Temperature and sea level are predicted to rise with climate change, bringing an urgency to evaluating future viability of native fish. Lamprey are confronted with widespread habitat degradation, migratory barriers, and episodes of environmental change projected to be commonplace in the future. In California, range contraction likely shifted lamprey rearing downstream, but the extent and physiological constraints that restrict estuarine rearing are unclear. We used a single-season occupancy model to describe juvenile lamprey estuarine distribution and found occupancy was regionally variable and constrained by temperature. Habitat and hydrology providing thermal refugia may be critical for future persistence. K E Y W O R D Sammocoete, climate change, Entosphenus sp., Lampetra sp., single-season occupancy model, temperature

Early one morning, a scientist collecting fish in the San Francisco Estuary was surprised to find a young lamprey. She knew lamprey were ancient fish without jaws, bones, or scales, and she wondered to herself: Where else are young lamprey in the estuary? She discovered that although lamprey existed long before dinosaurs, little is known about their lives in estuaries. She and her team decided to gather data from scientists who accidentally caught lamprey while studying other animals. This process of reusing data from unrelated studies, called data synthesis, is an important tool for answering unsolved mysteries. The team uncovered differences in where young lamprey live and that three species live in the San Francisco Estuary, including two species that live half of their lives in the ocean and the other half in freshwater. Knowing where young lamprey live can help scientists protect the habitats lamprey need to develop.