SOME BIOLOGICAL ASPECTS OF THRYSSA HAMILTONII AND THRYSSA MYSTAX IN KHOR AL-ZUBAIR, NORTHWEST ARABIAN GULF N. A. HussAiN AND T. S. ALI Marine Science Centre, University of Basrah, Basrah. Iraq, ABSTRACT Study on 826 specimens of T. hamiltonii (Gray) and 439 specimens of T. mystax (Bloch and Schneider) collected from Khor al-Zubair during the period from July 1983 to June 1984 has shown that the dominant length range for both the species is 15.0-19.9 cm, representing 62.7% of the total number of T. hamiltonii and 58% of that of T. mystax. Based on GST, both species have single, prolonged spawning, extending from December to April, with peaV in March. Length-weight relationship and 'K' factor have been calculated for both species. The nature of food has proved that both are carnivorous, depending mainly on shrimp and animal remains. However, T. hamiltonii is the dominant species from September to December whereas T. mystax is dominant from Fe­ bruary to August. But the peak feeding for both the species is during September and December, and, according to the Morisita index (C), high competition exists between the two species. INTRODUCTION Anchovies are widely distributed throughout the Indian Ocean, the Ara­ bian sea and the Red sea (Kuronuma & Abe 1972). They contribute largely to the fishery industry of the Arabian -Gulf (Lamboeuf & Simmonds 1981). Of the six species of anchovies recorded from the Arabian Gulf (Relyea 1981), Thryssa hamiltonii and T. mystax are among the common species caught in the coastal regions. Most of the previous studies on anchovies are mainly concerned with spawning period, maturation cycle and, to a less extent, with length-weight re­ lationship and food and feeding habits (Venkataraman 1956, Dharmamba 1959, Masurekar & Rege 1960, Shamsul-Hoda 1976, 1982, 1983). But, despite flie commercial importance of T. hamiltonii and T. mystax in the northern Arabian Gulf, no attention has so far been paid to the study of their biology. NEUROCRANIUM OF FLATFISH 151 REFERENCES DAS, M. 1981. Coastal water ichthyofauna of two districts (Cuttack and Bdasore) of Orissa with observation on the feeding and reproductive biology of some flatfishes. Ph.D. thesis, Calcutta University. DATTA, N. C. AND M. D««. 1984a. Cranial anatomy of some Indian Cynoglossids. A study in adaptation. Part^I: CHteology, Zool. Jb. Anal., 112: 33-42. DATTA, N. C. AND M. DAS. 1984b. Cranial anatomy of some Indian Cynoglossids. A study in adaptation. Par-II Myology, Zool. Jb. Anat., 112: 43-50. DoBBEN, W. H. VAN. 1937. Uber • <}en Kiefermechanismus der Knochenfische. Archs. neerl, Zool., 2: 1-72. KAPOOR, A. S. AND P. P. OJHA, 1973. The olfactory apparatus in the flatilsh Cyno­ glossus oligolepis. Trans. Amer. Micros. Soc., 92(2): 298-304. KYLE, H. M. 1923. The asymmetry, metamorphosis and origin of flatfishes. Phil. Trans. R. Soc. (B), 211: 75-129. MENON, A. G. K. 1977. A systematic monograph of the tongue soles of the genus Cyno­ glossus HAMILTON-BUCHANAN (Pisccs: CynogloSsidae). Smithson. Contrib. Zool, 238: 1-129. NELSON, E. M. 1963. A preparation of standard teleost study skull. Turtox News, 41: 72-74. OcHiAi, A. 1966. Studies on the comparative morphology and ecology of the Japanese soles. Spec. Rep. Misaki. mar. biol. inst. (Kyoto University), 3: 1-97. SAXENA, P. K. AND R. K. RASTOOI. 1968. The cranial nerves of the flat fish, Cynoglossus bilineatus. Annotnes Zool. Jap,, 41: 70-76. TAYLOR, W. R. 1967. An enzyme method for clearing and staining small vertebrates. Proc. V.S. natn. Mus., \22: 1-17. TRAQUAIR, R. H. 1865. On the asymmetry of the Pleuronectidae, as elucidated by an examination of the skeleton in the turbot, halibut and plaice. Trans. Linn. Soc. I^ndon, 25: 263-296. YAZDANI, G. M. 1969. Adaptation in the jaws of flatfish (Pleuronectriformes). J. Zool., London, 159: 181-222. BIOLOGICAL ASPECTS OF ANCHOVIES 153 MATERIAL AND METHODS Fishes were collected duiriiig the period from July 1983 to June 1984 from Khor al-Zubair (Fig. 1), using four fixed giU nets of diflferent mesh sizes, 1.0, 1.5, 2.0 and 2.5 cm^., and classified after Al-Daham (1977). Specimen^ were analysed individually for total length (T.L.), weight (W) and gonad weight. Stomach contents were examined for food composition after Hynes (1950) and Windell (1968). Similarity of food items between the two specie$ was ascertained by the Morisita iiidex (Cx): s XI Yi C X = 2 L s xi2 + I Yi^ where (s) is the total number of food items, (xi) is the proportion of food items in the stomach of T. hamiltonii and (Yi) the proportion of these items in the stomach of T. mystax. (i) represents % of the total points allotted. 