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A. Al-Nahdi, Al-Marzouqi A, and S. Al-Shajibi – Length-weight relationships, maturity and reproductive season of the fringelip mullet Crenimugil crenilabis from Arabian Sea coast of Oman. Sultan Qaboos University Journal

A. Al-Nahdi, Al-Marzouqi A, and S. Al-Shajibi – Length-weight relationships, maturity and reproductive season of the fringelip mullet Crenimugil crenilabis from Arabian Sea coast of Oman. Sultan Qaboos University Journal
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   Agricultural and Marine Sciences , 13:23-32 (2008)©2008 Sultan Qaboos University 23 *Corresponding author. E-mail: alnahdi22@hotmail.com Length-Weight Relationships, Maturity, and Reproductive Season of the Fringelip Mullet Crenimugil crenilabis  (Forssk å l, 1775) from the Arabian Sea Coast of Oman 1 Marine Science and Fisheries Centre, P.O. Box 227 PC 100, Muscat, Sultanate of Oman  2  Ministry of Fisheries Wealth, P.O. Box 427 PC 100, Muscat, Sultanate of Oman ABSTRACT: Fringelip mullet ( Crenimugil crenilabis ) caught at Al-Duqum and Salalah in the Arabian Sea region of Oman were sampled monthly between January 2001 and December 2002. The total length (TL) of mullet caught with gill- and seine nets ranged from 12 to 54.5 cm (32.4 cm mean ± SD; n = 2445) but <2% of specimens had a TL<24 cm, suggesting gear selectivity. Females dominated samples (1 : 0.36) and reached L 50  at 32.8 cm TL (32.2 cm in males). Length weight relationships were non-linear and conversion factors between TL, fork length (FL) and standard length (SL) were calculated. Proportionately, most females had ripe gonads in September and October, and males in September to November. Both female and male gonadosomatic indices (GSI) peaked in September and October and were lowest between May and August. The  peak in reproductive activity coincides with the fall intermonsoon, when the Arabian Sea upwellings caused by the onshore winds of the SW monsoon season (June to August/September) retreat. During this intermonsoon period the inshore water temperatures begin to rise after reaching a minimum in August/September. Some reproductive activity also occurs in other months, particularly February, when water temperatures are seasonally lower. Keywords: Crenimugil crenilabis , reproductive season, length-weight relationships, Oman. Abdullah Al-Nahdi* 1 , Abdulaziz Al-Marzouqi 1 , Said Al-Shajibi 1  and Ahmed Al-Hosni 2   ﺏﺮ   ﺍ   ﺮ    ﺎ    ﻲﻓﺡﺎﻴ   ﺍﺔﻜ    ﺮﺛﺎﻜﺘ   ﺍ  ﻮ   ﻭﻲ   ﺎﻨﺘ   ﺍﺞﻀﻨ   ﺍﻭ ﻥﺯﻮ   ﺎ    ﻝﻮﻄ   ﺍ   ﺕﺎﻗﻼ   ﻥﺎ    ﺔﻨﻄ   ﻲﻨ   ﻮ   ﺍ   ﺪ   ﺃﻭﻲ   ﻴﺠﺸ   ﺍﺪﻴ    ﻲﻗﻭﺯﺮ   ﺍﺰﻳﺰ   ﺍﺪ    ﻱﺪﻬﻨ   ﺍ   ﺍﺪ   ﻝ   ﻳﺮﻬ    ﺏﺮ   ﺍ   ﺮ    ﻰ    ﻪ   ﻭ  ﺪ   ﺍﻲ    ﻲ   ( Crenimugil crenilabis )  ﺡﺎﻴ   ﺍﺔ    ﺕﺎ   ﻴ    ﺕﺬ   ﺃ  : ﺔ   ﺍﻙﺎ   ﻭ   ﺔﻴ   ﻮ   ﻴ   ﺍﻙﺎ   ﺎ   ﺩﺎ    ﻲ   ﺍﺡﺎﻴ   ﺍﻙﺎ   ﻷﻲ   ﺍﻝﻮ   ﺍﻥﺃ  ﺍﻭ . 2002  ﺮ   ﻳﺩ   ﺮﻬ    ﻰ   ﺇ 2001 ﺮﻳﺎ   ﻳ   ﺓﺮ   ﺍﻝﻮ    ﺎﻬ    ﺕﺎ   ﻴ   ﺍ  %2   ﺃﻥﺃ  ﺍﻭ .( ﺔ   ﻴ   2445  ﺩﺪ    32.4  ﻝﻮ   ﺍ   ﻮ  )  54.5  ﻰ   ﺇ 12  ﻴ    ﺎ    ﺡﻭﺍﺮ    ﻲ   ﺍﻮ   ﺍﺔ    ﺭﻮ   ﺬ   ﺍ   ﻰ    ﻉﻮ   ﺍﺍﺬﻬ    ﺙﺎ   ﺍﺩﺍﺪ   ﺃﺕﺩﺎ   ﻭ . ﺪﻴ   ﺍ  ﺋﺎ   ﻭ   ﻉﻮ   ﺍﺍﺬﻫﺔﻴﺋﺎ   ﺍﻰ    ﺍﺮ   ﺆ    ﻲ   ﻳﺎ    24   ﺃﻲ   ﺡﺎﻴ   ﺍﺔ    ﻥﺯﻮ   ﺎ   ﻝﻮ   ﺍﺔ    ﻥﺃ  ﻴ   . 