Marine Region 11 Arabian Seas - AJH Environmental

33 Pages·2002·0.24 MB·English

MARINE REGION 11
Arabian Seas
Anthony W. Chiffings
BIOGEOGRAPHY AND BIODIVERSITY gin to be mined (Hanna 1983a; Nawabi
1983).
The Arabian Seas Marine Region includes Ecological problems also result from the
marine areas from Djibouti to Pakistan, in- loss and degradation of productive coastal
cluding the northern part of Somalia, the habitats caused by coastal landfill, dredg-
Red Sea, the Gulf, and parts of the Arabian ing, and sedimentation. In some Gulf States
Sea.1 (for example, Saudi Arabia), 40 percent of
Coastal and marine environments the coastline has now been developed, and
throughout the Arabian Seas Region are a significant proportion of the shoreline of
subject to increasing human pressures, countries such as Kuwait and Bahrain is arti-
many of which appear to have resulted in ficial. Loss of habitat extends to other parts
harmful environmental effects. Oil, phos- of the region and to the wider Indian Ocean
phate mining (Hanna 1982, 1983a), and do- where approximately 50 percent of man-
mestic, urban and industrial pollutants are a grove forests may have been lost over the
problem in several areas in the Gulf and the last 20 years (IUCN/UNEP 1985c). Overfish-
Gulf of Aqaba and have resulted in local ing is a major concern in all areas of the
habitat degradation, eutrophication and al- region.
gal blooms. Throughout much of the Ara- Degradation of coral reefs from heavy col-
bian Seas the coastal zone is becoming a lecting and other recreational and tourism
repository for solid wastes. In the Red Sea, uses is becoming widespread, particularly
the ecological effects from oil exploitation, in the Red Sea (Hanna and Ormond 1982;
phosphate mining (Hanna 1982, 1983a,b), Ormond 1981; Hanna 1991). Fishing and
and industrial inputs (such as from mining) hunting of adult turtles and birds (and their
are of increasing concern, and will become eggs) is extensive in some areas (Hanna
more so if deep-sea metalliferous mud be- 1994). The effects of land-based activities
such as nutrient and sediment runoff from
39
40 A Global Representative System of Marine Protected Areas
phosphate mining (Hanna 1982; Hanna and rainfall may amount to as little as 10–15 mil-
Ormond 1982), agriculture and grazing, and limeters per year, whereas along the coast-
reductions in freshwater seepage due to line its estimated range is from a few
groundwater extraction are also contribut- millimeters per year along the northern part
ing to degradation of coastal environments. of the western shore, gradually increasing
to 180 millimeters at Suakin (19°N) (Ed-
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:4)(cid:9)(cid:10)(cid:11) wards 1987). These unique features have
contributed to making the Red Sea vulner-
The Arabian Seas Marine Region is a natu- able to the impacts of human activities.
ral unit created by the geological history The Gulf, in marked contrast, is a shallow
that formed the Arabian peninsula and its sedimentary basin with an average depth of
associated regional seas and the maritime 31 meters. Freshwater is supplied by the
climate that is dictated primarily by the sea- Shatt el Arab and some Iranian rivers. Sur-
sonal monsoons associated with the Asian face water temperatures range from 32–
continent. 33°C in summer to 22–24°C in winter in the
Map 11 shows ideal summer and winter south but only 16°C in the north. Salinities
wind distribution and surface circulation pat- generally range from about 38–40o/oo, in-
terns for the Indian Ocean. Wyrtki (1973) creasing to more than 60o/oo in areas such
and Shepherd, Price, and Roberts (1992) as the Gulf of Salwa, but falling to around
provide concise descriptions of the physical 37o/oo in the Gulf of Oman. Tides in the Ku-
oceanography of the Indian Ocean, includ- wait region range up to 3.5–4.0 meters,
ing the monsoon gyre. whereas south of Al Kobar, on the Saudi
The Red Sea, a product of deep-ocean coast, they are is less than 1 meter. Even
rifting, extends for 2,100 kilometers from so, strong tidal flows occur throughout the
Suez to the Strait of Bab el-Mandeb, which Gulf.
