ABSTRACT
This study was conducted to meet regulatory requirements under the Fisheries Act in Canada, specifically for a hydroelectric facility on the Yellowknife River in the Northwest Territories. The research focused on annual snorkel surveys of adfluvial fish and their spawning habitat below the facility. Initial observations of egg mortality, potentially due to overcrowding, prompted the investigation of natural and enhanced habitat for spawning Lake trout (Salvelinus namaycush ), lake whitefish (Coregonus clupeaformis ) and cisco (Coregonus artedi ) from 2016 to 2019. The design and composition of the installed habitat were based on fish utilization of the natural channel below the hydro facility and design principles from previous habitat rehabilitation projects for anadromous fishes. Pre- and post-enhancement data on egg density and survival were collected using 1 m2 plots on both natural and artificially enhanced substrates. Three years of post-enhancement monitoring indicated higher egg densities and a greater proportion of live eggs in the artificially enhanced habitat compared to the natural habitat, with more pronounced trends observed for coregonids (lake whitefish and cisco) compared to lake trout. These findings suggest that habitat enhancement has the potential to enhance juvenile recruitment for adfluvial fish. A critical factor in the design was the substrate composition, providing adequate interstitial spaces for egg development and protection. This study represents the first documented attempt at habitat improvement in a regulated sub-Arctic river in Canada. The findings offer valuable guidance for stakeholders involved in new or existing development projects that require conservation actions to maintain fisheries productivity.
INTRODUCTION
Habitat enhancements in freshwater ecosystems have the potential to be a viable tool for fisheries conservation and management in northern regions of Canada where effects of climate change, overfishing, and industrial works have the potential to limit or impair the productivity of fisheries (McPherson, Cott, Lewis, Swanson, & Poesch, 2023). Habitat enhancement can also be a legal requirement in many countries, including Canada, where proponents are responsible to implement project mitigation or conservation offsets for new developments or existing operating facilities (DCCEEW, 2012; DFO, 2019; Favaro & Olszynski, 2017; zu Ermgassen et al., 2019). Indeed, habitat enhancement is a widely practiced restoration method in many developed regions around the world where high valued recreational fisheries are in decline, however, designing and implementing suitable habitat enhancements in northern (e.g., Arctic and sub-Arctic regions) ecosystems is challenging due to their remote nature, the difficulty in monitoring the effectiveness of outcomes, and the paucity of available information and documented records on past successes or failures in habitat enhancement approaches.
Lake trout (Salvelinus namaycush ), lake whitefish (Coregonus clupeaformis ), and cisco or lake herring (Coregonus artedi ) are widely distributed salmonid species found in the northern regions of North America. They inhabit areas that were previously glaciated, ranging from the Atlantic watersheds westward throughout the Canadian provinces to British Columbia. They can also be found in the Arctic watersheds across the Northwest Territories and Nunavut (Evans, Reist, & Minns, 2002; Richardson, Reist, & Minns, 2001; Scott & Crossman, 1998). These three species are found in water bodies of various sizes, ranging from smaller inland lakes (McAughey & Gunn, 1995) to the largest lakes in North America, including the Laurentian Great Lakes (Lake Superior, Huron, Michigan, Erie, and Ontario) and the Mackenzie Great Lakes (Great Bear and Great Slave Lake) (Evans et al., 2002; MacKenzie, Fortin, & E., 2021). They hold significant biological, cultural, commercial, and recreational importance throughout their distribution range (MacKenzie et al., 2021; Mohr & Nalepa, 2005).
Habitat enhancements in northern ecosystems often focus on general principles of improving habitat characteristics (Theis, Ruppert, & Poesch, 2023; Theis, Ruppert, Shirton, & Poesch, 2022), but with a specific focus on improving spawning substrates habitat (Gatch, Höök, Roseman, & Koenigbauer, 2020; Selgeby et al., 1995), based on the assumption that it will improve egg survivorship and ultimately increase fish abundance. However, the success of enhancing spawning substrate is heavily dependent on the life history types of the targeted species, and there is a significant disparity in available research between lacustrine (lake) and adfluvial (river) spawning life history types. For example, there is a substantial body of research on the life history, spawning behavior, and spawning habitat requirements of lacustrine lake trout (Callaghan, Blanchfield, & Cott, 2016; Fitzsimons, 1995; Gunn, 1995; Richardson et al., 2001). Similarly, extensive literature exists on the spawning behavior and habitat of lacustrine lake whitefish and cisco, mainly in relation to fisheries management objectives for commercial fisheries in the Laurentian Great Lakes (Fischer et al., 2018; Gatch et al., 2020; George, 2019). Although lake trout, lake whitefish, and cisco can be found in northern adfluvial systems, our understanding of the extent to which they use riverine habitats for spawning is limited. There is a lack of information regarding the frequency and characteristics of watercourse utilization for spawning, highlighting the need for further research on the utilization of riverine areas by these three adfluvial species (Evans et al., 2002; Jones, Parna, Parna, & Chong, 2018; Richardson et al., 2001).
