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Water Temperature Suitability for Steelhead Trout (Oncorhynchus mykiss) in Codornices Creek


Codornices Creek is a small urban stream in Alameda County, California, that flows along the border between the cities of Albany and Berkeley. The creek provides habitat for Oncorhynchus mykiss, a species of salmonid known as steelhead or rainbow trout, depending on whether populations have an ocean-going life stage. Steelhead trout are a species of interest in Codornices Creek because they are listed as threatened under the Endangered Species Act. Populations in the creek have fluctuated over the past 30 years due to impacts from restoration projects, urban pollution, and climatic conditions. Previous reports have suggested that water temperature may be limiting steelhead trout success in some reaches, prompting our investigation into water temperature and potential drivers in Codornices Creek.We analyzed spatiotemporal trends in water and air temperature recorded at eight sites along Codornices Creek during a 5-month period (June - November 2022), including potential drivers for water temperature trends such as rainfall and canopy cover. We analyzed canopy cover using spatial analysis and field observations. We obtained water temperature data from long-term monitors, a local stream gauge, and a thermocouple for instant field measurements. We downloaded air temperature and rainfall data from the National Weather Service weather station located at the Oakland International Airport (June - November 2022) and additional rainfall data from a local rainfall gauge (October - November 2022). We then calculatedlong-term water temperature suitability as maximum weekly average temperature and maximum weekly maximum temperatures. We compared water temperature to air temperature, rainfall, and canopy cover to determine primary drivers of water temperature. To evaluate canopy cover, we brought National Agriculture Imagery Program satellite imagery into ArcGIS and calculated the normalized difference vegetation index within a 30-meter buffer along open stream sections of Codornices Creek. We supplemented this analysis with field observations at each study site using a densiometer.  Our investigation indicated that air temperature affected long-term trends in water temperature, rainfall affected short-term water temperature trends, and canopy cover tended to moderate the effects of air temperature. Despite water temperatures in the upper half of the watershed being slightly cooler than downstream temperatures, water temperatures throughout Codornices Creek were generally within suitable range for steelhead trout. Only one location downstream reached temperatures stressful for steelhead trout. This location may require further investigation, as air temperature, rainfall and canopy cover do not explain elevated temperatures. Locating and addressing the source of elevated temperature may benefit steelhead trout populations.

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