Almost every year since 1995, Pescadero Estuary in San Mateo County has experienced mysterious die-offs of fish, including federally protected steelhead trout. These fish kills always occur after the sandbar across the lagoon mouth has been breached (due to the build-up of freshwater behind it) and the lagoon becomes opened to tidal flushing. Why would tidal flushing kill fish? It is speculated that the initial outflow of stagnant freshwater stirs up sulfur-containing sediments on the bottom, which, by changing the water’s sulfur chemistry, cause a potentially lethal drop in the water’s dissolved oxygen content. This project seeks to test these ideas by quantifying sulfur cycling in the estuary, as it opens and closes and its marshlands are flooded and drained. The scientists will also characterize the physical dynamics of the estuary during open and closed states and during transitions from each. The project’s third objective is to characterize how changes in geochemical and physical water conditions influence fish movement patterns, especially how fish attempt to escape anoxic (suffocating) conditions. For this component of the project, fish will be acoustically tagged and tracked with an existing array of receivers. Findings, which will be shared at two workshops for decision-makers, may help identify restoration activities that could reduce fish kills in the estuary and protect habitat quality for the commercially important Dungeness crab fishery. The project is a partnership with NOAA Fisheries and state and federal agencies participating in the Pescadero Estuary Science Panel.