connects it with the Gulf of Aden and the In- The Gulf of Oman is deep and largely
dian Ocean. The Sea has an average depth open to the influence of the northern Indian
of 500 meters, with a maximum of over Ocean (Arabian Sea). The oceanic coasts
2,000 meters, and is noted for some of the of Yemen and Oman are completely open
hottest and most saline seawater in the to the influence of the northern Indian
world. In the south, surface water tempera- Ocean and are, in parts, subject to large
tures exceed 30°C in summer, while salinity oceanic upwellings that bring nutrientrich
is 40o/oo in the north in winter and up to water to the surface, causing high productiv-
46o/oo in the middle of the Suez Canal. ity.
The Red Sea has a number of unique fea- The coast of Pakistan constitutes the
tures. It is the warmest and most saline of northern boundary of the Arabian Sea, with
the world’s seas. It has no permanent in- oceanic influences dominating over those
flowing coastal rivers or streams, only sud- of the continent, which is essentially a sub-
den brief torrents, and mostly northwesterly tropical dessert. River flows are monsoonal,
prevailing winds, which contribute material with the only major freshwater input coming
to the sea floor. The nature and distribution from the Indus, at the eastern extremity that
of sediments are unlike those in other seas. discharges some 200,000 gallons of water
The Red Sea is partially isolated from the and 450 million tons of suspended sedi-
open ocean (Hanna, personal communica- ment annually and forms the Indus cone, a
tion). It is located in an arid, tropical zone. subaqueous delta 1,500–2,000 kilometers
Rainfall in general is sparse and varies long (Pernetta 1993).
widely, with particular areas receiving no Currents in the Gulf and the Red Sea
rainfall for months or years. Over the sea, largely result from density gradients in the
Marine Region 11: Arabian Seas 41
water column. Evaporation in the Gulf changes in water level that are more signifi-
causes movement of water inward through cant.
the Strait of Hormuz. Surface salinity is high- The Arabian Sea coastline of the Arabian
est in the two large southern Gulf embay- peninsula has a tidal range of 1.5–2.5 me-
ments where greatest evaporation occurs. ters over most of its length, but these tides
Denser water formed in these regions sinks are thought to be subordinate to the persist-
toward the Strait of Hormuz while less- ent impact of high-energy waves (Shep-
dense water enters along the surface and pard, Price, and Roberts 1992). The
northern edge of the Strait, moving in a Pakistani coast has tides of up to a 3.5-me-
counterclockwise surface current and leav- ter range (Pernetta 1993).
ing the Gulf in the deeper and southern part
of the Strait (Sheppard, Price, and Roberts (cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:3)(cid:6)(cid:8)(cid:9)(cid:2)(cid:10)(cid:11)(cid:3)(cid:12)(cid:13)(cid:14) (cid:3)(cid:15)(cid:16) (cid:8)(cid:9)(cid:2)(cid:6)(cid:2)(cid:10)(cid:14)
1992).
Currents in the Arabian Sea result from The main body of the Red Sea lies in a rift
the removal of surface water during the valley separating the African and Arabian
summer monsoon and its replacement by plates (Drake and Girdler 1964; Quennel
cooler upwelling water (Sheppard, Price, 1958). Plateaus and mountains rise steeply
and Roberts 1992). to more than 1,000 meters above sea level
Surface water density in the Red Sea north of Jeddah and 3,660 meters in
rises with a fall in water temperature to the Yemen. The coastal plain is from 2–50 kilo-
north and evaporation causing increased meters wide and slopes up gently to the
salinity. Decreasing temperatures and east until it meets the mountains. The moun-
evaporation in the Gulf of Suez result in the tains are deeply cut by valleys but streams
formation of dense water that turns under flowing in the uplands fail to cross the
and is returned southward in the deep Red coastal plain to reach the sea (Schwartz
Sea (Sheppard, Price, and Roberts 1992). 1982).
