A deep scattering layer under the North Pole pack ice

Randi B. Ingvaldsen,Rolf Korneliussen,Larry Mayer,Pauline Snoeijs-Leijonmalm,Hein Rune Skjoldal,Harald Gjøsæter,Katarina Gårdfeldt,Christian Stranne,Tor Knutsen,Martin Jakobsson,Egil Ona

doi:10.1016/j.pocean.2021.102560

Abstract

The 3.3 million km2 marine ecosystem around the North Pole, defined as the Central Arctic Ocean (CAO), is a blind spot on the map of the world’s fish stocks. The CAO essentially comprises the permanently ice-covered deep basins and ridges outside the continental shelves, and is only accessible by ice-breakers. Traditional trawling for assessing fish stocks is impossible under the thick pack ice, and coherent hydroacoustic surveys are unachievable due to ice-breaking noise. Consequently, nothing is known about the existence of any pelagic fish stocks in the CAO, although juveniles of Boreogadus saida richly occur at the surface associated with the sea ice and ice-associated Arctogadus glacialis has been reported as well. We here present a first indication of a possible mesopelagic fish stock in the CAO. We had the opportunity to analyse a geophysical hydroacoustic data set with 13 time windows of usable acoustic data over a transect from 84.4 °N in the Nansen Basin, across the North Pole (90.0 °N), to 82.4 °N in the Canada Basin. We discovered a deep scattering layer (DSL), suggesting the presence of zooplankton and fish, at 300–600 m of depth in the Atlantic water layer of the CAO. Maximum possible fish abundance and biomass was very low; values of ca. 2,000 individuals km−2 and ca. 50 kg km−2 were calculated for the DSL in the North-Pole area according to a model assuming that all acoustic backscatter represents 15-cm long B. saida and/or A. glacialis. The true abundance and biomass of fish is even lower than this, but cannot be quantified from this dataset due to possible backscatter originating from pneumatophores of physonect siphonophores that are known to occur in the area. Further studies on the DSL of the CAO should include sampling and identification of the backscattering organisms. From our study we can conclude that if the central Arctic DSL contains fish, their biomass is currently too low for any sustainable fishery.

Highlights

The “Central Arctic Ocean” (CAO) is defined as the permanently icecovered deep basins and ridges outside the continental shelves of the Arctic Ocean (Fig. 1; PAME, 2013)
It is evident that physonect siphonophores inhabit waters that overlap with the deep scattering layer (DSL) we discovered in the CAO, and that presently it cannot be excluded that these siphonophores could contribute with acoustic backscatter of similar target strength (TS) as small mesopelagic fish
Despite great reduction of the sea-ice cover in the CAO, the area is still inaccessible for fishing vessels, preventing both commercial har vesting of fish and standard data collection for assessment of its fish stocks

Summary

Introduction

The “Central Arctic Ocean” (CAO) is defined as the permanently icecovered deep basins and ridges outside the continental shelves of the Arctic Ocean (Fig. 1; PAME, 2013). For the CAO Large Marine Ecosystem (LME), there is an almost complete absence of knowledge of the occurrence of pelagic fish as a key ecological link between zooplankton and seals, whales and polar bears (Van Pelt et al, 2017). The reason for the absence of data from the CAO LME is related to the difficulty of accessing this remote cold area for on-site research due to its 2–3 m thick year-round sea-ice cover. Traditional trawling for assessing fish stocks is impossible under the pack ice. The area is only accessible by icebreakers, and acoustic data targeting the water column cannot be used if acquired while ice-breaking due to the high noise level. The fish stocks of the (partly) seasonally ice-covered Arctic shelf seas are relatively unknown as well, apart from those of the well-investigated, monitored and managed Barents Sea (Hansen et al, 2019; ICES, 2019a,b)

Methods

Results

Discussion

Conclusion