Ontogeny influences sensitivity to climate change stressors in an endangered fish strongly tied to California conservation

Background/Question/Methods

Coastal ecosystems are among the most human-impacted habitats globally, and their management is often critically linked to recovery of declining native species. In the San Francisco Estuary (SFE) the Delta Smelt, Hypomesus transpacificus, is an endemic, endangered fish strongly tied to California conservation planning. The complex life history of Delta Smelt combined with dynamic seasonal and spatial abiotic conditions results in dissimilar environments experienced among ontogenetic stages. This may yield stage-specific susceptibility to abiotic stressors and, by extension, climate change induced shifts in these environmental factors. Climate change is forecasted to increase SFE water temperature and salinity, two key factors influencing the survival and performance of fishes. Therefore, understanding influences of ontogeny and phenotypic plasticity on tolerance to these critical environmental parameters is particularly important for Delta Smelt and other SFE fishes. We experimentally assessed thermal and salinity limits in several ontogenetic stages and acclimation states of Delta Smelt across multiple timescales, and paired these data with in situ environmental data as well as climate change model projections for the SFE to evaluate sensitivity to climate change stressors.

Results/Conclusions

Thermal tolerance decreased among successive stages, with larval fish exhibiting the highest tolerance and post-spawning adults having the lowest. Delta Smelt had limited capacity to increase tolerance through thermal acclimation, and comparisons to field temperature data revealed that juvenile tolerance limits are the closest to current environmental conditions, which may make this stage especially susceptible to future climate warming. Maximum water temperatures observed in situ exceeded tolerance limits of juvenile and adults. Although these temperature events are currently rare, if they increase in frequency as predicted, it could result in habitat loss at these locations despite other favorable conditions for Delta Smelt. In contrast, Delta Smelt tolerated salinities spanning the range of expected environmental conditions for each ontogenetic stage, but salinity did impact survival in juvenile and adult stages in exposures over acute timescales. Our results underscore the importance of considering ontogeny and phenotypic plasticity in assessing climate change impacts, particularly for species adapted to spatially and temporally heterogeneous environments.