MANGROVE STUDY YIELDS NEW INFORMATION ON FISH BREEDING

the country’s Gulf of Oman coast there is only one mangrove forest, lining

the creek at Khor Kalba, which is the home of the only endemic sub-species

of bird yet identified in the UAE: Halcyon chloris kalbaensis, a sub-species

of the white-collared kingfisher.

Along much of the UAE’s 450 kilometre long Arabian Gulf coast however,

mangroves can be found from the north, near Ra’s al Khaimah, westward to

the Sila’a Peninsula, adjacent to the Khor al Odaid, as well as in sheltered

waters around many of the country’s over 200 offshore islands, the bulk

of which lie in the waters of the largest Emirate, Abu Dhabi. On the instructions

of UAE President, Sheikh Zayed bin Sultan al Nahyan, existing mangrove

forests are protected, and, during recent years, large areas of inshore

tidal flats have been successfully planted.

One of the key purposes of the University’s study was to evaluate the

health of the mangroves themselves, which are sensitive to environmental

pollution. Apart from some damage near an industrial centre at Khor Khuwair,

in the northern emirate of Ra’s al Khaimah, Al Ghais found the mangroves

to be in generally good health along the remainder of the coastline.

Investigations in the past into mangroves, albeit carried out by amateurs

rather than qualified scientists, have suggested that they are important

breeding grounds for many of the species of fish which are commercially

caught in the Arabian Gulf. In recent years, as the number of commercial

fishermen in the UAE has rapidly increased, the average catch per fisherman

has steadily declined, and Al Ghais has been in the forefront of officials

and scientists warning of possible irreversible depletion of fish stocks.

Dr Al Ghais began his study by selecting a number of field monitoring

stations in areas of mature mangroves, stretching from Khor Khuwair in

the north-east to the lagoon of Khor al Beida, in Umm al Qaiwain, and then

to the east of the island of Abu Dhabi. These were visited at regular intervals

in order to net fish and other small marine fauna, and to collect water

samples. Access to the sites, most of which cannot be reached by land,

was facilitated by the UAE Coastguard, the first time that the force has

become involved in a scientific study of this nature.

In thirty-two different catches made with beach seine nets on the edge

of the mangroves, Al Ghais and his colleagues caught a total of 10,600

fish. Identifying a sample of the catch to the level of family and genera,

Al Ghais identified a total of 29 different forms with the catch dominated

by two distinct species, the Arabian killifish, Aphanius dispar, and the

slender spine mojarra, Gerres oyena. Of these the killifish was found at

all stages of development, indicating according to Dr Al Ghais, that the

species most likely spends its entire life-cycle among the mangroves. Whilst

it has no commercial value in its own right it serves as a prey species

for other food fishes that hunt among the mangroves.

The mojarra, a member of the sand perch family, is of commercial importance,

although only one of the three species known from the Arabian Gulf was

collected. The slenderspine mojarra grows to a length of around 32cms but

the maximum length of those collected in the study was around 10cm, suggesting

that the mangroves serve as a nursery for this species.

Other commercially important fish to be found, again in the form of

juveniles, included snappers, of which the most abundant was Lutjanus fulviflamma;

mullets represented by Liza macrolepis; sea breams, represented by Sparus

sarba; and flatfish represented by Pseudorhombus javanicus. ” Many

of these commercial species are predatory,” says Al Ghais. “It

appears that they find the mangroves and associated creeks an ideal nursery

ground for their development.”

On occasion a large catch of Clupeidae, (herrings and sardines) was

made in the seine nets. More commonly associated with the open sea, these

relatively small fish feed upon zooplankton, and the abundance of this

food source in and near the mangroves may account for their presence, Al

Ghais believes.

“It is obvious from the initial work undertaken on the fish population

of the UAE’s mangroves that these areas provide an important habitat for

various stages in the life-cycle of a large number of fish” says Al

Ghais.

The study also examined the phytoplankton composition of the water samples

collected in and around the mangroves. A total of 43 species were identified,

a majority of which, as expected, were diatoms, with the dominant order,

Centrales, providing 23 species.

Besides the task of studying the fish population of the mangroves of

the UAE coast, Al Ghais has also examined the population of crabs. Two

species were found to be most common, the blue crab, Portunus pelagicus,

and the fiddler crab, Uca lactea annulipes. Other species collected included

Metograpsus messor, Macrophthalmus depressus, and Sesarma (Parasesarma)

plicatum, with a single specimen of Thalamita crenata being collected at

one of the stations in Umm al Qaiwain, but nowhere else.

Water and sediment samples collected in the various stations were also

analysed for salinity content, and for a number of other parameters, including

the presence of phosphorus and petroleum hydrocarbons. While the work on

the presence of hydrocarbons is still to be completed, the results so far

have indicated, according to Al Ghais, that the water samples from the

mangrove ecosystems generally have a higher salinity than the average value

from nearby creeks.

Al Ghais’s study is one of a series of investigations currently being

undertaken into the health and wildlife of the UAE’s mangroves. Ever since

an oil spill in the early nineteen eighties, the Abu Dhabi Company for

Onshore Oil Operations, ADCO, in which Shell has a minority shareholding,

has carried out twice-yearly monitoring in a number of coastal areas to

assess the health of the mangroves, while bodies like the Abu Dhabi-based

National Avian Research Centre are collecting information on the migrant

and resident birds to be found breeding in, or feeding near, the mangroves.

Unlike mangroves in many parts of the world, those in the UAE do not

face a major threat from fire-wood collection by local inhabitants, while

the extensive Government-backed planting programme has meant that the total

area of inter-tidal flats now covered by mangroves is presently increasing,

rather than declining.

The University study by Al Ghais and his colleagues, however, has provided

the first scientific evidence of the important role that the eco-system

of the mangroves plays in ensuring the continued health of the country’s

fish-stocks.