Linking Water Flows to Chinook Salmon in the Nicola River
A SFU‒DFO collaborative research project using time-series models investigated how the Nicola's flow regime has affected Chinook salmon productivity over the past 22 years.
Summary of Findings
Informing Environmental Flows for Fish
Water flows in rivers are controlled by many different human activities, such as water withdrawals for agriculture and other uses, dam operations, forestry, and climate variability. The seasonal patterns of water flow can impact the survival and productivity of fishes. For example, low flows during summer can decrease rearing habitat for young Pacific salmon, such as stream-type Chinook salmon. Yet, it can be difficult to know how much water the fish in a particular river need in order for the population to sustain itself, recover, or support fisheries. This watershed-specific information can help guide management of environmental flows, such as minimum instream flow targets.
Studies often inform environmental flow needs by studying river habitats at specific locations across modeled or observed flow variation. While an important and prevalent approach, this method does not address the actual response of fish to different flows; fish may move, use different habitats, or otherwise respond unpredictably. Thus, studies such as this, which examine the population-level response of fish to flows, could help inform environmental flow decisions in systems with multiple demands on water systems, such as the Nicola River.
We examined the observed response of Chinook populations to flow variation over the past 22-years. We used a state-of-the art time-series modeling approach with Chinook salmon data from DFO, and data on water flows from the Nicola River just upstream of its confluence with the Thompson River. Specifically, we examined how variation in river flows influences early summer-run Chinook salmon productivity, while accounting for density-dependence and early ocean survival. We focused on the number of adult salmon that return for each parent salmon (recruits per spawner)—if this productivity is greater than one, then the population is increasing; if it is less than one, then the population is declining. We tested the effects of different parts of the flow regime, including fall floods, winter ice cover, low summer flows during spawning and migration, and low summer flows during juvenile rearing. This rigorous approach revealed which factors are driving the ups and downs of Chinook salmon productivity.
After accounting for ocean survival and density dependence, we discovered that August flow during juvenile rearing was the most important predictor of their survival. We also found strong evidence that lower flows in the August during adult migration are associated with lower productivity. Based on this best model, we predict that cohorts that experienced 50% below average flow in the August of spawning and rearing have 29% lower productivity.
We predict that, for an average number of spawning adults, mean August flows need to be at least 10.6 m³s ̄¹ for the population to be able to maintain itself, when all other tested flow conditions and ocean survival are at average levels.
The current minimum flow target for the Nicola mainstem is 5.66 m³s ̄¹. We estimate that in a year with typical ocean survival and other average conditions, 100 spawners would only produce 72 recruits to the new generation, leading to a decline of the population.
We also found that Chinook productivity is lower when there are large fall floods when Chinook eggs are incubating. Given the extreme flooding in 2021 and predictions of more extreme precipitation due to climate change, this result is relevant and timely.
There is substantial uncertainty in these predictions (transparent bands. In addition, shifting habitat conditions and oncoming climate change may further modify these relationships. Regardless, this scientific study shows how summer flows affect Chinook salmon.
Warkentin, L., C. Parken, R. Bailey, and J.W. Moore. 2021. Shifting flow regimes erode the productivity of imperiled Chinook salmon. Ecological Solutions and Evidence. https://besjournals.onlinelibrary.wiley.com/doi/10.1002/2688-8319.12124
A collaborative project between SFU and DFO investigated how water flows in the Nicola River affect early summer-run Chinook salmon. We fit a state-of-the-art time series model to understand 22 years of variation in Chinook salmon productivity.
After accounting for ocean survival and density dependence, we discovered that the flow during the August when Chinook are rearing as juveniles was the most important predictor of productivity. Higher summer flows during adult spawning and migration also likely boost productivity. On average, cohorts that experienced 50% below average flows in the August of spawning and rearing had 29% lower productivity. Over the past 100 years, Nicola River flows in August have decreased by 26%, on average, based on analyses of long-term flow data.
Given the current minimum flow target for the Nicola mainstem of 5.66 m³s, we estimate that in a year with typical ocean survival and other average conditions, 100 spawners would only produce 63 recruits to the next generation, leading to rapid decline of the population.