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.