Circulation and exchange in the Red Sea The Gulf of Aqaba is 170 kilometers long
is not well understood, but is thought to be and 14–26 kilometers wide and forms part
quite complex as a product of the low-en- of the Afro-Syrian Rift System with steep
ergy regimes involved. In simple terms walls dropping to great depths (2,000 me-
though, in winter (September–June) sur- ters in places). It is separated from the Red
face water is driven into the Red Sea by pre- Sea by the 6-kilometer wide Straits of Tiran.
vailing winds from the southeast, beneath Fringing reefs lie along most of the Sinai
which there is a deep outward flow of more Peninsula shoreline (Hanna, personal com-
dense, saline water. During summer, pre- munication).
vailing winds change to the northwest, driv- The Gulf of Suez is a wide, shallow basin
ing the upper water layer out of the Red with an average depth of 20–30 meters,
Sea over a deeper inward flow from the greater depths in the central trough, and a
Gulf of Aden. Net outflows are 10 percent depth of 90 meters at the entrance to the
higher in salinity, and balance the effects of Red Sea. The fringing reefs in the Gulf of
evaporation in the Red Sea. Suez are not as well developed as those in
Tides in the two regional seas are in the the Gulf of Aqaba.
range of 0.5–1.5 meters. In both the Red The Red Sea Barrier Reef is located 10–
Sea and the Gulf, tidal movement provides 40 kilometers off the coast of Saudi Arabia
nutrients necessary for the vigorous growth and is about 400 kilometers long and sev-
of benthic biota. The central Red Sea is al- eral kilometers wide. The platform surface
most tideless and has wind-driven seasonal is 30–60 meters deep, on which sit many
steep-sided patch reefs. Almost continuous
42 A Global Representative System of Marine Protected Areas
marginal coral reefs occur along much of short, sandy beaches backing on to high,
the coast from the Gulf of Aqaba to the near-vertical cliffs or sometimes sandy
Strait of Bab al-Mandeb. A similar descrip- plains or dunes. Much of the area is volcani-
tion of the reef morphology also applies to cally active. Eastward, greater variation oc-
the African side of the Red Sea. The main- curs, from steep cliffs, up to 145 meters
land coast along the southern Red Sea has high, to tidal lagoons, including tidal deltas,
experienced less uplift than other areas, and mud volcanoes. The Indus Delta, with
with some subsidence in recent times. The its extensive mudflats, tidal channels and
Farasan and Dahlak Archipelagos are rel- mangroves, has already been described
icts of an ancient carbonate platform a few (see Pernetta 1993).
hundred meters thick that grew on ancient
evaporitic salt deposits. (cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:4)(cid:6)(cid:7)(cid:8)(cid:10)(cid:11)(cid:12)(cid:7)(cid:13)(cid:4)(cid:11)(cid:6)(cid:5)
The Gulf lies between the Arabian and
Iranian plates, and there is a strong con- The following discussion is adapted from
trast between the sheer eastern coast and Sheppard, Price, and Roberts (1992) and
the flat, low western coast. The Arabian Hanna (1991 and 1994).
western coast is generally low, flat and
sandy. Beach sands may be cemented into Sabkha
beach rock. Often a sandbar overtopped by
dunes isolates large lagoons flooded in win- Sabkha is a widespread inter- and supra-
ter but dry and covered by salt or gypsum tidal habitat, measuring many kilometers
for the rest of the year. Extensive algal and across in places. It forms flat plains, with
intertidal flats occur south of the Bahrain ar- crusts of sodium chloride and gypsum, with
chipelago. The coast of the United Arab important “algal mats” a few centimeters
Emirates is characterized by a number of thick, beneath which is a black reducing
broad, sandy flats and lagoons and edged layer. The mats are complex associations
with barrier and fringing reefs. of cyanophytes, bacteria and diatoms.
