Acoustic visualization of large scale macroplankton and micronekton distributions across the Norwegian shelf and slope of the Norwegian Sea.

Abstract

We present results from acoustical (38 kHz split beam) surveys, biological sampling (trawling, zooplankton nets), and measurements of physical parameters (salinity, temperature, currents) across and along the shelf off Norway (62-70°N). Major recurrent structures were apparent both geographically and with time. Off the shelf, two deep scattering layers prevailed; one of 50-100 m thickness where the upper border by day fluctuated between 100 and 200 m depth, and one located deeper between 300-500 m. The upper layer was mainly composed of mesopelagic fish (Maurolicus muelleri) and krill (Meganyctiphanes norvegica), while the lower layer consisted of krill (Meganyctiphanes norvegica), mesopelagic fish (Maurolicus muelleri, Benthosema glaciale), shrimps (Sergestes arcticus, Pasiphaea multidenta), and jellyfish (Periphylla periphylla). During winter, these two layers roughly comprise 95% of the backscattering volume (biomass) in the upper 500 m. The shallow layer partly intrudes onto the continental shelf, where the bottom topography exerts strong impact on its distribution. In June/July an additional scattering layer was apparent in the upper 20-30 m throughout most of the study area, though integrated backscattering biomass varied by a factor of 50. In the south the layer was associated with water masses of salinity <35 (i.e. with coastal characteristics). Further north the layer was found off the shelf in water with stronger oceanic characteristics as well. Hydrographic features indicated that coastal water and biomass was transported off the shelf in connection with gyres over the banks. Trawl catches showed that this structure was composed of 0- group herring, fish (mainly seith), and krill. Backscattering volume was positively correlated with abundance of 0-group herring caught in trawl, but was not correlated with the meso-zooplankton biomass (mainly Calanus finmarchicus), or other components of the trawl catches. The lack of positive correlations between acoustic backscattering volume, and biomass from net and trawl samples probably reflected differences in selectivity of the sampling methods.