Reproductive Biology of the Mullet, Rhinomugil corsula (Hamilton, 1822) (Mugiliformes: Mugilidae) in the Sitakunda Coast of Bay of Bengal, Chattogram, Bangladesh

Abstract

Reproductive biology of the Corsula Mullet, Rhinomugil corsula (Hamilton, 1822) (Mugiliformes: Mugilidae) was carried out based on 1166 (394 male and 772 female) fish samples collected monthly from the fishermen’s catch of Sitakunda coast of the Bay of Bengal, Bangladesh, from December 2016 to November 2017. Yearly male and female sex ratio were 1 : 1.96 ($P<0.01$). In the male fish, immature, mature, and spent gonads were recorded as 69%, 27%, and 4%, respectively, while in the female fish, immature, maturing, mature, ripe, and spent gonads were recorded as 72%, 9% 11%, 5%, and 3%, respectively. First sexual maturity in male and female fish was attained at 16 cm and 21.5 cm length, respectively. The fish was a prolonged breeder with a peak in late monsoon. Highest GSI and GLI of male and female were recorded in September. Spawning season extended from July to November, and the peak was in September. Ova-diameter ranged from 0.01 mm to 0.81 mm. Fecundity varied from 31488 to 251880 eggs/fish. Fecundity factor varied between 1099.09 and 2808.23 eggs/g of body weight. Relationships between fecundity and total body length, total body weight, ovary length, and ovary weight were highly significant ($P<0.01$).

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1. Introduction

Mullets are widely distributed euryhaline fish inhabit in different ecosystems such as inshore sea, estuaries, brackish waters, and freshwater areas of all tropical and temperate seas [1]. The Corsula Mullet, Rhinomugil corsula belong to the order Mugiliformes under the family Mugilidae is one of the commercially important grey mullets which inhabit both coastal brackish water and freshwater habitats of Bangladesh [2]. The fish is considered as a least concern of the family Mugilidae and has high demand as food fish in Asian countries. Natural population of this fish has declined due to over-exploitation and various ecological changes in its natural habitats to an alarming condition and deserves high conservation importance [3]. It can tolerate a wide range of salinity and temperature [2]. It has a good potential for culture fisheries, in mono- and poly-culture, although, so far, the fish cultivators have not been successful regarding the culture of this fish species [4]. Knowledge of the reproductive biology of this species is essential for conservation, successful management, breeding, and successful culture.

Lots of works have been done on the reproductive biology of mullets in different parts of the world [5–9]. Some works have also been done on the reproductive biology of Corsula in the reservoirs of India [10–12] and in the rivers of Bangladesh [4, 12]. In Bangladesh, some scanty works are found on the reproductive biology of Corsula [4, 12], which contains very insufficient information and all these works have been done from the freshwater rivers.

So far, no information is available on the reproductive biology of R. corsula from the Sitakunda coast or any other coastal waters of Bangladesh. Hence, this investigation was undertaken to study the reproductive biology of R. corsula from Sitakunda coast of the Bay of Bengal with the following objectives: to find out the sex ratio, spawning periodicity, length at first maturity, cycle of gonadal maturation, and fecundity. Findings of this study might be helpful to know the reproductive pattern and breeding performance of this Corsula Mullet in the coastal brackish water of Bangladesh, which are needed for maintaining a sustainable population, artificial propagation and culture of this species.

