The East Asian Copepod Diet:: A New Low Carbon Craze Taking San Francisco Estuary Fish by Storm

Declines in several species of planktivorous fish in the San Francisco Estuary (SFE) have been correlated to changes in the abundance and distribution of their zooplankton prey. These correlations provide indirect evidence that changes in food supply are contributing to the decrease in fish abundance. 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 1/20^th the biomass of the historically dominant calanoid species (i.e., 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 gape size and detection limits, the accessibility of prey can be significantly influenced by its size. To examine how changes in the prey assemblage translate into the population success of planktivorous fish in the SFE, we quantified the relative consumption of different copepod species by larval delta smelt (Hypomesus transpacificus) and striped bass (Morone saxatilis) in laboratory feeding experiments. We also 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 the copepodite stages of E. affinis, P. forbesi and L. tetraspina in approximately the same proportion to their abundance in the prey assemblage. This contradicts prevailing views that L. tetraspina avoids predation by planktivorous fish because of its small size, and suggests that L. tetraspina may be an important prey item for larval planktivorous fish in the SFE. However, prey selection of later stage larvae shifted towards the larger calanoid species indicating that this may only be true for early stage larvae. Larval delta smelt grew faster when fed P. forbesi than L. tetraspina. However, larval survival did not differ significantly among diets. Unlike, the information on prey selection, this information support the prevailing view that high abundances of L. tetraspina may provide suboptimal nutrition to the delta smelt population, and that the shift in prey composition may be contributing to the decline of pelagic organisms in the SFE.