Small-scale spatial distribution of the euphausiid Euphausia pacifica and overlap with planktivorous fishes

The spatial distribution of zooplankton and planktivorous fish in Saanich Inlet, British Columbia was characterized with a 445 kHz multibeam sonar capable of localizing individual animals in three dimensions. Numerical simulation of data processing procedures indicates that this instrument has sufficient resolution to characterize zooplankton spatial distributions using Lloyd’s index of patchiness and nearest neighbour distance (NND) measures. Zooplankters at this site were dominated by the euphausiid Euphausia pacifica which underwent diel vertical migration (DVM). Fish (mainly herring and walleye pollock) were only abundant at this deep-water station at night. Analysis of 4 m3 volumes over scales of ∼100 m indicates that both euphausiids and fish were strongly aggregated in the vertical dimension, but not in the horizontal dimension. In the horizontal dimension, fish distributions were independent of zooplankton abundance, indicating that the fish were not exploiting small-scale horizontal prey aggregations. Analysis of NND at the centimetre scale indicates that euphausiids were not aggregated, but rather, they were approximately randomly distributed. These observations indicate that in this environment euphausiids do not form subsurface social aggregations as has been documented in other locations. DVM and social aggregation are both predator avoidance behaviours that may be redundant, as populations that exhibit pronounced DVM may derive little additional benefit from routine social aggregation.