Diel vertical distribution of fish larvae their prey in nearshore waters of southern California

Abstract

Ichthyoplankton was sampled with bongos and microplankton was sampled with a pump at 3 to 5 discrete depths over isobaths of 8, 22 and 30 m, during a winter cruise in Santa Monica Bay, California. Although no thermocline was present, fish larvae were significantly stratified (p < 0.05) by sampling depth during daylight hours and were relatively more dispersed at night. Larvae of northern anchovy Engraulis mordax and white croaker Genyonemus lineatus showed length-specific vertical distributions. Many large northern anchovy (312 mm in length) apparently migrated to the sea surface at night, but large white croaker (>6 mm in length) did not. Only 9% of northern anchovy larvae contained at least 1 food particle (i.e. 9 % feeding incidence), compared to 76 % for white croaker larvae. Early life stages of copepods were important foods for both fish species. Bivalve veligers and tintinids occurred relatively frequently in the guts of white croaker but not northern anchovy. There was little relation between daytime feeding distribution of fish larvae and esculent microzooplankton, except that distributions of white croaker and bivalve veligers were significantly correlated (p < 0.05). The low densities of bivalve veligers in samples (2 = 3 I-'), and paradoxically, their frequent occurrence in the guts of white croaker, are discussed in relation to their availability as food for fish larvae. The die1 vertical positions of neritic fish larvae may, in part, be an expedient strategy for their retention in shallow waters.