Abundance and community composition of micronekton across a front off Southern California

Abstract

Concurrent net and acoustic data were collected across a front off SouthernCalifornia to assess its influence on the abundance, biomass and community com-position of the micronekton. Higher abundance and biomass of vertically migra-tory mesopelagic fish (VMF) and euphausiids were found on the coolermesotrophic (north) side of the front, whereas larval fish abundance was higher onthe warmer oligotrophic (south) side. The biomass and community composition ofnon-VMF did not differ between the two sides of the front. The north was domi-nated by the euphausiid Euphausia pacifica and the myctophid Diaphus theta, whereasthe south was dominated by E. gibboides, E. recurva and a combination of VMFspecies with warm water affinities. The larval fish assemblage in the north wasdominated by the nearshore species Engraulis mordax and Citharichthys spp., whereasmesopelagic fish larvae dominated the south. Acoustic data revealed differences inthe distribution of fish, euphausiids and the deep-scattering layer across the front.In particular, epipelagic fish were aggregated directly at the frontal interface in thearea of enhanced plankton concentrations and may have contributed to higherplankton mortality and turnover rates at the front. In addition to being an area ofhigh biological abundance, the front may be acting as a distributional boundaryfor several migratory species, as well as a locus of enhanced prey forage and theactivity of epipelagic species.KEYWORDS: micronekton; A-Front; fish; krill; mesopelagic; California Current