I’ve allways noticed a texture/color difference in some of the mahi, mainly with ones of different size, the larger ones seem to have more of a white flesh like your mentioning. I don’t thnk it has anything to do with icing em down, but more of age of fish. I allways restack our fish box to ensure ice is on all sides of fish and add some salt water to make a bit of a slurry in box and still notice the meat difference in some, no taste diff tho.
Have had that problem with the dolphin more than other fish. Have been playing with diffrent meat box tech. lately and determined it is in the care of the fish. Plenty of ice and no slush seems to work best. I like to ice real good a few time during the trip or even re-stack the fish one the tail shakes are done. Drain the box and apply fresh ice. Seems the more slush you have in the box the worse the meat is, or from what I can tell. Up until recently I have never liked dolphin cause it was white and chewy(and it’s still down at the end of my list, right there with fresh water ccat-fish).
I haven’t tried ike jime with dolphin cuz’ I never get to the blue water anymore. I have done it with reds and it seems to work well. http://en.wikipedia.org/wiki/Ike_jime. The Japs know a lot about fish!