Acoustic micronektonic distribution and density is structured by macroscale oceanographic processes across 17-48° N latitudes in the North Atlantic Ocean.

This study investigates the large-scale distribution patterns of the acoustic scattering layers and micronekton density across the Northeast Atlantic Ocean during daylight hours. A research cruise on board R/V "Kronprins Haakon" was conducted during May 2019 from Cape Verde to Bay of Biscay. Hydrological data were obtained at 20 conductivity-temperature-depth sensor (CTD) stations. To estimate the micronekton densities in front of the trawl, an autonomous echo sounder (120 or 200 kHz) on the headrope of the macroplankton trawl was used. Acoustic data were also collected along the cruise track using ship-mounted multi-frequency echo sounders (backscatter at 18 and 38 kHz was analyzed). Acoustic observations (both at 18, 38 and 120/200 kHz) showed clear patterns in the horizontal distribution of the micronekton during daytime with higher backscatter and echo densities in the south of the study area (from 17 to 37° N), and the absence of high backscatter in the surface from 37 to 45° N. Backscatter and echo densities were found to be significantly influenced by: temperature, salinity, and oxygen, as well as depth and time of the day.

Mesopelagic fish gas bladder elongation, as estimated from wideband acoustic backscattering measurements.

Backscattered acoustic energy from a target varies with frequency and carries information about its material properties, size, shape, and orientation. Gas-bearing organisms are strong reflectors of acoustic energy at the commonly used frequencies (∼18-450 kHz) in fishery surveys, but lack of knowledge of their acoustic properties creates large uncertainties in mesopelagic biomass estimates. Improved knowledge about the volume and elongation (i.e., longest to shortest dimension) of swimbladders of mesopelagic fishes has been identified as an important factor to reduce the overall uncertainties in acoustic survey estimates of mesopelagic biomass. In this paper, a finite element approach was used to model gas-filled objects, revealing the structure of the backscattering, also at frequencies well above the main resonance frequency. Similar scattering features were observed in measured broadband backscattering of several individual mesopelagic organisms. A method is suggested for estimating the elongation of a gas-bubble using these features. The method is applied to the in situ measured wideband (33-380 kHz) target strength (TS) of single mesopelagic gas-bearing organisms from two stations in the North Atlantic (NA) and Norwegian Sea (NS). For the selected targets, the method suggested that the average elongation of gas-bladder at the NA and NS stations are 1.49 ± 0.52 and 2.86 ± 0.50, respectively.

Estimating target strength and physical characteristics of gas-bearing mesopelagic fish from wideband in situ echoes using a viscous-elastic scattering model.

Wideband (38 and 50-260 kHz) target strength of organisms were measured in situ using a towed platform in mesopelagic (200-1000 m depth) layers. Organisms with a gas-inclusion are strong scatterers of sound and acoustically distinct from organisms lacking one. In the mesopelagic zone, some of the fish species and physonect siphonophores have a gas-inclusion. Trawl and multinet biological sampling as well as photographic evidence indicate that in the study area (eastern mid-Atlantic Ocean) the majority of the gas-bearing organisms were fish. Subsequently, using a two-layer viscous-elastic spherical gas backscattering model, physical characteristics such as gas-bladder features and body flesh properties were deduced from the measured backscattering signal of individual gas-bearing fish. Acoustic techniques are non-extractive, can be used for the monitoring and quantification of marine organisms in a time- and cost-effective manner, and suit studies of the mesopelagic zone, which is logistically challenging. Vessel-mounted acoustics, widely used for epipelagic studies, has limitations for mesopelagic studies as the deep organisms are inaccessible to high-frequency (≳100 kHz) acoustic pulses transmitted from the surface due to absorption. Therefore, a towed platform equipped with wideband acoustics has several features that can be utilized for monitoring the mesopelagic dense scattering layers containing mixed species.

Genetics redraws pelagic biogeography of Calanus.

Planktonic copepods of the genus Calanus play a central role in North Atlantic/Arctic marine food webs. Here, using molecular markers, we redrew the distributional ranges of Calanus species inhabiting the North Atlantic and Arctic Oceans and revealed much wider and more broadly overlapping distributions than previously described. The Arctic shelf species, C. glacialis, dominated the zooplankton assemblage of many Norwegian fjords, where only C. finmarchicus has been reported previously. In these fjords, high occurrences of the Arctic species C. hyperboreus were also found. Molecular markers revealed that the most common method of species identification, prosome length, cannot reliably discriminate the species in Norwegian fjords. Differences in degree of genetic differentiation among fjord populations of the two species suggested that C. glacialis is a more permanent resident of the fjords than C. finmarchicus We found no evidence of hybridization between the species. Our results indicate a critical need for the wider use of molecular markers to reliably identify and discriminate these morphologically similar copepod species, which serve as important indicators of climate responses.