Present throughout Australian waters where sea surface temperatures exceed 17°C - from east of Torres Strait to 43°S and from approximately 140°E in the Great Australian Bight to the North West Shelf (Ref. 6390).

Stock structure: Few biological studies of bigeye tuna have been conducted and there is uncertainty regarding stock structure. Limited tag-recapture results indicate that bigeye tuna are capable of long-range movements of several thousand nautical miles, but separate Pacific Ocean and Indian Ocean stocks are assumed for the purpose of stock assessment. Bigeye tuna taken in Australian waters are members of larger, possibly trans-oceanic stocks.

Commercial fishery: Bigeye tuna are an important target species of Japanese, Korean and, more recently, Taiwanese fleets using longlines in the tropical waters of the Pacific, Indian and Atlantic oceans. Bigeye tuna also form a significant bycatch of purse seining for yellowfin tuna in the Western Indian Ocean and for yellowfin tuna and skipjack tuna in the western Pacific.

Longlining using drifting lines accounts for almost all of the catch of bigeye tuna in Australian waters. During October-December in some years, bigeye tuna are also caught from Japanese vessels with pole-and-line and handline off north Queensland (Ref. 30331, 30314).

The Australian longline fishery for tunas off south-eastern Australia has been in operation since the 1950s. With the successful air-freighting of yellowfin tuna and bigeye tuna to Japan in 1984, a longline fishery for these tunas grew rapidly, stabilising during the late 1980s. Domestic longliners, mostly 10-20 m long, commence fishing off southern Queensland and northern New South Wales in about August. Many vessels follow the movement of the warm East Australian Current southwards along the coast, fishing off southern New South Wales until May and sometimes as late as June. During the 1980s, there were several attempts to establish small-scale longline operations for bigeye tuna off south-western Australia. These operations were unsuccessful and now few Australian fish for bigeye tuna off Western Australia.

The Japanese commenced longlining off eastern and western Australia during the early 1950s. Before the declaration of the Australian 200-mile fishing zone in 1979, Japanese longline vessels would commence fishing off southern New South Wales during July and gradually move north over August-September to areas such as off Fraser Island and Lord Howe Island. Since 1979 however, access restrictions to the Zone have modified Japanese fishing patterns in Australian waters. Off eastern Australia, the Japanese vessels, usually 40-45 m long, fish further offshore and usually over more northerly waters than do the Australian vessels. Some Japanese vessels fish off the east coast between 25°S and 34°S from July to October while others operate in more northerly waters between October and January. Most vessels leave the Australian Fishing Zone by January although a few remain until February or March. On average, each longliner spends up to 2 months in the Zone during its 6-18 month global fishing campaign.

Longlines used by Australian fishers are 10-30 km long and are set each day with 250-800 baited hooks. Japanese vessels daily set a line 80-100 km long bearing 2500-3250 hooks. Both Japanese and Australian fishers target yellowfin tuna, setting the lines to fish between 50 m and 150 m depth. In tropical waters outside the Australian Fishing Zone, the Japanese use mostly deeper sets to 250-300 m to target bigeye tuna (Ref. 30327), but such deep sets are not used by either fleet within the Zone unless other catches in the area suggest the presence of bigeye tuna.

In north-eastern Australian waters the Japanese target adult yellowfin tuna, bigeye tuna, broadbill swordfish (Xiphias gladius) and striped marlin (Tetrapturus audax). Albacore (Thunnus alalunga), black marlin (Makaira indica) and blue marlin (M. mazara) are also taken. Off south-western Australia, the Japanese specifically target bigeye tuna during October-April as an adjunct to fishing for southern bluefin tuna (Thunnus maccoyii). Whereas bigeye tuna comprise only 5-10% of the catch off the east coast, they account for over 70% of the catch in the south-west. Off north-western Australia and around the Cocos-Keeling and Christmas islands during the summer, a small bycatch of bigeye tuna is taken by the Japanese targeting yellowfin tuna and striped marlin.

Bigeye tuna is a highly valued item on the ‘sashimi’ (raw fish) market in Japan, second only to southern bluefin tuna and northern bluefin tuna (Thunnus thynnus). Most bigeye tuna caught in Australian waters are sold fresh (Australian vessels) or frozen (Japanese vessels) to such markets. Bigeye tuna from cooler waters south of 30°S are often large and particularly high in quality.

Recreational fishery: Bigeye tuna are rarely taken by recreational anglers. According to records of the Game Fishing Association of Australia, up to 1993, the record bigeye tuna caught weighed 120 kg and was taken off Bermagui in New South Wales.

Resource status: Longline catch rates of bigeye tuna declined from the late 1950s to the late 1960s, then stabilised in the 1970s as the Japanese commenced targeting bigeye tuna (Ref. 30327, 30332). Preliminary assessments (Ref. 30332) suggest a fairly constant stock size in the Pacific Ocean over this time.

The South Pacific Commission estimated that over 32,000 t of bigeye tuna were caught by longline in the western Pacific in 1990 (Ref. 30315). No bigeye tuna are reported to have been caught by purse seining, yet it may go undetected. Port sampling suggests that bigeye tuna may comprise a significant portion - perhaps as much as 10-15% - of the 177,000 t of yellowfin tuna reported by purse seiners. The ability of the resource to support these catches is not known. Uncertainty over links between bigeye tuna of the eastern Australian Fishing Zone and the wider western Pacific hamper assessment of the condition of local groups off eastern Australia. Also Ref. 2334.