Habitat and Ecosystem – Pelagics

Water temperatures in the Pacific vary from freezing in the poleward areas to about 86° F near the equator. Circulation of Pacific waters is generally clockwise in the Northern Hemisphere (the North Pacific gyre) and counter-clockwise in the Southern Hemisphere. The North Equatorial Current, driven westward along latitude 15° N by the trade winds, turns north near the Philippines to become the warm Japan or Kuroshio Current. Turning eastward at about 45° N, the Kuroshio forks and some waters move northward as the Aleutian Current, while the rest turn southward to rejoin the North Equatorial Current. The Aleutian Current branches as it approaches North America and forms the base of a counter-clockwise circulation in the Bering Sea. Its southern arm becomes the chilled, slow, south-flowing California Current.

The waters that form the surface layer of the tropical west and central Pacific enter into the transpacific inter-tropical circulation from the eastern boundaries of two subtropical anti-cyclonic gyres, where the coastal upwelling of California and Peru provide enrichment of nutrient rich subsurface waters. The waters remain on the surface and the thickness well established thermocline. As these waters move from east to west they grow warmer and more impoverished as nutrients are consumed by photosynthesis and particulate materials are sedimented.

Limited primary production continues on the basis of partial re-mineralization within the isolated upper surface layer of the water column. Nutrient-depletion leads to very clear blue oceanic water in which suspended particles are depleted and living organisms are scarce. The term oceanic desert has been used by to describe these nutrient poor-waters. Upwelling is one mechanism by which impoverished tropical waters can be enriched with nutrients from the subsurface waters and this has been observed at the equator. Another mechanism whereby subsurface nutrient-rich waters reach the euphotic zone involves shallowing of the thermocline at 10° N and 10° S, at the edge of the equatorial counter currents.

In the South Pacific, nutrient inputs from precipitation and runoff are of major significance only in the waters surrounding the large island archipelagos of Melanesia where highlands are extensive and rainfall is very heavy. Not surprisingly, the highest oceanic primary productivities in the region are found on the shelf area of the Gulf of Papua which receives much of the drainage from PNG highlands region. Combination of various physical factors results in the accumulation in the tropical Pacific of a thick surface layer of warm water west of 180 deg.

This accumulation forms one of the pre-conditions necessary for the generation of cyclones or hurricanes that are a common meteorological phenomenon in the South Pacific. The second pre-condition is the existence of a cyclonic-like convergence in the lower layers of the atmosphere that can be found in the western tropical Pacific between the equatorial monsoon winds from the west and the easterly trade winds. In the northwest tropical Pacific, cyclones form most frequently between June and November, and are most frequent in August/September, with an average of 18 per year. South of the equator, cyclones occur from December to April and are less frequent than in the northwest, with an average of four per year.

Although named Mare Pacificum (peaceful sea) by the Portuguese explorer Ferdinand Magellan, many tropical cyclones (i.e., typhoons, the equivalent of Atlantic hurricanes), batter the islands of the Pacific. The lands around the Pacific Rim are full of volcanoes and often affected by earthquakes. Tsunamis, caused by underwater earthquakes, have devastated many islands and destroyed entire towns.

The South Equatorial Current, flowing west along the equator, swings southward east of New Guinea, turns east at about 50° S, and joins the main westerly circulation of the Southern Pacific, which includes the Earth-circling Antarctic Circumpolar Current. As it approaches the Chilean coast, the South Equatorial Current divides; one branch flows around Cape Horn and the other turns north to form the Peru or Humboldt Current large-scale oceanic events such as the El Niño Southern Oscillation (ENSO) also influence the coastal marine environment of the South Pacific islands. The Southern Oscillation Index is the difference in atmospheric pressure between Tahiti and Darwin, which is usually positive due to the low pressure area over Indonesia and Australia. During an ENSO episode, the pressure gradient reverses and becomes negative for a prolonged period with a consequent shift in climatic and oceanographic conditions. The easterly trade winds weaken and westerly winds are observed over parts of the equatorial western Pacific. The area of warm water usually associated with the western tropical Pacific is displaced eastward over the central and eastern Pacific region and the ocean waters of the western Pacific cool. This phenomenon results in the appearance of an anomalous warm ocean current off the coasts of Peru and Ecuador around the Christmas season and hence was named by Peruvian fishermen El Niño, the familiar diminutive Spanish term for the infant Christ.

This major climatic shift produces unseasonal droughts in the western Pacific and unseasonal rains in the central and eastern Pacific. Information from commercial tuna fisheries in the South Pacific and pelagic and demersal fisheries in South America suggests that ENSO events can, depending on species, have both negative and positive effects on catch ability and apparent abundance. In the western and tropical Pacific, the abundance of surface skipjack and yellowfin tuna stocks shifts eastwards during an ENSO episode. This can be inferred from the concentration of fishing effort by tuna purse-seine vessels, which during normal years concentrate to the West of 160°E line of longitude and to the east of this line during an ENSO event. Little is known at present about how ENSO events affect coastal fish and invertebrate stocks in the South Pacific due to the lack of any suitable time series of data. It is likely, however, that such a large scale anomaly will have an influence on productivity and recruitment, especially in those species with long oceanic pelagic larval stages, and those reef species that are sensitive to anomalous water levels during spawning or recruitment.

There are other long-term climatic cycles in the Pacific region that will influence the productivity and abundance of marine organisms. The Pacific Decadal Oscillation (PDO) is a pattern of Pacific climate variability that shifts phases on at least inter-decadal time scale, usually about 20 to 30 years. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20° N. During a “warm” or “positive” phase, the west Pacific becomes cool and part of the eastern ocean warms; during a “cool” or “negative” phase, the opposite pattern occurs. The mechanism by which the pattern lasts over several years has not been identified; one suggestion is that a thin layer of warm water during summer may shield deeper cold waters. The Inter-decadal Pacific Oscillation (IPO or ID) display similar Sea Surface Temperature (SST) and Sea Level Pressure (SLP) patterns, with a cycle of 15–30 years, but affects both the north and south Pacific. In the tropical Pacific, maximum SST anomalies are found away from the equator. This is quite different from the quasi-decadal oscillation (QDO) with a period of 8-to-12 years and maximum SST anomalies straddling the equator, thus resembling the El Niño-Southern Oscillation (ENSO).