for management practices. Policies for harvesting have been implemented based on

Transcription

1 Chapter II Seasonal ovarian changes in channa gachua Introduction Knowledge on reproductive biology of cultivable fish is an important phenomenon for management practices. Policies for harvesting have been implemented based on information on reproductive biology viz: size at sexual maturity, duration and periodicity of spawning (Shapiro, 1987; Wootton, 1984). From an aquaculture perspective, market forces demand well-planned production of gametes and offspring to ensure year-round production of fish (Bromage, 1995; Patino, 1997). Manipulation of fish reproductive system under controlled condition requires an understanding of natural spawning patterns and other influential factors. Munro (1990) reported the environmental factors that influence teleost reproduction and has categorised them as either ultimate or proximate. Ultimate factors relate to offspring survival and growth, while proximate factors are concerned with coordination of gonadal cycles with the environment so as to maximise an individual s potential fecundity and reproductive fitness. Teleosts have inherent mechanisms for correlating gonadal structure and function with environmental factors. This results in temporal and spatial spawning seasons. This strategy allows spawning when conditions are optimal for survival and growth of the progeny (Bye, 1984; Munro, 1990). The correlation between reproduction and environment is mediated through the neuroendocrine system, which perceives environmental cues and transduces signals that influence gonadal structure and their function (Peter and Yu, 1997). 27

2 The reproductive cycle must ensure a sufficient quantity of mature egg cells, which is possible only within the regular process of the oogenesis. The oogenesis is a very dynamic process in the ovaries, in which the oocyte passes through various phases of development similar in different fish species. The development of the piscine gonads has been described in terms of stages of maturity (Treasurer, 1990; Ha and Kinzie, 1996). The ovaries of the fishes have been classified into five types according to the pattern of the oocyte development viz: Immature, Matruring, Ripening, Ripe, Spent (Selman and Wallace, 1989). In the case of synchronic oogenesis, all the oocytes develop at the same time, ovulation also being simultaneous. The synchronous ovary consists at least two populations of the oocytes at different developmental stages: teleosts with this type of ovary generally spawn once a year and have a relatively short breeding season. In the case of asynchronic ovulation, different development stages of the oocyte maturation and ovulation in groups may be found within the ovaries (Nagahama, 1983; Nejedli et al., 2004). The spawning biomass is employed routinely in stock assessments of fishes as indicators of reproductive potential. For proper management of a fishery, a thorough study of maturation cycles and depletion of gonads is important, since such a study is aimed at understanding and predicting the annual changes of the population (Thorpe et al., 1990, Jobling et al., 2002, Tomkiewicz et al., 2003, Shein et al., 2004). Fish ontogeny is usually divided into different stages. They are larval, pre-reproductive, reproductive and postreproductive periods (Rass, 1989 and Balon, 1984). The pre-reproductive period is characterized by the formation and maturation of oocytes. This is followed by ovulation and fertilization. The teleost oocytes as in other vertebrates are surrounded by two major cell layers as an outher thecal layer and an inner granulosa. As the oocytes grow, the follicle cells multiply and form a continuous follicular 28

