Marine ecosystem effects of eutrophication: Interactions between small pelagic fish and predators in low oxygen waters

The starting hypothesis for this proposal is that clupeid fish (Sprat) are more tolerant to low oxygen waters than their gadoid predators. Low oxygen waters (albeit above a critical level) thus represent a resource as a predatory refuge. This hypothesis w ill be addressed during winter in a fjord branch in the Oslofjord, characterized by steep vertical oxygen gradients that vary through time related to oxygen consumption and partial water renewals. The distribution and behaviour of both sprat and predators will be studied in situ using acoustic methods that will provide information on size, 3-D swimming patterns and swimming speeds of both sprat and their predators. The overlap between predator and prey will be assessed throughout the diel cycle, and durin g varying oxygen conditions. Sampling, acoustic and experimental physiological studies will be conducted during repetitive field- and experimental campaigns, while an echosounder placed in an upward facing mode on the bottom will provide continuous regist rations throughout winter. In addressing the physiological status of sprat and predators we will ask for the rate of oxygen consumption of sprat swimming at speeds seen in nature, at what level of hypoxia and swimming speed the sprat have to turn on anaer obic metabolism (revealed by lactate build up), what lactate levels forces sprat out of hypoxic water and to what extent the oxygen store in the swim bladder can be used for extending the time that can be spent in hypoxic water. The alternative hypothesis that overwintering habitats are selected based on favourable feeding conditions will also be addressed.