3100 Jd'ao 3(fot ARABIAN GULF FIG. 1. The map of nortKern Arabian gulf showing the sampling area. 154 HUSSAIN AND ALI RESULTS A total of 826 specimens of T. hamiltonii and 439 of T. mystax were collected during the sampling period. Length-Frequency Distribution The distribution of length frequency for T. hamiltonii is represented in Fig 2a. The main size range, 15-19.9 cm, was predominant during the period from September to May, forming 62.7% of the total number. The second im­ portant size group was 20-24.9 cm, which appeared in samples from Septem­ ber to June with a peak in February. The size group 10-14.9 cm was dominant during the summer months, when the Juveniles < 10 cm also appeared in the samples. FIG. 2. Length-frequency distribution of T. hamiltonii (above) and T. mystax (below). The length-frequency distribution of T. mystax is shown in Fig. 2b. Here also the dominant length range was 15-19.9 cm, found during September to BIOLOGICAL ASPECTS OF ANCHOVIES 155 May, forming about 58% of the total number. The second important size group was 10-14.9 cm, dominating in the suminer months. The size group < 10 cm was absent round the year. The Gonado-Somatic Index (GSI) The GSI results (Fig. 3) have indicated that, for both the species, there was a prolonged spawning period, extending from December to April (reach­ ing the nutxwnnm in March). 4.^0 1..00 3.80 J>0 3i<.0 i20 3.00 ?j60 ?M zao ^ wo O no 1,40 1.20 1.00 aeo O.bO 0.1.0 0,20 Jul Au9. Sept Oct., Nov. Get, Jan F«t «or. Apr May JUn FIG. 3 The gonado somatic index (GSI) of T. hamiltonii (unbroken line) and T. mystax (broken line). Length-Weight Relationship Length-weight relationship for both species was calculated using the wbole Siample. The Results, shown in Fig. 4 and Fig. 5, are best represented by 156 HUSSAIN AND ALI the following equations: r. hamiltonii W = 0.00275 L^'^^^ r. mystax W = 0.00309 L 3.264 The rate of change (n) of weight relative to length in the equation for r. hamiltonii was a little different from that of T. mystax, the rates being 3.286 and 3.264 respectively. This indicates that T. hamiltonii was marginally more robust than T. mystax of the same length and both species grew at a rate higher than that of cube total length. w X • 1.1 • 0,9 • 1 Tl 17 n I' '^ f 7 1. log I. L^9 U FIG. 4. Length-weight relationship of FIG. 5, Length-weight relationship of T. hamiltonii. T. mystax. The Relative Condition Factor (K) Calculated K values for T. hamiltonii and T. mystax are shown in Fig. 6. Two peaks were noticed for both species, the first at 14 cm (K = 1.111 for T. hamiltonii, K = 1.115 for T. mystax) and the second at 20 cm (K = 1.114 for T. hamiltonii, K = 1.120 for T. mystax), respectively representing the minimiun and maximimi maturation lengths. Food and Feeding Habits For the study of food, the stomachs of 660 T. hamiltonii and 410 T. mystax were examined. The results of the stomach analysis, expressed by both points and occurrance methods, are shown in Table 1 and Table 2. Animal remains were the dominant food item of T. hamiltonii, forming 47.15% of the total points, whereas shrimps (Parapenaeus stylifera and Meta- penaeus ^affinis) constituted the most dominant food item of T. mystax, form­ ing 43.03% of the total points. BIOLOGECAL ASPECTS OF ANCHOVIES 157 Shrimps were the important food item of t. hamiltonii during the period from August to December and of T. mystax during August to January. Animal remains were of major importance during the period from February to July for 0 9 K> » tt « l< ^ 16 17 m-'tf X 21 22 23 X 25 FIG. 6. Variation of Condition factor (K) in relation to length (cm) in T. hamiltonii (unbroken line) and T. mystax (broken line). both species. Fishes (Mugil spp. and Otolithes spp.) were the third important food item, forming 7.85% and 7.50%, respectively for T. ha­ miltonii and T. mystax, with two peaks, in October and in March. Crabs (Petrolisthes lamarckii, P. indicus and Macrophthalmus depressus) formed tiie fourth important food item and were found in 4.73% and 5.66% respectively of the T. hamiltonii and T. mystax examined, with a peak in April for the former and February for the latter. Molluscs (Dosinia laminata) were found only in less than 1% of both species,. Generally, the feeding intensity of bofli species was high during the period from September to January and decreased sharply during the summer months. The stomachs were empty in 2.6% of T. hamiltonii and 1.7% of T. mystax examined.