The effect of temperature on embryo survival and development in pallid sturgeon Scaphirhynchus albus (Forbes & Richardson 1905) and shovelnose sturgeon S. platorynchus (Rafinesque, 1820)

The thermal response of pallid sturgeon Scaphirhynchus albus and shovelnose sturgeon S. platorynchus embryos was determined at incubation temperatures from 8 to 26°C and 8 to 28°C, respectively. The upper and lower temperatures with 100% (LT₁₀₀) embryo mortality were 8 and 26°C for pallid sturgeon and 8 and 28°C for shovelnose sturgeon. It was concluded that 12–24°C is the approximate thermal niche for embryos of both species. Generalized additive and additive‐mixed models were used to analyze survival, developmental rate and dry weight data, and predict an optimal temperature for embryo incubation. Pallid sturgeon and shovelnose sturgeon embryo survival rates were different in intermediate and extreme temperatures. The estimated optimal temperature for embryo survival was 17–18°C for both species. A significant interaction between rate of development and temperature was found in each species. No evidence was found for a difference in timing of blastopore, neural tube closure, or formation of an S‐shaped heart between species at similar temperatures. The estimated effects of temperature on developmental rate ranged from linear to exponential shapes. The relationship for rate of development to temperature was relatively linear from 12°C to 20°C and increasingly curvilinear at temperatures exceeding 20°C, suggesting an optimal temperature near 20°C. Though significant differences in mean dry weights between species were observed, both predicted maximum weights occurred at approximately 18°C, suggesting a temperature optimum near 18°C for metabolic processes. Using thermal optimums and tolerances of embryos as a proxy to estimate spawning distributions of adults in a river with a naturally vernalized thermal regime, it is predicted that pallid sturgeon and shovelnose sturgeon spawn in the wild from 12°C to 24°C, with mass spawning likely occurring from 16°C to 20°C and with fewer individuals spawning from 12 to 15°C and 21 to 24°C. Hypolimnetic releases from Missouri River dams were examined; it was concluded that the cooler water has the potential to inhibit and delay sturgeon spawning and impede embryo incubation in areas downstream of the dams. Further investigations into this area, including potential mitigative solutions, are warranted.