At the northern end of the Gulf is the vast Pools are a special feature of sabkha.
deltaic plain of the Euphrates, Tigris and Those with subterranean connection with
Karun rivers that is formed of swamps, the sea have a relatively high benthic diver-
sandbars, spits and islands with fluctuating sity. With increasing isolation from the sea,
boundaries. The eastern coast is a region diversity falls and the persistent microbial bi-
of extensive continental sedimentation. It is ota then forms a typical mat. These are
flat and low as far as Bushehr, then rocky highly productive and fix nitrogen. When
and cliffed. In front of Ras Musandam the desiccated in summer, mats become dry
coast forms a large recess at the Strait of and crisp, breaking into characteristic poly-
Hormuz, with two main islands—Queshm gons.
and Hormuz. Along the north shore, cliffs Large areas of Sabkha are found in the
and deltaic plains alternate. United Arab Emirates, the Bar al Hiskmann
The open, oceanic coast of Oman and peninsula in Oman, along the shores of the
Yemen includes sandy and rocky stretches Gulf of Suez, and much of the Saudi Red
with ragged cliffs. The southern coast of the Sea coast.
Arabian peninsular is composed of rocky
headlands with cliffs alternating with shores Marshes and Wetlands
of fine sands buffeted by oceanic swells.
The Baluchistan coast (700 kilometers, Salt marshes have been greatly reduced or
75 percent of which is in Pakistan) is steep eliminated throughout much of Arabia, al-
with rugged outcrops and is dominated by though marshes of the Shatt al Arab still
Marine Region 11: Arabian Seas 43
cover approximately 18,500 square kilome- Rocky Shores
ters. Eleven halophytic community types
have been described, largely on the basis The diversity of rocky shores is significantly
of elevation and periodicity of immersion. In higher than that of sandy beaches or mud,
places, reed vegetation is strongly en- although biomass may be less. Hanna
hanced by sewage enrichment, when it (1994) found that about 20 percent of the
may reach heights of up to 5 meters. In the Egyptian Red Sea coast is formed by rocky,
Gulf and parts of the Red Sea (not the erosional, wave-cut cliffs. Basson and oth-
northern Red Sea), many new marsh com- ers (1977) note that the rocky intertidal in
munities are appearing as a result of sew- the Gulf is much less productive than sandy
age outfalls along the coast of Saudi Arabia intertidal areas, attributing this to intense
and near Port Sudan. Enrichment not only heating at low tides in summer. The rocky
stimulates marsh development, but in the shores of Oman show a general increase in
case of the Red Sea, also adds significant diversity southward, although little taxo-
nutrient loads. These areas act as a focus nomic work has been done in this area
for numerous species of birds, especially mi- (Jones 1986; Campbell 1988).
grants. Detailed descriptions of the Red Sea are
About 3 percent of the Egyptian coast is provided by Jones, Ghamrawy, and
comprised of saltmarsh (Hanna 1994). Salt- Wahbeh (1987). Much of the available
marshes in the northwestern Red Sea are rocky intertidal zone of the north occurs in
located around Ras-Mohammed, Abu-Mon- erosion notches of fossil cliffs. These pro-
qar, Wadi El-Gemal islands and Gebel Elba. vide a more moist and sheltered habitat
Numerous deltas and estuaries with ex- than do the horizontal expanses of intertidal
tensive intertidal mudflats and their associ- rock that are common in the south and sup-
ated wetlands occur along the Pakistani port a greater range of the fauna.
coast. The Indus Delta has an estimated Rocky shores are a major feature of the
3,000 square kilometers of delta marshes. Gulf of Oman and the Pakistani coastline,
as well as the Arabian Sea shore of Oman
Sandy Shores and Yemen.
Most sandy shores in the northern Red Sea Mangroves
are narrow beaches adjoining coral reef
flats, which themselves are usually narrow. The mangroves of Arabia include communi-
Broader beaches occur where the reef ties that grow on soft-bottom and hard-bot-
edge swings further out from shore, leaving tomed substrates, the latter being more
a lagoon. Sand beaches are most important prevalent in the northern Red Sea. Com-
and extensive in the Arabian Sea. A study pared with other Indian Ocean mangroves,
by McCain (1984) found that in the Gulf the number of mangrove and associated
every square meter could contain 400,000 species in the Arabian Seas Marine Region
animals, each over 0.5 millimeters in size, is low, although most of the characteristic
with faunal abundance significantly corre- faunal zones are still present. Low diversity
lated to slope. The total number of species is attributed to the generally severe climatic
found in this study was 147, much less than and environmental conditions (such as high
on the rocky shores. Sandy beaches are salinity), in conjunction with the more lim-
also a feature of parts of the Gulf of Oman ited range of suitable habitats and niches.
