The physiological response of the Arctic key species, Boreogadus saida, to environmental hypoxia: critical oxygen level and swimming performance

Sarah.Kempf [ at ]


Since the beginning of the industrialization, uncontrolled greenhouse gas emission led to a distinct temperature increase on earth. Arctic environments are projected to experience the most severe changes due to climate change. Higher atmospheric temperatures caused already various environmental changes, for example a decrease in Arctic sea ice of 49 % (1979-2000) and increasing carbon dioxide concentrations which reduced the sea surface pH. A reduced sea ice formation will strengthen the summer stratification of warm, oxygen poor on top of cold, oxygen rich water masses, which may consequently cause local hypoxia in ground water layers. As a result, the deep cold water layers do not receive oxygen-rich water and oxygen consumption extends over more than one season. This can lead to local hypoxia in the ground water layers of the protected fjords. Especially endangered of this long-lasting stratification in winter are the deep fjord systems of the Svalbard archipelago. In this region, the change of winter temperatures from 1961–90 corresponded to an increase of 0.6 °C per decade. Corresponding, an additional increase of 0.9 °C per decade is projected for 2071–2100. Thus, the present study investigates the hypoxia tolerance of Polar cod, Boreogadus saida, one of the main Arctic key species. Therefore, different performance parameters were determined. The respiratory capacity as well as the swimming performance under declining oxygen concentrations were measured in two different experimental setups. A sample size of 30 Polar cod with similar body length and weight were chosen. All individuals were used several times during the experiments. First, the routine (RMR) and standard metabolic rate (SMR) were determined via flow-through respirometry. The calculated SMR for Polar cod accounted 0.44 μmol O2/g∙h. The RMR followed an oxygen regulating pattern, indicating that aerobic metabolic pathways such as lipid oxidation were used, rather than anaerobic pathways. This implies a relatively small contribution of anaerobic metabolism to the energy production in B. saida. This was confirmed in the swim tunnel experiments. However, Ugait (the speed at which the fish changed to anaerobically fuelled swimming) was not significantly affected by hypoxia, the total number of bursts (p = 0.025) and total active swimming time (p = 0.017) significantly decreased with decreasing oxygen saturation. The loss of anaerobic swimming capacity due to hypoxia may endanger this species in regard to predator-prey-interactions and loss of escape reactions. Under exercise Polar cod was able to up-regulate its maximum metabolic rate (MMR) until a threshold of 45 % PO2 was reached. Afterwards, the oxygen consumption significantly decreased with decreasing oxygen concentrations. Throughout both experiments neither RMR nor MMR decreased below SMR level. Furthermore, the present study revealed that Polar cod is an extremely hypoxia tolerant fish species, which is able to handle oxygen saturations down to a Pcrit of 4.81 % PO2. This outstanding capability could give the otherwise rather disadvantaged fish species an advantage under changing climate conditions.

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Kempf, S. (2020): The physiological response of the Arctic key species, Boreogadus saida, to environmental hypoxia: critical oxygen level and swimming performance , Master thesis, Bremen University.

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