Cold Shock would be the cause of death in your experiment. Cold shock for a trout is a change of -7 degrees or more in Less than a hour. Cold Shock is what is responsible for most winter fish kills. Generally the fish would have to be trapped in swallow water during a very strong, very cold winter front. Mostly you see redfish die this way due to getting caught in shallow marshes as the tide quickly falls. Trout rarely die this way in Big Lake due to the proximity of deeper water for them to move to as the temperature quickly drops in the shallow waters. The deeper water will drop in temperature also but at a slower rate thus giving the trout time to acclimate to the lower temperature. So in conclusion, yes the trout you drop in a cooler with 40 degree water in it will die but Cold Shock caused its death.
Here is some reading on Cold Shock
http://8811166979549632204-a-1802744...attredirects=0