BIOLOGICAL ASPECTS OF TWOBAR SEABREAM ACANTHOPAGRUS BIFASCIATUS (FORSSKÅL, 1775) IN WATER SURROUNDING SAMMALIAH ISLAND – ABU DHABI

SUMMARY \A total of 346 fish of Acanthopagrus bifasciatus were collected from water surrounding Sammaliah Island during September 2001 to May 2004. Lengths and weights for all fish were taken and relationship equation was obtained. Age of fishes were determined depend on scales reading and found 10 age groups from 0-group to 9 + group. Fish growth was represent by von Bertalanffy equation as follow (L t = 535 [1-e -0.1983(t+1.0828) ]) and determine 535 mm as a maximum total length reached by this fish. Sex of 153 fish was determined and gonado somatic index was measured. It was found according to GSI and relative condition factor values that spawning period during January and February. Sex ratio was [females 93 (60.8%), males 12 (7.8%) and hermaphrodites 48 (31.4%)]. Food habits of 212 fish were studied. The main food for this fish is snails followed by crabs. It seems that feeding intensity and relative condition factor are higher during winter months than summer months.


Introduction
The Arabian Gulf is an extension of the Indian Ocean and has an area of about 24832 km 2 , while the volume of it's water is around 6000 km 3 , and it's average depth is 35 m (4).The Arabian Gulf waters are generally warm because of their geographical location and for their shallow depths and for the absence of cool circulating sea currents (13).The United Arab Emirates is a coastal country located on the southern side of Arabian Gulf having extensive coastlines on its west and east facing the Arabian Gulf and the Gulf of Oman respectively.The long of coastlines of UAE is 1400 km involved the coasts of islands.Al Sammaliah Island is located between 54 longitude and 24 latitude and approximately 12 km for Abu Dhabi City (Figure , 1).The Island has an area of 13.448 km 2 .
Acanthopagrus bifasciatus belongs to Sparidae family (porgies) that occurs in tropical and temperate regions of Atlantic, Indian and Pacific Oceans.Fishes of this family are represented by 35 genera that involve 112 species (6).Ten species of this family are recorded in UAE waters (22; 5).Twobar seabream occurred in small groups and fished in UAE shallow coastal waters by gargoors, seine nets and lines.It is common in UAE fish markets and has commercial importance (16).

Materials and Methods
Fish samples were collected from water surrounding Sammaliah Island during a period extended from September 2001 to May 2004 by cast and seine nets.A total of 346 fish were collected and examined.Measurements of total length (TL) and total weight (W), were taken for all fishes.Ages of 341 fish are also determined by reading their scales.The digestive canals of 212 fish were examined for feeding study.Sex determined for 153 fish and the gonads were weighted.
Linear equation Y = a + n × X used to express relationships between total length and weight where X represented total length and Y represented other measurements, a and n are constants calculated from data (14).
The famous equation of von Bertalanffy used to represent the growth study, which has the general formula, L t = L ∞ [ 1 -e -k (t -to) ], Where the time unit t represents the age of the fish measured in years from a starting time t o , L t is the length of the fish at time t, L ∞ is maximum length reached by this fish and k is the growth coefficient.The values of K and L ∞ are measured by using draft of ( 9) and (26), while the value of t o be measured by the following equation that derived from von Bertalanffy equation: t 0 = { ln (L ∞ -L t ) + K tln L ∞ } / K Instantaneous growth rate (G) was calculated using the following formula of (19), G = ln (W t+1 / W t ), Where W t is the weight at time t and W t+1 is the weight at time (t +1 ).Relative Condition factor (Kn) was calculated according to the following Lecren formula (14): Where W is total observed weight in gram and W is total calculated weight in gram.Sex was determined and gonado somatic index (GSI) was calculated according to following equation ( 25), GSI = (W g / W) × 100, Where W g is gonad weight in gram and W is total fish weight in gram.
Both intestinal and stomach contents were examined to determine food materials.Point's method (12) was used, where points range from 0 (empty canals) to 25 (filled canals).In addition to points the frequency of kinds of food also used to give more information about food materials.Feeding intensity was calculated according to the following equation of (10): Feeding Intensity = Sum of feeding points / No. of feeding fish Feeding activity was calculated according to the following equation (7): Feeding Activity = (No. of feeding fish / No. of examined fish) X 100.

Results and Discussion 1-Length distribution and length weight relationship
The distribution of total length frequency for all 346 fish was shown in figure (2).Table (1) illustrates that there are 28 length groups with dominant group of 280-289 mm (12.7% of total fish), followed by group 270-279 mm (10.7% of total fish).Total length of fish range from 159 to 420 mm and weight from 73 to 1520 g.Length frequency study of twobar seabream in Gulf of Aden pointed that fork length range is 187-417 mm (8).The survey of (5) stated the smallest fish for twobar seabream and yellowfin seabream is 190 mm and for king soldierbream is 160 mm. Figure (3) show the relationship between total length and weight, where slope value was 3.1339.Length-weight equation for twobar seabream in Kuwaiti waters was W = 0.0176 + 3.001 × TL (20), while (8) found that this relationship for the same species in Gulf of Aden is (W = 0.039 + 2.91 × TL).

