Aggregation of euphausiids and Pacific hake (Merluccius productus) along the outer continental shelf off Vancouver Island

Abstract

From spring through autumn, euphausiids and Pacific hake (Merluccius productus) form dense aggregations off the outer coast of British Columbia along regions of steeply sloping bathymetry such as the continental shelf break. Their spatial overlap is ecologically significant because of their very strong prey-predator interaction. We analyze high-resolution measurements of shelf-edge spatial distributions of euphausiid and Pacific hake biomass (by echo integration), water properties (from surface measurements and conductivity-temperature-depth (CTD)/Rosette profiles), and current patterns (from acoustic Doppler current profiler, CTD profiles, and nearby current meter moorings). Both euphausiids and hake share similar horizontal distributions, although separated vertically by tens of meters during daylight hours. Bathymetric and water property patterns provide good coarse-scale prediction of aggregation location. However, details of patch location are better explained by flow-field indices such as cross-shore position of the shelf break current, intensity of cross-isobath flow and upwelling at the depth of the euphausiid scattering layer, and doming of isopycnals. Under prevailing summer oceanographic conditions along the British Columbia coast, likely ecological consequences of aggregation in and beneath upwelling water include access to high food density in the overlying surface layer, reduced alongshore transport, and horizontal segregation between adult and larval euphausiids.