Interactions of migrating zooplankton with shallow topography: predation by rockfishes and intensification of patchiness

Abstract

Stratified net tows, multiple-frequency sonar and drifter drogues, combined with analysis of rockfish stomachs, were used to investigate the processes taking place when vertically migrating plankton are swept onto a shallow bank in the Southern California Bight. Euphausia pacifica, the dominant migrating plankter near the bank, was also a dominat item in the stomachs of Sebastes spp. when currents in predawn hours advected water from deep water toward the bank. In the morning, schools of 12 kHz scatterers, apparently rockfish, ascended tens of meters above the bottom toward the descending plankton. Net tow estimates of euphausiid biomass and the acoustic backscattering at 200 and 520 Hz were much lower above the bank than in a nearby deep-water area, particularly during early evening. At night and during predawn hours, a relatively high abundance of zooplankton, with densities similar to the deep-water area, was usually found near the edge of the bank. Causes of the observed recurrent lack of migrating plankton above a seamount in early evening could be diurnal predation by epibenthic fish or daytime advection of migrators around the seamount, thereby creating a “hole” above it. The formation of such “holes” can intensify the regional patchiness of plankton near shallow seamounts. Historical data of CalCOFI night-time tows show that the biomass of euphausiids, but not that of the weakly migrating chaetognaths, is significantly more variable (patchy) at stations near offshore shallow topography than at nearby deep-water stations. The similarity in the mean biomass of euphausiids at the two station sets suggests that the contribution of predation to the observed intensifications of patchiness may be insignificant.