and the Pakistani coastline. Avicennia marina grows in both types of
substrates and is the dominant mangrove
species, tolerating the high salinities (40–
44 A Global Representative System of Marine Protected Areas
50o/oo) and extremes of water temperature Coral Reefs
(12–35°C) associated with the Arabian
Seas region. In the Red Sea, three other The diverse and spectacular coral reefs for
species are known but are uncommon. The which the Red Sea is renowned are found
northern latitudinal limit (27–28°N) of natu- only in its central and northern half. North of
rally occurring mangal ecosystems in both 20°N reefs are typically well developed and
the Red Sea and Gulf is attributed largely to drop steeply into deepwater; south of 20°N,
cold winter temperatures. reefs occur in a shallow, turbid environment
Mangroves are tallest (5–7 meters) in the and are less developed. Greatest develop-
southern Red Sea, where the continental ment occurs in offshore barrier reefs and in
shelf is wider and the intertidal slopes more reefs fringing 1–7 kilometers wide alluvial
gradual, allowing development of better plains on the mainland. Thinner reefs cover
sedimentary conditions. In parts of Oman, the Gulf of Aqaba and other northern
Avicennia reaches 6 meters, whereas in the shores. The continental shelf widens to the
Gulf it is poorly developed and often south and mainland shores are dominated
stunted (1–2 meters), at least along west- by mangrove and sand beaches. Well-de-
ern shores. Productivity of mangroves in veloped reefs occur around the Farasan
the region is considered to be generally and Dahlak Islands, which also support ex-
low, with the exception of the Indus delta tensive mangroves.
(see below), although few quantitative stud- The Gulf of Aden has very poor reefs be-
ies have been undertaken. cause of upwelling water and sandy shore-
Interactions between mangroves and ad- lines, and this condition continues down the
jacent ecosystems are probably greatest in coast of Somalia for 500 kilometers and
the southern Red Sea and undoubtedly north to the Muscat area of Oman. Further
make a significant contribution to coastal north, Musandam has the most diverse
productivity. Important mechanisms include reefs, while Iran probably has the most de-
transfer of nutrients and energy, aided by veloped. The coast of the United Arab Emir-
movements of fauna. Stabilization of shore- ates is low lying and mostly swampy.
line sediments is also enhanced by man- Offshore the water is very shallow and rich
groves, particularly in the southern Red in seagrasses, and while it is generally
Sea. muddy and unsuitable for most corals, there
The Indus Delta has an estimated 44 per- are numerous patch reefs.
cent of its intertidal area (260,000 hectares) Algal reefs occur in the southern Red
covered in mangroves of four species, Sea in low-energy conditions. They support
Avicennia officinalis, Rhizophora conjugata, dense brown algal cover and provide impor-
Ceriops tagal, and Salsola foetida, that tant hard substrate in otherwise sandy ar-
reach heights of 12–15 meters. These spe- eas.
cies line the tidal channels that extend in- A longitudinal series of coral reefs lie
land for considerable distances. Tidal along the axis of the Red Sea on ridges re-
influence reaches 97 kilometers upstream sulting from normal faulting and upward
as far as Tatta. These mangrove systems movement of underlying salt deposits.
are extensively studied (Pernetta 1993). These are widespread in the Red Sea. At-
Mangrove areas are also found at other olls are also numerous and are found
sites along the Pakistani coast. mostly on the ridges. Diverse reefs are
found between Ras Shukhei and Quseir
along the Egyptian coast and in the area of
Ras Mohammad and the Straits of Tiran
(Hanna, personal communication).
