Circulation and behavior of euphausiids in two Norwegian sub-Arctic fjords

Abstract

We conducted 2 cruises in 2 subarctic fjords, Ullsfjorden and Sorfjorden, northern Norway, to investigate the role of coupled advection and individual behavior of euphausiids in determining their distribution and retention. The surveys examined the seasonal change in buoyancy-driven circulation in fjords due to surface cooling and ice formation during the fall and winter, and surface heating, freshwater runoff and a deep-water intrusion during the spring, as well as seasonal variations in euphausiid distribution and migration. The causes leading to vertical migration of euphausiids were studied by light and food measurements. Euphausiid transport was examined on the basis of timing and duration of euphausiid migration, as well as variations in water currents. Our results demonstrate the entrapment of euphausiids in these fjords. To understand the aggregation mechanisms, euphausiid swimming behavior was studied in terms of cruising speed, escape velocity, deterministic swimming and random walking. Existing stochastic aggregation theories were used to interpret the observations of swimming behavior. The mismatch between theories and observations found in this study led us to construct a new model configuration between aggregation, deterministic vs. random swimming, and dispersion.