Fecundity and Temporal Reproductive Cycle of Four Finger Threadfin (Eleutheronema tetradactylum) in Malaysian Coastal Water

MATERIALS AND METHODS

Collection of specimens: The sampling for our study was carried out on monthly basis in the coastal waters of Sebatu, Malacca (02°06.002’ N; 102°28.004’ E), Peninsular Malaysia (Fig. 1) from August 2009 to October 2010. A total of 162 specimens of E. tetradactylum were collected using gill net. The mess sizes of the gill nets were 7-12.5 cm and net lengths were 800-1200 m. Nets were set at depth ranging from 2-7 m at early morning between 5.00 and 7.00 h, then raised and the fish collected between 8.00 and 9.00 h. The identification of fish’s specimen for our study was taken care of following on external markings together with meristic and morphometric features (Mohsin and Ambak, 1996; Motomura et al., 2002; Motomura, 2004). In addition, following on the standard procedure our fish’s specimens were located in ice slurry and shifted to the laboratory same day.

Fig. 1:	Map showing the study area (in box) where four finger threadfin was sampled at coastal water of Sebatu, Malacca, Malaysia

Laboratory procedures and data collection: All fish were measured to the nearest 1 mm in the Total Length (TL) and weighted to the nearest 1 g. Gonads of the fish were examined under a dissecting microscope for its external features such as turgidity and colour in order to determine a maturity stage. The sex ratio also calculated in this study (i.e., No. of males/No. of females (Simon et al., 2012). The female was determined by the macroscopic observation of matured ovary (Laevastu, 1965a).

Fecundity estimates and relative fecundity: Fecundity by definition is measured in an ovary of a female by the total number of nonprimary growth oocytes with determinate fecundity (Hunter et al., 1992). This standard measured of estimation was used in our study from 15 females E. tetradactylum captured during sampling period. To obtain the best result, the fresh ovarian tissue was carefully weighed to the nearest 0.01 g using the best practice, dissected cautiously and preserved in Gilson’s fluid (Laevastu, 1965b; Sreenivasan, 1978). In addition, sub-samples were dissection in three different parts for of the each ovary; particularly apical regions, near the base, mid and weighed cautiously. Oocytes were counted and loosened from trabeculae using best practice. The total number of oocytes was estimated in the ovary and reported as well in our sample to find out the fecundity (Clavier, 1992; Simon et al., 2012). Total fecundity ‘F’ was considered as the number of vitellogenic oocytes at any time in ovary development (Hunter et al., 1992; Iversen, 1996; Simon et al., 2012; Salcedo-Bojorquez and Arreguin-Sanchez, 2011). Relative fecundity is the number of eggs per unit of body weight of female sample was also calculated (Simon et al., 2012). Finally, we utilized correlation analysis to find the relationships of Fecundity (F) of the sampling Body Length (BL), Body Weight (BW), Eviscerated Weight (EW) and Gonad Weight (GW) (Simon et al., 2009, 2012).

Temporal reproductive cycle: Paired gonads were weighed individually in our study to the nearest 0.01 g and the mean of Gonadosomatic Index (GSI) was determined monthly. GSI was calculated as:

where, GW is gonads weight and SW is somatic weight (represents the BW without GW) (Solomon and Ramnarine, 2007; Simon et al., 2009; Ozvarol et al., 2010). In this study, GSI of both male and female were combined as this species was identified as hermaphrodite protandry fish. Specimens with mature ovary were identified with maturity stages. Maturity classification of our study specimens is followed of seven macroscopic stages adapted from Laevastu (1965b), Patnaik (1970) and Selman and Wallace (1989). Stage I and II considered as immature, stage III and IV maturing or developing, stage V and VI mature/spawning and stage VII spent. Ova were measured from each stage to determine the average diameter.

RESULTS AND DISCUSSION

Sex ratio and size distribution: A total of 162 specimens of E. tetradactylum were collected during the study. Among the 162 samples, 147 were males and 15 were females with a sex ratio of 9.8 males/females (Fig. 2). Males ranged in TL from 14.8-39.0 cm and BW ranged from 26.3-644 g, conversely, in female TL ranged from 35.0-55.5 cm and BW ranged from 450.0-1700.0 g. It was observed in the present study that among the collected samples males were predominated in number compared to their female samples (Table 1). Overall the total catch of four finger threadfin male population was highest round the year except in July.

Fig. 2:	E. tetradactylum male and female size distribution collected from Malacca coastal waters, Malaysia from August 2009-September 2010

Table 1:	Monthly distribution of E. tetradactylum collected at study area from August 2009- October 2010

In month of September, the occurrences of males were highest whereas females were observed more frequently during June, July and August. This might be due to different timing of sexual maturity, habitat partitioning or sampling bias (Bagenal, 1978).

