Introduction

The Greater amberjack Seriola dumerili (Risso, 1810) is widely distributed and prefers coastal epibenthic and pelagic areas. This fish is one of the most common in the coastal waters of the Mediterranean and Adriatic seas, where supports a commercially important local fishery (Sley et al., 2014). This species is common in the Tunisian waters of the Mediterranean Sea (Sley, 2010) and it is may be caught by many fishing methods including haul seines, lampara nets, purse seines, gill nets, and by hook and line, however, its commercial catch has decreased relatively in recent years in the Gulf of Gabes (Sley, 2010).

Several biological aspects of the greater amberjack have been documented throughout its geographical range (Masuma et al., 1990; Tachiaraet al., 1993; Marino et al., 1995; Manooch and Potts, 1997; Jerez et al., 2006; Sley et al., 2014).

The speciation measures are the most important issues in any systematic study. These measures can be traced by studying genetic differences between organisms, which can be analyzed either directly at the level of nucleic acids or indirectly through visible modifications of morphological structures (Wägele, 2005). Morphometric and meristic characters are important in providing accurate information about the identity of the fish species as do the genetic studies, so cannot be ignored as valuable for species recognition (Wägele,2005; Harrisonet al., 2007; Ibañezet al., 2007; González- Castro et al., 2008). The morphometric and meristic procedures are known to be quicker, more practical, and less expensive than molecular studies, thus allowing many individuals to be screened in the field (Ibañez et al., 2007).

The morphometric characters are genetically (Narrow range), intermediate (Moderate range) and environmentally (Vast range) controlled (Johal et al., 1996).

Analyses of countable body features (meristic) have been widely used for fish identification and in studying stock structure of fishes. The most commonly meristic characters have been external, including number of fin spines and fin rays, gill rakers and scales (Waldman, 2005). Ecophenotypic variation of meristic expression has been clearly demonstrated in many species of fishes. The morphometric and meristic characteristics are often analyzed together for the purpose of population structure analysis.

The information about the morphometrics and length-weight relationship given in Fishbase (Froese and Pauly, 2016) for this species are for areas other than Tunisia. The aim of this study was to present some morphometric and meristic parameters and length-weight relationship of Greater amberjack Seriola dumerili from Gulf of Gabes, Tunisia. Results of this study may help fishery biologists of this species to plan further strategy for the male and female of this fish.

1 Material and Methods

The fish specimens of S. dumerili (total 267, 66 male, 83 female, 118 unidentified sexes) were collected from landing site at Gulf of Gabes (Figure1). Southern Tunisia’ between June 2004 to May 2006 by gill and purse seine nets. The specimens were brought to the laboratory fresh and on ice. The stretched meshes of the nets and their accessories measured at least 20 mm. Fish were examined shortly after landing while still fresh. Total length (TL), fork length (FL), standard length (SL), 1^stpredorsal length (PD1); 2^ndpredorsal length (Pre D2); length of 1^st dorsal (LD1); length of 2^nd dorsal fin (LD2); head length ( HL); pectoral fin length (PFL); body depth at pectoral fin (BDPF); body depth at 2^nd dorsal fin (BDD2) were measured in mm using digital caliper (Figure 2). Morphometric characters were recorded and presented in Table 1. The specimens were deposited in the fish collection of the National Institute of Marine Science and Technologies of Tunisia, Sfax, Tunisia. Sex was determined macroscopically and data of each sex kept separated. The indices of the morphological characters were calculated using the following formula:

I = C/TL x 100

Where, I = morphological character index; C = morphological character measurement in mm; TL = total length of the fish. The relationship between fish total length and different morphometric characters studied was calculated using the following formula:

Y = a X^b

Where, Y = morphological characters; X = Fish total length; a, b = constants. According to the law of the allometry, "b" would take a value close to 1. To test this value, Student test "t" was used.

