The best identifier is huge size, since small Bluefins have always been almost non-existent in Florida and the tropics. The massive body is dark blue above, shading to silvery on the lower sides. All fins and finlets are steely blue.
SIZE: In the past, runs in the Bahamas (and, unpredictably, in Florida waters) consisted of fish weighing from about 200 to 600 pounds. Presently, most run over 500 pounds and many push 900. The increase in average size is indicative of the declining stock. Giant Bluefins are in trouble worldwide. World record 1,496 pounds. Bluefin are the largest tuna and can live up to 40 years. They migrate across oceans and can dive more than 4,000 feet. Bluefin tuna are made for speed: built like torpedoes, have retractable fins and their eyes are set flush to their body. They are tremendous predators from the moment they hatch, seeking out schools of fish like herring, mackerel and even eels. They hunt by sight and have the sharpest vision of any bony fish. There are three species of bluefin: Atlantic (the largest and most endangered), Pacific, and Southern. Most catches of the Atlantic bluefin tuna are taken from the Mediterranean Sea, which is the most important bluefin tuna fishery in the world. The bluefin tuna has a large, torpedo-shaped body that is nearly circular in cross-section. They are the largest of the tuna species and can reach up to 13 feet and 2,000 pounds. Bluefin tuna are dark blue-black on the back and white on the lower sides and belly. Live bluefin have colorless lines alternating with rows of colorless spots on their lower sides. The second fin on their back (dorsal fin) is reddish brown, and they have short pectoral fins. These characteristics separate this species from other members of the tuna genus, Thunnus. Dorsal spines (total): 12 - 14; Dorsal soft rays (total): 13-15; Anal spines: 0; Anal soft rays: 13 - 16; Vertebrae: 39. A very large species, deepest near the middle of the first dorsal fin base. The second dorsal fin higher than the first; the pectoral fins are very short, less than 80% of head length. Swim bladder present. Lower sides and belly silvery white with colorless transverse lines alternated with rows of colorless dots. The first dorsal fin is yellow or bluish; the second reddish-brown; the anal fin and finlets dusky yellow and edged with black; the median caudal keel is black in adults. May be confused with several other tunas, these are typically much smaller and easily distinguished by specific patterns of stripes, bands or dots.