Fecundity and gonado-somatic index of wild freshwater spiny eel Mastacembelus armatus ( Lacepede ) from Kishoreganj region of Bangladesh

Fecundity and gonado-somatic index (GSI) of wild freshwater spiny eel Mastacembelus armatus were estimated from haor region of Kishoreganj district of Bangladesh. Estimated fecundity ranged from 2,235 to 19,493 for the fish measuring from 25 to 54 cm and weighing from 79 to 345 g. The minimum, median and maximum ova diameter was 0.20, 1.90 and 2.20 mm, respectively. The average numbers of ova present per g body weight were 45.41±8.85, 46.93±15.00 and 47.52±8.32 while the average numbers of ova present per g ovary were 369.64±40.66, 351.29±30.90 and 328.36±33.00 in the months of April, May and June, respectively. Maximum GSI for female spiny eel was 14.40±1.48 in the month of June and the minimum GSI for the same sex was 0.44±0.06 in September. The regression of fecundity on total length, body weight and gonad weight of female spiny eels were Log F = -0.875 + 2.957 × Log TL (r = 0.874), Log F = 0.630 + 1.461 × Log BW (r = 0.917) and Log F = 2.425 + 1.087 × Log GW (r = 0.977), respectively. Fecundity-gonad weight gave a better relationship when compared to fecundity-body weight and fecundity-total length relationship.


Introduction
Baim (Mastacembelus armatus) is one of the most sought after fishes.People like its flesh for special flavour and characteristic texture.Among the available freshwater eel species, M. armatus has lucrative size, important production potentials and high protein contents.The caloric value of eel flesh is as high as 303 cal/100 g compared to 110 cal/100 g in other average fishes (Nasar, 1997).The freshwater spiny eel belongs to a family Mastacembelidae of order Synbranchiformes is also known as Bam or Baim.It commonly occurs in the freshwater of this sub-continent (Talwar and Jhingran, 1991).Baim is the inhabitant of rivers, canals, beels, ponds and inundated fields (Rahman, 1989).Its production from beels, floodplain and Kaptai lake was 234 ton in the year 2006-07 (FRSS, 2006(FRSS, -2007)), 206 ton in the year 2005-2006(FRSS, 2005-2006) ) and 283 ton in the year 2004-2005(FRSS, 2004-2005)).
Mastacembelus armatus is a very popular fish with high market demand, having about double the market value compared to the carp fishes in the country.In spite of its delicious taste, flavour and market demand, very scanty systematic attempt has been made to study the biology and culture potential of this fish (Narejo, 2003).
Knowledge on reproductive biology of fish is essential for evaluating the commercial potentialities of its stock, life history, culture practice and management of its fishery (Islam et al., 2012).Reproductive potential of a population is one of the basic exigencies to designate the individuals of that population in respect to their gonadal conditions (Akter et al., 2012).In order to achieve success in fish culture, it is important to assess the breeding cycle with fecundity of cultivable fishes.Knowledge on the fecundity of a fish species is important for determining: (a) spawning potential and its success (Das et al., 1989;Karim and Hossain, 1992); (b) fluctuations in the egg production potential of individual stock related to life processes such as age and growth (Shaheena, 2012); (c) effects of environmental factors (Bromage et al., 1992); and (d) formulating the commercial management of fishery (Lagler, 1956).Reddy (1979) mentioned that determination of breeding season is an essential part of biological investigations of fishes.Narejo (2003) and Taslim et al. (2002) worked on the reproductive biology of some freshwater eels of Mymensingh region.But there is a serious shortage of some basic scientific information on the biology, especially fecundity of the spiny eel (M.armatus).In view of the above, present investigation was aimed to study the reproductive biology based on fecundity, ova diameter and gonado-somatic index of wild freshwater spiny eel, M. armatus, of haor region of Kishoreganj district of Bangladesh.The objective also included the study to establish the empirical relationship between the fecundity with total length, body weight and ovary weight of this species.

Collection of fish samples
The experiment was conducted for a period of 12 months from January to December 2012.114 female spiny eels were collected from natural habitat of haor region in Kishoreganj district through the fishermen in order to determine gonado-somatic index (GSI) and 42 wild fish of the same species for fecundity study.

