Ontogeny influences sensitivity to climate change stressors in an endangered fish

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.

Keywords: Conservation management; estuarine fishes; salinity tolerance; thermal tolerance.

Figure 1:

Map of the San Francisco Estuary depicting estimated habitat regions for different ontogenetic stages of Delta Smelt, where abiotic conditions, including salinity and temperature, fluctuate spatially and temporally.

Figure 2:

Thermal tolerance of Delta Smelt. (a) Estimates of chronic lethal thermal maximum (CLT_max; continuous lines) and 95% confidence intervals (dashed lines) for juvenile (blue), adult (red) and post-spawning Delta Smelt (black). Points denote the CLT_max50 and CLT_max95 estimated mean and 95% confidence interval for each ontogenetic stage on the respective curves. (b) Critical thermal maximum (CT_max) temperatures (means ± SEM) for ontogenetic stages of Delta Smelt at different acclimation temperatures, with jitter (0.05) added to each point to avoid overlapping. Capital letters denote significantly different groups across all ontogenetic stages at medium acclimation temperature only; lower case letters denote significantly different groups among stages of non-larval fish at low, medium and high acclimation temperatures. Within each case, groups not sharing a common letter are significantly different at an adjusted α level of 0.05 as determined by a linear model and pairwise comparisons. PS-Adult denotes post-spawning Delta Smelt.

Figure 3:

Habitat temperature profiles for each ontogenetic stage of Delta Smelt examined and the corresponding thermal tolerances determined in this study. The histograms depict constructed data sets of water temperature distributions relevant for each ontogenetic stage; red dashed lines indicate the mean CT_max for each stage of fish at medium acclimation temperature; dashed blue and green lines depict the CLT_max50 and CLT_max95, respectively, for each stage in post-larval fish.

Figure 4:

Cumulative proportional survival of juvenile and adult Delta Smelt in acute salinity maximum over 2 weeks of exposure (low salinity, 2.3 ppt, medium salinity, 18.5 ppt; and high salinity, 34.0 ppt). Late-larval data are not shown because they did not exhibit any significant difference in survival across salinity treatments.