Studies on the Reproductive Biology of Freshwater Spiny Eel, Mastacembalus armatus (Lacepede) Reared in the Cemented Cisterns of BAU, Mymensingh, Bangladesh

MATERIALS AND METHODS

The above study was conducted for a period of twelve months from February 2001 to January 2002. From the examination 120 specimens of M. armatus were collected from cultural stock maintained in the cemented cisterns of Bangladesh Agricultural University (BAU) Mymensingh. Ten mature fish (five males five females) were collected randomly during each month to determine the total length and body weight. The male and female fish were differentiated and data was recorded after dissecting out the gonads of the individual fish. The condition of the gonads was noted and weighed on electronic balance before being preserved in a labeled vial containing 5% buffered formalin for subsequent studies. From each ovary, diameter of about 100 ova was measured by using a graticule under the dissecting microscope. Gonadosomatic index (GSI) of the male and female was determined separately by using the following method.

For fecundity estimation gravimetric method was applied in the present studies. The gravimetric method has been successfully used as described by Blay (1981), Dewan and Doha (1979). For fecundity estimation 15 matured fish ranged from 245-605 mm (in TL) were used. Samples taken from the anterior, middle and posterior region of both ovaries were weighed separately and the number of ova present within each sample was counted, fecundity was estimated on the basis of total weight of the ovaries. The fecundity of the fish was obtained by using the following formula:

F= N x Gonad weight / sample weight.

Where, F is the fecundity and N is the number of eggs in the sample. The relationship between various parameters viz., fecundity-total length, fecundity –body weight and fecundity- gonad weight were determined by the method of least square.

RESULTS

Sex dimorphism: The fish Mastacembalus armatus is diecious but there are no secondary sex characters. The sex of M. armatus could not be recognize easily outside the spawning season except by dissection. During the breeding season the male became active and brighter in colour while female became more pot- bailed and dull in colour. A light press on the belly of ripe fish had brought the whitish milk from males and the eggs from females. Females grew faster than the males and so attained a larger size.

Gonadosomatic index (GSI): Gonadosomatic index was ranged from 0.76–8.40 in males and 0.82-10.58 in females and also showed one peak in July (Table 1). During the present study the higher values of GSI were observed from May to July it ranges from 4.80–8.40 and 6.89 to 10.58 with mean 6.60 ± 1.80 and 8.61±1.51 for males and females respectively (Fig. 1). After extrusion or evacuation of ripe ova, the gonads were suddenly reduced in size and weight.

Table 1:	Month-wise changes in ova diameter and gonadosomatic index (GSI) of males and females of M. armatus

Table 2:	Data on fecundity to total length (TL), body weight (BW) and gonad weight (GW) of M. armatus

Fig. 1:	Changes of gonadosomatic index (GSI) in males and females of Mastacembalus armatus.

There were no ova from August to November (Table 1) because the percentage of GSI declined rapidly after spawning to minimum indices during August, 0.76 ± 0.20 and 0.82 ± 0.10, respectively. Therefore it was observed that the fish spawned once in a year with one spawning peak highest in the month of July as indicated by the values of both ova diameter (1.00 mm In July) and gonadosomatic index (6.60 and 8.61) respectively.

Maturation of ova: The data on ova diameter and changes in GSI in males and females of M. armatus from the cemented cisterns of BAU, Mymensingh is presented in Table 1. During the period of twelve months (February, 2001- January, 2002) diameter of ova found in the ovaries of M. armatus was recorded. The diameter of ova ranged from 0.100 to 1.00 mm during December to July.

Fig. 2:	Logarithmic relationship between fecundity and body length of female M. armatus.

Fig. 3:	Logarithmic relationship between fecundity and body weight of female M. armatus.

All the ova (100) were measured and found to be spherical and uniform in diameter, it indicates that the eggs were shed in a single batch during the peak period of spawning May to July. Thus the eggs shed in a season developed simultaneously. No evidence was found to show that the resting oocytes in mature females would reach maturity during the current spawning season.

Fecundity: The estimate of fecundity in present study was based on 15 specimens of mature females of M. armatus sampled during April to July 2001. These individuals ranged in size from 245- 605 mm in length (TL) and 34.7– 355.8 g in weight. The estimation of ova, in the present study ranged from 580-10980 eggs. The maximum fecundity was observed from a fish measuring 605 mm (TL) in length and 355.8 g in weight and the minimum from a fish measuring 245 mm (TL) in length and 34.7 g in weight. The number of ova present per gram of body weight was 29.38 and the number of ova present per gram of ovary was 407.57.

Fig. 4:	Logarithmic relationship between fecundity and gonad weight of female M. armatus

The fecundity increases with the increase in length and weight of the fish. The equations of regression coefficient between total length (TL), body weight (BW) and gonad weight (GW) versus fecundity (F) (Fig. 2,3 and 4) are given below.

Log F= -1.74+ 3.30 x Log L (Fecundity- Total length relationship)
Log F= - 0.53+1.39xLogW (Fecundity- Body weight relationship)
Log F=1.87+1.69xLog Gw (Fecundity- Gonad weight relationship)

As may be noted from the above regression equations it would appear that the fecundity has a linear relationship with total length and body weight. It is clear that the fecundity of this fish was mainly dependent on their total length and body weight. The fecundity- total length and fecundity body weight relationship was found to be more dependable than the fecundity- gonad weight relationship.

DISCUSSION

The reproductive biology of freshwater spiny eel, Mastacembalus armatus were described for twelve months from February 2001 to January 2002. There were no secondary sex characters in M. armatus and female became more pot-bailed during the spawning season. Narejo et al. (1998) reported similar observations, no secondary sex characters in Tenualosa ilisha. Berra (1984) observed milk and egg on applying slight pressure on the belly of Prototractes maraena male and female respectively. It was observed during the present studies that the fish have only one breeding season in summer and spawn during May to July with peak in July, similar observations have been reported by Nabi and Hossain (1996) in freshwater spiny eel, Macrognathus aculeatus. The maximum size of the mature egg found during the present investigations was 1.00 mm. The eggs were spherical and uniform in diameter. Similar findings were also reported by Nabi and Hussain (1996) in the spiny eel, Macrognathus cualeatus. The gonadosomatic index (GSI) during the present investigations shows one peak in summer and the mean GSI 8.61 ± 1.51 and 6.60 ± 1.80 in females and males, respectively during the months from May to July, it indicates that all the mature ova were released at once during the spawning season. Nabi and Hossian, (1996) also reported the single spawning season from May to July and all the mature ova were released during the breeding season in spiny eel Macrognathus aculeatus. The fecundity estimation during present study ranged from 580 to 10980 eggs and the size ranged from 245 to 605 mm (TL). The fecundity increased with the increase in length and weight of the fish. The results of the present study are very similar to those obtained by Kabir et al. (1998) in Gudusia chapra. During the present investigations the fecundity was plotted against the body length, body weight and gonad weight. It was observed that the fecundity body length and fecundity body weight relationship gives a better relationship as compared to fecundity-gonad weight relationship. Similar observations were reported by many authors like Kabir et al.(1998) in Gudusia chapra. Faruq et al. (1996) in Clarias batrachus. Azadi and Siddique, (1986) and Das et al. (1989) in Heteropneustes fossilis, Danwattana and Nakorn (1985) in Channa striatus and Azadi et al. (1987) in Mystus vittatus. The same linear relationship was also observed between the two variables i.e. fecundity- total length and fecundity- body weight relationship.