Deep Scattering Layers at the Svalbard Gateway to the Arctic Ocean.

Abstract

As part of the Norwegian SI-ARCTIC Program, in late summer of 2014 and 2015 acoustic data (18, 38 and 120 kHz) for the estimation of the distribution and abundance of zooplankton and fish were collected from regions west and north of Svalbard, to examine high latitude epipelagic and mesopelagic scattering structures. The deep scattering layer biological constituents were determined from vertical and oblique hauls with zooplankton nets and pelagic trawls. There was strong patchy scattering in the upper part of the epipelagic zone (<50 m) throughout the area due to 0-group fish that were particularly abundant west of the Spitsbergen Archipelago and by copepods, krill, and amphipods. The distinct Off-shelf deep scattering layer (DSL) occurred between 200 and 600 m and contained a range of larger longer lived organisms (mesopelagic fish and macrozooplankton). In eastern Fram Strait, the DSL also included larger fish close to the shelf/slope break that were associated with Warm Atlantic Water moving north towards the Arctic Ocean, but switched to dominance by species having weaker scattering signatures further offshore. The Weighted Mean Depths of the DSL were deeper (WMD >440 m) in the Arctic habitat north of Svalbard compared to those south in the Fram Strait west of Svalbard (WMD ~400 m) and the mesopelagic nautical area scattering coefficient was a factor of approximately 6-10 lower around Svalbard compared to the areas in the south-eastern part of the Norwegian Sea ~62º30’N. The DSL displayed a clear ascending and descending diel movement. The high-light WMD with respect to backscattered energy was statistically deeper than the low-light WMD for the locations studied. This behavior of the DSL was consistent both when the sun was continuously above the horizon and after it started to set on 1 September, and both in open water and sea ice covered waters.