Arabian Sea

Category: Marine and Oceanic Biomes.

Geographic Location: Northwestern part of the Indian Ocean.

Summary: A large marine ecosystem notably enriched with high productivity supporting fisheries; also a place of great seasonality in weather patterns: an engine of biodiversity.

The Arabian Sea composes the northwestern part of the Indian Ocean, lying west of India and covering a total area of approximately 1,491,000 square miles (3,862,000 square kilometers), and has formed part of the principal sea route between Europe and India over centuries. It is bounded to the west by the Horn of Africa and the Arabian Peninsula, with entrances to the Gulf of Aden/Red Sea and Gulf of Oman/Persian Gulf respectively; to the north by Iran and Pakistan; to the east by India; and to the south by the remainder of the Indian Ocean. The largest river flowing into the Arabian Sea is the Indus River; others include the Netravathi, Sharavathi, Narmada, Tapti, Mahi, and the numerous rivers of the Indian state of Kerala.

The Arabian Sea is a Large Marine Ecosystem (LME), as defined by the U.S. National Oceanic and Atmospheric Administration (NOAA): “relatively large areas of ocean of ocean space of approximately 200,000 square kilometers [77,000 square miles] or greater, adjacent to the continents in coastal waters where primary productivity is generally higher than in open ocean areas.” The Arabian Sea LME is considered a Class I, highly productive ecosystem based on SeaWiFS global primary productivity estimates. It has an average depth of 8,970 feet (2,734 meters).

The Arabian Sea has some of the most extreme climatic regimes, due to seasonal fluctuations in air and water temperatures. The sea's average temperature, which used to be 72 to 81 degrees F (22 to 27 degrees C) until the 1980s, is now 81 to 90 degrees F (27 to 32 degrees C), resulting in occurrence of some stronger cyclones. An example is Cyclone Gonu, occurring in 2007 and the strongest cyclone ever recorded in the northern Indian Ocean; it attained a peak wind intensity of 165 miles per hour (270 kilometers per hour). Recent increases in the average intensity of tropical cyclones in the Arabian Sea may be a side effect of increasing air pollution over the Indian subcontinent as well as the higher water temperatures in the sea itself.

The Arabian Sea contributes to a monsoon climate in the surrounding region by providing the water necessary for wet storms. In summer, strong winds blow from the southwest to the northeast, bringing rain to the Indian subcontinent. During the winter, the winds are milder and blow in the opposite direction, from the northeast to the southwest.

During the monsoon season, the generally southwest winds are particularly cold, and are so strong that they succeed in sweeping away some of the salt content of the upper levels of seawater. In monsoon season, the upper waters are less then 35 practical salinity units (psu), while in the non-monsoon season salinity is over 36 psu. Monsoons are characteristic of the Arabian Sea and are responsible for the yearly cycling of its waters.

Species and Economy

Scientists have found that because the Arabian Sea has naturally high productivity, it also has one of the thickest zones of oxygen-depleted water below 330-feet (100-meters) deep. Depending on monsoon winds, local topography, the width and depth of the continental shelf, and drainage of coastal areas, there are three coastal ecosystems in the Arabian Sea, each characterized by its own productivity and species distribution. The Arabian Sea's predictable, seasonally reversing monsoons drive one of the most energetic current systems in the world. The strength of the monsoon winds is regulated; during the monsoon season (May to October), the southwesterly winds in this region are from the southwest, inducing a great deal of evaporation from the warm waters of the Arabian Sea, and heavy rainfall along the coy, a thermal gradient that develops from different heating of land and sea. The winds blow toward India and cause upwelling of low-oxygen waters. There is a concentration of fish in nearshore areas at that time. During the other half of the year, the winds blow in the opposite direction, and not as strongly.

The LMEs produce about 80 percent of the annual world's marine fisheries catch. The United Nations Food and Agriculture Organization (FAO) 10-year trend shows an increase in capture trends; from 1.9 million tons in 1990 to 2.2 million tons in 1999. There are catches of herrings, sardines, anchovies and crustaceans. India's southwest coast is fished for oil sardines (Sardinella longiceps), mackerels (Rastrelliger kanagurta), and tunas (Euthynnus affinis and Auxis thazard). Most of this catch comes from a narrow six- to 10-mile (10- to 15-kilometer) coastal belt, and accounts for 23.6 percent of India's fish catch.

The dominant fish species off India's central west coast ecosystem are Sciaenids (Pseudosciaena diacanthus), Carangidae (Caranx spp.), and Engraulidae (anchovies). The dominant species off India's northwest coast are prawn, Sciaenids (Pseudosciaena diacanthus), and Carangidae. Small tuna (Euthynnus affinis) migrates to this area to breed. There are also catches of herrings, sardines, anchovies, crustaceans and spiny lobster (P. homarus megasculptus), with zonal abundance in relation to surface circulation patterns.

A wide variety of invertebrates and algae exist, including Sargassopsis zanardinii, a marine species that is endemic to the Arabian coastline. The endemic (locally evolved and unique to a particular biome) fishes include certain members of the barracuda (Sphyraena), wrass (Labridae), and damselfish (Pomacentridae) families. Also found in the Arabian Sea are dugong (Dugong dugon: order Sirenia vulnerable International Union for Conservation of Nature [IUCN] 2.3) and several species of turtles, including the green turtle (Chelonia mydas: IUCN-listed endangered species), hawksbill turtle (Eretmochelys imbricata, critically endangered IUCN), and olive Ridley turtle (Lepidochelys olivacea vulnerable, IUCN 2.3).