3 layer called the granulose cell layer. The fish oocyte development can be divided into oocyte growth and oocyte maturation. Vitellogenesis plays an important role in the oocyte growth. Germinal vesicle migration and breakdown, coalescence of lipid droplets and yolk globules, and release of the 1st polar body are the characteristic event in the precess of maturation (Nagahama, 1983; Yueh and Chang 2000). The reproductive cycles in teleosts occurs during a particular phase; some breed once in a year as annual breeders and others as monsoon breeders (Zuckerman 1962). Cyclic changes in the gonads (ovaries and testes) have been examined in a few species viz. Channa marulius (Hamilton, 1822) (Parameswaran and Murugesan 1976), C. punctata (Srivastav and Srivastav 1998), C. striata (Haniffa et al.,. 2000), Puntius filamentosus (Mannar Mannan, 2010, C. batrachus (Fagbenro et al., 1992), Leiognathus brevirostris (Jayawardane and Dayaratne 1998), Nemipterus randalli (Rao 2003) and Micropogonias furnieri (Vicentini and Araújo, 2003). Cytology-based classification systems provide the most accurate description of transformations taking place in the gonads. Fisheries management uses information pertaining to size at sexual maturity, patterns of gonadal recrudescence, spawning seasonality and synchronicity and fecundity. These data are, however, only available for a small number of teleost species of commercial importance (Tyler and Sumpter, 1996). In labeines, reproductive studies have been primarily macroscopic in focus (Cadwalladr, 1965b; Siddiqui et al., 1976; Gaigher, 1983; Van Zyl et al., 1995; Weyl and Booth, 1999) with only two studies investigating ovarian histomorphology (van der Merwe et al., 1988; Booth and Weyl, 2000). Histological studies provide precise information on oocyte development but are unfortunately slow to undertake and are expensive because they involve complex laboratory techniques (West, 1992). The 29

4 importance of histological description of gametogenesis was emphasised by Booth and Weyl (2000) who noted that macroscopic staging must be validated if errors in the estimation of maturity and reproductive seasonality are to be minimised. In male fish, knowledge of spermatozoon ultrastructure was used in systematics (Mattei, 1991) and found to be important in assessment of milt quality (Billard, 1978). Structural variations have been reported in several fish taxa (Mattei, 1991; Lahnsteiner et al., 1995) including those within the Cyprinidae (Baccetti at al., 1984). Murrels being seasonal breeders exhibit clear changes in the gonads during breeding season. Haniffa et al., (2002) indicated that a marked seasonal periodicity of the ovary in C.punctatus. Changes in the gonads of grouper fish Epinephelus septemfasciatus were investigated by Shein et al., (2004). C.gachua breeds in any type of freshwater viz., lakes, reservoirs, rivers, ponds, ditches, swamps and paddy fields. Its breeding season is not confined to a definite period but varies geographically. The variation in the reproductive biology of the dwarf snakehead C.gachua in different localities may be attributed to the difference in climatic conditions. Information concerning reproductive cycle and histomorphological changes of ovary in captive conditions is yet to be known. Therefore, studies were conducted on the influence of seasonal changes in gonado somatic index and fecundity of brood stock of C.gachua under captive condition. 30

5 Materials and Methods C.gachua ranging in total length from cm and weight g were collected from Tamirabarani river during the months of December February 2009 and were introduced into CARE earthen pond ( 3 x 3 x 1m). Fresh chicken viscera was collected from local market, cleaned and sliced into small pieces and supplied to the fish ad libitum. For the study of maturation and gonad development monthly samples of 4-5 female fish were collected from the pond using drag net during the period April 2009 to March Visual morphological observations and common experience helped in identifying the gravid females. Test fish were brought to the laboratory in fresh condition, measured for their length and total body weight and recorded to the nearest millimeter and gram respectively. After dissection, the gonads were taken out and then moisture was thoroughly wiped out from the ovaries using a filter paper and weighed used a electronic balance (Anamed electronic balance; sensitivity 0.001g). Total length of the ovaries was also recorded using a measuring scale and a divider. The ovary was preserved in 4% formalin for detailed examination of ova diameter and one or two pieces of ovary were fixed in Bouin s fixative solution. These tissues were embedded in paraffin wax and sectioned at 5-6 µ m in thickness and stained with hematoxylyn and eosin. Gonadosomatic Index Gonadosomatic index was estimated by the following equation GSI GW = TW 100 Where GW represents the weight of the gonads and TW is the total weight of the fish. 31