Marine Region 11: Arabian Seas 45
There is a fairly distinct Arabian coral spe- Pakistani coast is not well described at this
cies grouping. Within it, there is a single, time.
principal division into a Red Sea group, and Studies on seagrass standing crop have
a Gulf of Oman/Arabian Sea group, which been undertaken mostly in the northern
then fuses with the Gulf. In the Red Sea Red Sea. Highest biomass is associated
there are 13 principal coral communities, with Thalassodendron ciliatum, Thalassia
some of which can be subdivided further hemprichii andSyringodium isoetifolium.
into a total of 22 recognizable units. Most Biomass of Halodule uninervis–dominated
show considerable localization, correlated communities in the Gulf are comparable to
with latitude but linked with gross changes figures for similar species in the Red Sea
in coastal bathymetry and morphology. On and elsewhere.
any one reef in the Red Sea, the general Seagrasses provide a mostly indirect
pattern of coral diversity with depth follows food source and habitat for both resident
that of most Indo-Pacific reefs, rising to a fauna and temporary visitors, including com-
maximum at 5–20 meters deep before de- mercially important fish and crustaceans
clining. About two-thirds of species have a (for example, Penaeus semisulcatus). De-
depth distribution that is not significantly spite regional variation, available data sug-
skewed to deep or shallow water. Coral gest that both species richness and
cover is usually less than 50 percent, but in abundance of fauna are greater in the Gulf
sheltered areas one or two species, espe- than in the Red Sea, at least in its northern
cially Porites, may cover 80 percent of the parts. Benthic fauna (within seagrasses and
substrate. sand or silt) in the Gulf are principally sus-
In the Gulf, fewer coral communities ex- pension feeders, that utilize more abundant
ist: only five are recorded from Bahrain. Ku- organic particulates than occur in the
wait, Qatar, Bahrain and the United Arab clearer waters of the northern Red Sea.
Emirates have 30 species or less. Despite
this, coral cover is high. The richest reefs Upwellings
known surround Saudi Arabian coral cays.
At the entrance to the Gulf, Musandam con- In summer, prevailing winds flow down the
tains reefs dominated by Porites and Acro- Red Sea for its entire length, reinforcing the
pora. In the Capital Area of Oman, clockwise airflow in the Arabian Sea. This
substantial monospecific reefs of Pocil- generates strong southwesterly winds, lead-
lopora damicornis occur. Coral-dominated ing to cool, nutrient-rich upwelling. Up-
communities become rare further south. wellings result in higher nutrients and the
development of Ecklonia kelp beds in
Seagrasses places, inhibiting the development of coral
reefs.
Eleven seagrass species are known for the
Arabian Seas Marine Region, of which Ha- (cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:3)(cid:6)(cid:8)(cid:5)(cid:9)(cid:3)(cid:10)(cid:6)(cid:5)(cid:11)(cid:12)
lodule uninervis and Halophila ovalis are
the most prevalent. Diversity is greatest in The following discussion is adapted from
the Red Sea proper (10 species) and low- Sheppard, Price, and Roberts (1992).
est in southeast Arabia (4 species) and the
Gulf (4 species). Seagrass beds also attain Plants
greatest development in the Red Sea, par-
ticularly toward the south, despite the re- Seaweeds
verse trend shown by certain species. The There are extensive areas dominated by
seagrass distribution and diversity of the macroalgae on both reefs and other sub-
46 A Global Representative System of Marine Protected Areas
strates. Brown algae are mostly of small Gulf, plankton densities are much greater
species, although large forms occur on reef than in the Red Sea, most being diatoms.
crests and in the Arabian Sea where up- Phytoplankton in the Arabian Sea is also
welling is important. Green and red types dominated by diatoms as is typical of tropi-
are ubiquitous, the latter including some cal upwelling areas. This area is the most
species that grow deeper in the Red Sea fertile part of the Arabian Seas Marine Re-
than anywhere else due to their utilization gion
of blue light and energy-conserving growth Pelagic primary productivity of the Red
patterns. Calcareous red algae are mainly Sea is highest in the south. In the very clear
restricted to very shallow areas. For many Gulf of Aqaba, significant production contin-
species, vigorous water movement is essen- ues to depth of about 200 meters (com-
tial, although dense algal growth also oc- pared to 40 meters in the Arabian Sea).
curs on unconsolidated substrate, helping Summer values are approximately half
to stabilize it. In a list derived from numer- those of winter.
