Salmon and trout species are important to California’s ecology, culture and economy. But climate change threatens these vital species, some of which are already endangered or threatened. Drier and hotter summers are bringing lower stream flows and heat stress. Both of these things are directly related to low oxygen conditions in streams, which can further stress fish populations.
Coastal fog events help to keep coastal regions cool by reducing solar radiation, which likely influences water temperature in streams. However, few studies to date have investigated how fog events affect stream temperature and fish survival.
This project shed light on the relationships between coastal fog and stream temperatures in coastal California streams, increasing the understanding of where cool areas form that can serve as refuges for fish to survive hot and dry weather. The researchers measured and compared stream temperature variation originating from three sources: coastal fog, riparian shading and physical stream habitat.
The researchers collected data from two central California stream systems: the Russian River and Pescadero Creek watersheds. This work was completed in collaboration with the California Sea Grant Extension Program and the Resource Conservation District of San Mateo County.
The results showed that drought negatively impacts juvenile salmonid habitat. Water levels dropped to the point that pools became disconnected, and many of them dried completely by mid-summer. Since these pools support juvenile salmonids, this directly impacts the wellbeing of the fish. Surprisingly, isolated pools were cooler on average than during a non-drought year when pools were connected (2019). The researchers hypothesized that these pools were more strongly influenced by cooler groundwater than surface water during drought in comparison to non-drought periods.
This study was one of the first to incorporate coastal fog events into predictions of stream temperature on seasonal and diurnal timescales, and will better inform management strategies at sub-watershed scales with the potential to improve the sustainability of the coastal California salmonid fishery.