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

Concurrent net and acoustic data were collected across a front off Southern California to assess its influence on the abundance, biomass and community composition of the micronekton. Higher abundance and biomass of vertically migratory mesopelagic fish (VMF) and euphausiids were found on the cooler mesotrophic (north) side of the front, whereas larval fish abundance was higher on the warmer oligotrophic (south) side. The biomass and community composition of non-VMF did not differ between the two sides of the front. The north was dominated by the euphausiid Euphausia pacifica and the myctophid Diaphus theta, whereas the south was dominated by E. gibboides, E. recurva and a combination of VMF species with warm water affinities. The larval fish assemblage in the north was dominated by the nearshore species Engraulis mordax and Citharichthys spp., whereas mesopelagic fish larvae dominated the south. Acoustic data revealed differences in the distribution of fish, euphausiids and the deep-scattering layer across the front. In particular, epipelagic fish were aggregated directly at the frontal interface in the area of enhanced plankton concentrations and may have contributed to higher plankton mortality and turnover rates at the front. In addition to being an area of high biological abundance, the front may be acting as a distributional boundary for several migratory species, as well as a locus of enhanced prey forage and the activity of epipelagic species.