The development of habitat enhancements to improve spawning substrate based on life history characteristics of multiple adfluvial species provides the potential to help prioritize conservation measures for the benefit of multiple species. To do this, we require an understanding of life history types and similarities and differences of these advluvial species, their habitat use, and spawning characteristics, based on empirical information. Lake trout, lake whitefish, and cisco have unique life histories and spawning behaviors, but they also share certain similarities, which may allow for some synergism in developing habitat enhancements. For example, all three species are members of the Salmonidae family with lake trout belonging to the Salmoninae sub-family and lake whitefish and cisco belonging to the Coregoninae sub-family (Evans et al., 2002; Richardson et al., 2001; Roberge, Hume, Minns, & Slaney, 2002). Secondly, all three species are fall-spawning, lithophilic broadcast spawners, meaning they deposit their eggs in granular substrates (Roberge et al., 2002; Scott & Crossman, 1998; Stewart, 1997). Thirdly, while their lacustrine and/or anadromous populations have been extensively studied, the adfluvial populations of these species, which migrate to rivers for spawning, have received less research attention regarding their life history, spawning behavior, and habitat requirements (Evans et al., 2002; Richardson et al., 2001). Finally, all three species play significant roles in commercial, Indigenous subsistence, and recreational sport fisheries in Canada (MacKenzie et al., 2021; Mohr & Nalepa, 2005), which make them highly suitable candidates for habitat enhancements measures and offsetting in general.
The availability of suitable spawning habitat plays a crucial role in the recruitment of spawning salmonid populations. Numerous studies have highlighted the significance of spawning habitat availability for freshwater fishes and its impact on year class strength and population variability (Fitzsimons, 1995; Gunn, 1995; Milner et al., 2003). Unfortunately, anthropogenic activities have significantly impacted the spawning habitat of anadromous salmonids (McPherson et al., 2023; Poesch, Chavarie, Chu, Pandit, & Tonn, 2016), resulting in degradation and fragmentation. Factors such as habitat degradation, hydroelectric dams, and overfishing have contributed to these impacts (Cowx & Welcomme, 1998; Whiteway, Biron, Zimmermann, Venter, & Grant, 2010; Zeug et al., 2014). To address these challenges and restore or enhance fisheries production, government agencies and non-profit organizations have dedicated substantial resources to the restoration of degraded spawning habitat. Efforts have included the installation or creation of artificial spawning habitat and the evaluation of different designs and approaches for rehabilitating anadromous salmonid populations (Murchie et al., 2008; Pulg, Vollset, & Lennox, 2019; Roni, Hanson, & Beechie, 2008). Various rivers and waterways along the Pacific coast of North America, as well as the Atlantic coast of North America and Europe, have seen the construction and assessment of artificial spawning habitat for anadromous salmonid species, including chinook salmon, steelhead trout, Atlantic salmon, and brown trout (Barlaup, Gabrielsen, Skoglund, & Wiers, 2008; Pedersen, Kristensen, Kronvang, & Thodsen, 2009; Whiteway et al., 2010). Overall, these efforts highlight the importance of addressing the impacts on spawning habitat and the implementation of restoration measures to support the sustainability of salmonid populations.
The Great Slave Lake fishery is home to adfluvial species such as lake trout, lake whitefish, and cisco. It heavily relies on the major tributaries surrounding the lake for essential flows, nutrients, and energy for food webs. These tributaries also serve as critical spawning and egg incubation locations for fall-spawning adfluvial salmonids (MacKenzie et al., 2021; Rawson, 1951; Stewart, 1997). Located at the outlet of Bluefish Lake on the Yellowknife River, approximately 18 km upstream from where the Yellowknife River meets Great Slave Lake, is the Bluefish Hydroelectric Facility (hereafter ‘Bluefish’) (KCB, 2017). In 2011, the Northwest Territories Power Corporation (NTPC) received a Fisheries Act Authorization (FAA) from Fisheries and Oceans Canada (DFO) for the construction of a new primary impoundment dam and spillway for the facility (Golder, 2019). The FAA mandated monitoring, including confirming the spawning of fall-spawning salmonid species (lake trout and coregonids), in the lower spillway of the historical Yellowknife River channel and downstream of Bluefish. Observations of spawning adults and fertilized eggs in the area provided direct evidence of adfluvial lake trout, lake whitefish, and cisco spawning in the Yellowknife River below Bluefish (Golder, 2019) (Figure 1).
