title: Vertical distribution and diel vertical migration of krill beneath snow-covered ice and in ice-free waters.
abstract: A bottom mounted upward looking Simrad EK60 120-kHz echo sounder was used to study scattering layers (SLs) and individuals of the krillMeganyctiphanes norvegica. The mooring was situated at 150-m depth in the Oslofjord, connected with an onshore cable for power and transmission of digitized data. Records spanned 5 months from late autumn to spring. A current meter and CTD was associated with the acoustic mooring and a shore-based webcam monitored ice conditions in the fjord. The continuous measurements were supplemented with intermittent krill sampling campaigns and their physical and biological environment. The krill carried out diel vertical migration (DVM) throughout the winter, regardless of the distribution of potential prey. The fjord froze over in mid-winter and the daytime distribution of a mid-water SL of krill immediately became shallower associated with snow fall after freezing, likely related to reduction of light intensities. Still, a fraction of the population always descended all the way to the bottom, so that the krill population by day seemed to inhabit waters with light levels spanning up to six orders of magnitude. Deep-living krill ascended in synchrony with the rest of the population in the afternoon, but individuals consistently reappeared in near-bottom waters already <1 h after the ascent. Thereafter, the krill appeared to undertake asynchronous migrations, with some krill always being present in near-bottom waters even though the entire population appeared to undertake DVM.
authors.affiliation: King Abdullah University of Science and Technology, Red Sea Research Center , Thuwal 23955-6900 , Saudi Arabia.
authors.forename: Hege
authors.initials: H
authors.lastname: Vestheim
authors.affiliation: King Abdullah University of Science and Technology, Red Sea Research Center , Thuwal 23955-6900 , Saudi Arabia ; Department of Biosciences , University of Oslo, Norway , PO Box 1066 Blindern, 0316 Oslo , Norway.
authors.forename: Anders
authors.initials: A
authors.lastname: Røstad
authors.affiliation: King Abdullah University of Science and Technology, Red Sea Research Center , Thuwal 23955-6900 , Saudi Arabia ; Department of Biosciences , University of Oslo, Norway , PO Box 1066 Blindern, 0316 Oslo , Norway.
authors.forename: Thor A
authors.initials: TA
authors.lastname: Klevjer
authors.affiliation: King Abdullah University of Science and Technology, Red Sea Research Center , Thuwal 23955-6900 , Saudi Arabia.
authors.forename: Ingrid
authors.initials: I
authors.lastname: Solberg
authors.affiliation: King Abdullah University of Science and Technology, Red Sea Research Center , Thuwal 23955-6900 , Saudi Arabia ; Department of Biosciences , University of Oslo, Norway , PO Box 1066 Blindern, 0316 Oslo , Norway.
authors.forename: Stein
authors.initials: S
authors.lastname: Kaartvedt
keywords: Meganyctiphanes norvegica; Norway; sea ice; stationary acoustics; synchronous and asynchronous DVM