Williston Reservoir is the seventh largest reservoir in the world by volume, and the largest in British Columbia. To increase productivity of the reservoir and to improve harvesting opportunities for larger fish, the Province of B.C. stocked approximately 3.5 million kokanee in tributaries of the Williston Reservoir between 1990 and 1998. Although kokanee were found in the reservoir watershed prior to stocking, they were only spawning in a handful of tributaries and in relatively small numbers. There are now an estimated nine million kokanee in Williston Reservoir.
Kokanee are a forage species – relatively small fish that can feed larger piscivores like bull trout and lake trout. And when abundant, they have the capacity to influence other species and habitats by redistributing nutrients in the ecosystem. They feed and grow in the reservoir and, when they migrate to the tributaries to spawn, it is possible that the nutrients are dispersed. This can happen in a variety of ways: their eggs can be eaten by other fish; their bodies consumed by animals or birds, their decomposing carcasses can provide nutrients to river or creek sediment, or indeed to riparian areas alongside.
Currently, a team of researchers is trying to determine the influence of those nutrients in the tributaries where kokanee spawn. The team is led by the Natural Resources and Environmental Studies Institute of UNBC, in partnership with Chu Cho Environmental, Kwadacha and Tsay Keh Dene Nations, and the McLeod Lake Indian Band.
The researchers are currently looking at four main metrics to measure the influence of the nutrients the kokanee may be adding to Williston Reservoir tributaries. The metrics are: 1) the diversity and abundance of aquatic invertebrates; 2) the abundance of other fish, such as sculpin; 3) species diversity of lichen in riparian areas; and 4) the flow of nutrient such as carbon and nitrogen through the food chain.
In each case, the results for the control tributaries, where kokanee are not present, will be compared against those tributaries where there is a low, medium, and high abundance of kokanee, to see if certain patterns emerge.
The work is funded by the Fish & Wildlife Compensation Program (FWCP) – a partnership between BC Hydro, Fisheries and Oceans Canada, the Province of B.C., First Nations and Public Stakeholders that conserves and enhances fish and wildlife in watersheds impacted by BC Hydro dams.
The project aligns with the FWCP’s Reservoirs Action Plans, and is one of its priorities because the effects the stocked kokanee have on the reservoir’s ecology, as well as their interactions with other species, are largely unknown.
Year two of the three-year project has just been completed and much of the data has yet to be analysed. It is therefore too early to draw concrete conclusions, but from the 2016 analysis, findings strongly suggest that kokanee provide a significant source of nutrients to tributary streams where they spawn.
One interesting observation has been made. While salmon are well-known to exhibit strong homing instincts – to return to the very same tributaries where they emerged from the gravel – the kokanee that were stocked in Williston Reservoir did not. Over successive generations they have “strayed,” and are now primarily found spawning in tributaries different from those where they were originally introduced.
The results of this project will provide information on how kokanee are influencing the stream and riparian ecosystem and how their influence may be affecting other species in the areas where kokanee spawn. Having this understanding will support future management of kokanee in the Williston Reservoir watershed.
Chelsea Coady is the Peace Region manager for the Fish and Wildlife Compensation Program. Have a question? Email her at email@example.com.