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Currents Patterns of water currents are of major importance in understanding the distribution, abundance and movements of large pelagic fishes. Areas where currents meet and converge or diverge tend to have concentrations of food. These features create local areas of high food availability in otherwise barren tropical oceans. Although marlins and tunas can swim against almost any current, they tend to swim with the direction of the current. Seasonal and year-to-year variations in current patterns can thus have a major effect on the distributions of large pelagic fishes. Currents in the study area, particularly in the southern and central parts (Solomon and Coral Seas) are poorly known. However, with the recent recognition of the critical importance of the area just to the north of the Solomons and Papua New Guinea in determining global climate change (this is the largest pool of very warm water in the world), major advances in our understanding of these currents should come in the next few years. Marlin spend most of their time in the top one hundred metres or so of the water column, above the thermocline where temperature rapidly decreases with depth. Relatively few detailed studies of the swimming depth of marlin have been carried out, however, and the depth of the thermocline varies from one part of the ocean to another. Further, the top 10m or so of the surface layer is strongly affected by the wind. This is the current experienced by a fishing boat but may not be the current where the sought after fish spend most of their time. For the interests of anglers we present here two charts of the wind-driven surface currents in the region, based primarily on information in the Pacific Island Pilot. These are the currents likely to be experienced by gamefishing boats and fish very close to the surface. The charts also show our present understanding of the major subsurface currents (at approximately 10-100m depth) in the region. Major gaps in the charts indicate areas of highly variable currents or areas where patterns are unclear. Note: The broad arrows plotted show currents in waters deeper than 200m in the open ocean. Close to land masses, strong local effects, such as eddies, will occur due to tidal currents and coastal topography. Sub-surface currents Major shifts in regional currents occur with the changes between the South East Trade Wind and North West Monsoon seasons. During the South East Trades, the major subsurface flow into the region is the South Equatorial Current (SEC), moving from east to west. Also known as the Trade Wind Drift, the southern component of the SEC is more variable in strength and direction than the equatorial part of the SEC. It enters the Coral Sea south of the Solomon Islands and bifurcates off the northeast Australian coast between Princess Charlotte Bay, north of Lizard Island, and Townsville. The southern branch of this bifurcation becomes the East Australian Current (EAC) and the northern arm becomes the New Guinea Coastal Undercurrent (NGCU). The NGCU is called an undercurrent because, unlike the EAC, its maximum speed is mostly found well below the surface, generally around 200-300m deep, and in parts of the region it is below surface currents flowing in different directions. Although called an undercurrent, for much of its length it also pushes through to the surface, particularly during the South East Trades. During this season subsurface currents in the eastern Solomon Sea show signs of forming a gyre which moves clockwise. During the North West Monsoon, the southern edge of the North Equatorial Counter Current (NECC) enters the region from the north-west. The NECC flows from west to east right across the Pacific Ocean between the SEC and the westward flowing North Equatorial Current (NEC). Over most of its length it flows in the northern hemisphere, north of 5°N. The intrusion of the NECC causes a reversal in the sub-surface current along the north coast of Papua New Guinea. The other major change in subsurface currents during the North West Monsoon is the formation of the eastward flowing South Equatorial Counter Current (SECC), splitting the SEC into two distinct branches. This results in a south-eastward flow of subsurface waters along both sides of the Solomon Islands. However, currents around the Solomon Islands, in general, tend to be highly variable. Surface currents Surface currents in the eastern sector of the region generally flow from east to west during the South East Trade season, deflecting to the north-west along the coasts of the Solomon Islands, Bougainville and New Ireland. The flow of the SEC is relatively constant and averages 0.5 to 1.5 knots. A strong (averaging 1 to 2 knots) and constant South East Monsoon current runs along the north coast of Papua New Guinea. Surface flow in the western Solomon Sea is towards the northwest, while in the eastern Solomon Sea it is variable, but most commonly to the southwest. During the North West Monsoon the SECC forms, and the flows reverse along the northeasterly side of the Solomons, the western Solomon Sea and the north coast of Papua New Guinea. Year-to-year variability Awareness of large year-to-year variations in strength and direction of tropical ocean currents has been highlighted only over the last decade with increasing studies of global climate change and the El Niño-Southern Oscillation (ENSO) phenomenon. The ENSO is an extremely complex phenomenon. One of its general effects, however, is that during El Niño years there is a tendency for tropical surface waters of the eastern Pacific to be warmer than average while at the same time surface waters of the western Pacific are cooler than average. In La Niña (anti-El Niño) years, the reverse occurs, with western Pacific waters tending to be warmer than average, while eastern Pacific waters are cooler. Another general observation is that tropical currents running eastward, such as the SECC and the NECC, tend to be stronger during El Niño years. Thus, considerable variability in currents in the region should be expected from year-to-year, particularly during El Niño and La Niña events, and this is likely to significantly affect the occurrence and distribution of billfishes and other large pelagic fishes. Some evidence of this already exists. For example, the occurrence of good catch rates of blue marlin in the gamefishery off Brisbane (southern Queensland) appears related to the occurrence of La Niña. Preliminary data suggest that exceptionally good years for the light tackle fishery for black marlin off Queensland may be associated with El Niño. In recent years, the largest El Niño of this century occurred in 1982/83, with smaller ones in 1986/87 and 1991/93. The only La Niña event since 1974/75 occurred in 1988/89.
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