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MATURATION and SPAWNING of the SMALLSPOTTED GRUNT Pomadasys Commersonnii (LACEPÈDE, 1801) in the ARABIAN SEA OFF OMAN

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MATURATION and SPAWNING of the SMALLSPOTTED GRUNT Pomadasys Commersonnii (LACEPÈDE, 1801) in the ARABIAN SEA OFF OMAN Thalassas, 27 (1): 67-79 An International Journal of Marine Sciences MATURATION AND SPAWNING OF THE SMALLSPOTTED GRUNT Pomadasys commersonnii (LACEPÈDE, 1801) IN THE ARABIAN SEA OFF OMAN A. AL-NAHDI, A. AL-MARZOUQI, N. JAYABALAN(1) & S. AL-HABSI Key words: Pomadasys commersonnii, Lm, GSI, relative condition, spawning, fecundity, sex-ratio, Arabian Sea, Oman ABSTRACT INTRODUCTION Observations on the maturation and spawning Fishes of the genus Pomadasys (Family: of the smallspotted grunt Pomadasys commersonnii Haemulidae) are represented by 11 species in caught from the Arabian Sea off Oman were made Omani waters (Fouda et al., 1997). The species between April 2005 and March 2007. While, males P. commersonnii popularly called smallspotted matured (50% maturity) at 44.2 cm TL, females grunt is one of the commonest fishes caught by matured at 42 cm TL. The population of P. the artisanal and industrial trawlers from Omani commersonnii spawned during June-December. The waters. The species is distributed along the northwest monthly gonado-somatic indices and the occurrence coast of India to the southern Oman, in the south of spent gonads coincided with the peak spawning to Seychelles, Madagascar and South Africa in the season. However, the monthly relative condition was coastal waters, estuaries, tidal fissures and even can not indicative of the spawning activity. Individual fish tolerate fresh water (Smith and Heemstra, 1986). spawned more than once a year. Fecundity ranged Besides forming a good recreational fishery (Smith from 214,510 to 1,421,520 eggs. Sex-ratio indicated and McKay, 1986; Mann et al., 2002), the species has homogeneous distribution of males and females in also been identified as a potent candidate for intensive the population. marine fish farming in South Africa (Deacon and Hecht, 1995; Mperdempes and Hecht, 2002). The estimated contribution of P. commersonnii Marine Science and Fisheries Centre from the Arabian Sea coast of Oman for the years Ministry of Fisheries Wealth, P.O. Box 427; P.C. 100, Muscat Sultanate of Oman 2005-2006 and 2006-2007 stood at 507 t and (1)E-mail: [email protected] 443 t respectively (GoSO, 2005; 2006; MSFC, 67 A. AL-NAHDI, A. AL-MARZOUQI, N. JAYABALAN & S. AL-HABSI Figure 1: Map showing the sampling locations. unpub. data). In spite of its commercial and culture gillnet, trap and handlines landed at Lakbi and based fisheries importance of P. commersonnii, Salalah operated in the coastal waters of the Arabian no information is available on the biology and Sea along the coast of Oman (Fig. 1). Altogether, population characteristics from Omani waters. 838 specimens of P. commersonnii were used for the The present paper deals with the maturation and study. The fish were brought to the laboratory in ice spawning of P. commersonnii from the Arabian Sea boxes and after washing, each fish was measured coast of Oman. to its total length (TL) to the nearest 1 mm using a fish measuring board and the total wet weight (TW) MATERIAL AND METHODS was recorded to the nearest 1 g using an electronic balance. Then the fish was cut open and the sex Sampling protocol and maturity stage were recorded. The gonads were removed and weighed to the nearest 0.001 g using Monthly random samples of fish were obtained an electronic balance. Ovaries were preserved in between April 2005 and March 2007 from the 5% neutral formalin for subsequent analyses of commercial catches of the artisanal gears such as maturation and spawning. 68 MATURATION AND SPAWNING OF THE SMALLSPOTTED GRUNT Pomadasys commersonnii (LACEPÈDE, 1801) IN THE ARABIAN SEA OFF OMAN Maturation and spawning Maturity stages The female maturity stages were fixed by macroscopic observation of ovaries and by the microscopic structure of the eggs. The male maturity stages were determined by macroscopic structure of testes in the body cavity. Six stages of maturity (I-Immature; II-Maturing 1; III-Maturing 2; IV-Mature; V-Ripe/Running and VI-Spent) were recognized in females and males. Development of ova to maturity A total of 36 ovaries from various stages excepting stage V were used for measurement of intraovarian eggs. The development of ova from immature to mature/ripe was studied by measuring the intraovarian eggs using an ocular micrometer fitted to the eye piece of the microscope. Each ocular micrometer division (m.d.) was equal to 12μ. The method of measurement of ova diameters was similar to that of Clark (1934); Prabhu (1956) and Jayabalan (1986). Ova diameters from ovaries of same stage