32.2  ﺭﻮ   ﺬ   ﻭ  32.8  ﻰ   ﺇﺙﺎ    ﻲ   ﺎ     ﻝﻭﺃ   ﺪ    ﻝﻮ   ﺍ  ﻳﻭ .(0.36:1)  ﻴ    ﺔ   ﺎ   ﺙﺎ   ﺍ  ﺃﻥﻮ    ﻡﺎ    ﻭ  . ﻱﺭﺎﻴ   ﺍﻭﻲ   ﻮ   ﺍﻝﻮ   ﺍﻭﻲ   ﺍﻝﻮ   ﺍ   ﻴ    ﻳﻮ   ﺍ  ﺎ    ﺩﺎ   ﻳﺇ   ﺪ   ﻭ . ﺔﻴ    ﺮﻴ   ﻝ  ﺎ   ﺮ    ﻴ   ﺍ  (GSI)  ﻲ   ﺍ  ﺎ   ﺍﻥﺯﻭ   ﻥﺎ   ﻭ . ﺮ   ﻮ    ﻰ   ﺇ   ﺮ     ﺭﻮ   ﺬ   ﺍﻭﺮ   ﻮ   ﺃ   ﻰ   ﺇﺮ     ﺓﺮ   ﺍﻝ  ﺔﻴ   ﺮ   ﺍﺔﻴ   ﻮ   ﺍﺡﺎﻳﺮ   ﺍ  ﻮ     ﺮﺛﺎ   ﺍ   ﻁﺎ   ﻲ    ﻉﺎ   ﺭ  ﺍ  ﺍﺰ   ﻳﻭ . ﺃﻭﻮﻳﺎ    ﻴ    ﻷﺍ   ﺎ    ﺎ   ﻴ    ﺮ   ﻮ   ﺃﻭﺮ    ﻱﺮﻬ   ﺔ   ﺭﺩﺃﺪ   ﺔﻴ   ﻮ   ﺍﺡﺎﻳﺮ   ﺍﺓﺮ    ﻝ  ﻭ . ﺮ   / ﺃﻰ   ﺇﻮﻴ   ﻮﻳ   ﺓﺮ   ﺍﻝ   ﻩﺎﻴ    ﻲ   ﺎ   ﺍﺙﺎ   ﺍﺙﺪ   ﻳ   ﺎ   ﺪ    ﺏﺮ   ﺍﺮ   ﺮﺛﺎ   ﺍﺔ   ﺃ    ﺬ    ﺙﺪ   ﻳﻭ . ﺮ   ﻭ  ﺃﻱﺮﻬ    ﻲ    ﺎﻬ   ﺎﻳﻮ    ﺃﻰ   ﺇ   ﺎ    ﺪ    ﻉﺎ   ﺭ  ﺎ    ﺔ   ﺍﻩﺎﻴ   ﺍﺓﺭﺍﺮ  .  ﺎ   ﺍ   ﺍﻮ    ﻲ    ﺓﺭﺍﺮ   ﺍﺔ   ﺭﺩ   ﺎ   ﺪ    ﺮﻳﺍﺮ    ﻲ    ﺔ   ﺎ   ﻭﻯﺮ   ﺃﺮﻬ   ﺃ   ﻲ    ﻲ   ﺍ  Al-Nahdi and others24 Material and Methods Catches made by traditional fishers at Al-Duqum and Salalah (Fig. 1) were sampled by Marine Science and Fisheries Center (MSFC) staff between January 2001 and December 2002. Random samples from Al-Duqum were measured to determine length-weight relationships and conversion factors. Length measurements were made to the nearest 0.1 cm of total length and all mean values were reported ± SD. (TL; from the anterior tip of the snout to the posterior tip of the caudal fin, with the caudal fin lying straight and not in its natural position), standard length (SL; anterior tip of the snout to the anterior base of the caudal fin rays) and fork length (FL; anterior tip of the snout to the anterior margin of the fork in the caudal fin), and total weight was determined to the nearest 1 g on an electronic balance. Non-linear length weight regressions (TW = aTL  b ) were calculated for males, females and both sexes combined using a least squares fit, and regressions were compared using ANCOVA test (Zar, 1984). Linear regressions of TL versus SL and TL versus FL were calculated to determine conversion factors. Monthly subsamples (approximately 40 specimens, randomly selected) were collected at Al-Duqum, and the gonads removed, sexed and weighed to the nearest 0.01 g. These gonads were classified into six macroscopically distinguishable stages for females and four stages for males (Table 1). To calculate the size at which 50% of females reach sexual maturity (L 50 ), the proportions of females with ovaries in stages 2-5 per 2-cm size class were fitted to a logistic equation of the form: P = 1 (1 + exp (a-bTL) where P is the proportion of mature females in each length class, TL is total length, and a and  b are constants. The best fit model was obtained by minimizing the sum of squares, and the length at 50% maturity was determined by back-calculation. The same method was used to calculate the male L 50 , using the grouped stages 2-4 as the mature proportion in each length class. A gonadosomatic index (GSI; mean ± SD) was calculated for females and males respectively for each month as follows: GSI = GW / TW ×100. The liver of each fish was weighed (LW ± 0.01g) and the hepato- Introduction The fringelip mullet Crenimugil crenilabis (Forsskål, 1775) is a coastal species from the tropical waters of the Indo-Pacific, mainly between 32 o S and 32 o  N, and