The Transcriptome-Wide Effects of High Water Temperature on Two Threatened California Fishes

Climate change is one of the most serious issues faced by resource managers in the San Francisco Bay-Delta, California, and predicted future increases in water temperature may greatly influence the ability of some fishes to persist in the system. Two species of concern are the critically endangered Delta Smelt (Hypomesus transpacificus) and the threatened Longfin Smelt (Spirinchus thaleichthys); however, little is known about the molecular mechanisms involved in thermal tolerance and whether these species can cope with predicted increases in water temperature. We conducted a comparative RNA-sequencing study to identify transcriptomic responses to an environmentally-relevant high water temperature of 20°C in 50 day old developing larval fishes acclimated to 14°C. Longfin Smelt showed an upregulation of genes involved in a thermal cellular stress response at 20°C and decreased expression of genes involved in osmoregulation and growth, responses that were not detected in the relatively more thermally-tolerant Delta Smelt. These results suggest that Longfin Smelt may be particularly sensitive to moderate future increases in water temperatures from current conditions. This work generated the first data on the effects of water temperatures on Longfin Smelt and provides molecular tools to aid in conservation efforts and management of these two important Bay-Delta species.