2-Age and Growth
Table (2) shows the average measurements for 9 age groups (from 1+ group to 9+ group).Figure (4) show Ford and Walford draft that represented the relationship between lengths at time t and lengths at time t +1 , so growth of this species were represented by the following von Bertallanfy equation: L t = 535 [1-e -0.1983(t+1.0828)] The value of L ∞ for A. bifasciatus was 535 mm, while the previous studies indicate different values.Maximum total length of 339 mm for this species in UAE waters mentioned by (5), while (21) mentioned maximum total length of 500 mm in Somalia waters.Maximum total length of 318 mm for A. bifasciatus in Gulf of Aden recorded by ( 8), while Pauly (18) recorded maximum total length of 481 mm in Gulf of Aden at depth 50 m, and (20) recorded 349 mm as maximum total length in Kuwaiti waters.Maximum total length recorded for relatives seabreams are 900 mm for A. berda, 800 mm for Rhabdosargus sarba, 700 mm for Argyrops spinifer, 500 mm for A. datnia, A. latus, Sparidentex hasta and 300 mm for Crenidens crenidens and Diplodus sargus kotschyi (11).The value of calculated lengths from above growth equation and related value of calculated weights from length-weight logarithmic equation are shown in table (3).Maximum value for instantaneous growth rate (0.9451) was recorded between age 1+ and age 2+, then the values decreased with age reaching to the minimum (0.1096) between age 8+ and age 9+ (table, 3).It is well known that most organisms have rapid growth rate during early stages of life, which became slowly or stopped at all during progress stages of life.Figure (5) shows the differences in the values of relative condition factor for different length groups of A. bifasciatus.These values range from (0.8050-1.0340), with an average of 0.9917.Generally there aren't differences between relative condition factors with the size of fish.
Table (3) shows the values of total length, observed weight, calculated weight and relative condition factor for different months.It seems that the value of relative condition factors is higher in winter months than in summer months (Figure,13).This result may be related to very high temperature (36 ْ◌C) for more than two months during summer, which retard feeding process and growth.It is necessary also to mentions here that this very high temperature is very close to lethal temperatures for most fishes around the world.Too much mortalities occur during summer months for twobar seabream and other UAE fishes reared in floating cages at the same area of fish sampling (Personal Observations).It is concluded from average value (1.0004) of relative condition factor of different months that this fish are well being in the water surrounding Sammaliah Island.Relative condition factor also related to reproduction cycle where it is increased at maturation period and decreased after spawning.Values of relative condition factor begin to increase from October and reach maximum value (1.0565) at December, and then begin to decrease after spawning till reaching the minimum value (0.9416) at September.It is supposed from these results that spawning of twobar seabream may be beginning at January.

3-Food Habits
Table (4) shows monthly differences in feeding activity and feeding intensity (Points/fish) of twobar seabream.Feeding activity values for most months are 100%, except for May, July and September where the values decreased to (94.6, 95 and 85.7) % respectively.Maximum value (11.4) of feeding intensity occurs during November and December, while the minimum (7.1) occurs during September, with an average of 9.6.It seem that feeding intensity are high during winter months (November-February) and low during summer  months (May-September), with moderates values of 9.2 and 9.4 during March and April respectively.This result may be related also (as we mentioned about relative condition factor) to very high temperature (above 30 ْ◌C) at summer months, which retard feeding process and reduced feeding intensity.This result can be supported also by twobar seabream fish behavior at obvious shallow waters in Sammaliah Island during very hot summer months, where they didn't bite live crabs in the hooks (Personal Observations).Table (5) illustrates monthly differences in feeding ratio and frequency of twobar seabream.Main food of twobar seabream is snails that comprise 39.5% of food materials, followed by crabs that comprise 18.7%, with low importance starfish that comprise only 4.4% of food materials.This result may be related to the indigestive calcareous shells of snails that accumulated in fish stomachs.The main foods of juveniles and adults twobar seabream are zoobenthos, benthic crustaceans, mollusks and worms (17).Mollusks are recorded as the first main food in king soldierbream and goldlined seabream, and also as important food in black seabream, yellowfin seabream and natal pandora (11).It seem that the ratio of crabs in fish stomachs are high during hot summer 44.2, 33.8 and 31.1 for July, August and October respectively, except for September 5.9, while it is low during winter months from November 17.8 to April 6.1 (Table,5).At the same time it is noticed that frequency of crabs in fish stomachs are higher during summer months than winter months.These results may be related to high abundance of crabs in
Table (6) shows monthly differences in gonado somatic index (GSI) for different sexes of twobar seabream.There is a decreased in GSI of females from 1.6264 at December to 0.7476 in January, followed by suddenly decreased to 0.16 at February.Also there is a suddenly decreased in GSI of males from 1.3435 at December to 0.5253 at January.It may be concluded from these differences in GSI of females and males that spawning of twobar seabream occurs during January and February.This fact supported by previous conclusion of relative condition factor values, that spawning begins at January.It has been stated that major spawning period for sobaity seabream in Kuwaiti waters extending from January to March, while GSI values increased dramatically in January, reaches its beak value in February, remains at the high levels in March and then decreases in April (1), while (15) stated that spawning of sobaity seabream in Kuwaiti waters occurs during February to March.Major spawning period for Pomadasys argenteus in Kuwaiti waters extending from February to April (2).The spawning period for picnic seabream in Australia extending from June to September (23), while spawning periods in South Africa for picnic seabream extending from May to August, and for goldlined seabream from July to November (25).