http://www.daybreakfishing.com/fishw...efin%20weights
Take a look at the tuna length/weight chart. Your fish would have had to be 60 pounds or over to be legal. Given the picture, and the mates estimate on the fish's weight, your fish was short and should not have been kept.
On a positive note, congratulations on catching a tuna! That's something to be proud of! I just started fishing for them this year and it is an addiction. It's just unfortunate that your captain gaffed and kept a sublegal fish. Bluefin tuna are endangered and breaking the regs is simply unacceptable.