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  • University of Gothenburg, Sweden

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Employment of Data Storage Tags on individual Atlantic cod (Gadus morhua) in the Bornholm Basin has shown that some fish migrate towards the deeper basin centre, presumably to feed. During these voluntary dives, fish expose themselves to oxygen saturations as low as 10 % and many individuals spend a third of their total time at oxygen saturation <50 %. This behavior may either be a result of an optimum foraging strategy or a necessity due to limited or less accessible prey in the cod´s optimal habitats. 

Feeding induces several cardio-respiratory changes to enable the animal to efficiently digest, absorb, and redistribute the nutrients. Gastrointestinal blood flow (GBF) increases profoundly to facilitate digestion and absorption of the food. However, under circumstances of low oxygen availability, regional blood flow must be altered and in unfed fish there is a redistribution of blood away from the gastrointestinal area. If GBF is curtailed while the fish search for and ingest prey in the hypoxic water, this may implicate impaired digestive capacity following the return to well-oxygenated waters, either by prolonging gastric evacuation time or by reducing the assimilation efficiency. Furthermore, if the fish face the limitations of a reduced metabolic scope the onset of anaerobic energy production will result in an oxygen debt which has to be paid back with subsequent return to more well-oxygenated water. Using state of the art methods (including both ‘hardwired’ and biotelemetry equipped animals) the aim is to investigate whether (1) ingestion of prey and initiation of digestion in hypoxic water results in an oxygen debt which has to be paid back following return to more well-oxygenated water, and whether (2) GBF will be spared or curtailed under the same circumstances, and if the latter, whether this will influence of the digestive processes subsequent return to normal oxygen conditions.

We found that fish after 48hr of post-surgical recovery following surgical implantation of a dummy biotelemery blood flow device had reduced gastric evacuation rate as compared to controls; 24-48 hrs recovery is normal standard allowed for post-surgical recovery for fish when they are ‘hard-wired’. However after ten days of recovery gastric evacuation rate was comparable to that of control fish – this long recovery time is possible only when using biotelemetry systems.

Furthermore, then exposed to oxygen conditions simulating the low levels in the Bornholm Basin (the Baltic Sea) into which the fish voluntarily undertake feeding migrations, we found that GBF is not negatively impacted, as long as the fish subsequently is allowed to return to more well-oxygenated waters. We thus suggest that short feeding migrations into hypoxic bottom waters is a viable strategy under conditions where food is limited in the more well-oxygenated water layers. 

The project was coordinated by DTU Aqua.



  • Research areas: Marine Populations and Ecosystem Dynamics & Fish Biology & Marine Living Resources
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ID: 2290066