Assessing the fate of returning Upper Yukon River Chinook Salmon

Upper Yukon River Chinook Salmon populations (defined for the purpose of this study as fish that terminate in the mainstem Yukon River or its tributaries above the confluence with the Teslin River) have experienced similar declines to other Yukon River populations in recent years. Greater declines probably occurred much earlier in the past century. Historic reports from First Nations along with a biologist and RCMP officer indicate that ~10,000 Chinook Salmon were harvested annually in the M’Clintock River system (Cox 1997); however, returns counted at the Whitehorse Rapids Generating Facility (WRGF) fish ladder have averaged only ~1200 fish over the past 15 years, and no active fisheries are able to exploit the population.

The fate of many Chinook Salmon after they pass the fish ladder is unknown. Previous radio telemetry studies (Cleugh and Russel 1980; Matthews 1999) showed that 74% to 81% of these Chinook Salmon traveled to the M’Clintock/Michie system, though sample sizes were small. The majority are believed to spawn in Michie Creek, between Michie Lake and Byng Creek (de Graff 2015). Understanding whether Chinook Salmon spawn elsewhere in the M’Clintock River system will inform further efforts to recover the stock. Other spawning locations may represent genetically unique stocks that would benefit from restoration, or habitats that would benefit from improved access (e.g., log jam removal). Perhaps more importantly, the fate of the ~25% of Chinook Salmon that pass the WRGF fish ladder but do not terminate in the M’Clintock River system is unknown. These fish could spawn in unknown locations in the Southern Lakes or the mainstem Yukon River above the WRGF, or they may expire before reaching the spawning grounds. In either case, stock or habitat restoration actions could be identified that would benefit these depleted stocks, once their terminal location is known.

The role of the WRGF in limiting Chinook Salmon population recovery is largely unknown. Returns have oscillated considerably around a relatively stable mean since the dam and fish ladder were built in 1958 and 1959, respectively. Beginning with the first release of hatchery-reared fry in 1985, returns have been maintained in part by hatchery-origin fish, which represent ~50% of the return. Each year, ~1200 fish successfully pass via the fish ladder; however, the proportion that fail to pass the WRGF remains unknown.

Similarly, little is known about delays, stress, or energetic costs of fish passage at the WRGF. More than five decades of successful passage and subsequent spawning in the M’Clintock River system provide clear evidence of individual passage success. However, sub-lethal and population-level consequences of passage are unclear. Cleugh and Russel (1980) assessed passage success and delays at the WRGF using radio telemetry, but only managed to tag two fish. One fish was delayed for 10 hours, the other for 9 days during this study, which was conducted before a fourth turbine was installed at the dam. Passage delays can increase disease incidence and pre-spawning mortality (Hinch et al. 2012). Frequent exertion above critical swimming speeds (i.e., anaerobic activity) below fishways can lead to passage failure (Hinch and Bratty 2000). Fish attempting to pass the dam expend energy and can experience stress and exhaustion, and swimming speeds are often higher in tailraces than in fishways (Hinch and Bratty 2000). Stress associated with exhaustion can lead to increased energy expenditure (Barton and Schrek 1987), suppression of reproductive hormones (Kubokawa et al. 2001) and mortality (Wood et al. 1983). Research methods have been developed to evaluate each of these effects, and fish passage structures can be modified to increase passage speed and efficiency if necessary. The first step is understanding whether passage failure is occurring at the WRGF and whether delays are rare and short or more severe.

This project has two primary goals. The first is to identify depleted stocks that are candidates for restoration, along with potential spawning restoration sites. Specific objectives for this proposal associated with this goal are to assess:

1) Where salmon spawn in the M’Clintock River system;
2) What other terminal locations exist above Lake Laberge aside from the Takhini River, Wolf Creek and the M’Clintock River;
3) Whether some fish that pass the WRGF fail to reach Marsh Lake (and to subsequently assess whether these fish spawn successfully in the mainstem Yukon River or experience pre-spawning mortality).

The second goal is to assess whether challenges associated with passage at the WRGF are limiting production of Upper Yukon River Chinook stocks. Specific objectives for this proposal associated with this goal are to assess:

4) What proportion of fish approaching the WRGF successfully pass it;
5) The extent to which fish are delayed at the WRGF before passing;
6) What proportion of fish return downstream after passing the WRGF.

This proposal requests funding for year 1 of a four-year project. The experiment will be replicated each year. This will ensure that only a small proportion of the population is manipulated for experimentation each year, reducing the risk of negatively affecting the already depleted population while ultimately providing a sufficient sample size. This will also allow us to control for confounding factors such as inter-annual variability in flow, temperature, run timing, or hydropower operations. Given that previous research on this topic has failed to put disagreements to rest regarding the fate of Chinook Salmon after they pass the WRGF, this large sample size coupled with the ability to control for confounding factors will improve confidence in our results.

As deliverables, we will provide an annual report and a final report at the completion of the four-year project. At least two peer-reviewed journal articles will be published based on the results of this study. Results will also be presented at a minimum of three scientific conferences.