John Fleng Steffensen.

1st International FitFish Workshop on the Swimming Physiology of Fish, Barcelona 2010.

Oxygen consumption of swimming fish is well known to increase as a power function or exponentially as swimming speed increase. When a fi sh swim through the water they consume oxygen and hence less oxygen will be available behind the fish. Similarly oxygen concentration will decrease from front to rear in a school of swimming fish. McFarland and Moss (1967) showed that the oxygen saturation decreased about 30 % from front to rear in an approximately
150 m long school of swimming mullets.
Based on measurements of oxygen consumption in Atlantic herring (C/llpea harengus) swimming at a variety of forced speeds in a swimming respirometer, a model describing the decline in oxygen conditions through a fish school was constructed. With the model the effects of swimming speed, environmental hypoxia and nearest neighbour distance, etc. on oxygen conditions through a fi sh school as well as the maximum length of the school can be predicted.
Do we have any data that confinn the model? Recently we did some observations in a sea cage containing approximately 50.000 1.5 kg rainbow trout (Oncorhynchus mykiss). 3 individuals were instrumented with micro oxygen electrodes (Oxyguard, Denmark) and acoustic transmitters. For more than two months oxygen conditions very close t.o the dorsal side of the fish was collected by Vemco VR2 data loggers. Simultaneously oxygen and temperature was measured with a logging YSI CTD from a fixed position on the net cage at a depth of one meter. The data did not show lower oxygen levels in the vicinity oflhe fish compared to a position on the edge of the pen.
In the near future we hope to be able to verify the model by measuring oxygen levels in large schools of Atlantic herring with a ROV instrumented with cameras and a logging YSJ CTD as well as an acoustic Oxyguard oxygen transmitter.