ous earlier collections, Papenfuss (1968) re-
corded nearly 500 species of algae from Zooplankton
the Red Sea. Basson and others (1977) Zooplankton diversity declines westward.
and Basson (1979) recorded only about Peak numbers throughout the Red Sea lag
one-tenth of this number of fleshy algae in a few weeks behind those of phytoplankton.
the Gulf, and there has been little system- Calanoid copepods are the most important
atic collecting subsequently to indicate what group with 300 species in the Arabian Sea,
the true numbers are. 60 in the southern Red Sea, and 46 in the
In the Gulf there is usually a gradation north. Euphausiids are important in terms of
from coral to algal domination on limestone biomass, although of 22 Indian Ocean spe-
platforms as stress increases, causing a de- cies, only 10 occur in the Red Sea. In the
mise of both corals and reef growth. In Gulf, zooplankton shows marked temporal
these conditions, usually termed “marginal,” and geographic variation; diversity is less
algal dominance arises from shading, than in the Arabian Sea but is similar to that
greater tolerance of temperatures below in the Red Sea with 33–45 species per cu-
18°C, and high levels of dissolved nutrients. bic meter near offshore islands. Densities
are high and as many as 3,000 individuals
Phytoplankton per cubic meter have been recorded.
Many species of plankton found in the In- There is marked vertical stratification of
dian Ocean are absent from the Gulf and zooplankton in the Red Sea. Peak diversity
the Red Sea. Of 452 known Indian Ocean and abundance remain within the photic
dinoflagellates, 130 have been recorded in zone, but a secondary maximum is found
the Arabian Sea, 88 in the Red Sea, and near the oxygen minimum layer at 400-me-
fewer still in the Gulf. The decline westward ters. Close to shore, demersal zooplankton
in the Red Sea is partly compensated by and larvae of reproducing invertebrates
the presence of several endemics and by tend to dominate measurements of num-
blooms of Oscillatoria erythraeum. Cell den- bers and productivity.
sities similarly decline westward. While the
trend is similar in winter and summer, win- Fish
ter cell counts are one to two orders of mag-
nitude greater. There is a large input of There are marked differences throughout
mesoplankton from the Gulf of Aden at the Arabian Seas Region in the structure
peak times of influx, though most do not sur- and composition of fish assemblages, re-
vive beyond the central Red Sea. In the
Marine Region 11: Arabian Seas 47
flecting the heterogeneous nature of the en- habitat and the extreme environmental con-
vironment. ditions.
The most diverse assemblages occur Although the region is relatively rich in
within the Red Sea with a total of approxi- terms of commercial finfish and shellfish
mately 1,000 species present (including species, the fisheries sector plays only a mi-
nonreef species). The Gulf supports only nor role in most national economies. In
about 200 species in total, of which at least most countries the contribution of the fisher-
125 are found on reefs. Within the Red Sea ies sector to the gross domestic product is
there are major differences in assemblage less than 1 percent. However, in the Sultan-
composition between areas north and south ate of Oman revenue from fish was equiva-
of latitude 20°N. This may be due to the dif- lent to 36.5 percent of the total oil export
ferences in reef habitat between these ar- revenue for 1984, and fish currently are the
eas or differences in water quality, most important export product after petro-
particularly the limit of penetration by nutri- leum. In general, fisheries of the region
ent-rich water from the Gulf of Aden (which seem to be suffering from overexploitation.
occurs at around 20°N), and north–south This is particularly true in the case of the
gradients in temperature, salinity and turbid- shrimp fisheries. Aside from overexploita-
ity. tion resulting from inadequate fisheries
The Gulfs of Aqaba and Suez support dis- management, degradation of the environ-
tinctive fish assemblages. Those of the Gulf ment is probably a major cause of the de-
of Suez share greater affinities with south- cline in fish and shrimp catches. This
ern Red Sea assemblages than with the degradation includes the elimination of im-
Gulf of Aqaba, probably due to its shallow, portant nursery areas (especially for
turbid nature. The Gulf of Aden marks a divi- shrimp) by land reclamation and dredging
sion between a fauna dominated by Red in the coastal areas, destruction of feeding
Sea species to one dominated by Indian and breeding habitats by bottom trawling,
Ocean species in the Gulf of Oman and the and increased marine pollution by dis-
Gulf. Upwelling of cold water in the Arabian charge of liquid and solid wastes into the
Sea appears to provide this major bio- marine environment. In addition, selective
geographic barrier. Compared with the Red fishing for species of predator fish may
Sea, reefs of the Gulf and Gulf of Oman have upset the balance between the differ-
support low-diversity fish assemblages. ent species.