Fecundity estimation
There are several methods for the estimation of fecundity of fish of which the actual counting method was found to be the most accurate one.Among the methods as outlined by Lagler (1956), gravimetric method is considered to be more efficient and gives fairly accurate result.This method has also been successfully used by Shafi and Quddus (1978); Dewan and Doha (1979); Mustafa et al. (1980) and Blay-Jr (1981).In this method, eels were anaesthetized with 0.02% clove oil (Keene et al., 1998).Individual eel was then measured for total length to the nearest cm and body weight to the nearest g.The ovaries were dissected out by a pair of scissors.The external connective tissues were removed from the surface of each pair of ovaries.The moisture of the ovaries was removed with the help of a blotting paper.The weight of the ovaries of each fish was recorded with the help of an electronic balance.Then 1.00 g of ovarian part was taken separately from anterior, middle and posterior portions of each ovarian lobe accurately and gonad samples were fixed at 10% buffered formalin (gonad and fixative ratio being 1:10) for subsequent studies.The gonad samples were swollen heavily, their membrane became transparent and the number of mature and maturing eggs from each portion was found out separately by actual counting.The mean number of eggs in 1.00 g was determined and then multiplied by the total weight of the ovary, which gave the total number of eggs i.e. the fecundity of respective fish.This was done by the following formula:

Measurement of ova diameter
When an ovary of a fish was dissected out, a small representative part from anterior, posterior and middle portion of the same was taken separately.The ova of the samples were separated keeping in a physiological saline solution (0.85% NaCl) and spread on a glass slide to measure the diameter of ova under a microscope with an ocular micrometer.However, at least 100 ova of a single sample were measured and observed values for ova diameter were expressed in the unit value of the ocular micrometer according to recommended method given by LeCren (1951).

Ganado-somatic index (GSI)
Gonado-somatic index (GSI) is frequently applied to determine the spawning frequency of fishes.The GSI was calculated according to the following formula: weight Body weight Gonad GSI

Statistical analysis
Several morphological characters were analyzed by Microsoft Excel (MS Excel) computer package as descriptive values such as mean and percentage.Determination of linear relationship and correlation coefficient (r) between total length and fecundity, body weight and fecundity, gonad weight and fecundity was performed using MS Excel.The statistical data analysis was carried out with the aid of the computer software SPSS version 11.5 (Statistical Package for Social Science).

Fecundity
One hundred fourteen specimens of mature female spiny eel sampled during January to December 2012 ranged from 25.40 to 54.20 cm in total length (TL) and 78.50 to 345.00 g in weight.The number of ova ranged from 2,235 to 19,493.Maximum fecundity was recorded from a fish measuring 54.20 cm in TL and 345.00 g in weight and the minimum, from a fish measuring 25.40 cm in TL and 78.50 g in weight (Table 1).The average numbers of ova present per g of body weight were 45.41±8.85,46.93±15.00and 47.52±8.32 while the average numbers of ova present per g ovary were 369.64±40.66,351.29±30.90 and 328.36±33.00 in the months of April, May and June, respectively (Table 2).It was also noted that the fecundity increased with the increase in total length and body weight of the fish.The equations of regression coefficient between total length (TL), body weight (BW) and gonad weight (GW) versus fecundity (F) are given below.All these relationships have been shown graphically in Figs. 1, 2 & 3. Fecundity of the 42 freshwater spiny eels increased symmetrically with total length, body weight and gonad weight as characterized by the relations: Log F = -0.875+ 2.957 × Log TL (Fecundity -Total length) (r = 0.874) Log F = 0.630 + 1.461 × Log BW (Fecundity -Body weight) (r = 0.917) and Log F = 2.425 + 1.087 × Log GW (Fecundity -Gonad weight) (r = 0.977), respectively, where F = total number of eggs in an individual, TL = total length (cm), BW = Body weight (g) and GW = gonad weight (g).

Ova diameter
Measurements of ova diameters were made to determine the relative sexual maturity of M. armatus.The minimum, median and maximum ova diameter was 0.20, 1.90 and 2.20 mm, respectively (Table 1).All the ova (100 from anterior, middle and posterior region of each ovary) were measured and found to be spherical and uniform in diameter.

