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Effects of incubation temperature on the upper thermal tolerance of the imperiled longfin smelt (Spirinchus thaleichthys)

Yanagitsuru, Yuzo1; Mauduit, Florian1; Lundquist, Alexis1; Lewis, Levi1; Hobbs, James2; Hung, Tien-Chieh1; Connon, Richard1; Fangue, Nann1

Published Oct 23, 2023 on Dryad. https://doi.org/10.25338/B8H06D

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Oct 23, 2023 version files 459.95 KB

Abstract

Upper thermal limits in many fish species are limited, in part, by the heart’s ability to meet increased oxygen demand during high temperatures. Cardiac plasticity induced by developmental temperatures can therefore influence thermal tolerance. Here, we determined how incubation temperatures during the embryonic stage influence cardiac performance across temperatures during the sensitive larval stage of the imperiled longfin smelt. We transposed a cardiac assay for larger fish to newly hatched larvae that were incubated at 9, 12, or 15 ℃. We measured heart rate over increases in temperature to identify the Arrhenius breakpoint temperature (TAB), a proxy for thermal optimum, and two upper thermal limit metrics: temperature when heart rate is maximized (Tpeak) and when cardiac arrhythmia occurs (TArr­). Higher incubation temperatures increased TAB, Tpeak, and TArr­, but high interindividual variation in all three metrics resulted in great overlap of individuals at TAB, Tpeak, and TArr­ across temperatures. We found that the temperature where 10% of individuals reached Tpeak or TArr­, and temperature where number of individuals at TAB relative to Tpeak (ΔN(TAB, Tpeak)) is maximized correlated more closely with upper thermal limits and thermal optima inferred from previous studies compared to mean values of the three cardiac metrics, respectively. Larvae of embryos incubated at 15 ℃ did not have a higher temperature where ΔN(TAB, Tpeak) is maximized compared to larvae incubated at 9 or 12 ℃, suggesting that incubation temperatures beyond 12–15 ℃ are unlikely to shift larval thermal optima to match warming waters. Overall, by measuring cardiac performance across temperatures, we defined upper thermal limits (10% thresholds, Tpeak: 14.4–17.5 ℃; TArr­: 16.9–20.2 ℃) and optima (ΔN(TAB, Tpeak): 12.4–14.4 ℃) that can guide conservation strategies for longfin smelt, and demonstrated the potential of this cardiac assay for informing conservation plans for the early life stages of fish.