A preliminary study of zooplankton sound scattering layers in Balsfjorden: Structure, energetics, and migrations

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Abstract
An investigation of zooplankton generated Sound Scattering Layers (SSLs) using both echosounders and net sampling methods, was started in Balsfjorden, a Norwegian fjord situated within the Arctic circle, in April 1977. The results of some of the preliminary work over a 24 h period from 5–6 April are presented. Samples were taken in and out of the SSL with a Beyer's Low Speed Midwater Trawl, and demonstrated that the SSL was mainly due to concentrations of euphausiids, chaetognaths, and calanoid copepods. The results are also presented of analyses of dry weight, lipid, protein, carbon, nitrogen, ash, and caloric value showing that these factors were up to 30 times more concentrated in the SSL than outside it. The species composition and abundance of zooplankters in the SSL changed diurnally indicating the dynamic nature of the assemblage. Acoustic profiles taken with a 120 kHz precision echosounder, showed the SSL to have a text-book pattern of diurnal migration, apparently related to changes in incident and sub-surface radiation. Migration speeds of the SSL ranged from 8.1–24.2 m per h. Hydrography and phytoplankton are examined as possible factors influencing migration. The implications of zooplankton SSLs in fjords are considered in terms of ecological energetics as well as resource biology.