Abstract
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, the Delta Smelt (Hypomesus transpacificus) is an endemic, endangered fish strongly tied to Californian conservation planning. The complex life history of Delta Smelt combined with dynamic seasonal and spatial abiotic conditions result in dissimilar environments experienced among ontogenetic stages, which may yield stage-specific susceptibility to abiotic stressors. Climate change is forecasted to increase San Francisco Estuary water temperature and salinity; therefore, understanding the influences of ontogeny and phenotypic plasticity on tolerance to these critical environmental parameters is particularly important for Delta Smelt and other San Francisco Estuary fishes. We assessed thermal and salinity limits in several ontogenetic stages and acclimation states of Delta Smelt, and paired these data with environmental data to evaluate sensitivity to climate-change stressors. 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 with 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. Maximal water temperatures observed in situ exceeded tolerance limits of juveniles 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 favourable 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 time scales. Our results underscore the importance of considering ontogeny and phenotypic plasticity in assessing the impacts of climate change, particularly for species adapted to spatially and temporally heterogeneous environments.
Highlights
Coastal ecosystems are among the most human-impacted habitats globally (Lotze et al, 2006), and climate change is predicted to interact with existing stressors to generate effects spanning multiple physical and biological scales (Harley et al, 2006; Crain et al, 2008)
Acute thermal tolerance of Delta Smelt (CTmax) across all stages examined at the medium acclimation temperature was highest in larval fish (Fig. 2b, Table 1 and Table S2)
The critical thermal maximum (CTmax) decreased with each subsequent ontogenetic stage, with the exceptions of between adults and juveniles and between adults and late-larval fish
Summary
Coastal ecosystems are among the most human-impacted habitats globally (Lotze et al, 2006), and climate change is predicted to interact with existing stressors to generate effects spanning multiple physical and biological scales (Harley et al, 2006; Crain et al, 2008). Like many other species inhabiting human-altered ecosystems, the decline of Delta Smelt has been associated with numerous stressors, such as habitat loss, entrainment at water pumping stations (i.e. fish drawn through intakes; Castillo et al, 2012), competition and predation from nonnative species, food limitation due to changes in the plankton community, altered abiotic conditions and contaminants (Sommer et al, 2007; Winder and Jassby, 2011; Brooks et al, 2012; Cloern and Jassby, 2012) Listed under both the Federal Endangered Species Act and California Endangered Species Act and an indicator of SFE ecological health, recovery of Delta Smelt is a critical component of statewide management efforts to balance ecosystem restoration with ecosystem services. Determining the best management actions to accomplish these sometimes conflicting objectives has been politically and publically debated in California for over a decade, making the scientific understanding of stressor impacts on Delta Smelt important to effective conservation, and to statewide environmental policies
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