Blue marlin are distributed throughout eastern and western Australian waters and, depending on the extent of warm currents, as far south as northeastern Tasmania and Rottnest Island (Ref. 6390). Blue marlin rarely, if ever, occur in continental shelf waters and have not been recorded from northern Australia (Ref. 6390).

Stock structure: A single stock in each of the Pacific and Indian oceans is assumed, based on the homogeneous distribution of larvae and adults (Ref. 30354, 30364).

The identity of the blue marlin in the Indian and Pacific oceans remains an issue of contention. Some authors (Ref. 43) consider it distinct from the Atlantic Ocean blue marlin, whilst others (Ref. 30399, 30371) consider the 2 are 1 species. These nominal species can be distinguished only by differences in the lateral line pattern (Ref. 43), although very recent mitochondrial DNA work on blue marlin suggests isolation between the Pacific and Atlantic populations.

Commercial fishery: Unlike black marlin (Makaira indica) and striped marlin (Tetrapturus audax), blue marlin are not targeted by any commercial fishery. However, they are caught as bycatch or incidental catch of longliners targeting other billfish, bigeye tuna (Thunnus obesus) and yellowfin tuna (T. albacares), and purse seiners targeting yellowfin tuna and skipjack tuna (Katsuwonus pelamis) throughout the Indian and Pacific oceans. In the Western Pacific purse seine fishery individual blue marlin are frequently caught when sets are made around floating logs (Ref. 30375).

Japanese drifting longline fisheries began in both the western Coral Sea (off north-eastern Australia) and off north-western Australia in 1953. Since the declaration of the 200-mile Australian Fishing Zone in 1979, access of this fleet to Australian waters has been subject to bilateral agreements. The Japanese boats range from 200 t to 350 t and are 35-45 m long. They spend about 60 days in the Zone during their long fishing campaign in the south-west Pacific.

Highest catch rates of blue marlin by Japanese vessels in both the Coral Sea and off the north-west of Australia occur between January and March and further offshore than for black marlin. In both the Pacific and Indian oceans the size of blue marlin tends to increase from east to west (Ref. 30354, 30377). The usual size of blue marlin in the Japanese fishery on the east coast during the 1980s was 40-100 kg processed weight, with most between 50 kg and 70 kg. Pre-1965 data from the longline fishery off north-western Australia indicate modal sizes between 50-90 kg north of 20°S and 105-160 kg south of 20°S (Ref. 30377).

The longline catch of blue marlin is exported to Japan, where it is not a high value fish. Although black marlin and blue marlin fetched similar prices for fresh fish in 1970 - to 500 yen per kg - blue marlin was worth less than half the price of black marlin by 1986 (Ref. 30378).

Recreational fishery: Blue marlin are caught less frequently by recreational anglers in Australian waters than are black marlin.

The fishery is centred over continental slope waters 150-300 m deep. In recent years, exceptional catch rates of blue marlin have been made from grounds off south-eastern Queensland from Mooloolaba and Tweed Heads beyond the 200 m line. Blue marlin are caught in these depths generally, all the way from Cape Moreton to Bermagui - wherever serious attempts have been made. Other grounds fished by recreational anglers are (eastern Australia) off Cape Byron and from Port Macquarie to Jervis Bay; (Western Australia) Rottnest Trench west of Perth, the Houtman Abrolhos, Point Cloates, north of North West Cape to Muiron Islands, and off Rowley Shoals.

The recreational fishery for blue marlin has grown dramatically in the last 4-5 years. They were rare in the catch 10 years ago, and now are quite common. This is partly a result of boats fishing further offshore and using Hawaiian-type straight running lures.

The fishery targets fish to 350 kg (Ref. 27128), but some individuals as small as 50 kg were caught off New South Wales in 1989. The Game Fishing Association of Australia’s blue marlin record up to 1993, was 364 kg for a fish landed off Cape Moreton, Queensland in 1989. Recreational fishers in eastern Australia landed approximately 600 marlin (3 species, most of them black marlin) in 1988-89, 80% of the caught by gamefishing club members. In the same period, almost 1300 marlin were tagged and released by recreational anglers in Queensland and New South Wales. In the 19 years to mid 1992, 586 blue marlin have been tagged under the New South Wales Fisheries Research Institute’s gamefish tagging program.

Resource status: The Japanese longline catch of blue marlin dominates in the Pacific Ocean. Total catch peaked in the early 1960s, exhibited a decreasing trend in the mid 1970s and was followed by a moderate recovery in the early 1980s (Ref. 30364). The longline catch rate has shown a comparable trend. A surplus production model based on the equatorial Western Pacific led to the conclusion (Ref. 30379) that the Pacific blue marlin stock is overfished (ie more effort is expended than needed to take maximum sustainable yield) but that some recovery of the stock from 1975 to 1980 is indicated. Despite this, overfishing remains a possibility.

Lack of appropriate data prevents an accurate assessment of the Indian Ocean blue marlin stock.

Also Ref. 2334.