In addition to a reduction in the rate of protein synthesis, it appears that some species of hypoxia-tolerant fish conserve energy by employing Hochachka's ion channel arrest hypothesis.  This hypothesis makes two predictions:
# Hypoxia-tolerant animals naturally have low membrane permeabilities
# Membrane permeability decreases even more during hypoxic conditions (ion channel arrest)
The first prediction holds true.  When membrane permeability to Na+ and K+ ions was compared between reptiles and mammals, reptile membranes were discovered to be five times less leaky.  The second prediction has been more difficult to prove experimentally, however, indirect measures have showed a decrease in Na+/K+-ATPase activity in eel and trout hepatocytes during hypoxic conditions.  Results seem to be tissue-specific, as crucian carp exposed to hypoxia do not undergo a reduction in Na+/K+ ATPase activity in their brain.  Although evidence is limited, ion channel arrest enables organisms to maintain ion channel concentration gradients and membrane potentials without consuming large amounts of ATP.