6 Fecundity determination Altogether 50 females were examined mostly by observing slightly bulged abdomen of the gravid fish, which could be easily distinguished during the month of the July- December The length, weight of the dissected ovary from each fish was taken and the ovary was fixed in 4% formalin. Gravimetric method was used for the estimation of fecundity. Three sectional samples weighing 100 mg each were removed with accompanying membrances from anterior, middle and posterior regions of the two lobes of ovaries of each fish (Lagler, 1956). The total number of eggs for each individual was calculated from the sample mean and the total weight of the ovaries. To establish the relationship between fecundity and total length, and total body weight, gonadal length and gonadal weight, regression coefficients were calculated. The point of interception and coefficients of correlation were estimated by the least square method (Jhingran, 1961; Swarup, 1961). 32

7 Results Reproductive seasonality The gonadosomatic indices of females varied significantly between different months for Tamirabarani River populations. The minimum observed GSI was in April (0.79 ± 2.15%) and May (0.9 ± 1.02%), while the maximum was recorded in December (3.61 ± 1.85%). The rise of GSI from June to August indicated the preparatory cum active phase or maturing phase of the fish (Table. 2.1). During this period the gonadosomatic index value varied between June (1.87%), July (1.97%) and August (2.01%) respectively. The high gonadosomatic index during September to February clearly indicated vittellogenic period or active cum quiescent period. In December the index suddenly raised from 2.42% to 3.61% as the ovaries mature, and was maximum in December (3.61%), when the ovaries were in ripe condition. The period December February indicates the peak season of the spawning of the fish. The highest proportion of spent gonads was noticed in March, April and May. The GSI value declined rapidly from 3.31 to 0.79 after spawning. Therefore it was confirmed that the fish spawned once in year with one spawning peak during December to February. Fecundity The number of eggs varied from 925 for an individual measuring 11.2cm length and 25g body weight to 3450 for individual of 15.5cm and 53gm. The mean number of eggs was 1750 ± for mean total length of ± 1.09 cm and mean body weight 34.8 ± 3.28g. The regression equation for the fecundity of C.gachua studied is shown in Table 2.2. There was a significant difference in fecundity total length and standard length, indicating non homogenecity. In the present investigation it has been found that the number of eggs 33

8 increased linearly with the increase of body length, body weight, standard length, gonadal length and gonadal weight. Table 2.2. Relationship between fecundity (ordinate) and total length (TL), standard length (SL), gonadal length (GL), total weight (TL) and gonadal weight (GW) of c.gachua. Fecundity Ver. TL (cm) SL (cm) TW (g) GL (cm) GW (g) A B r Significance of r at 5% and 1% level Significance,,,,,, Macromorphological description of the ovary The ovaries of C.gachua are paired organs situated in the peritoneal cavity and suspended on either side of the mid-line by a mesovarium. The spindle shaped ovary on the left side is generally greater in length than that of the right side and both remain separated from one another throughout their length. The length, weight and colour of the ovaries change seasonally according to the degree of maturity they have attained. The size and shape of the ovaries varied with the stage of development (Table 2.3). The matured ovary is orange yellow in colour with distinct granular appearance during the breeding season. Ova are visible through the ovarian wall, to a naked eye. They do not ooze out through the genital aperture when a striping is applied. In spent fish, the ovaries were thin straight and translucent and as oogenesis progressed, they enlarge to become lobed, sac-like, greenish cream organs filling the abdominal cavity. 34

9 In this study six different morphological stages of ovary (I to VI) were observed and classified according to the colour and blood vascular pattern of the ovary (Arockiyaraj et al., 2004). The percentage weight of ovary at six different morphological stages in relation to total body weight of C.gachua is presented in Table 2.3. In the present study based on the shape, size, changes in the nuclear and cytoplasmic components, six different developmental stages of oocyte have been classified under three major categories viz., i) immature ii) maturing oocyte and iii) mature oocyte. Light microscopic description of oogenesis The process of oogenesis was classified according to oocyte location and size, staining characteristics, number of nucleoli, presence of the follicular layer, and the distribution of cytoplasmic inclusions. According to these criteria oogenesis was found to proceed through six stages viz; oogonia, chromatin nucleolus, perinuclear oocytes, primary yolk vesicle oocytes, secondary yolk vesicle oocytes and tertiary yolk vesicle oocytes. Fig 2.2. Transverse section through the ovary illustrating oogenesis. Ovigerous lamellae (ol) from the tunic albuginea (ta),. 35