The main goal of this study was to provide a comprehensive summary of the collected data on spawning activity at Bluefish and contribute to the limited existing research on adfluvial lake trout, lake whitefish, and cisco spawning behavior and habitat requirements, both for natural and artificially enhanced substrates in northern freshwater ecosystems.
STUDY AREA
The study area is located on the Yellowknife River, situated downstream of Bluefish Lake and upstream of Prosperous Lake, in the northern region of Yellowknife, Northwest Territories (refer to Figure 1). The Yellowknife River drainage area, just downstream of the study area at the entrance to Prosperous Lake, spans approximately 11,300 km2 (ECCC, 2020). Further downstream, at the outlet where the Yellowknife River meets Great Slave Lake, the total drainage area expands to around 16,000 km2 (Spence, 2001). The Yellowknife River basin falls within the sub-Arctic Canadian Shield physiographic region, specifically within the Taiga Shield Ecozone. This basin is characterized by shallow and exposed Precambrian bedrock, an open black spruce forest, and a multitude of lakes, with surface water covering approximately 25% of the basin area (Spence, 2001).
The study area encompasses the lower portion of the historic Yellowknife River channel, situated downstream of the Bluefish Lake Dam, which serves as a natural spillway for Bluefish. The construction of the Bluefish Lake Dam took place in 1940 at the Yellowknife River’s outlet, primarily to facilitate storage for the G1 generating plant (KCB, 2017). Subsequently, in 1994, a second generating plant (G2) was constructed approximately 20 meters west of the G1 plant. In 2012, a new dam and spillway were built around 400 meters downstream from the original dam (KCB, 2017). The discharge of flow from the G1 and G2 plants occurs into the main Yellowknife River channel, situated 80 meters east of the spillway outlet, within a section known as the Tailrace Area (Golder, 2019). The Tailrace Area spans approximately 110 meters in width and exhibits depths reaching around 4 meters.
The spillway, which carries water not utilized by the generating plants, incorporates the historic Yellowknife River channel (Golder, 2019). Within this channel, the Instream Flow Gate was installed in 2012 to ensure minimum flows for fall-spawning fish, with a regulated minimum flow of 0.75 m^3/s. Prior to the installation of the gate, flow in the spillway was maintained through leakage from the old dam, spanning from 1942 to 2010 (Golder, 2019). The downstream section of the spillway, providing habitat for fall-spawning salmonids, is referred to as Reach 1. It is characterized by a natural waterfall at the upstream end and extends to the confluence with the Tailrace Area, where the flow enters the main Yellowknife River channel at the downstream end. Reach 1 measures 178 meters in length and exhibits water depths of up to 1.5 meters. The substrate composition is predominantly boulders and cobbles, with exposed bedrock areas interspersed (Golder, 2019). Adfluvial fall-spawning fish species, such as lake trout, lake whitefish, and cisco, migrate annually from either Prosperous Lake or Great Slave Lake to spawn in Reach 1 and the Tailrace Area (Golder, 2019; MacKenzie et al., 2021; Stewart, 1997).
METHODS
The study was conducted over a 4-year period, including Pre-Habitat Enhancement Construction Monitoring (2016), Habitat Enhancement Construction (2017 & 2018) and Post-Habitat Enhancement Construction (2017-2019) during the months of September through October. Annual monitoring of spawning fish abundance and distribution was conducted in the Yellowknife River below Bluefish as part of the regulatory requirements for Bluefish operations under the FAA for the new impoundment dam since 2013 (Golder, 2019). Spawning fish counts were made during six to eight snorkel surveys conducted from September to early November, timed to coincide with suitable temperatures for the spawning of lake trout, lake whitefish, and cisco (Scott & Crossman, 1998). During the surveys, one observer was in the water enumerating spawning, while a recorder on the shore documented the observations. It is important to note that surveys in 2019 began in early October, missing the peak arrival of lake trout, and therefore, the snorkel data for lake trout in that year are not included in the analysis.
To assist with the interpretation of trends across study years, we summarized supporting environmental data on daily water temperature and river flow discharge, from the Water Survey of Canada (WSC) hydrometric gauges in the Yellowknife River (ECCC, 2020). Daily discharge data were obtained from station no. 07SB003, located below the Tailrace Area (Figure 1), while daily water temperature data were obtained from station no. 07SB015 in Bluefish Lake, upstream of the dam. Historical ”baselines” were defined on data collected from 1991 to 2020.