The low-salinity zone in the San Francisco Estuary as a proxy for delta smelt habitat: A case study in the misuse of surrogates in conservation planning

Abstract

Abstract The use of surrogates in conservation planning for at-risk species is both a necessary and a fraught practice. Here we assess the use of the position of the low-salinity zone in California’s Sacramento-San Joaquin Delta as a surrogate for the extent and quality of habitat available to the imperiled delta smelt. The U.S. Fish and Wildlife Service issued a biological opinion and incidental take statement under the Endangered Species Act analyzing the impacts of ongoing operation of two large water infrastructure projects on the delta smelt. The Service’s analysis and the conservation actions it imposed are based on the assumption that the low-salinity zone can serve as a “surrogate indicator” for delta smelt habitat. We demonstrate that available scientific information on the species countermands use of the low-salinity zone to represent delta smelt habitat in conservation planning for the species, and that the U.S. Fish and Wildlife Service erred by assuming the existence of a surrogate relationship absent validation using the best available scientific information. Notably, large expanses within the low-salinity zone are unoccupied by delta smelt and the species consistently occurs outside of it. This case serves to remind scientists of the dysfunction between a consensus in the scientific literature that calls for analytical validation prior to the use of species surrogates and habitat proxies, and the commonplace practice of using surrogates and proxies based on surmise and assertion.