10 Oogonia Primary and secondary oogonia were discernable in ovarian nests where they attached together with follicle cells. Primary oogonia were the smallest germ cells noticeable and were 10.4±0.6µm in diameter. Primary oogonium was characterised by a large nucleus: Secondary oogonia showed high nucleus cytoplasm ratio and one nucleolus but were larger than primary oogonia and their nucleus was filled with basophilic chromatin threads. Chromatin nucleolus stage Chromatin nucleolar oocytes were characterised by a large centrally located nucleus compared to the cell size with clumps of basophilic chromatin on the nuclear wall and surrounded by a light basophilic cytoplasm (Fig. 2.2) and showed a high nuclear:cell diameter ratio. Fig Primary oogonia (po), Secondary oogonia (so) and chromatin nucleolar oocytes (cno) adhered in ovarian nests together with follicle cells (fc). Pre-perinucleolar oocytes (ppo) were outside the nests 36

11 Peri-nucleolar stage Growth of the chromatin nucleolar oocytes to the peri-nucleolar oocyte stage was accompanied by migration out of the germ cell nests. Three types of peri-nucleolar oocytes were recognised. i) Pre-perinucleolar oocytes were close to the nests, polygonal in shape, and contained multiple nucleoli of varying sizes in the nucleus. Their cytoplasm was basophilic. ii) Early peri-nucleolar oocytes were also polygonal in shape but had three to four large nucleoli and several smaller ones. Increase in the size of these cells was accompanied by the cells becoming spherical and less basophilic in Haematoxylin and Eosin (H&E). iii) The late peri-nucleolar oocytes were the least basophilic and spherical in shape. Late perinucleolar oocytes were characterised by numerous nucleoli neatly arranged on the nuclear wall (Fig. 2.3). All peri-nucleolar stage oocytes had acellular zona radiata and were surrounded by two follicle layers - a theca and a granulosa. Yolk vesicles, or cortical alveoli, started to appear in the late peri-nucleolar oocytes marking the end of the primary growth phase. Fig Late peri-nuclear oocyte (lpo), early perinuclear oocyte (epo) was less ovoid 37

12 Primary yolk vesicle stage During this stage the cortical alveoli formed at the periphery of the oocyte and increased in number to fill the whole of the cytoplasm. The zona radiata and the follicular layer increased in thickness together with an increase in the number of nucleoli (Fig. 2.4). Fig Cortical alveoli (ca) in the primary yolk vesicle oocyte (1oyvo) Secondary yolk vesicle oocyte stage Oocytes in this stage were characterised by initial appearance of acidophilic yolk granules, staining red in H&E, in the cytoplasm (Fig. 2.5). The prevalence of oocytes in this development stage was low when compared to other stages. Secondary yolk vesicle oocytes had similar follicular layers as the primary yolk vesicle oocytes. Fig Yolk granules (yg) and the cortical alveoli in secondary yolk vesicle oocyte (20yv0) 38

13 Tertiary yolk vesicle oocyte stage In tertiary oocytes, the yolk granules were initially at the periphery of the cytoplasm and increased in size to form globules and occupied the entire central section of the cytoplasm. Cortical alveoli were seen only at the periphery of the cytoplasm, forming a ring around the ooplasm. The nucleus at this stage was centrally positioned and irregular in shape with several nucleoli on its membrane. The chromatin in the nucleus was no longer visible at this stage. As the oocyte developed further, the nucleus migrated from the centre to the periphery of the cell (Fig. 2.6). Fig Transverse section through a tertiary yolk vesicle illustrating the central location of the yolk globules (YG), cortical alveoli (CA) and an eccentric germinal disk (GD). 39

