The effectiveness of conservation aquaculture in meeting recovery objectives for an endangered White Sturgeon population

Documenting the effects of hatchery practices on vital rates and abundance of hatchery-origin fish in the wild is important for adaptive management of aquaculture programs aiming to improve the status of wild populations. Here, we synthesize outcomes from a long-running (2001–2021) conservation aquaculture program for White Sturgeon Acipenser transmontanus in the Canada (CDN)–USA transboundary reach of the upper Columbia River. A Bayesian multistate Cormack–Jolly–Seber model was used to estimate abundance, survival, movement, and harvest rates for 14 year-classes of hatchery-origin White Sturgeon. The model was fitted to 20 years of mark–recapture data from more than 140,000 PIT-tagged juveniles released from hatcheries in CDN and the United States between 2001 and 2014. There was substantive variation in survival and movement rates among year-classes, which in CDN was driven in part by increased competition from hatchery-origin fish. Survival rates of age-1 fish in CDN declined by fourfold over 7 years during a period when biomass of hatchery fish increased from 0 to 20 kg/ha. Movement rates out of CDN over this period increased by a similar amount. Releasing fish at 30 cm compared to 20 cm led to a fivefold increase in the survival rate to age 5. Age-1 survival rates for fish released in spring or summer were 6- and 27-fold higher compared to survival rates of fish released in fall or winter in CDN and the USA, respectively. Maternal family effects on survival rates were generally weak. Harvest fisheries in the USA reduced the abundance of over-represented year-classes by 44–92% between 2017 and 2021, but the extent of depletions was uncertain due to the limited number of PIT tag recoveries. In 2021, abundance of hatchery-origin fish was 60% higher in CDN (8,900) than in the USA (5,700) due to a combination of (1) higher stocking rates in CDN in the early years of the program and (2) removal of fish in the USA from recreational and tribal fisheries. Evaluating the success of sturgeon conservation aquaculture programs requires decades of effort, and adaptive management is challenged by the much shorter time frames that decision makers are used to working with. Our study showed that conservation aquaculture for White Sturgeon in the transboundary reach has achieved some of its original goals in the first two decades of implementation by reducing the risk of extirpation and maintaining genetic diversity of the wild population. High stocking rates during the early years of the program resulted in lower survival of hatchery-origin fish in CDN, higher rates of movement of hatchery-origin fish from CDN to the USA, and lower somatic growth rates of hatchery- and wild-origin fish in both countries. These effects will likely persist for decades owing to the long lifespan of White Sturgeon. Natural spawning of hatchery-origin fish has yet to be documented but will likely begin about three decades after the start of the program. It will take at least four decades from the start of the program to determine whether hatchery-origin fish are increasing natural recruitment to the wild population. Our study underscores the importance of using a rigorous long-term adaptive management framework to evaluate recovery actions and potentially modify them over time.