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Species Synopses BLACK MARLIN (Makaira indica) Black marlin are distributed throughout the tropical and subtropical Indian and Pacific Oceans. In common with sailfish, black marlin are more closely associated with land masses than either blue or striped marlin. The possibility of at least three stocks has been suggested: southwest Pacific, eastern Pacific and Indian Ocean (including East and South China Sea). The lack of significant spawning activity in the eastern Pacific and the recapture of a fish tagged in the eastern Pacific off New Zealand, indicates the likelihood of a single stock between the southwest and eastern Pacific. Five juvenile black marlin tagged off eastern Australia have been recaptured large distances to the east in the central Pacific (four off Kiribati, one east of the Marquesas Islands). All five fish were recaptured during El Niño years when eastward-flowing equatorial currents (North and South Equatorial Counter Currents) are particularly strong. Together with the lack of significant black marlin spawning activity in the eastern Pacific, these observations suggest that the eastern Pacific "stock" may simply be an outlying group of southwest Pacific fish transported eastward during El Niño years. Just as considerable movement probably occurs from the southwest Pacific to the eastern Pacific, it is also possible that interchange occurs between the Indian and Pacific Oceans via the Banda Sea and the north coast of Papua New Guinea. Occasional movements of juvenile fish through the shallow Torres Straits may also occur. Recent black marlin tagged off eastern Australia and recaptured from Irian Jaya, the southern Philippines (two fish) and Sulawesi, strengthen the argument that there may only be a single Indo-Pacific stock of black marlin. Black marlin are highly mobile, and while seasonal changes in the distribution of catch-rates do occur, there is as yet, no evidence that they follow clear migratory routes. Major seasonal concentrations occur in the northwest Coral Sea (September - December), the northeast Indian Ocean (November - March), the eastern Banda Sea west of Irian Jaya (January - April), and the East China Sea between Taiwan and Japan (June - November). Fish congregate in the northwest Coral Sea, in the Queensland Trough (Cairns - Lizard Island region) off north Queensland, to spawn from September to December. Subsequently they disperse. Seasonal catch-rates by long-liners in Australia's eastern AFZ and in waters around the Solomon Islands are inversely related, suggesting that during April to August, when catch-rates are very low off eastern Australia, the area of maximum catch-rates shifts to the general area of the Solomons and eastern Papua New Guinea. However, further information on the size and sex composition of the fish in the latter area is required to determine whether these changes in distributions of catch-rates reflect movements of the fish from the Queensland Trough; movements of only part of this group or another part of the population entirely. The heavy-tackle gamefishery off northeast Australia is based on the aggregations of fish preparing to spawn. A light-tackle fishery for juvenile fish of 10-50kg occurs in the shallower waters (<100m deep) on the continental shelf. In northern Queensland this fishery runs from June to October. Further south, off Brisbane, the light-tackle fishery peaks in December - February. As the warm East Australian Current moves south, black marlin of a wide size range are caught from January to March on the continental slope off northern and central New South Wales. Although it is possible that large females may not disperse as far as smaller fish, tagging studies off eastern Australia show that smaller fish can disperse very large distances. The largest distance recorded to date is over 8,000 km from Port Stephens in New South Wales to the east of the Marquesas Islands in French Polynesia. Fish under 100kg tagged off eastern Australia and recaptured outside Australian waters include the following: seven in the Milne Bay Province of southeast Papua New Guinea; two off Bougainville; four in the Solomon Islands; two off Irian Jaya; one 1,000km north of Guadalcanal; two off Mindanao in the southern Philippines; one off Sulawesi in Indonesia; two in the Federated States of Micronesia; one off the Cook Islands; four off Kiribati; one off the Marquesas Islands and one off New Zealand. Although the proportion of tagged fish recaptured is similar for large and small fish, only three fish weighing more than 100kg have been recaptured outside Australian waters: one off Irian Jaya; one off New Zealand and one off the Line Islands. All other recaptures of fish over 100kg have been in the Queensland or Townsville Troughs. There are strong indications that the distributions and movements of male and female black marlin may differ. Although there are equal numbers of males and females caught in the light-tackle fishery on the Queensland continental shelf, males outnumber females by almost ten to one in the Coral Sea longline fishery. Almost all fish in the longline fishery under 180kg are males and almost all of those over 180kg are females. Detailed studies have proved that black marlin do not change sex. There is, however, at least one area outside the southwest Pacific (adjacent to the East China Sea, between Taiwan and Japan) where the fish caught on longlines are a similar size to those caught in the Coral Sea (excluding the very large females) but where females greatly outnumber males. Detailed information on the sex structure of black marlin caught around the Coral Sea Rim, particularly in the areas influenced by the New Guinea Coastal Undercurrent, could greatly help to solve the possibility of sex-dependent migration patterns. Black marlin females grow in excess of 700kg, whereas males rarely exceed 200kg. Available information on age indicates that fish may reach 15kg in their first year, 30kg in their second year, and a 450kg female is over 10 years old. Males probably reach similar ages as females, but with a slower growth rate. web@aims.gov.au
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