Vertical distribution and trophic interactions of krill, sprat and gadoids in the inner Oslofjord during winter

ABSTRACT Vertical distribution and trophic interactions of zooplankton and fish were studied at a 150 m deep station in Bunnefjorden – the innermost part of the Oslofjord - during winter 2005/2006. Focus was on the krill Meganyctiphanes norvegica, the small clupeid fish sprat (Sprattus sprattus) and gadoids, mainly whiting (Merlangius merlangus). Bunnefjorden is characterized by hypoxy in the bottom water, and the study was part of more long-lasting investigations addressing how hypoxy may influence the vertical distribution and the trophic interactions among the pelagic fauna in the fjord. An upward looking EK60 120 kHz echo sounder deployed on the sea bed was used for collection of acoustic data of krill and fish. Krill and fish were sampled by trawling both day and night, to verify acoustic recordings, to establish size distributions and to examine feeding behavior of krill and fish. A CTD equipped with water bottles was used to obtain hydrographical data and water for oxygen and chlorophyll a measurements. Mesozooplankton was sampled using a WP2 net. Measurements of pigment content of stomach and hindgut of krill were examined to quantify the herbivore diet, while the carnivore diet was assessed microscopically by quantifying copepod mandible in the krill guts. Fish stomachs were dissected out, and analyzed for prey. During daytime, acoustic scattering layers of the krill, Meganyctiphanes norvegica, and sprat, Sprattus sprattus were restricted to waters below 75 m and 130 m respectively. Krill and sprat ascended at dusk, a few migrated all the way to the surface, however upward migration was mainly arrested at 20-30 m for krill and 40-60 m for sprat. Sprat and krill were scattered throughout the water column at night. Haddock (Merlanogrammus aeglefinus) and whiting, both foraging on krill, were found in the upper and middle part of krill scattering layer during the day and in upper water layers at night. Whiting were also found to 2 predate on sprat. Although found in much less quantity in whiting stomachs than krill, 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 below sill depth were fairly homogenous by depth, and did not have any explanatory power for the vertical distribution of krill and fish during the day, or the night distributions in deeper waters. Sprat and krill are fairly tolerable to low oxygen concentrations and did not seem to affect their distribution in this study, where oxygen levels never got below 1 ml O2 l-1. However, it has been proposed that sprat 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 for gadoids to be able to exploit sprat in deep water. This could add to the antipredatory benefit of darkness at depth. Gut content of krill was higher during the night, yet both ambient chl a and gut pigment levels were low. Feeding on algae by krill increased in the upper layers at night, while krill appeared to forage on copepods both day and night and throughout the water column. Feeding on phytoplankton and copepods makes krill less transparent and more vulnerable to visual predators, suggesting an advantage of feeding in the dark. Sprat foraged on copepods in the upper water layers 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 to the upper layers at night to feed and in the middle layer during the day where light intensity may be sufficient to detect its prey while at the same time being sufficiently low to give shelter towards own predators. There appears to be a trade-off between food intake and predation risks for both krill and sprat. Smaller sized sprat did feed significantly more than larger sprat. Smaller sprat migrated to the surface waters at night, while larger appeared to stay in deeper 3 waters, suggesting that small sprat need more energy to survive and maybe take higher risks to reach maturity faster and hence prioritized feeding during winter. Krill and sprat feeding was related to prey abundance, and possibly also prey size and movements, which will affect both detectability and prey avoidance reactions