High densities of hatchery-origin white sturgeon suppress somatic growth rates of an endangered wild population.

Success in conserving or recovering endangered species requires a detailed understanding of vital rates that control population trends. Sturgeons (Acipenseridae) are the most imperiled taxon globally, with ongoing population declines despite significant conservation and restoration efforts in recent decades. Here, a long-term dataset is analyzed to describe growth rates of endangered white sturgeon (Acipenser transmontanus) in the transboundary section of the upper Columbia River over a 32-year period where conservation aquaculture was initiated in year 12 to prevent extirpation. The extent of spatial (Canada and United States), temporal (1990-2021), and seasonal variation (summer and winter) in growth dynamics was modeled to understand the potential impact of increased competition due to stocking on somatic growth of wild-origin fish. von Bertalanffy growth models were fit to observations of growth in length and mass from recaptures of tagged fish. Overall, growth rates of wild- and hatchery-origin white sturgeon were similar, with growth in length higher in summer than in winter, especially in the United States. Growth in mass was similar in winter and summer for wild- and hatchery-origin fish weighing less than ~30 kg, but growth was considerably higher in winter than summer for heavier fish. Examination of temporal random effects showed an abrupt decrease in growth partway through the time series. To evaluate the magnitude and uncertainty of the change, we fit a Bayesian changepoint model and found evidence for suppressed growth in length and mass of wild fish in years following the peak in densities of hatchery-origin sturgeon released from conservation aquaculture. Results provide an understanding of how an endangered species responds to a conservation action commonly applied for imperiled fish species worldwide and may provide insight into other important population dynamics (e.g., reproductive metrics) that are critical for recovery.