14 Post-spawned Ovaries In the post-spawned ovary, cohorts of basophilic developing oocytes were visible in spent ovaries (Fig. 2.7). The theca and granulosa layers remained and hypertrophied to form the post-ovulatory follicles. The granulosa cells that were squamous prior to ovulation became cuboidal to columnar in structure with ovoid basophilic heterochromatin. The thecal layer became increasingly vascularised (Fig. 2.8). The post-ovulatory follicles were later invaded by macrophages to form melano-macrophage centres.. Fig Transverse section through the ovary of Channa gachua illustrating postovulatory changes. A. Post ovulatory follicles (POF), atretic oocytes Type (I &II) and a cohort of previtellogenic oocytes (PVO) 40

15 Oocyte atresia Oocyte atresia was common in the spent fish and three forms were noted. Type I atresia was characterised by fast fragmentation of the zona radiata and dissolution of the cytoplasm contents. This type of atresia was common in tertiary oocytes of successfully spawned ovaries. Type II atresia also occurred in vitellogenic oocytes and was characterised by the breakdown of yolk globules, to smaller granules, together with vacuolar degeneration of the cytoplasm with an intact zona radiata. The presence of Type II atresia indicated spawning failure. In both spawning scenarios, the ovaries were cleared of the matured oocytes to facilitate gonadal recrudescence. In this case, ovigerous lamellae with the germ cells surrounded, and formed a ring of pre-vitellogenic stages around, the unspawned oocyte prior to onset of Type II atresia. The seasonal changes in the ovary of C.gachua during an annual reproductive cycle has shown the following phases. 1. Quiescent cum preparatory phase, 2. Preparatory cum active phase and 3. Active cum Quiescent phase. The Quiescent cum preparatory phase extended from June to November. The oocytes recorded during this period were immature, maturing and oocyts of different states of resorption. In Quiescent cum preparatory phase the ovary proceeds to the next phase. This phase extended from March to May. In active phase the oocyte were mature although few immature oocytes were seen. This phase extended from December to February. 41

16 Discussion The gonad morphology and histology of the mouth brooding dwarf snakehead Channa gachua has not been already described. This study forms the first detailed attempt to investigate the gonad morphology and histology of this fish. The C.gachua is a maternal mouth brooder in which the female churns the eggs in her mouth. Parental care occurs in 21% of families of teleost fishes (Gross and Sargent 1985). Most parental care (more than 95%) involves guarding of the eggs by the male (Specker and Kishida 2000). Reproduction is a seasonal event among a majority of teleostean fish species and the environment has a profound effect on growth and reproduction (De vlaming, 1974). Reproductive similarities and differences between the study of C.gachua were clearly noticeable. Seyit Oymak et al., (2008) reported that the oocyte maturation in Capoeta trutta was associated mainly principally with ecological and climatic conditions. It is generally accepted that these factors are co ordinate by the hypothalamus which inturn controls the gonadotropin activity from the pituitary to gonad axis (Arockiaraj et al., 2004). The fish spawns and the spawning burst depends upon the availability of appropriate environmental stimuli (Wijeyaratne,1994). Therefore environmental factors play important role in the developing patterns of the reproductive cycle (Wang et al., 2001). In the present investigation it has been found that the number of eggs increase linearly with the increase of body length, body weight, standard length, gonadal length and gonadal weight. Similar results were also reported by different workers (Praween et al.,2000; Alam et al., 2002) for other species. The variation in fecundity is not only due to fish length and weight but also due to malnutrition, lack of vitamins and adverse environmental factors 42

