Prey selection of larval and juvenile planktivorous fish: impacts of introduced prey

Changes in the abundance and composition of zooplankton resulting from species introductions may be contributing to a decline in the abundance of planktivorous fish in the San Francisco Estuary (SFE). To examine how changes in the prey assemblage may translate into feeding success of planktivorous fish, we quantified the relative consumption of native and introduced copepods by larval and juvenile delta smelt, Hypomesus transpacificus, and larval striped bass, Morone saxatilis, in laboratory feeding experiments. Larvae ≤20 days post-hatch (dph) of both fish consumed the copepodite stages of ~1 mm calanoid copepods, native Eurytemora affinis and introduced Pseudodiaptomus forbesi, and the smaller (~0.5 mm) introduced cyclopoid copepod, Limnoithona tetraspina, in approximately the same proportion as their abundance in the assemblage. Conversely, prey selection of larvae >20 dph shifted towards the larger calanoid species, E. affinis and P. forbesi. The timing of this shift differed between fish species, occurring earlier in striped bass, and varied between calanoid species, occurring later for P. forbesi than E. affinis. These data suggest that the impacts of introduced prey on planktivorous fish depend not only on the prey being introduced, but also on the resident predators. Additionally, even small differences in size and behavior between native and introduced prey can influence prey selection of planktivorous fish. Changes in prey selection of planktivorous fish resulting from species introductions could directly impact fish recruitment. As a result, species introductions could significantly alter food web function in the SFE and other invaded aquatic ecosystems.