Acoustic scattering layers reveal a faunal connection across the Fram Strait

Harald Gjøsæter,Randi Ingvaldsen,Jørgen S Christiansen

doi:10.1016/j.pocean.2020.102348

Abstract

Acoustic scattering layers (SL) at various depths are common phenomena in most oceans, but the organisms that make up these layers vary and so does their density, and hence the backscattered energy. During two crossings of the deep Fram Strait between the shelves at Svalbard and Northeast Greenland at latitudes 77°N and 79°N, we registered epipelagic and mesopelagic SL across the entire Fram Strait and quantified their acoustic backscattered energy. In addition, one pelagic trawl haul was made at each crossing together with a CTD cast at the northern crossing. The epipelagic SL was present at 0–200 m depth, whereas the mesopelagic SL was located at 300–500 m depth during day and at shallower depths during night indicating diel vertical migrations. The epipelagic SL consisted of young-of-the-year fishes, mostly redfish Sebastes sp. No trawl hauls were made in the mesopelagic SL, and the identity of organisms is unknown. Few strong echoes from single targets at mesopelagic depths stood out from the rest of the targets and were interpreted as adult Atlantic cod Gadus morhua. This is the first report of scattering layers covering the whole distance of the deep parts of the Fram Strait, and strengthen the assumption about an east–west connection of organisms and young-of-the-year fishes originating from the spawning grounds along the Norwegian coast and the western Barents Sea towards Northeast Greenland.

Highlights

Marine organisms like zooplankton and fishes often aggregate in the water column and are detected by echo sounders as scattering layers at given depths
We address the following research questions: do the scattering layers and the corresponding organisms such as young-of-the-year fishes previously observed close to Svalbard extend across the deep Fram Strait westwards to the Northeast Greenland (NEG) shelf? And if so, do the same organisms make up the layers near the Svalbard coast and over deep water? Does the distribution of organisms give evidence for a transport
For both TUNU-crossings, the West Spitsbergen Current (WSC) was evident with near-surface temperatures above 6 °C east of about longitude 6°E (Fig. 2A)

Summary

Introduction

Marine organisms like zooplankton and fishes often aggregate in the water column and are detected by echo sounders as scattering layers at given depths. Layers at epipelagic depths (0–200 m) often consist of zooplankton feeding on phytoplankton in the photic zone where photosynthetic production takes place and fishes feed on the zooplankton Layers at mesopelagic depths (200–1000 m) may consist of a mixture of mesopelagic fishes, crustaceans, cephalopods and gelatinous plankton (Irigoien et al, 2014). The depth of such scattering layers is most often determined by the light regime, but hydrographic features may play a role (Kaartvedt, 2008; Klevjer et al, 2016). Mesopelagic layers are more permanent but some of the organisms may undertake diurnal vertical migrations (DVM), hiding from predators in darker depths at day-time and ascend towards the surface to feed during night-time (Kaartvedt, 2008)

Methods

Results

Conclusion