shallower than 20 m depth (Fischer and Bianchi, 1984). Single individuals or small schools occur over coral reefs and reef flats, and in lagoons with sandy or muddy substrata and harbours, and juveniles are sometimes found in tide-pools (Al-Abdessalaam, 1995; Randall, 1995). Crenimugil crenilabis  is one of at least 11 mullet species known from the coastal waters of Oman (Al-Abdessalaam, 1995), where they are caught by fishers using non-selective gill- and seine nets, often from small fiberglass boats with outboard engines. Government statistics show that the annual catches of mullets have increased from 123t in 2000 to 839t in 2005, however detailed catch-by-species statistics are unavailable for the Mugilidae (Anon, 2006).  Nevertheless, C. crenilabis  is common in fish markets and in biological samples collected by fisheries observers at landing sites, and it therefore makes up a substantial proportion of mullet catches. Of the total mullet catch from Oman in 2005, 58% came from the Gulf of Oman and the rest from the Arabian Sea (Anon, 2006); the latter fishery is seasonal, declining sharply over the SW monsoon period (May-August). Very little information is available on the general  biology of C. crenilabis  in the primary literature. It attains a maximum size of 50–55 cm (Al-Abdessalaam, 1995; Randall, 1995), and feeds on detritus containing algae and microscopic animals by scooping up the upper sand or mud layers and filtering through the gills . Reproduction is oviparous and eggs are  pelagic and non-adhesive (Breder, and Rosen, 1966). Helfrich and Allen (1975) observed spawning in large aggregations near the surface at night, at the beginning of an ebb tide in June. This observation was made over shallow open areas of a lagoon slope at Enewetok atoll in the Marshall Islands. Apart from the above, the basic biological parameters that are fundamental to the management of exploited fish populations are unknown for C. crenilabis  in Oman. The aims of this study were to determine the length-weight relationships, conversion factors among various length measurements, size composition of captured fish, size at reaching sexual maturity and reproductive season of C. crenilabis  from the Arabian Sea coast of Oman.  25  Length-weight relationships, maturity and reproductive season of the fringelip mullet  Figure 1. The sampling sites along the Arabian Sea coastline.     A   r   a    b    i   a   n     S   e   a  Al-Nahdi and others26size of 32.4 ± 4.4 cm (S.D) (n = 2445). Less than 2% of samples comprised fish < 24 cm TL, suggesting that the gear used to catch mullets select fishes above that size (Fig. 1). There was no significant difference in size  between females (37.3 ± 7.0 cm, n = 329) and males (36.7 ± 6.4 cm, n = 117; t = 0.79, df = 444, p>0.05) in the samples taken at Al-Duqum, and in these samples females dominated at a ratio of 1 female to 0.36 males (χ  2 = 100.77, p<0.001). During April, June, July and December, no male individuals could be collected. Table 1. Macroscopically distinguishable stages of female and male gonads. Sex Stage Characteristics Female 0 Immature or resting – thin, short, transparent and no eggs visible1 Developing – cream/yellow, longer in appearance, eggs not visible2 Maturing – developed, yellow to opaque eggs visible to naked eye3 Ripe and spawning – well-developed and swollen, hydrated oocytes4 Spent– Translucent, pale violet with obvious blood vessels5 Post-spent – Empty, shrunk, violet, flaccidMale 1 Developing – small, occupying about 1/3 of body cavity, transparent/pale2 Maturing – whitish/translucent, occupying about 1/2 of body cavity3 Ripe – creamy white, occupying about ¾ of body cavity4 Spent/atrophied – flabby, reddish, about 1/2 of body cavity                                                                          Figure 2.  Length composition (TL in cm) of C. crenilabis caught by artisanal fishers using gill and seine nets at Al-Duqum and Salalah during 2001.somatic index (HSI; mean ± SD) calculated as HSI = LW / TW × 100 for each month. Results and Discussion The largest female was 51.4 cm TL and weighed 1611 g, and the largest male was 54.5 cm, and weighed 1684 g. These measurements are consistent with the maximum size of 51 – 55 cm TL provided by Randall (1995) and Al-Abdussalaam (1995). The TL of all fish measured ranged from 12 cm to 54.5 cm with a mean  27  Length-weight relationships, maturity and reproductive season of the fringelip mullet  This indicates that there may be segregation among sexes. However, this has to be verified by subsequent studies.The length-weight relationships were TW females  = 0.0222×TL 2.801  (n = 329, r  2  = 0.9499), TW males   Table 2. Linear regressions of TL versus SL and FL for C. crenilabis females (n = 329; 14 – 52 cm TL), males (n = 117; 25 – 54 cm TL) and sexes combined, and conversion factors among length measurements.Sex Equation R  2 F SL = 0.772 TL + 0.2284 0.964M SL = 0.780 TL – 0.0619 0.971All SL = 0.774 TL + 0.1732 0.966F FL = 0.870 TL + 0.8806 0.984M FL = 0.864 TL + 1.0546 0.982All FL = 0.869 TL + 0.9093 0.984= 0.0126×TL 2.9441 (n = 117, r  2  = 0.9473), and TW  both sexes  = 0.0247×TL 2.7693 (n = 446, r  2  = 0.9426) (Fig. 2). The log-transformed data fitted best to linear models (logTW males  = 2.9441×logTL – 1.9004 and logTW females  = 2.801×logTL – 1.6541) and the regression slopes differed significantly between sexes (ANCOVA, F = 1.31, p>0.05), females being slightly heavier than males of the same TL, except at the largest sizes. The linear regressions of TL versus FL and SL respectively fitted the data with r  2 -values > 0.96 in all cases, and the equations and conversion factors are shown in Table 2. The logistic models fitted to the mature  proportions of females (n = 337) and males (n = 117) resulted in minimized sums of squares of 0.18 and 0.042, respectively (Fig. 3). The smallest female with a developing ovary (stage 1) had a TL of 25.5 cm, and 50% of females reached sexual maturity at                                                                                                                                                                                                              Figure 3.  Total length and total weight relationships of C. crenilabis  females (a), males (b) and both sexes combined (c).
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