Pre-Screen Loss and Fish Facility Efficiency for Delta Smelt at the South Deltas State Water Project, California

Abstract

Water exports have been implicated in the decline of fish populations in the upper San Francisco Estuary, California. We evaluated the relation between delta smelt salvage at the John E. Skinner Delta Fish Protective Facility (SFF) and underlying entrainment losses at the State Water Project (SWP, south Delta). We used cultured delta smelt in mark–recapture experiments in February and March 2009 (adults) and June 2009 (juveniles) to estimate: (1) the percent of fish recaptured at the SFF of the total released at the entrance of the SFF (fish facility efficiency), (2) the percent of fish recaptured at the SFF of the total released in Clifton Court Forebay (CCF), a reservoir for SWP exports, and (3) the fish losses in CCF and before the SFF (pre-screen loss). Mean fish facility efficiency was lower in successive releases: February (53.2%), March (44.0%) and June (24.0%). The mean percent recapture of fish released at the CCF entrance was also lower over time: February (3.01%); March (0.41%) and June (0.03%). Correspondingly higher mean pre-screen losses occurred over time: February (94.3%); March (99.1%) and June (99.9%). We concluded that: (1) entrainment losses of delta smelt could be higher at times, compared to other species previously studied at the SWP; (2) pre-screen loss was the largest source of mortality for delta smelt; (3) increased release distance from the SFF and residence time in CCF—and decreased exports—resulted in a lower percentage of recaptured fish at the SFF; and (4) salvage of delta smelt at the SWP does not seem to be a consistent index of entrainment.