Increased water temperatures from four climate-change scenarios: Implications for selected fishes of the San Francisco estuary and watershed

We evaluated the possible effects on fishes of changes in water temperatures generated by four 100-year scenarios of climate change for the San Francisco estuary and watershed.  Input data used to model water temperatures were from two regional climate models at two levels of greenhouse gas emissions.  There were significant increasing trends for water temperature for all scenarios in the rivers and the Sacramento-San Joaquin Delta (Delta), but only in some months—generally summer and early fall.  In the Sacramento River, we counted the number of years the mean monthly water temperature (MMWT) exceeded 15.5ºC in the spawning area of endangered winter-run Chinook salmon.  A MMWT>15.5ºC would likely result in high mortality of incubating eggs and pre-emergent fry.  Most exceedances occurred during the final third of the warmest scenario in August and September, when salmon are emerging.  In the Delta, habitat available for the threatened delta smelt decreased substantially under  the moderate and warmest scenarios because  of lethal water temperatures (>25 ºC) in the northern region.  Projections like these provide managers with a useful tool for anticipating long-term challenges to managing fish populations and possibly adapting water management to ameliorate those challenges in the San Francisco Estuary and watershed.