To fillet a tuna into loins it is best to partially thaw the fish. Ice crystals remaining in the cells keep the fish from becoming mushy.
How to filet a tuna
Cut straight down to the horizontal bones, following cut lines in illustration #1.
Pry the loin away from the bones carefully and trim off the loin along the dorsal fin edge.
Then cut the bottom or ventral loin.
Turn the fish over and repeat.