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Distribution Global Black Marlin are distributed throughout the tropical and sub-tropical waters of the Indian and Pacific Oceans within preferred Sea Surface Temperatures (SSTs) of 20 - 30oC: N.Pacific to 35-40oN; S.Pacific to 45oS in west, 30-35oS in east; Indian Ocean to 25oN, 45oS. They generally do not occur in the Atlantic Ocean. Australian Fishing Zone Black marlin are more closely associated with land masses than are blue or striped marlin. In contrast to the latter two species, catch-rates of blacks in the eastern Australian Fishing Zone (AFZ) are higher close to the continental slope and in the East Australian Current than further offshore (Figure 1). Black marlin can be found in almost any depth of water. Young juveniles are often found in shallow waters of the continental shelf (as shallow as 5m in Harvey Bay, Qld.). Juveniles have been caught on the north Australian shelf as far east as the Wessel Is. (137oE). Black marlin are present in the western Gulf of Carpentaria to south of Groote Eylandt but their presence has not been confirmed in the eastern Gulf. There are authenticated reports of black marlin in the Torres Straits but these are believed to indicate only occasional occurrences of these fish in that strait and not regular migrations in numbers from the Pacific to the Indian Ocean or vice-versa. They are not usually found south of Mallacoota in Victoria or Albany in W.A. SEASONAL CONCENTRATIONS AND MIGRATIONS Most of our knowledge of the distribution and movements of black marlin, as of most billfish, is based on catch data from the Japanese longline fleet. Because of changes in fishing techniques, such as "deep" longlining (see below) and changes in fleet behaviour caused by the introduction of 200nm Economic Fishing Zones (EFZs), it is believed that the data most accurately reflecting the natural distributions of billfishes is that from the early history of the fishery. Black marlin are highly mobile. Seasonal changes in distributions of catch-rates do occur but individuals do not follow clear migration routes. Major seasonal concentrations of black marlin occur in the NW Coral Sea, the NE Indian Ocean, the eastern Banda Sea and the East China Sea.
NW Coral Sea Fish congregate in the NW Coral Sea, in the Cairns - Lizard Island region off north Queensland, to spawn between September and December. It is not known whether this congregation includes only mature fish or whether immature fish are also present. Following these seasonal aggregations, fish are found progressively further south, apparently in association with the southward seasonal progression of the East Australian Current (EAC). A southward directed migration of one to two year-old fish from northern Queensland to central NSW is indicated (by length-frequency data and validated age estimates) and one or two older groups also undertake the southward movement. Post-spawning, fish are believed to disperse eastward and southward. Seasonal catch-rates (by longliners) in the eastern AFZ and in waters around the Solomon Islands are inversely related, suggesting that during April to August, when catch-rates are very low in the eastern AFZ, the centre of distribution of the population shifts to the waters around the Solomons and western PNG. Tag-recaptures support the movements inferred from longline catch-rates. Tagged fish (mostly tagged in the Coral Sea but also a small number in the Eastern Pacific) covering a wide size-range have moved large distances (up to 7,200km in 359 days) but recaptures of fish near their points of release after 1, 2, 3 or 4 years strongly suggest annual homing of at least part of the population (to the NW Coral Sea). NE Indian Ocean Catch-rates of black marlin off North-Western Australia are highest between November and March. Catch-rates south of Java and the Lesser Sunda Islands are highest between May and September. These observations suggest that fish migrate north and south between these areas with October and April, respectively, being transition periods. These months are also the times of seasonal shifts from South-West Monsoon to North-East Monsoon (October) and vice-versa. Relatively small fish of 20-50kg dominate the catches in May to September, south of the Indonesian islands. These juveniles are of a similar size to those caught in Queensland in January-February and the modal size of fish caught between Cape Moreton and Lake Macquarie in January-March. A wider size-range of fish including larger fish, 30-140kg, comprise the majority of the catch in October to March. Banda Sea Catch-rates in the Aru Basin (where the deep Banda Sea meets the continental shelf of the Arafura Sea) are at a maximum in January to April (NE monsoon), leading to the suggestion of a connection between fish in this area and those south of Java (and hence NW Australia). An alternative hypothesis is that seasonal concentrations in the Aru Basin and Banda Sea are primarily fish that have migrated southward from the East China Sea (see below). During the period of concentration around the Aru Basin the size-distribution of fish covers a wide range, similar to that at the same time in the NE Indian Ocean. The size range of fish in the Banda Sea during the SW monsoon is also similar to that of the NE Indian Ocean at this time, being dominated by smaller fish. The Timor sea has been suggested as a transition area between North-West Australia and the Banda Sea. East China Sea Seasonal concentrations of black marlin in the East China Sea are at a peak from June to November (when they are at a minimum in the Banda Sea). Fish in this area cover a wide size-range up to more than 450kg but the dominant group is 30-120kg. Seasonal shifts in the size-structure of the population are well documented. Juveniles of 25-60kg move into the area in June to September (as in the area south of Java and the Lesser Sunda Islands). The average size increases through to the following year. Fish in the East China Sea are believed to comprise a "feeding group" rather than a spawning group. This group moves northward, away from the equator, with the Kurushio and Tsusima currents during spring and summer, and southward away from the cooling waters in autumn. "By February, fish have nearly disappeared from this area [East China Sea] on their southward migration to other areas". The route taken by fish moving from the East China Sea southward to the Aru Basin, if such movement does occur, is unclear because of the lack of fishing data from the deep waters east of the Philippines and from the South China Sea, the two likely routes. Adjacent, and to the west of the South China Sea, concentrations of juvenile black marlin (5-80kg but mostly 10-25kg) occur along the Malay peninsular in the Gulf of Thailand, particularly near Samui Island during June to September. These fish are of similar size to those found on the continental shelf of North Queensland at the same time of year. The origin and fate of these Gulf of Thailand fish is unknown. Western Indian Ocean Peak black marlin abundance occurs in Mauritian waters in November, and declines as water temperature increases and blue marlin become more prevalent.
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