The delta smelt, endemic to the Sacramento-San Joaquin Delta and adjacent upper San Francisco Estuary, has experienced record low abundances since 2004. We address three questions in this investigation: What is the relative importance of environmental factors with direct effects on abundance? Do factors that may have indirect effects provide an explanation of abundance changes? Are effects of environmental factors better accounted for individually or as criteria defining the volume of water with suitable abiotic attributes? Strong evidence was found of density-dependent population regulation. The density of prey was the most important environmental factor explaining variation in delta smelt abundance from 1972 to 2006 and over the recent period of decline in the fish’s abundance. We found evidence of predation on delta smelt and water temperature showed probable effects. Entrainment of delta smelt at south Delta pumping plants showed statistically significant effects on adult-to-juvenile survival, but not over the fish’s full life cycle. Neither the volume of water with suitable abiotic attributes nor other factors with indirect effects, including the location of the 2 ppt isohaline in the Delta in the previous fall (“Fall X2”), explained delta smelt population trends beyond those accounted for by prey density.
Miller WJ, Manly BF, Murphy DD, Fullerton D, Ramey RR. 2012. An investigation of factors affecting the decline of delta smelt (Hypomesus transpacificus) in the Sacramento-San Joaquin Estuary. Reviews in Fisheries Science 20:1–19.