Gonado-somatic index (GSI)
The GSI of the fish of M. armatus (female) increased as the fish reached toward maturity and declined with the start of spawning activities.It was calculated during January to December 2012.Month wise changes in mean GSI values are presented in Table 2 and Fig. 4. The values of GSI ranged from 0.44±0.06 to 14.40±1.48 in female M. armatus during January to December 2012.The higher values of GSI were observed during April to June ranging from 12.21±1.48to 14.40±1.48.The GSI increased in every month from March to June.After spawning activities, GSI declined suddenly in the month of July and the lowest GSI (0.44±0.06) was recorded in the month of September.

Discussion
The fecundity of freshwater spiny eel, M. armatus was estimated from samples of 3 months i.e.April to June 2012.The fecundity estimates during the present study ranged from 2235 to 19493 eggs and the size of fish varied from 25.40 to 54.20 cm (TL).The average numbers of ova present per g of body weight were 45.41±8.85,46.93±15.00and 47.52±8.32 while the average numbers of ova present per g ovary were 369.64±40.66,351.29±30.90 and 328.36±33.00 in the months of April, May and June, respectively.Nasar (1989) calculated the fecundity of Monopterus cuchia to range from 118 to 687.Narejo (2003) reported the fecundity of M. cuchia to range from 260 to 5890 and M. armatus to range from 580-10980 respectively.The authors also reported that average number of ova present per g of body weight was GSI 4.61 and per g of ovary weight was 74.27 in case of M. cuchia and average number of ova present per g of body weight was 29.38 and per g of ovary weight was 407.57 in case of M. armatus respectively.Estimates of fecundity from mean counts of aliquots for an individual American eel (Anguilla rostrata) ranged from 1.84 million to 19.92 million eggs in the length range from 45.2 to 113.3 cm (Barbin and McCleave, 1997).It was suggested that perhaps the variation was due to different temperature and feeding regimes, producing variation in fat metabolism (Svedang et al., 1996), although fat content was not measured.The productivity of local waters may thus produce variation in size at maturity and it may also produce variation in size-fecundity relation.The estimate of fecundity in the present study was much higher than the estimate given by Nasar (1989) and Narejo (2003), and was much lower than the estimate given by Barbin and McCleave (1997).The difference in the fecundity estimation could be due to different environmental conditions in which the populations live.The fecundity also varied with the seasons, climatic conditions and environmental habitat, nutritional status and genetic potential (Bromage et al., 1992).
During the present investigation, the fecundity was plotted against the total length, body weight and gonad weight.Correlation coefficient (r) between fecundity and total length was 0.874; fecundity and body weight was 0.917; and fecundity and gonad weight was 0.977 for M. armatus.The relationship with fecundity was stronger (r=0.917) in case of body weight than total length (r=0.874)where as the strongest correlation was found between fecundity and gonad weight (r=0.977).Similar observations were reported by many authors like Khan et al. (2002) for the canine catfish, Plotosus canius; Rehman et al. (2002) for grey mullet, Liza parsia; and Nabi et al. (2007) for Glossogobius giuris.
In the present experiment, the minimum, median and maximum ova diameters were 0.20, 1.90 and 2.20 mm, respectively.Narejo et al. (2002) observed that M. armatus has only one breeding season during May to July with a peak in July and the maximum size of the mature ova was 1.00 mm progressively increasing in the ova diameter during December to July.The results of the present study are closed to those obtained by Narejo et al. (2002) who worked on fish that were reared in the cemented cisterns.
The fecundity of the fish was inversely proportional to the ova diameter (fecundity increases with the decrease of ova diameter).Various workers in different fish species have reported higher fecundity with small ova diameter (Pathak and Jhingran, 1977;Nabi and Hossain, 1996;Narejo et al., 1998;Afroz et al., 1990).Similarly fecundity decreases with the increase in ova diameter that was reported by Das et al. (1989); Shaima et al. (1992); and Faruq et al. (1996, 1998).
The GSI, which is indicative of the breeding season of the fish, was calculated from January to December.There was a spectacular rise in the values in April to June (12.21±1.48 to 14.40±1.48).The highest gonado-somatic index of female M. armatus was 14.40±1.48 in June and the lowest was 0.44±0.06 in September.The present experiment indicated that the fish spawn once in a year during April to June with a peak in June.The results of the present study closely relate to those obtained by Narejo et al. (2002).Similar observations were reported by Nabi and Hossain (1996) in freshwater spiny eel, Macrognathus aculeatus.
Fecundity of fish; and N = Number of eggs in sample