When anaerobic pathways are turned on, glycogen stores are depleted and accumulation of acidic waste products occurs.  This is known as a Pasteur effect.  A challenge hypoxia-tolerant fish face is how to produce ATP anaerobically without creating a significant Pasteur effect.  Along with a reduction in metabolism, some fish have adapted traits to avoid accumulation of lactate.  For example, the crucian carp, a highly hypoxia-tolerant fish,  has evolved to survive months of anoxic waters.  A key adaptation is the ability to convert lactate to ethanol in the muscle and excrete it out of their gills.  Although this process is energetically costly is it crucial to their survival in hypoxic waters.