Of the baleen whales, the following species have been recorded: Bryde's whales (Balaenoptera edeni, data deficient IUCN 3.1), minke whales (B. acutorostrata, least concern IUCN 3.1), fin whales (B. physalus, endangered IUCN 3.1), blue whales (B. musculus, endangered IUCN 3.1), and humpback whales (Megaptera novaeangliae, least concern IUCN 3.1). Toothed whales include sperm (Physeter macrocephalus: vulnerable A1d), orca (Orcinus orca, data deficient IUCN 3.1), and false killer (Pseudorca crassidens, data deficient IUCN 3.1). At least a dozen species of dolphins (family Delphinidae) as well as the finless porpoise (Neophocaena phocaenoides, vulnerable IUCN 3.1) exist in the Arabian Sea.

Pressures on Biota

The Arabian Sea, due to its seasonal weather fluctuations, offers excellent examples of biological adaptation to environment. However, the diverse aquatic habitat is currently under threat from the oil industry, which uses the sea as a shipping lane. As a result, oil spills, anchor damage, sedimentation, and other pollution effects are severe threats and have long-standing effects. Mining operations, fishing pressures, destructive fish collecting practices (e.g., dynamiting), residential and commercial development, and effluent discharge have resulted in altered species composition in many areas. Recreation and tourism also contribute to eutrophication and reef degradation. War-related activities provide another source of environmental damage.

Oil from accidents and bilge washings can reach the coast and impact coastal ecosystems. On August 7, 2010, a spill of about 88,040 tons of lubricant oil was poured in the Arabian Sea due to a collision off two cargo ships; this was identified as a cause of tangible damage to mangrove plants and other pelagic organisms. Current fishing methods have resulted in the overexploitation of coastal fishery resources such as prawns, sardines, pomfrets and mackerel. However, most of the fish stocks breed in deeper offshore waters so there is an opportunity to rebuild stocks. The over-exploitation is mostly because of large fishing vessels that fish illegally near the coast.

Population expansion in India will continue to put enormous pressure on the living and non-living coastal resources. This population pressure also creates major pollution problems, like untreated organic waste as well as sewage, which contribute to the nutrient loading of nearshore areas. Nutrient loading influences productivity cycles and depletes dissolved oxygen supplies. Rapid industrialization is also contributing pollution from industrial wastes and industrial effluents. Heavy metals such as cadmium, lead, manganese, and zinc have been found in core sediment samples to depths up to 18 inches (45 centimeters). Large amounts of pesticides are also deposited in the coastal areas, particularly off the Mumbai coast. This situation would require the dredging and removal of contaminated sediments for proper mitigation. Currently, plans are being considered that would protect the delicate wildlife of the Arabian Sea, particularly the turtle and coral populations.

Sixty-five percent of fish landings in the Arabian Sea derive from artisanal (small scale) fisheries. Coastal populations have traditionally relied on nonmotorized boats. Along its Arabian Sea coast, India operates about 180,000 country crafts, 26,000 motored traditional vessels, 34,000 mechanized boats, and a few large boats. The most important export is prawn. It is uneconomical to harvest the deepwater fisheries here because of low market price, except for tuna, which commands a good price.

Climate change is an interconnected event based on long-term change of regional weather that is thought to be caused by an increase in atmospheric carbon. It is thought that climate change is causing global warming. In the Arabian Sea, it has been recently suggested that climate change is also causing intensified monsoon winds and, in turn, intensified upwellings. This results in huge algal blooms in the western half of the Arabian Sea causing serious changes in productivity. This has detrimental effects by causing oxygen depletion at depth. In addition, incidence of fish mortalities is recorded in association with algal blooms.

Bibliography

Dwivedi, S. N. and A.K. Choubey. “Indian Ocean Large Marine Ecosystems: Need for National and Regional Framework for Conservation and Sustainable Development.” In Kenneth Sherman, E. Okemwa, and M. Ntiba, eds. Large Marine Ecosystems of the Indian Ocean: Assessment, Sustainability, and Management. Cambridge, MA: Blackwell Science, 1998.

Food and Agriculture Organization (FAO) of the United Nations. Trends in Oceanic Captures and Clustering of Large Marine Ecosystems—Two Studies Based on the FAO Capture Database. Rome: FAO, 2003.

Goes, Joaquim I., Helga Gomes, Prasad Thoppil, Prabhu Matondkar, and Adnan Al Azri. Eurasian Warming—Hydrography and Biological Productivity in the Arabian Sea. East Boothbay, ME: Bigelow Laboratory for Ocean Sciences, 2007.

Pollock, David E. “Spiny Lobsters in the Indian Ocean: Speciation in Relation to Oceanographic Ecosystems.” In Kenneth Sherman, E. Okemwa, and M. Ntiba, eds. Large Marine Ecosystems of the Indian Ocean: Assessment, Sustainability, and Management. Cambridge, MA: Blackwell Science, 1998.