ABSTRACTVertical distribution and trophic interactions of zooplankton and fish were studiedat a 150 m deep station in Bunnefjorden – the innermost part of the Oslofjord -during winter 2005/2006. Focus was on the krill Meganyctiphanes norvegica, thesmall clupeid fish sprat (Sprattus sprattus) and gadoids, mainly whiting(Merlangius merlangus). Bunnefjorden is characterized by hypoxy in the bottomwater, and the study was part of more long-lasting investigations addressing howhypoxy may influence the vertical distribution and the trophic interactions amongthe pelagic fauna in the fjord.An upward looking EK60 120 kHz echo sounder deployed on the sea bed wasused for collection of acoustic data of krill and fish. Krill and fish were sampled bytrawling both day and night, to verify acoustic recordings, to establish sizedistributions and to examine feeding behavior of krill and fish. A CTD equippedwith water bottles was used to obtain hydrographical data and water for oxygenand chlorophyll a measurements. Mesozooplankton was sampled using a WP2net. Measurements of pigment content of stomach and hindgut of krill wereexamined to quantify the herbivore diet, while the carnivore diet was assessedmicroscopically by quantifying copepod mandible in the krill guts. Fish stomachswere dissected out, and analyzed for prey.During daytime, acoustic scattering layers of the krill, Meganyctiphanesnorvegica, and sprat, Sprattus sprattus were restricted to waters below 75 m and130 m respectively. Krill and sprat ascended at dusk, a few migrated all the wayto the surface, however upward migration was mainly arrested at 20-30 m for krilland 40-60 m for sprat. Sprat and krill were scattered throughout the watercolumn at night. Haddock (Merlanogrammus aeglefinus) and whiting, bothforaging on krill, were found in the upper and middle part of krill scattering layerduring the day and in upper water layers at night. Whiting were also found to2predate on sprat. Although found in much less quantity in whiting stomachs thankrill, one sprat do have a greater energy value than one krill.The apparent avoidance of the surface layer by krill and sprat, even at night,could be a response to predators. It could not be explained by temperature,salinity, oxygen or chlorophyll a concentrations. Temperature and salinity belowsill depth were fairly homogenous by depth, and did not have any explanatorypower for the vertical distribution of krill and fish during the day, or the nightdistributions in deeper waters. Sprat and krill are fairly tolerable to low oxygenconcentrations and did not seem to affect their distribution in this study, whereoxygen levels never got below 1 ml O2 l-1. However, it has been proposed thatsprat use the inner Oslofjord as a refuge for their gadoid predators during winter.In the case of this study, oxygen levels appeared to be just at the limit forgadoids to be able to exploit sprat in deep water. This could add to theantipredatory benefit of darkness at depth.Gut content of krill was higher during the night, yet both ambient chl a and gutpigment levels were low. Feeding on algae by krill increased in the upper layersat night, while krill appeared to forage on copepods both day and night andthroughout the water column. Feeding on phytoplankton and copepods makeskrill less transparent and more vulnerable to visual predators, suggesting anadvantage of feeding in the dark. Sprat foraged on copepods in the upper waterlayers during the night and in mid-water during the day. Sprat is a visual feeder,itself having visual predators, and this might be the reason why sprat migrates tothe upper layers at night to feed and in the middle layer during the day wherelight intensity may be sufficient to detect its prey while at the same time beingsufficiently low to give shelter towards own predators. There appears to be atrade-off between food intake and predation risks for both krill and sprat.Smaller sized sprat did feed significantly more than larger sprat. Smaller spratmigrated to the surface waters at night, while larger appeared to stay in deeper3waters, suggesting that small sprat need more energy to survive and maybe takehigher risks to reach maturity faster and hence prioritized feeding during winter.Krill and sprat feeding was related to prey abundance, and possibly also preysize and movements, which will affect both detectability and prey avoidancereactions.