This probably reflects the scarcity of reef
Table 11.1 Breeding Turtle Records and Counts around Arabia
Location Green Hawksbill Loggerhead Olive Ridley
Gulf islands 750–1,000 100–300
Daymaniyat Islands +
Capital Area, Oman + +
Rass al Hadd 6,000
Masirah Island + 80 30,000 230
Gulf of Kutch +
Gulf of Aden —
Red Sea + +
— Data not available.
48 A Global Representative System of Marine Protected Areas
Marine Reptiles actively hunted and are also caught acci-
dentally (Preen 1989).
Sea Snakes
A recent review of snakes of Arabia (Gas- Whales and Dolphins
peretti 1988) records that of the 55 species There has been no systematic survey of
of sea snakes, 9 or 10 occur in some whales or dolphins in any of the coastal wa-
coastal waters of Arabia. All but one spe- ters of the Arabian Seas. The greatest num-
cies of sea snake are found in shallow ber of records come from the Arabian Sea,
coastal waters, which may be turbid and where both dolphins (and toothed whales)
where there is organically rich substrate. and several baleen whales have been re-
Notably, sea snakes do not occur in the ported. At least a dozen species of dolphin,
Red Sea. and finless porpoise, have been recorded
for the Arabian Sea and coastal waters.
Turtles Fourteen species of cetacean, including
Table 11.1 gives details of breeding turtle three species of great whale (blue, Bryde’s
records and counts around Arabia (Miller, and sperm), have been recorded in the Gulf
personal communication; Symens, personal of Aden (Smith and Smith 1991).
communication; Sheppard, Price, and Gallagher (1991) lists 14 species of
Roberts 1992). toothed whale and dolphin from Oman,
The region is now very important for sev- based on a collection of stranded car-
eral species of turtles. A significant reduc- casses. Humpback whales are believed to
tion in numbers of turtles from breed off Oman, and the highly productive
overexploitation has taken place (Miller upwellings that occur along this coast are
1989; Frazier, Bertram, and Evans 1987). thought to be an important feeding area for
The most important part of the region for tur- this endangered species (Reeves, Leather-
tles is the Arabian Sea both in terms of num- wood, and Papastravrou 1991). Papas-
bers of breeding species and abundance of tavrou and Salm (1991) describe a
nesting individuals. Present levels of turtle small-scale marine mammal fishery in
populations are clearly reduced, providing a Oman.
focus for conservation efforts in the region Basson and others (1977) reported that
(Clarke and others 1986). several species of dolphin occur in the Gulf,
some in schools of hundreds. Four small-ce-
Marine Mammals tacean species are known to occur in the
Gulf, including the finless porpoise (Neopho-
Dugong caena phocanoides). Three to four species
The dugong occurs in both the Gulf and of great whales have also been recorded,
Red Seas. It has not been recorded along although it is probable that these animals
the Arabian shores of the Arabian Sea are not resident but strand after becoming
where very few sites of suitable habit occur. trapped (Preen, personal communication).
In Oman, there have been no confirmed Concern about the long-term survival of
sightings of dugong. The estimated Gulf marine mammals in the Gulf has arisen as
population is 7,310 (about 1,300) individu- a result of a series of die-offs. In 1983 at
als, making this the most important area for least 38 dugong and 33 dolphins stranded
the species in the western part of its range, along the Saudi Gulf and Bahrain. This die-
and second in global importance only to off was coincidental with the Nowruz oil
Australia. In the Red Sea dugong are esti- spill, but there was no direct evidence to
mated to number about 4,000. Dugong are link the two events (Preen 1989). In 1986
over 500 dolphins died in Saudi Arabia,