Black Marlin

FISHERIES OCEANOGRAPHY

Ironically, less is known of the physical and biological oceanography of the NW Coral Sea than any of the other localities where black marlin are known to concentrate. It is presently being studied by oceanographers at AIMS. One area of particular interest is the influence of the Queensland and Townsville Troughs on current patterns because black marlin and other large pelagic fishes appear to concentrate in these troughs (Figure 2). Another is the significance of the bifurcation of the South Equatorial Current (SEC) (from which the EAC develops) which also appears to broadly coincide with the general area of concentration of black marlin.

Figure 2. Concentration of Japanese longline effort in and near the Townsville Trough in 1990. (Bifurcation of the South Equatorial and East Australian currents are shaded pink).

The NE Indian Ocean is an oceanographically complex and heterogeneous area with well documented upwelling. It appears to be an area favoured for spawning by many species of large pelagic fish. The "Oki" fishing ground is the only known spawning ground of the Southern Bluefin Tuna.

The Banda Sea and the Aru Basin in particular are also areas of extensive upwelling. The upwelling occurs during the SW monsoon, however, and the fish concentrate during the NE Monsoon when the Aru Basin is an area of strong convergence, rather than divergence. During the latter season, current flow in the SE Asia region is predominantly from the East China Sea, south through the South China Sea into the Java Sea, eastward to the Banda Sea, exiting the Banda Sea to the NW of Iran Jaya and to the south into the Timor Sea. This general flow is reversed in the SW monsoon although mean flow through the Timor Sea is from NE to SW all year-round.

The productive fishing ground for black marlin in the East China Sea is found around the "contact zone" between the Kuroshio and Tsushima currents and the Yellow Sea water mass. Here the seasonal movements of black marlin coincide with those of the commercial mackerel grounds.

The Gulf of Thailand concentrations are located at the boundary of a high salinity water mass extending from the South China Sea along the Malay Peninsular. The convergence of this water with the coastal water mass results in steep gradients (fronts) of surface water properties such as salinity, nutrients and phytoplankton. These fronts are particularly strong in the vicinity of Samui Island where the black marlin concentrate.

Inter-Annual Variability

Significant seasonal and interannual variability in the distribution of catch-rates of black marlin occurred in the Western Coral Sea from 1954-1958. There were significant differences in the timing of the season in 1955 and 1956 and significantly lower catch-rates over all the fishing area in 1957 and 1958 than in the previous three years.

1990 was reportedly the "best season in at least 10 years" for the Cairns-Lizard Island Gamefishery (The Lizard Island Angler March 1991). "Perhaps there were seasons in the seventies with a higher percentage of big fish, but I can't recall a season with a greater abundance of marlin than 1990" .

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Last updated - 28 October 98

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