Feeding Ecology of During a Seasonal Pulse of Turbidity William Hilton1,2, Aaron Johnson2, Wim Kimmerer2 1California State University Stanislaus, Turlock Ca. 2Romberg Tiburon Center For Environmental Studies, San Francisco State University, Tiburon Ca. Background and Introduction Sample Analysis Winter 2012 Preliminary Results The delta smelt (Hypomesus transpacificus) is a small pelagic fish that is • Delta smelt were dissected at the UC Davis Aquatic Health Program Lab Relative Abundance of Prey Species •Gut contents and prey samples were counted and identified to the lowest endemic to the San Francisco Estuary (SFE). Once abundant in the SFE, Caught Using Different Sampling Methods dramatic declines in delta smelt abundance have prompted their protection possible taxon under the federal and state endangered species acts. • Prey was grouped into larger taxonomic groups (e.g., calanoid, cyclopoid, or harpacticoid copepods, cladocerans, amphipods, and mysids ) Delta Smelt Abundance by Year • Total weights of prey in guts were estimated from published length-to-weight ratios Pseudodiaptomus forbesi Daphnia spp.

Photo: Department of Water Resources This project is part of the Delta Smelt Turbidity Study (DSTS), an intensive Photo: Tony Ignoffo four-year monitoring effort to determine the extent to which changes in Photo: Lindsay Sullivan freshwater flow, salinity, and turbidity trigger the spawning migration of delta Photo: Anne Slaughter smelt. Although delta smelt migration is thought to be associated with turbid Winter 2010 Results water, migration patterns may also vary with other environmental conditions such as prey availability. The goal of this portion of the DSTS is to evaluate Importance of the role of prey availability in the timing of the delta smelt migration. To this Gut Contents by Number Gut Contents by weight Amphipods end, we examined the feeding ecology of adult delta smelt during their spawning migration in the winters of 2010 and 2012. Amphipod Field Sampling Methods Cladoceran Calanoid Copepod • We sampled delta smelt with hourly Kodiak trawls for several days before Cyclopoid Copepod Other and several days after the first seasonal pulse of turbidity in the winters 2010 Discussion/Conclusion and 2012 •Delta smelt prey were sampled at same time and location as smelt sampling Previous studies of delta smelt feeding at earlier life stages have emphasized calanoid copepods as their main food source, whereas the most frequent prey •Prey samples were immediately concentrated and preserved in 10% organisms ingested by delta smelt sampled in this study (in 2010) were buffered formalin Prey Sampling Methods cladocerans and epibenthic amphipods. This implies a very different foraging strategy and foraging habitat for adult delta smelt than for earlier life stages. 2010 Relative Abundance of Amphipod To identify the foraging habitat utilized by migrating adults, prey sampling in 2012 2012 was modified to adequately sample the entire water column including Species in Gut Samples benthic habitats.

Preliminary data from 2012 prey samples indicates that only mysids and one species of amphipod (Gammarus daberi) are adequately sampled by the new sampling methods. This means that other amphipods in the delta smelt diet (Hyallela azteca, Crangoyx floridanus, Americorophrium spinicorne) must be coming from other habitats (likely submerged and floating aquatic vegetation). Future field sampling will be modified to sample these habitats to give a Sampling Sites complete picture of prey availability for the delta smelt during migration. Cache Slough San Francisco Estuary Acknowledgements

Sacramento RiverRi We thank the Kimmerer Lab, Romberg Tiburon Center, and San Francisco State University. We thank UC Davis for the San Joaquin RiRiver preparation of delta smelt gut samples. We thank all those from the USGS Seattle office for assistance in the field. This work is part of the Delta Smelt Turbidity Study funded by an Interagency Ecological Program grant. (Cooperative agreement # G11AC20206) This material is based upon work supported by the S.D. Bechtel, Jr. Foundation, the National Marine Sanctuary Foundation, the Carnegie Corporation of New York, and/or the National Science Foundation under Grant Nos. 0952013 and 0833353. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the funders. The STAR program is administered by the Cal Poly Center for Excellence in Science and Mathematics Education (CESaME) on behalf of the California State University.