The type of allometry was evaluated by testing the significance of the allometric coefficient “b” (b = 1, b>1 and b<1 for isometry, negative allometry and positive allometry respectively) that used as a measure for the intensity of differential increase in the morphological characters relative to a specific reference length (Van Snik et al., 1997). Confidence interval (CI) was calculated in order to find out the significance (t-test, p<0.05) of variations between the means of each character among the male and female.

Meristic characters were counted for each fish specimen. These counts are number of the 1^st dorsal fin spines and rays (D1), number of the 2^nd dorsal fin spines and rays (D2), number of pectoral fin soft rays (P), number of ventral fin rays (V), number of anal fin rays (A), number of gill rakes on the epibranchial portion of the left 1^st gill arch (LGR) and number of scales on the lateral line (LL).

2 Results

In total length, the specimens ranging in total length from 155 to 1660 mm were used in the present study. The total length of males and females ranges from 294 to 1480 and from 295 to 1480 mm respectively. The total length range of the undifferentiated specimens was 155 to 311 mm.

Ratios of the different morphological characters to the fish total length are shown in Table 1. For both male and female, the range and mean of these ratios were not significant (t-test; p>0.05).

For the male specimens, the non-linear power function coefficient ‘b’ of different variable characters (Y) on the total length (X) was highest in case of LPD2/TL, where the values are 1.03 and for the female was 1.01 in case of PD1 and PD2.

The present study revealed the highest correlation of the percentage on total length for the male (r = 0.99) in cases of LPD1/TL, PD1/TL and PD2/TL and the lowest value (r = 0.87) in case of LPD1/TL. For the female, the highest value was in the case of PD2/TL (r= 0.99) and the lowest was in the case of LPD1/TL (r = 0.91)

The growth of the eight morphometric characters are relative to the total length of male S. Dumerili shown isometric growth in the case of BDD1/TL, LPD2/TL, PD1/TL and PD2/TL. The remaining morphometric characters were shown to have positive allometry growth. For female, PD1/TL and PD2/TL were shown to have isometric growth and the remaining morphometric characters were shown to have positive allometry.

During the present studies, seven meristic characters have been counted. These characters have definite number and count, sometimes they variate and falls under some specific range. The specimens of S. dumerili collected from Gulf of Gabes, Tunisia have the following set of meristic characters: D1 VI-VII, D2 I/29-35, A III 18-21, P 19-20, V I 5, LGR 13-21 and LL 140-180.

The relationship between total, standard and fork lengths are given in Table 2 for S. dumerili. The values for coefficient of determination (R²) for all the length-length parameters of male, female, combined sexes and undifferentiated sexes were > 0.9.

3 Discussion

The results of the present study revealed that the variation between the means of all 8 morphometric characters studied of S. dumerili were found to be not significant (t-test; p>0.05) between male and female. Thus, it is not possible to observe phenotypic variation between the two sexes. Specific research works were performed regarding morphometric characteristics of male and female of different species of fish such as, Lashari et al. (2004), Narejo et al. (2000), Narejo (2010) and Dars et al. (2012) and their results found to be in agreement with our results. To accept the statement that morphometric can distinguish between different species of fish given by Minos et al. (1995), Cavalcanti et al. (1999), Sabadin et al. (2010), Díazde—Astrloa et al. (2011) and Zhan and Wagn (2012), morphometric data of another Seriola species need to be available for comparison.

In case of the male of S. dumerili the growth of LPD2, PD1 and PD2 were shown to have isometric growth. The decimal value of the coefficient b was very small and considered 1. On the other hand, the female showed isometric growth for the characters PD1 and PD2. The growth of the remaining morphometric characters of both male and female has shown to have positive allometry (b<1). No negative allometry growth was observed in the morphometric characters of both male and female of S. dumerili.

