Mesopelagic sound scattering layers and possible explanations for their diel variations in Bjørnafjorden, western Norway

Abstract

This thesis investigates the diel variations in mesopelagic acoustic scattering layers in Bjørnafjorden (60° 5' N 5° 23'E), western Norway. The main objective was to map these scattering layers and investigate possible explanations for their diel variations. The data on which this investigation is based was collected on a cruise directed by the Institute of Marine Research (IMR) (Knutsen, 2015: HI tokt 2015117). The cruise was conducted in Bjørnafjorden from the 15th to the 22nd of November 2015, with the research wessel G.O Sars. The data material consists of acoustic data, light measurements and light estimates, hydrographic measurements, vertical distribution of zooplankton biomass and stomach analysis and length measurements of the two mesopelagic fish species Benthosema glaciale (Northern lantern fish) and Maurolicus muelleri (Müellers pearlside). Acoustic data revealed two distinct backscattering layers, exhibiting different diel migration patterns. The shallowest layer (SSL1) had a mean daytime distribution at ~ 100 m, while the deeper layer (SSL2) had a mean daytime distribution at ~ 170 m. The SSL1 migrated towards the sea surface at dusk, while the SSL2 migrated to deeper waters. Shortly after the individuals of the SSL1 reached the surface at dusk, they descended to ~ 50 m and stayed there until dawn. At dawn the SSL1 undertook another migration to the surface before returning to their daytime depth. This can be described as normal diel vertical migration (DVM) with a concurrent midnight sinking. The SSL2 stayed in the deeper waters (~230-300 m) all night before ascending to their daytime distribution, exhibiting what is known as inverse diel vertical migration (IDVM). Measurements and estimates of light revealed that both the SSLs followed a preferred light comfort zone (LCZ) during daytime. Zooplankton biomass distribution showed that the zooplankton was mainly distributed below the depth of 150 m, overlapping the SSL2. This distribution of zooplankton might explain why the SSL2 did not migrate to the surface, but stayed at depth both day and night. Several hypotheses regarding the night time distributions of the SSLs were investigated, but no conclusions were made.