Diel vertical migration and feeding pattern of M. muelleri in Masfjorden in late autumn

The purpose of this study was to investigate diel vertical migration, feeding pattern and diet composition of mesopelagic fish Maurolicus muelleri in Masfjorden in late autumn and compare these with the previous studies. Sound scattering layers (SSLs) were obtained from an acoustic echo sounder device placed at the bottom which may give a better resolution and precision on the SSLs detection. The fish samples were taken by a pelagic midwater trawl equipped with 3 nets that can open and close while in the water at the different depths which may give better accuracy of the fish catches at each depth. The stomach content of pearlsides collected from different SSLs at different times of the day was analysed. M. muelleri comprised two SSLs located at approximately 25 to 200 m. The fish performed dusk and dawn feedings and the fish in the shallow layer more pronounced diel vertical migration than the fish below. Some large fish with lower condition factor also performed diel vertical migration in order to feed at dusk and dawn. The degree of stomach fullness and digestion varied through day and night. There was a size dependent trend in stomach fullness of M. muelleri as small fish had higher degree of stomach fullness than the large fish. Copepods were found to dominate zooplankton community in this study, and also comprised the main prey items of M. muelleri. The diel vertical migration and feeding pattern of the fish could be explained in relation to light regime, fish size, stomach dynamic, zooplankton distribution, environmental parameters and fish condition factor. The findings are in accordance with previous studies on M. muelleri