The growth pattern of certain morphometric characters was shown to be species-specific patterns such as the body depth allometry in S. dumerili, which possesses a deeper body. The profiles of the middle part of the body were always found to deepen, something that is necessary for maneuvering (quick starts and rapid turns) (Webb, 1984). Such results were obtained for several species of the family Sparidae by Niklioudakis et al. (2014). The noticeable growth in the abdominal region suggests greater development of the intestine (Elbal et al., 2004), a finding similar to other studies that have shown that the middle part of the body increases later throughout ontogeny (after head and tail) in bilateral species (Osse et al., 1997; Van Snik et al., 1997; Gozlan et al., 1999), as opposed to asymmetrical species (Paralichthys californicus, Gisbert et al., 2002). The other positive and negative allometric patterns of different morphometric characters obtained in this study are associated with changes in body form related to requirements of the carangiform swimming pattern that this species is used.

In the present results and in the case of male and female specimens, all morphometric indices shown high correlation value (above 0.9), while in the case of undifferentiated sexes, moderate correlation value was shown (0.7 to 0.95). When all specimens (male, female and undifferentiated) combined the correlation was high (>0.9).This indicates that the growth of S. dumerili in one area of the body do not grow as the same growth rate of the total length of the fish probably because when juvenile specimens were included in the account. Similar results were obtained by Oniye et al. (2006) and Safi et al. (2014) on Protopterus annectens and Pomadasys stridens respectively.

Niklioudakis et al. (2014) suggested that individual morphological characters was generally independent of species, but on the temperature of the environment the fish that developing in. Accordingly, the strength of the relationship (r² in Table 1) for the eight morphometric characters were higher for male and female specimens than those specimens contained undifferentiated sexes, implying that decreasing temperature induces increasing variability in morphological change, i.e. the change is less synchronized between characters. Since this group of specimens contains specimens of different ages and juvenile, therefore the strength of the relationship of the morphometric characters was less and it may possible that the morphometric characters is more synchronized with water temperature rather than with other morphometric characters (Niklioudakis et al., 2014).

The range of the total length given for Seriola dumerili at maturity is 800 to 1270 mm and the maximum and the common total length recorded are 1900and 1000 mm respectively (Froese and Pauly, 2016). The range of the specimens studied in this work is less than the range given for this species and the largest specimen (1660 mm TL) falls near the upper limit of the range given for this species in the literature.

Several specimens of Seriola dumerili were collected from different parts of the world having total length falls within the range of the total length of the specimens obtained in the present study from the Tunisian waters (Lin and Shao, 1999, 299 mm, Taiwan; Mazzola et al., 2000, 404 mm, Gulf of Castellammare, North West Sicily; Monteroa et al., 2004, 310-420 mm, Puertode Mazarrón, Spain).

Meristic characters of Seriola dumerili are in agreement with specimens of this species collected from other parts the Mediterranean Sea and the Black Sea by Fischer et al (1987) except for the upper range of the anal fin ray was lower (21 rays) by one ray than that reported by Fischer et al (1987).This difference in the upper range of anal fin rays indicates that different locations and environment have considerable impact on meristic characters where interaction between genetics and environment occurred(Marr, 1957; Swain and Foote,1999).

The LLRs values of male, female, combined sexes and undifferentiated sexes were highly significant (P< 0.001) (Tables 2) and they were compared with those available by Froese and Pauly (2016).The value of the coefficient “b” for the FL-SL obtained in the present study is higher than that given by Froese and Pauly, (2016) for combined sexes, but for TL-FL, the “b” value was slightly lower than that given by Froese and Pauly (2016). For such variations, the ecological conditions of the habits or variation in the physiology of animals, or both, are responsible (Le Cren, 1951).

Although S. dumerili is an important commercial fish, its conservation status has not been evaluated. Therefore, the fisheries managers should take this issue in consideration to maintain its stocks in the Mediterranean Sea.

In conclusion, this study provides additional basic information on LLRs for male and female of Seriola dumerili studied that would be useful for fishery biologists/ managers in Tunisia.

Acknowledgements

Our sincere thanks are due to Ana Ibáñez, Department of Hidrobiología, Universidad Autónoma Metropolitana-Iztapalapa, and México for Reading the manuscript for her valiable avise and sugestiones. Many thanks to Joäcim Naslund from Goteborg, Sweden for his technical assistance.

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