Department of Marine Ecology and COGCI Ph.D. School, Marine Biological Laboratory, University of Copenhagen, 1165 København, DENMARK.
Inner Danish waters have experienced seasonal oxygen depletion on an almost annual basis since the early 1980s. Oxygen is a key element in the metabolic processes of fish and the ecological effects of sub-lethal hypoxia can be profound. Sandeels are important in the food web, constituting a key prey item for many larger fish, marine mammals and sea birds. While sandeels school in the open water during daytime to feed, they bury into the sediment at night and during winter conditions. Thus, they are affected by pelagic and benthic conditions alike. The present study investigated basic metabolic and respiratory properties of lesser sandeel (A. tobianus), in particular how fish obtain oxygen when buried in anoxic sediment, and the effects of hypoxia (acute and chronic) on swimming speed and the fish’s diurnal activity pattern. Based on the experimentally determined critical oxygen tension (Pcrit) for sandeel, the coupling between low oxygen and habitat loss was predicted, both under present climate condition and expected future global warming. The results show that hypoxia indeed affects physiological and behavioral aspects of sandeel, though responses varied according to the rate and duration of hypoxic exposure. Sandeels have low minimum oxygen requirements but a hypoxia tolerance similar to other fish. While buried, the fish obtain oxygen by actively advecting water from above the sediment surface and across the gills. When exposed to acute hypoxia the fish remain buried, thus employing an energy saving strategy. Swimming fish on the contrary, exhibited neither reduced swimming speed nor showed an escape response when exposed to acute hypoxia. Prolonged moderate and severe hypoxia had a major influence on the diurnal activity pattern of the fish, affecting emergence and burying rates and the number of fish entering the water column during the day. Finally, up to one fourth of the suitable sandy seabed in which sandeels bury has been exposed to critical oxygen levels within recent years. Future global warning will exacerbate the situation considerably, leading to further possible habitat loss due to unfavorable oxygen conditions. In conclusion, hypoxia will likely exert negative effects on sandeels in the inner Danish waters. Further knowledge on any implications, whether direct or indirect, of climate change on sandeels population dynamics is of utmost importance, since a complete stock collapse is likely to have broad and severe effects on the entire ecosystem.