of maturity were pooled and used for percentage frequency curves. Ova were grouped into 2 ocular micrometer divisions (m.d.) size groups. Except in stage I, immature ova less than 5 m.d. were not measured in other maturity stages as they occurred in large numbers indicating the general egg stock. Length at first maturity To calculate the length at first maturity (Lm), fish were grouped sex-wise into 2 cm class intervals and fish from stage III onwards were considered as mature. A logistic function was fitted to the fraction of mature fish against length interval using the non- linear least square regression method as suggested by King (1995). The logistic equation used was: Figure 2: Development of ova to maturity in P. commersonnii. P = 100 / (1 + exp (-r (L-Lm))) 69 A. AL-NAHDI, A. AL-MARZOUQI, N. JAYABALAN & S. AL-HABSI 10 8 6 GSI 4 2 0 Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Months Figure 3: Monthly gonado-somatic indices (mean and SD) in male P. commersonnii. 10 8 6 GSI 4 2 0 Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Months Figure 4: Monthly gonado-somatic indices (mean and SD) in female P. commersonnii. 2,5 2 Kn 1,5 1 0,5 Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Months Figure 5: Monthly relative condition (mean and SD) in male P. commersonnii. 70 MATURATION AND SPAWNING OF THE SMALLSPOTTED GRUNT Pomadasys commersonnii (LACEPÈDE, 1801) IN THE ARABIAN SEA OFF OMAN 2,5 2 Kn 1,5 1 0,5 Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Months Figure 6: Monthly relative condition (mean and SD) in female P. commersonnii. 100 80 Ƒ Female 60 Lm = 42 cm ¨ Male ------- 40 Lm = 44.2 cm % maturity 20 0 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 Total length (cm) Figure 7: Length at first maturity in P. commersonnii. where GSI = GW/TW x 100 where GW is the gonad P = percent mature fish in length class L, weight and TW the total weight of fish. r = the width of the maturity curve and Lm = length at 50% maturity The monthly mean and standard deviation (SD) were estimated. Gonado-somatic Index Relative condition factor (kn) The monthly gonado-somatic index (GSI) was calculated separately for males and females for the The monthly relative condition factor (Kn) was pooled data of two years using the formula, estimated by pooling the data of 2005-06 and 2006- 71 A. AL-NAHDI, A. AL-MARZOUQI, N. JAYABALAN & S. AL-HABSI Figure 8: Distribution of monthly maturity stages of P. commersonnii (2005-06 & 2006-07 pooled). 72 MATURATION AND SPAWNING OF THE SMALLSPOTTED GRUNT Pomadasys commersonnii (LACEPÈDE, 1801) IN THE ARABIAN SEA OFF OMAN 07 to understand the condition of the fish following (F) were estimated by linear regression analysis based Le Cren (1951) as, on the equation, Log F = Log a +b Log X Kn = TW/aLb where TW = observed weight, aLb = calculated weight obtained from the length-weight Sex-ratio relationship. The monthly sex-ratio was estimated (male to Occurrence of maturity stages of gonads female) for 2005-06 and 2006-07. The monthly and annual sex-ratios were tested for the expected ratio of To determine the spawning season in the 1:1 by chi-square (X 2) analysis. species, occurrence of different maturity stages of males and females were estimated separately RESULTS AND DISCUSSION for various months of 2005-06 and 2006-07. Then the monthly data of maturity stages for both Maturation and spawning the years were pooled sex-wise and the average percentage occurrence of different maturity stages Maturity stages was calculated and plotted. Six stages of maturity (I-Immature; II-Maturing Spawning frequency 1; III-Maturing 2; IV-Mature; V-Ripe/Running and VI-Spent) could be established in both males and The spawning frequency in individual P. females of P. commersonnii (Table- 1). Microscopic commersonnii was determined based on the mature observation of ova in different stages of ovaries could and maturing modes of intraovarian eggs in advanced be clearly differentiated into immature, maturing stages of maturity. and mature ova. No male or female fish with stage V gonad occurred in the commercial catches during the Fecundity period of present study. Fecundity (F) was estimated gravimetrically from Development of ova to maturity 20 ovaries of stage IV preserved in ‘modified Gilson’s fluid’ (Bagenal and Brawn, 1971). A small portion of The intraovarian egg development from immature the ovary was removed and weighed to the nearest to mature stage was studied in all the stages of ovaries 0.001g in an electronic microbalance. The piece of excepting stage V (Fig. 2). In stage I ovary, the ova the ovary was taken to a counting chamber and all the measured from 1-2 m.d. to 8 m.d. (1-2 and 3-4 m.d. mature eggs in the piece of the ovary were counted size groups). This immature group of ova formed under a binocular microscope.
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