They are coming up for air
I'm sure the gar fishermen know the entire family (Lepisostidae
) of gars are obligatory air breathers. They have to come up for air or will eventually drown.
An unusual fish, it can breath air and/or water to provide oxygen to its organs but MUST take in air by gulping it on a regular basis to regulate its swim bladders. The combination of gills, dual swim bladders along with powerful circulatory and nervous systems allows the gar to some pretty amazing things and is part of why the group is so successful for so long (pre-date and outlasted the dinosaurs, and will likely outlast us) through several mass extinctions.
But you put one in a box plumb full to the top with water and close the lid, it will die.