Behrens, J.W. 2007. PhD thesis. Marine Biological Laboratory, <st1:place w:st="on"><st1:PlaceType w:st="on">University</st1:PlaceType> of <st1:PlaceName w:st="on">Copenhagen</st1:PlaceName></st1:place> and Department of Marine Ecology, NERI. National Environmental Research Institute, <st1:country-region w:st="on"><st1:place w:st="on">Denmark</st1:place></st1:country-region> . 46 pp.
Summary
Seasonal hypoxia has since the beginning of the 1980s been observed on an almost yearly basis in inner Danish waters. Oxygen is a key element in the metabolic processes of fish and the ecological responses of sublethal hypoxia can be profound. This thesis has its main focus on how short-term and prolonged hypoxia impacts the physiology and behaviour of lesser sandeel (Ammodytes tobianus), one of five species of sandeels inhabiting the inner Danish waters. These small fish constitute an important link in the food web being key prey items for many larger fish, marine mammals and sea birds. Sandeels school in the open water during daytime while feeding but bury into the sediment at night, when frightened and during winter. The burying behavior is thus of importance for the fitness of the fish, but also makes them exposed to bottom water conditions where oxygen conditions are often less favorable. Estimates of basic respiratory parameters revealed that lesser sandeel have low minimum oxygen requirements but an ordinary hypoxia tolerance. Fish buried in anoxic sediment obtained oxygen by actively advecting oxygen-rich water from above the sediment surface and over their gills. When exposed to acute hypoxia sandeels remained buried, thus employing an energy saving strategy. Swimming fish, on the contrary, exhibited neither reduced swimming speed nor showed a fleeing response when exposed to acute hypoxia. Prolonged moderate and severe hypoxia had major influence on the diurnal activity pattern of the fish, where moderate hypoxia affected emergence and burying rates while severe hypoxia prompted an increased number of fish to stay in the water column during daytime. Finally, modeling of monitoring data revealed that within recent years up to one fourth of the sandy seabed areas which sandeels potentially use to bury in, have been exposed to oxygen levels critical for the fish. Future global warning may exacerbate the situation considerably, leading to further habitat loss due to unfavorable oxygen conditions. In conclusion, hypoxia may have negative impact on sandeel populations of 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.
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