text: As oxygen levels decrease, fish may at first increase movements in an attempt to escape the hypoxic zone, but eventually they greatly reduce their activity levels, thus reducing their energetic (and therefore oxygen) demands. Atlantic herring show this exact pattern. Other examples of fishes that reduce their activity levels under hypoxia include the common sole, the guppy, the small-spotted catshark, and the viviparous eelpout.  Some sharks that ram-ventilate their gills may understandably increase their swimming speeds under hypoxia, to bring more water to the gills.