Selective Predation by Larval Striped Bass on Native and Introduced Copepods

Abstract

Year-class strength of striped bass Morone saxatilis is often determined by the success of first-feeding larvae. In the Sacramento–San Joaquin estuary, this success may be influenced by the recent invasion of exotic copepods that appear to be displacing native copepods important as food. We tested selection by larval striped bass on native copepods (Eurytemora affinis and Cyclops sp.) and on introduced copepods (Sinocalanus doerri and Pseudodiaptomus forbesi). Two-species and single-species tests established the following order of preference: Cyclops sp. (most preferred), E. affinis, P. forbesi, and S. doerri (rarely eaten). Tests to evaluate the underlying mechanism of prey selection showed that the presence of egg sacs and copepod size did not affect the larvae's choice of prey. Instead, selection by larval striped bass apparently involved differences in copepod swimming and escape behaviors.