The fish that dies if it stops moving is the Billfish and other fish that use a breathing method called ram ventilation.
Ram ventilation in fish is a breathing method that some fish use where they must swim forward continuously with their mouths open to force oxygen rich water over their gills.
Because the fish lack the muscles to pump water on their own, stopping movement means that the fish will suffocate pretty quickly and die.
Billfish, species like the Swordfish and Marlin fish require constant movement to push water over their gills.
Mackerel like tuna require constant forward motion to breathe or they will also die.
Certain sharks must also move to breathe, although not all sharks have to stay moving to stay alive and breathe.
But certain sharks like pelagic species, which include the Hammerhead shark, Mako shark, Whale shark and Great white shark also rely on ram ventilation to breathe and must stay moving to survive or they will die.
And tuna fish, like Atlantic bluefin, yellowfin and skipjack tuna fish also must keep moving or they die if they stop moving.
Tuna fish like the Atlantic bluefin, yellowfin and skipjack tuna fish will suffocate in just a matter of minutes if they stop swimming and stop moving.
Ram ventilation in fish is a passive respiratory mechanism where fish swim continuously with their mouths open, to force oxygen rich water over the fish's gills.
By relying on forward momentum, instead of muscular gill pumping, the fast swimming fish species like tuna and certain sharks save a significant amount of energy and they will suffocate if they are forced to stop moving.
Most fish use active pumping also known as Buccal Pumping where must fish draw water into their mouths actively and push the water over their gills using throat and gill cover or operculum muscles.
In ram ventilation, the fish opens it's mouth and swims forward and the forward motion physically forces the water through the fish's mouth, across their gills and out the gill slits.
Fish use the ram ventilation for high oxygen demand and energy efficiency.
Fast, active pelagic fish require massive amounts of oxygen, which ram ventilation provides the fish with efficiently.
And shifting the muscular work of breathing to the swimming muscles also reduces the energetic cost of respiration by around 10 percent, when compared to active pumping.