| IMPACTS OF INTRODUCED PREY ON THE GROWTH AND SURVIVAL OF LARVAL PLANKTIVOROUS FISH IN THE SAN FRANCISCO ESTUARY |
| SULLIVAN, L. J., Romberg Tiburon Center for Environmental Studies, San Francisco State University, 3152 Paradise Drive, Tiburon, CA, USA, 92920, ljswr@sfsu.edu; Kimmerer, W. J., Romberg Tiburon Center for Environmental Studies, San Francisco State University, 3152 Paradise Drive, Tiburon, CA, USA, 92920, kimmerer@sfsu.edu; and Lindberg, J., Department of Biology and Agricultural Engineering, University of California, One Shields Avenue, Davis, CA, USA, 95616, 01 209-830-9803, lindberg@steeper.us |
| Populations of pelagic fish in the San Francisco Estuary (SFE) are in a state of decline. Declines in several species of planktivorous fish have been correlated to changes in the abundance and distribution of their zooplankton prey. Over the past two decades, there has been a shift in the species composition of zooplankton in the SFE from a community dominated by numerous species of calanoid copepods to one dominated by a single introduced cyclopoid copepod, Limnoithona tetraspina. Since its introduction, L. tetraspina has become the most abundant copepod, at times outnumbering all other copepods by a factor of ten. Additionally, because L. tetraspina is approximately 1/20th the biomass of the historically dominant calanoid species (e.g., Eurytemora affinis and Pseudodiaptomus forbesi), there has been a corresponding decline in the total biomass of available prey. Since food intake by larval fish is governed in part by mouth size and detection limits, the accessibility of prey can be significantly influenced by its size. To examine how changes in the size composition of prey, resulting from species introductions, translate into the success of planktivorous fish in the SFE, we quantified the relative consumption of native and introduced copepods by larval delta smelt (Hypomesus transpacificus) and striped bass (Morone saxatilis) in laboratory feeding experiments. Additionally, we conducted long-term rearing trials with larval delta smelt to examine how differences in consumption translate into growth and survival. Results demonstrate that young larval delta smelt and striped bass consume native and introduced copepods in the same proportion to their abundance in the prey assemblage. This contradicts prevailing views that L. tetraspina avoids predation because of its small size, and suggests that L. tetraspina may be an important prey item for larval planktivorous fish in the SFE. Conversely, prey selection of later stage larvae shifted toward the larger calanoid species indicating that this may only be true for early stage larvae. Additionally, larval delta smelt grew faster when fed the larger calanoid species than L. tetraspina. This supports the prevailing view that high abundances of L. tetraspina may provide suboptimal nutrition, and that the shift in prey composition may be contributing to the decline of pelagic organisms in the SFE. |
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