Fish entrainment into California's State Water Project (SWP) and Central Valley Project (CVP) has been a source of mortality for native osmerid species, including Delta Smelt (Hypomesus transpacificus) and Longfin Smelt (Spirinchus thaleichthys). While entrained adult and juvenile fishes are collected, identified and enumerated, the facilities design does not allow for a quantifiable estimate of larval fishes being entrained. This knowledge gap makes it difficult for managers to make scientifically informed decisions regarding the endangered Longfin & Delta Smelt.
The Larval Entrainment Study (LES) aims to address this gap by providing a high-resolution dataset and insights into the variable abundances of Smelt at risk of entrainment, the community composition surrounding them, and associated water quality data. Our study design utilizes a variety of gear types and maximizes replicates to improve the understanding of larval Smelt abundances within the southern reach of the Sacramento and San Joaquin River Delta, the zone of entrainment for the SWP and CVP.
Sampling began in 2022 using 10-minute stepped oblique tows of a Smelt Larval Survey (SLS) sled mounted with a 500 μm net. Tows were conducted in West Canal, outside Clifton Court Forebay (CCF), where the risk of entrainment is highest. Sampling was conducted five days a week in 2022 and three days a week in the following years. . Beginning in mid-January 2023, a 940 μm mesh net with the same dimensions as the SLS net was added, with sampling alternating between the two. This provided an intermediate gear between SLS (500 μm) and 20mm (1600 μm) nets.
In 2024 we expanded to sample a transect of sites within the Old and Middle River (OMR) corridor. This provides broader spatial resolution and improves our ability to detect Osmerids within the South Delta. Sampling alternated between a high priority day (with intensive sampling in the lower San Joaquin River, SLS stations 809 & 812) and a longer transect day that ran the length of the OMR corridor( at SLS stations 809, 812, 902, 915 & 918). We also began to sample using a bongo sled, which allows two nets to sample side by side. Our 2024 data is still being processed but will be included in a future release. Finally, the dataset also includes special studies completed so far, including day-night comparisons in 2022 and 2024, and a gear comparison in 2023.
