I. Biological Description
Aku (Katsuwonus pelamis) is commonly known as skipjack tuna. Other names for this species include striped tuna, oceanic skipjack and katsuo. This near-surface schooling tuna is widely distributed across the Pacific Ocean.
II. Of Special Interest For Buying/Distributing
Availability and Seasonality: Aku is historically the most important single commercial fish species in terms of landed weight and value in Hawaii, as well as throughout much of the central and western Pacific. Hawaii’s aku fishery, however, is characterized by wide annual and seasonal fluctuations in landings. Aku caught in Hawaii routinely range between 4 and 15 pounds in round weight, but larger fish (16 to 30 pounds in round weight), move into Hawaiian waters during the summer season of increased abundance (April-September).
Fishing Methods: Most of the aku catch in Hawaii is landed by commercial pole-and-line fishermen who induce aku to bite on feathered hooks by chumming with live bait. The pole-and-line catch is sorted according to fish size and is initially stored and sold in tubs head down so that blood drains away from the flesh. Trollers and longline boats land the remainder of the aku catch.
Distribution: Troll-caught aku is marketed through fish auctions in Honolulu and Hilo, through intermediary buyers on all islands, and by peddlers from the roadside. The pole-and-line aku, fleet, which is centered on the island of Oahu, markets its catch through intermediaries who sell to fresh fish outlets.
Substitution: Although ahi are often the preferred species for sashimi, aku can be substituted and, in fact, is preferred by some. When cooked, the red-fleshed aku lightens considerably in color, so it is interchangeable with ahi and a‘u in broiled or fried forms. Aku, ahi, and a‘u are also interchangeable for dried and smoked products, but due to their larger size, ahi and a‘u offer better yields.
III. Of Special Interest For Preparation/Quality Control
Shelf Life And Quality Control: Even with the best care, aku has a relatively short shelf life as a high quality product and is generally consumed within 6-7 days after landing (See Table 3). Aku which has been caught by trolling or pole-and-line is fresher and, hence, has a longer shelf life than that caught by longline boats, which make longer fishing trips.
Aku keeps longer if it is stored whole (especially if head down) and is not filleted until shortly before use. Larger summer fish (16-30 pounds in round weight) keep better than smaller fish. The first evidence of deterioration is a transformation of the deep red color of the meat to a brownish-red or rainbow color, accompanied by loss of firm texture.
It is not uncommon to find small worms in the belly flaps of aku. Studies have shown that these parasites present little, if any, health hazard, and they can be easily removed or destroyed by cooking.
Product Forms And Yields: Aku is sold in various forms: whole fish, fillets, steaks, in raw fish preparations or as dried fish sticks. Much of the aku catch is sold fresh, but surpluses caught during the peak summer season are sometimes processed. Some of the excess summer fish are dried. The yield of fillet from whole fish varies from 45% for small aku to 60% for large aku (see Table 5).
Filleting Aku: Remove the dorsal fin, head, gills and guts. Cut into the fish from both sides to establish slits along the backbone. Join these slits at the narrowest part of the fish (the tail) and fillet along the bone all the way to the collar.
IV. Of Special Interest To Consumers/Food Service Personnel
Color, Taste, Texture: Good quality aku has firm flesh that is deep red in color. Flesh color varies with the size of the fish, with smaller fish having a lighter red color than larger fish; hence, larger aku are preferable for raw fish preparations requiring a red flesh. Larger aku have a greater fat content than smaller aku and this is another desirable attribute for raw fish dishes. Cooking causes the flesh to become lighter in color. Aku has a more pronounced taste than ahi or a‘u. This is an advantage in satisfying local ethnic taste preferences, but it may not be as desirable in other markets.
Preparations: Aku is the preferred species for many ethnic seafood dishes, especially poke, raw fish served in bite-sized pieces with various spices and condiments. Many Japanese and Hawaiian consumers prefer sashimi prepared from large aku to that from ahi. “Aku bone” (the backbone of a filleted fish which retains thin strips of flesh) is a favorite food among certain ethnic groups in Hawaii, as are aku roe and dried aku. Aku can be cooked in many different ways, but is usually broiled over hot coals, sauted or fried in a skillet. The meat cooks quickly and can easily dry out if overcooked.
V. Historical Note
Aku figures prominently in Hawaiian legends. According to one legend, while voyaging to settle in Hawaii from the south seas, a chief and his party were caught in a storm which threatened to swamp their canoes. In response to the prayers of the sailors, a school of aku appeared and calmed the rough waters. To honor this fish, it was forbidden for Hawaiians to eat aku for a few days each year.