Understanding links between flow regime and fish populations in the Saskatchewan River Delta

Successful spawning and survival to adulthood (i.e., recruitment) are essential to maintain sustainable fish populations. Abiotic environmental conditions can influence recruitment and the resulting ability to harvest adult fish. In rivers, the flow regime dictates water depths and velocities that subsequently trigger spawning and enable the survival of juveniles. However, anthropogenic activities such as the operation of hydroelectric generating stations can change the natural hydrograph with effects on physical and biological processes. I investigated the relationship between the river flow regime and sustainable fish population sizes for two species of economic and cultural importance (Walleye Sander vitreus and Lake Sturgeon Acipenser fulvescens) in the Saskatchewan River Delta. To estimate the annual recruitment of Walleye, aging structures were removed from fish sampled from the commercial fishery at Cumberland Lake, Saskatchewan. Cohort strength was then estimated by assigning the year of hatch to individual fish. The cohort strength was compared against discharge from a gauge below the E.B. Campbell hydroelectric generating station, located ~100 km upstream from Cumberland Lake. I found a significant effect of hydrology with an estimated 69% increase (28–105% credible interval) in recruitment with every 100 m3·s-1 increase in discharge over the fry growth period (weeks 30–42) in Walleye. Also, based on the estimated Bayesian posterior distribution, there was a very high probability (p > 0.99) that the effect was different from zero. To estimate long-term harvest numbers for Lake Sturgeon, data was drawn from multiple sources (Hudson Bay Company records, government commercial fishery records, and recent mark-recapture programs). During the pre-dam (1774 to 1960) and post-dam (1965 to 2019) eras, the annual total harvest of Lake Sturgeon was estimated and compared to determine if harvest levels differed before and after flow modification. I observed no significant difference between the pre- and post-dam eras when all data was combined, but using only a subset of the 20th-century commercial catch data revealed a significant difference in Lake Sturgeon catch before and after dams were built. Discharge during the first ten years of the Lake Sturgeon’s life before recruitment to the fishery was a significant predictor of catch, but only when backdating to the period 25–35 years prior to catch and only when using more recent gauge data rather than tree-ring records. I found an estimated 59% increase in Lake Sturgeon catch with every 100 m3·s-1 increase in mean annual discharge. The study of these two species strengthens our understanding of the relationship between interannual and multidecadal changes in flow and fish population sizes, with implications for maximum sustainable harvest levels in the Saskatchewan River Delta. As upstream hydropower operations and irrigation withdrawals continue to alter spring and summer flows, data suggest that re-naturalization of the flow regime could improve recruitment of Walleye and Lake Sturgeon.

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