boulder and bedrock habitats in the study reach of the Yellowknife River. This was attributed to the absence of suitable interstitial spaces for egg development and the presence of a cotton-like white substance, likely caused by the water mold Saprolegnia sp (Univ. of Saskatchewan, 1996; Brown & Bruno, 2002; Van Den Berg et al., 2013). To mitigate the high egg mortality in the study reach, the construction of artificially enhanced spawning habitat was initiated. The design principles drew inspiration from habitat installation and rehabilitation projects for anadromous salmonids in rivers, as well as artificial spawning reefs constructed in lakes for lacustrine populations of lake trout and lake whitefish (Claramunt, Jonas, Fitzsimons, & Marsden, 2005; Eshenroder, Bronte, & Peck, 1995; Evans et al., 2002; Fitzsimons, 2014; Gatch et al., 2020). Three site-specific actions were considered: structural placement, gravel injection, and designed gravel placement (Bunte, 2004). A crucial aspect of the design was the inclusion of substrates that aligned with the documented spawning shoals of lacustrine lake trout and whitefish. These substrates provided interstitial spaces for egg anchorage and protection from predation and strong currents that could otherwise dislodge the eggs (Marsden et al., 2016; Marsden, Casselman, et al., 1995; Sly, 1988).