The extent of the low-salinity zone has long been used as a measure of the health of the aquatic ecosystem in the upper San Francisco Estuary. Consistent with that concept, the US Fish and Wildlife Service in analyzing and responding to the impacts of ongoing water export operations on delta smelt has imposed conservation actions that assume that the low-salinity zone can serve as a “surrogate indicator” for the fish’s habitat. When freshwater is exported in autumns in years with greater freshwater input into the estuary, habitat for delta smelt is assumed to be reduced in extent and quality. But available scientific information countermands that finding. Notably large expanses of the low-salinity zone are not occupied by delta smelt and the fish is consistently found outside of the narrow range of salinity, 0.5 to 6.0 ppt, used by the Service to define the low-salinity zone. That foundational observation indicates that the availability of habitat for delta smelt and the resources that define it is not necessarily increased or decreased with the shifting lens of low-salinity conditions. Regulatory actions taken in response to the location and extent of the low-salinity zone cannot be expected to benefit the imperiled delta smelt.
Murphy DD, Weiland PS. 2019. The low-salinity zone in the San Fransisco Estuary as a proxy for delta smelt habitat: a case study in the misuse of surrogates in conservation planning. Ecological Indicators 105:29-35.