1 Introduction

Morphometric and meristic characters are frequently used to identify or define the purpose of stock, and thus is of strong interest in ichthyology (Tudela, 1999). To overcome the inherent weaknesses of traditional morphometric methods, a system of morphometric measurements entitled "the truss network system" was a conventional method performed in parallel with genetics because of easy operation, and it could be used to re-evaluate past study data and tested their reliability in genetic data (Strauss and Bookstein, 1982). The truss network system was a powerful tool for the analysis of shape, and designed to cover the animal’s body.

Length-weight relationships (LWRs) are used for estimating the weight corresponding to a given length. The LWR has been used in fisheries research since the beginning of the 20th century (Froese, 1998). And it could also be used to calculate the selected species condition index as well as the life history and morphological difference between populations from different regions (Peterakis and Stergiou, 1995; Goncalces et al., 1997).

Selaroides leptolepis (Cuvier, 1833) in Malaysia and Indonesia under the local name of “kuning” and its market price was within the average values for fish for human consumption (Sumaila et al., 2007). In the European Union, the S. leptolepis was imported as part of the scad fish group without a clear indication of a particular species. It is reported as trawling bycatch in Australia (Dell et al., 2009). In the Philippines, the S. leptolepis is defined as one of the most often caught species, apart from Decapterus macrosoma (Bleeker, 1851); Selaroides leptolepis.

Caranx kalla (Bloch, 1793) was Alepes, Carangidae, Perciformes which was broadly distributed over the tropical Eastern Indo-West Pacific region, not as widely distributed as most of the genus Alepes. The species has been recorded from the red sea in the west and its common distributed the Indian, South China Sea and Taiwan Strait, the East China Sea and other waters (Venkataramani and Natarajan 1984; Carpenter et al., 2001). The species inhabited inshore coastal environments, especially reef habitats.

Lactarius lactarius (Bloch and Schneider, 1801), also called false trevally, was species of fish in the family Lactariidae, currently the sole member of the family. This fish is colored silvery-grey on the upper parts with blue iridescence dorsally and a dusky black spot on the upper gill cover. Moreover, the underparts are colored silvery-white and the fins were pale yellow. Although this species most did not exceed 30 cm, it could reach a length of 40 cm.

Up to now, no reports on those three species were available in Pearl River Estuary. Therefore, in order to make up this gap, the present study measured basic morphological characters and estimated the length-weight relationship of wild collected S. leptolepis, C. kalla and L. lactarius from the Pearl River Estuary, South China. Results from this study could provide biological information to conserve the natural resource of these species in Pearl River Estuary.

2 Materials and methods

2.1 Fish collection

A total of 90 samples of three fish species were collected from Pearl River Estuary, South China in November (11^th to 17^th) 2015. Samples were kept in ice after collection and taken to the lab.

2.2 Measurement methods

Morphological studies of three species were immediately carried out. (Table 1) The body length (L) and wet weight (W) were measured with a digital caliper and a digital balance to the nearest 0.1 cm and 0.1 g, respectively. Length-weight relationships were typically represented by the function W = aL^b (where: W= weight, L = length, a = regression intercept, b = slope) (Pinheiro and Taddei, 2005; Pinheiro and Fiscarelli, 2009). A SONY (NEX-F3) digital camera was used to capture the images of the 90 fish samples which provided a complete archive of body shape and allowed a repeat of the measurements when necessary. The image analysis software of Matlab (R2011a) was used to perform the morphometric data analyses.

Morphometric character include (1) body length, (2) height, (3) head length, (4) eye cross, (5) snout length, (6) eye diameter, (7) caudal peduncle length, (8) caudal peduncle height. Moreover, the truss network protocol used for three species in the present study was based on 12 landmarks to describe the major features including (A) origin of the pectoral fin, (B) tip of maxillary, (C) origin of the pelvic fin, (D) top of operculum, (E) origin of the anal fin, (F1 or F) origin of the first dorsal fin, (F2) origin of the second dorsal fin, (G) end of the anal fin, (H1 or H) end of the first dorsal fin, (H2) end of the second dorsal fin, (I) ventral attachment of the caudal fin to the tail, and (j) dorsal attachment of the caudal fin to the tail.

2.3 Statistical analysis

The relationship between body length (L) and wet weight (W) were calculated by the power regression W=aL^b (Ricker, 1973; Jobling, 2008). All the truss measurements were log transformed and tested for normality using the SPSS 19.0.

3 Results and Discussion

3.1 Length-weight relationship

The length-weight relationship of S. leptolepis was found to be W_SL=0.28×L^0.48, while for C. kalla and L. lactarius was estimated as W_CK=2.40×10^-4×L^2.307 and W_LL=0.0095×L^1.67 respectively. (Table 2)

Values of the exponent b provide information of fish growth. When b=3, the increase in weight was isometric. When the exponent value of b>3, the weight increase was positive allometric, and when b<3 the weight increase was negative allometric. Previous research found that the b value of P. sextarius and Polydactylus sextarius was 2.90 and 1.46, and indicated that the fish followed a negative allometric growth pattern (b<3) (Wang et al., 2011; Wang et al., 2015). The result showed that we have the same conclusion with Wang et al. (2011; 2015), the b value of S. leptolepis, C. kalla and L.lactarius were lower than 3 in present studies. It was due to the analysis of narrow range of body length (10.19-12.84 cm, 4.22-7.42 cm, 7.44-14.16 cm from three species, respectively) and low fish number (30), which not contained a lot of size groups (Wang et al., 2015). Furthermore, the b values of our study were low that might be related to fish growth phase, seasonal effect, general fish condition and size selectivity of the sampling gear (Tesch et al., 1971). Our result could provide the basis of growth, proliferation and resource protection for three species.

3.2 Truss network analysis

Body length of S. leptolepis varied from 10.19 to 12.84 cm, and the wet weight ranged from 28.45 to 41.77 g. Body length of C. kalla varied from 4.22 to 7.42 cm, wet weight ranged from 12.77 to 22.21 g. Body length of L. lactarius varied from 7.44 to 14.16 cm, and wet weight ranged from 30.03 to 53.02 g. (Table 1)

All models of truss networks are presented. (Figure 1) Truss network measurements were a series of measurements calculated between landmarks that form a regular pattern of connected quadrilaterals or cells across the body form (Bagherian and Rahmani, 2009). It could be successfully used to investigate stock separation within a species. Moreover, morphometry based on truss network data has been used for species discrimination (Palma and Andrade 2002), ontogeny (Hard et al., 1999) and function morphology (Dean et al., 2006).

In our study, we measured S.leptolepis (36 characters), N. japonicus (38 characters), L.lactarius (39 characters); each having 12 landmarks from Pearl River Estuary for academic work. For detail measurement, all characters from significant loadings obtained, all of these might be useful as primary measurement characters. (Table 3)

Acknowledgements

This project was funded by the Science and Technology Infrastructure Construction Project of Guangdong Province (Grant Numbers: 2014A030305005 and 2015A030303008), National Infrastructure of Fishery Germplasm Resource Project (Grant Number: 2015DKA30470).

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