17 (Dube, 1993; Bagenal, 1957). There was a significant difference in fecundity total length and standard length, indicating non homogenecity. Doha and Hye (1970) reported that the variation of fecundity is very common and observed in fishes and the number of eggs produced by an individual female is dependent on several factors like size, age, and environmental conditions. Reproductive potential of the fishes is also influenced by availability of space and food (Mookeerjee and Mazumder, 1946). The present observation on fecundity reveals that C.gachua is less fecund fish when compared to carps, catfish and other channa species like C.striatus( ,000) and C.punctatus ( ).In the present study a maximum of 3450 and a minimum of 925 eggs were reported. Similar results are available in the literature. Breder and Rosen (1966) stated that spawning in India occurs with the female swimming upside down under the male, with eggs being released and fertilized in groups of every minute or two. Bhuiyan and Rahman (1982) reported fecundity of C.gachua ranging from C.gachua measuring 15.5 cm in total length, 53g in body weight and 0.52g in ovary weight produced 3450 eggs, whereas another fish of the same total length produced 2980 eggs. Similar kind of variation was also observed in other two fishes with the total length of 15.0 and 14.7 cm produced 2340 and 2050 eggs respectively. The same type of variation was also reported by Marimuthu et al., (2006) in spotted snakehead Channa punctatus and Musa and Bhuiyan (2007) in Mystus bleekeri. Gonadal development and reproductive strategy have been described in many teleost fish species in an effort to understand the time course and energetic consequences of reproductive effort. Oocyte growth follows a similar general pattern in most teleosts (Sundarabarathy et al.,2004; Encina and Lorencio, 1997; Stoumboudi etal., 1993; Dixit and 43

18 Agarwal, 1974). The highest values of CF and GSI were due to the active somatic energy accumulation and the lowest CF and GSI due to the somatic energy depletion (Encina and Lorencio 1997). Photoperiod also affect the CF and GSI as reported by Hansen et al., (2001) in Gadus morhua Linnaeus, Cambray and Bruton (1984) pointed out that the similarity in the CF and GSI, annual cycle and the fact that the seasonal pattern common among juveniles and adult fish, suggest that the seasonal variations encountered were more related to variations in food availability than to the reproductive cycle. Mishra (1991) described C.gacua mature (stage V) oöcytes ranging from 2.1 to 2.6 mm in diameter with the highest percentage of stage V oöcytes in July from specimens collected near Berhampur, Orissa, India. The highest gonadosomatic index was 3.61 and occurred in December. Estimated fecundity ranged from 2,539 to 7,194 in 15 mature specimens ranging from 13.4 to 17.2 cm in length Reproductive studies of the fishes require knowledge of the stage of the gonad development in the teleosts. The structural alterations were observed in the C.gachua oocytes during the oocyte development in the histological studies performed. In this study, the oocyte development of the C.gachua was divided to six stages. Again, the relationship between fecundity and length, as well as fecundity and body weight, was largely linear. In the majority of teleost fishes, the process of oogenesis may be divided to five, six or eight stages (Fishelson et al., 1996, Nagahama 1983, Ünal et al., 1999, West 1990, Gökçe et al.,2003, İşisağ 1996). Arockıaraj et al., (2004) described the morphological changes in the gonad of Mystus montanus and histologically divided into five stages. Gonadal development and reproductive strategy have been described in many teleost fish species in an effort to understand the time course and energetic consequences of reproductive effort. Oocyte growth 44

