I read a very detailed approach by the Japs on how to care for sushi grade fish. The key was to get it to the boat fast, then about 5-10cm behind the pec fin make a cut on each side. You will strike the liver or kidney I think and it will bleed like stink. Also make a cut near the tail. You want to do this while the fish's heart is still pounding. You can also cut the gills. Once blead, the quality of the meat will be much improved. Also gut and gill and pack the cavity with ice and keep out of the sun. I also heard you should rotate (flip him over) the fish every 15 min.
Here is the definitive link on the subject:
http://www.spc.org.nc/coastfish/Fish...%20a%20tuna%22
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