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).