This study was conducted in September–October 2008 in the deepest basin of Masfjorden (~60 ★2′N, ~5 ☂4′E), Norway, using R/V “Trygve Braarud” (for a map and further description of the locality, see Kaartvedt et al. A group performing IDVM was also recorded this year, and these fish are the main focus of this study. We here address this hypothesis by examining the vertical distribution of plankton and fish in combination with gut content analyses during the day and night the year subsequent to the study of Kaartvedt et al. ( 2009) hypothesized that glacier lanternfish with IDVM ascend to forage on overwintering Calanus finmarchicus in the better light conditions in the middle layers of the water column during the daytime. 2009), although feeding efficiency generally increases with available light. Mesopelagic fish with dark-adapted eyes may spot their plankton prey even at several hundred meters depth in daylight (Warrant and Locket 2004 Turner et al. thus overlaps with the observed IDVM group of glacier lanternfish. population is distributed in the depth intervals 0–50 and 150–250 m (Bagøien et al. During the autumn, the majority of the Calanus sp. In Norwegian fjords glacier lanternfish exert a strong predation pressure on overwintering Calanus finmarchicus (Bagøien et al. 2001), although other plankton are frequently observed in the stomach contents (Gjøsæter 1973b Roe and Badcock 1984 Sameoto 1988, 1989). Glacier lanternfish feed primarily on calanoid copepods, especially Calanus (Sameoto 1988, 1989 Balino and Aksnes 1993 Bagøien et al. One part of the population exhibited NDVM, one part IDVM and one part did not migrate (NoDVM) (Kaartvedt et al. ( 2009) observed three different modes of diel behavior in the population of glacier lanternfish. By means of a bottom-mounted echo sounder at ~400 m in Masfjorden, Kaartvedt et al. In Masfjorden, Norway, glacier lanternfish are mostly distributed below 200 m during the daytime, while the population is spread throughout the water column during the night (Kaartvedt et al. Glacier lanternfish ( Benthosema glaciale) is the most abundant species of myctophids (myctophidae) in the Atlantic Ocean north of 35°N and is together with pearlside (the Sternoptychidae Maurolicus muelleri) the dominating mesopelagic fish in fjords along the coast of Norway (Aksnes et al. 2011), and has only recently been documented in mesopelagic fishes (Kaartvedt et al. It has been described for fish (Neilson and Perry 1990), although rarely (Jensen et al. IDVM has commonly been ascribed to zooplankton species avoiding NDVM predators (Ohman et al. This behavior is characterized by organisms moving to shallower waters during the daytime and descending towards deeper waters during the night (Pearre 2003). Another, less common type of DVM is inverse DVM (IDVM). Under the normal DVM pattern (NDVM), the organisms forage on abundant plankton in upper waters at night and hide from visual predators at depth during the day (Pearre 2003 Kahilainen et al. Echo sounders have particularly been used in studies of diel vertical migration (DVM) of mesopelagic scattering layers, of which myctophids are a prevailing part (Valinassab et al. Mesopelagic fish form acoustic backscattering layers, and their behavior can thus be studied using echo sounders (Holton 1969 Godø et al. 2009) and marine mammals (Doksæter et al. 1990 Walker and Nichols 1993), sea birds (Hedd et al. 2009), and as prey for fish (Hansen and Pethon 1985 Giske et al. 2010) as predators on zooplankton (Gjøsæter 1973b Moku et al. They are important in marine food webs worldwide (Tyler and Pearcy 1975 Shreeve et al. Myctophidae is the most abundant family of mesopelagic fish (Moser and Ahlstrom 1974 Valinassab et al.
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