http://www.nmfs.noaa.gov/sfa/hms/Com...Guide_Tuna.pdf
Bluefin tuna
No person shall retain, or possess a bluefin tuna with the head removed that is less than 20 inches (51 cm) pectoral fin curved length.
Bigeye and yellowfin tuna
No person may remove the head of a bigeye tuna or yellowfin tuna if the remaining portion would be less than 27 inches (69 cm) from the fork of the tail to the forward edge of the cut (see image below).
Posted from my iPhone/Mobile device