Fecundity estimates and relative fecundity: A total of 12 gravid females of four finger threadfin fish were examined for its fecundity. Based on 15 specimens of female fish, 12 fishes contained mature ovary to estimate the fecundity. Eggs from three parts of matured ovary exhibit homogenous in shape and size suggested that four finger threadfin fish is a total spawner (Fig. 3). The estimated mean fecundity ranged from 341,358-1,114,757 eggs with a mean value of 663,078±76,935. Relative fecundity was calculated as an index of the capability of production with the fish body weight. The relative fecundity ranged from 393-1202 eggs g^-1 b.wt. with a mean value of 657±68.

The relationships between fecundity and total length revealed F = 25352 TL-541151 (r = 0.76; N = 12; p<0.001) (Fig. 4). This result means that fecundity increases with body length at a constant rate. The relationship of the Body Weight (BW), body Eviscerated Weight (EW) and Ovary Weight (OW) to fecundity also show the same results that fecundity increases with total Body Weight (BW) and Eviscerated Weight (EW) at a similar rate (Fig. 4).

E. tetradactylum at Malaysia coastal water seem to be highly fecund species, when consider that they have eggs which are dispersed by the coastal waters from the spawning ground. The results of this study agreed with the previous studies by Patnaik (1970) which reported estimate fecundity varied from 226,541-3,826,683 in Indian waters and Abdul Samad (1987) from 179,546-488,927 in Musi estuary, South Sumatra, Indonesia. The increase in fecundity with the weight and length is similar in nature by the observations done in two archer fishes Toxotes jaculatrix and Toxotes chatareus in Malaysian coastal waters (Simon et al., 2012) and also reported elsewhere for Myctophidae fish (Gartner, 1993).

Temporal reproductive cycle: The mean GSI (MGSI) of fish shows three peaks throughout a year (Fig. 5). The first peak was in month of March (MGSI = 1.38±1.09) followed by June-July (MGSI = 3.26±1.10) which the highest peak.

Fig. 3(a-c):	Eggs of E. tetradactylum from three parts of ovary, (a) Base, (b) Mid and (c) Apical region, Total length = 39.5 cm, The area of quadrate is 1x1 mm

Fig. 4 (a-d):	Relationship of fecundity with (a) Total length, (b) Body weight, (c) Ovary weight and (d) Eviscerated weight of E. tetradactylum females during August 2009-October 2010

Fig. 5:	Mean monthly gonadosomatic indices (GSI) ±SE of E. tetradactylum in Malacca coastal waters, Malaysia from August 2009-October 2010

Then the last peak which the second highest (MGSI = 1.85±2.68) was in September. This indicates the spawning season was began in March and continue until September which took about six months. Eight of nine female specimens collected during June to August were in spawning condition. In month of November, December, January and February, the mean GSI were decreased. This indicated that most female was released the eggs which undergone stage VI of maturity.

These study provides the first evidence that E. tetradactylum has a lengthy spawning period starting from March to September in Malacca coastal waters of Peninsular Malaysia. This lengthen period of spawning gives the opportunity of female fish to release all the eggs in one season. In west coast Peninsular Malaysia and Strait of Malacca area received maximum rainfall twice a year on April-May and October-November. Patnaik (1970) reported that E. tetradactylum at Chilka Lake, India spawns twice a year with the peak period in January and June, respectively. While Pember (2006) found that north-western Australia E. tetradactylum have the peak spawning season in October to December. This seasonal change in wind and rainfall affect the spawning season of the fish in terms of their higher fecundity.

Female reproductive maturity is commonly quantified base on GSI values, because using fish size is generally inaccurate and the size at maturity varies greatly not only within a species but even within populations (Lowe-McConnell, 1982). However, determination of reproductive maturity using only the GSI is not adequate because structures within the ovary that can be predictive, such as oocytes developmental stage and accumulation of yolk within the interstitial tissue, cannot be obtained from weight alone. Direct observation of histological architecture is the most accurate method to determine the stage of maturation of the ovary. The combined use of oocyte size distribution and GSI proved to be efficient to assess ovarian maturation in E. tetradactylum.

CONCLUSION

This study illustrates the reproductive biology of E. tetradactylum in Malacca coastal waters of Peninsular Malaysia. The analysis is done of our study such as GSI and maturity stages, define the spawning season of E. tetradactylum in the coastal water in Malacca. This study provides an insight of baseline knowledge of the reproductive biology of four finger threadfin for future studies of commercially important fish species. However, more in depth research is needed as histological observations of gonadal development are important to figure out the basic architecture of oocyte development and to elaborate the hermaphrodite protandrous phenomenon.

ACKNOWLEDGMENT

We thank the anonymous reviewers for their very useful comments. We acknowledge the volunteer fishermen from the Sebatu fishing village of Malacca, for their kind cooperation and support during sampling. The research was supported by Universiti Kebangsaan Malaysia though young researcher incentives grant “GGPM-2011-057”.