19 follows a similar pattern in most teleosts (Maddock and Burton 1999, Knuckey and Sivakumaran 2001). In the present investigation, C.gachua breeds from December to February. Similar observations were made in Gobioides rubicundus = Odontamblyopus rubicundus (Hamilton, 1822) by Kader et al., (1988). O. rubicundus mainly breeds in late January to early February and late June to early October. Based on these observations, it is proposed that development from oogonia to tertiary yolk vesicle oocyte takes four months in C.gachua. Oogonia developing from the ovigerous lamellae of ripe ovaries were not affected by post-ovulatory changes, with oocytes from the preceding cohort only becoming atretic. Oocytes from fish sampled two months after peak spawning periods were at the primary and/or secondary oocyte stage. The low prevalence of secondary oocytes was possibly due to rapid oocyte development to the tertiary oocyte stage. Ovaries of fish caught four months post-spawning included the new cohort of oocytes that had reached the tertiary yolk vesicle stage. Future studies involving repeated ovarian biopsy are, however, required to confirm this observation. In addition to the postovulatory follicles and atretic oocytes, seven stages of development are described based on the histological and ultrastructural characteristics of the oocytes (Munoz et al., 2002). According to Nejedli et al., (2004), the oogenesis in Sardines is manifested by a series of changes in the oocytes, which makes their division into four basic group. According to Fishelson et al., (1996) and West (1990), the nucleus was large in the primary growth phase, with 2-4 nucleolus situated in the centre of the nucleoplasm. Yolk vesicles were seen on the cytoplasm at the cortical alveolus phase. The nucleus membrane dissolved and the peripheral migration of the nucleus started in the mature oocyte phase. In 45

20 the present study, all the stages were identified in similar manner. Because of the sizes of the oocytes, the number of nucleoli may vary between the species (Fishelson et. al 1996). Histology revealed the reabsorption of both tertiary yolk stage oocytes and the other second growth phase oocytes. This resolves some contradictory reports found in earlier studies (Palmer et al., 1995). Daiber (1953) reported three distinct size classes of oocytes and concluded that the largest were released during spawning, the middle size were released in the next year, and the smallest were released in subsequent years. According to Yueh and Chang (2000), the morphological changes of the oocytes of black porgy during maturation were similar to those of other teleost fish. Vitellogenic oocytes proceed through final maturation with coalescence of the yolk globules and oil droplets. The oocyte diameter gradually increases. The sizes of the oocytes are evenly distributed in the range of oocyte diameter from 0 to 400 µm in the late vitellogenic stage. West (1990), Ünal et al., (1999,2005) reported that the vitellin membran appears commonly at the yolk vesicle stages and sometimes at the late perinucleolus or at the of the yolk vesicle stages. In this study, the vitellin membrane began to develop at the vitellogenic growth stage. In another study the macro- and microscopic ovary features of Hemiodus microlepis, H. ternetzi and H. unimaculatus were analyzed by Brandao et al., (2003). The microscopic analysis indicated a groupsynchronous oocyte development, common to the three species that were characterized as iteroparous synchronous spawners with a total spawning. According to Hibiya (1982) the contents of vitellogenic oocytes liquefy and diminish during atresia, a description that conformed to Type I atresia noted in tertiary oocytes, and also reported in the mullet, Mullus surmuletus (N da and Déniel, 1993). Tyler and Sumpter (1996) observed that it was difficult to distinguish between an atretic follicle that failed to be 46

21 ovulated in the previous spawning season, despite reaching full size, and a developing oocyte that becomes atretic before it reaches full size. The incidence of follicular atresia is reported to be high under sub-optimal conditions (Tyler and Sumpter, 1996). Unfortunately, those mechanisms that regulate follicular atresia in this process are not properly understood as various factors, including environmental conditions, influence follicular atresia in fishes (Bhagyashri and Saidapur, 1996). Results from this study underscore the importance of histology in the elucidation of the reproductive tactics adopted by fish species. Reproduction in fishes is known to revolve around trade-offs between immediate reproductive output and holding back some reproductive effort/resources for the future (Matthews, 1998). However, in the present study six oocytic stages (I to VI stages) were clearly defined. It can also be inferred from the observed findings that in Channa gachua there occurred atleast 3 phases in the annual ovarian activity. Based on the histological studies on the growing oocyte, the present study clearly distinguish the stage I and Stage II as immature, stage III,IV and V oocyte as maturing and stage VI as mature which is similar to that of classification adapted by sundararaj and Sehagal (1970) and Daham and Bhatti (1979). 47