I. Biological Description
Bigeye tuna (Thunnus obesus) is one of two species known in Hawaii simply as ahi. Similar in general appearance to yellowfin tuna (the other species known as ahi), the bigeye may be recognized by its plump body, its larger head and its unusually large eyes.
Adult bigeye tuna are the deepest occurring of all tuna species, with the depth range of greatest concentration at 150 to 250 fathoms. Smaller bigeye (20-30 pounds) may be encountered in shallower waters in the vicinity of seamounts or floating objects, including fish aggregation buoys.
II. Of Special Interest For Buying/Distributing
Availability And Seasonality: The availability of bigeye tuna in Hawaii has increased as a result of an expansion of the domestic longline fleet and an extension of the fleet's fishing range to as far as 800 nautical miles from port.
The peak in Hawaii's landings of bigeye tuna occurs during the winter season (October-April), which is the off-season for harvesting other tuna species.
Fishing Methods: Bigeye tuna is harvested in Hawaii primarily by longline boats which set hooks at the deep swimming depths of this species. Bigeye tuna is a minor component of the catch made by the small-boat handline (ika-shibi) fleet off the island of Hawaii. It is rarely caught by trollers.
Distribution: The longline catch of bigeye tuna is marketed primarily through the Honolulu fish auction. Most of the handline (ika-shibi) catch is sold through the fish auction in Hilo and through the intermediary buyers on the island of Hawaii. Virtually all bigeye is sold fresh. Substitution: Caught in deeper, cooler water, bigeye tuna typically has a higher fat content than yellowfin and is preferred over yellowfin by more discriminating sashimi buyers. For less discriminating raw fish consumers, the two species are interchangeable. They are also interchangeable with other tuna and marlin species for grilling purposes.
III. Of Special Interest For Preparation/Quality Control
Shelf Life And Quality Control: Bigeye tuna has a longer shelf life than yellowfin tuna, and the natural red flesh is slower to discolor after exposure to air. Longline-caught bigeye rarely develop the "burnt" flesh problems often found in yellowfin taken on handline and trolling gear.
Some longline boats which catch bigeye tuna remain at sea for up to 10-12 days, but with proper care, the fish will retain a high quality for over two weeks after capture (see Table 3). Although not as old when landed, the quality of handline-caught bigeye is more variable due to differences in handling by small-boat fishermen.
In bigeye which have been stored too long after harvest, the entrails burst, releasing stomach acids and bile into the gut cavity. The gut cavity is stained greenish-yellow as the acids attack the stomach wall and eventually cause the flesh to deteriorate.
Product Forms And Yields: Bigeye tuna landed in Hawaii range from 20 to over 200 pounds in round weight. The smallest fish are usually caught around fish aggregation buoys and over seamount summits, whereas the largest are usually caught in deep open ocean waters. The larger fish are preferred for their typically higher fat content and greater yields. These fish often enter the sashimi market. Smaller fish of good quality may also be used for sashimi, but there is a growing demand for fresh bigeye in the 20-50 pound size range for grilling in upscale restaurants on the U.S. mainland.
Fresh bigeye may be sold already prepared as sashimi, and it is also marketed as loins, loin sections, or steaks at fish markets or markets with fish counter service or self-service counters. It is usually exported in dressed (headed and gutted) form or as loins. The yield of fillet from a whole ahi varies from 55-65%, depending on fish size (see Table 5).
Quartering Ahi: Remove the ahi's head by sawing beneath the gill plate and through the bone. Next, with the tip of a sharp knife, cut through the thin layers of intermuscular bone laterally connected to the spine. Once these bones have been severed, a clean fillet can be made without tearing the flesh. Fillet cuts should be made as close to the bone as possible.
IV. Of Special Interest To Consumers/Foodservice Personnel
Color, Taste, Texture: Bigeye tuna of good quality has reddish-pinkish flesh color. When exposed to air, bigeye tuna flesh will begin to discolor (although at a rate slower than yellowfin flesh). For this reason, bigeye is usually not loined or filleted until shortly before use.
Larger bigeye typically have a higher fat content than smaller bigeye, but even a fish as small as 25-30 pounds may be rich in fat.
Preparations: Bigeye tuna is one of the preferred species for the preparation for sashimi. With a high fat content, bigeye is also among the most desirable species for grilling. Its mild flavor adapts well to numerous applications.
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