Mesoscale Aggregations of Euphausiid Sound Scattering Layers on the Continental Shelf of Vancouver Island

Abstract

High frequency acoustics (104 kHz) and stratified (BIONESS) net tows were used to map the vertical and horizontal distribution of macrozooplankton scattering layers on and seaward of the continental shelf off Vancouver Island, in June and August of 1986. The macrozooplankton scattering layers were dominated by euphausiids, primarily Euphausia pacifica and Thysanoessa spinifera. During both surveys, dense aggregations were found in the same two general regions. The first was along the shelf break, where highest biomass was observed (averages over a few kilometres of transect reaching to about 200 mg dry weight∙m−3 = about 40 individuals∙m−3 and > 4 g dry weight∙m−2). A second, broader and more diffuse aggregation occupied the deeper southeast end of the shelf (area about 200 km2, biomass 1–2 g∙m−2). The scattering layers showed a consistent diel vertical migration. Daytime depth was deeper at the shelf break (~ 150–200 m) than on the shelf (~ 100–150 m) and occasionally extended to the bottom. These day-depths corresponded to the California Undercurrent waters of 6–7 °C, a salinity of 33.75–34‰ and sigma-T > 26.5. Distribution patterns were coincident with those of upwelled regions, high phytoplankton biomass, and high hake and herring biomass. They were also associated with the shelf break jet and the recurrent Tully eddy. There is a strong possibility of aggregation in restricted areas through the coupling of advection and vertical swimming behavior of the animals.