2. Materials and Methods

2.1. Study Area

The geographical extent of Sitakunda, a south eastern coast of Bangladesh, is situated at the north-western part of Chattogram district between 22°34′ N and 22°43′ N latitude and 91°38′ E and 91°41′ E longitude (Figure 1). Country’s only ship breaking yard is situated in Sitakunda, which is world’s second-largest after Alang Ship Breaking Yard, India. Good numbers of Industries are also present in this coastal Upazila. Many small rivers, hilly streams and canals meet in the Bay of Bengal from the adjacent hilly land along this Sitakunda coastline. The coastal water of Sitakunda is enriched with marine, brackish and freshwater fishes due to influence of marine water in one side and pressure of freshwater from adjacent hilly areas in other side, mainly during rainy season. There are about 16 marine and coastal fish landing centers in Sitakunda area. Here, many local people are directly involved in fishing activities. There are 475 marine and coastal water fish species in Bangladesh. In the Sitakunda coast alone 124 marine and coastal finfish species including 7 Mullet species along with a reasonable number of Corsula, were recorded by Ara​ [13]. In this study reproductive biology of the commercial Corsula was carried out from Sitakunda coast of the Bay of Bengal.

[figure(s) omitted; refer to PDF]

2.2. Sample Collection and Analysis

Samples of Rhinomugil corsula were collected bimonthly from the fishermen catch harvested by three enclosure nets (length 161 m, mesh 3 to 4 mm) within 20 km area of Sitakunda coast. In each field trip all the corsula fish harvested by three enclosure nets, were purchased directly from the fishermen.

Samplings were continued for 12 months from December 2016 to November 2017. The collected Corsula samples were preserved in icebox and brought to the laboratory of the Department of Zoology of University of Chittagong for the study of different aspects of reproductive biology, i.e., sex ratio, length at first maturity, classification of gonads, gonadosomatic index (GSI), gonadal length index (GLI), ova-diameter frequency, and fecundity.

In the Laboratory collected fish samples were dissected to determine the sex on the basis of gonads; gonads were taken out and preserved in 70% alcohol for further study. Different external morphometric characters and gonad measurements i.e., total fish length (TL), total body weight (TW), gonad length (GL), and gonad weight (GW) were taken for each fish. Lengths were taken in centimeters (cm) by a measuring scale fitted with a wooden board. Weights were measured in gram (g) with the help of an electronic balance (ANDGULF Precision Electronic Balance, Mode: EK6001, Number: 14090602, Range: 0.01–620 g). Different methods used for the study of various aspects of reproductive biology of Corsula are given below.

2.3. Sex Ratio

Monthly variation in sex ratios was determined from the total number of two sexes of fish samples. The chi-square (χ²) test at 0.05 and 0.01 significance levels were performed to justify the difference from the expected 1 : 1 ratio.

2.4. Maturity and Classification of Gonads

Maturity stages of gonads were determined following Nikolosky [14]. Female gonads were classified as immature, maturing, mature, ripe, and spent ovaries, whereas male gonads were classified as immature, mature, and spent testes.

Gonado-somatic index (GSI) and Gonadal length index (GLI).

The GSI and GLI of each fish were calculated using the following formulae of King [15]:$$\begin{array}{c}\text{(1)}& \text{GSI}=\frac{\text{Gonad}\text{weight}\mathrm{g}}{\text{Total}\text{weight}\mathrm{g}}\times 100,\text{GLI}=\frac{\text{Gonad}\text{Length}\text{cm}}{\text{Total}\text{Length}\text{cm}}\times 100.\end{array}$$

2.5. Length Frequency Distribution and Length at First Maturity

Collected R. corsula were grouped into different length classes with 2 cm interval to determine the length frequency distribution. Length at first maturity was considered as the length at which 50% of R. corsula reached their sexual maturity, and was estimated by fitting the percentage maturity against mid lengths [15].

2.6. Size Frequency Distribution of the Intraovarian Oocytes

To determine size frequency distribution of intraovarian oocytes, samples (0.1 g) were taken from anterior, middle, and posterior portion of each of the left and right ovaries on a glass slide were measured separately with the help of an ocular micrometer, calibrated with a stage micrometer with the smallest unit of 0.01 mm, fitted with a binocular dissecting microscope [16].

2.7. Fecundity

Fecundity of R. corsula was estimated on the basis of counting of matured eggs of 38 ovaries collected during March to November 2017. Gravimetric method [17] was followed for the estimation of fecundity. After removing the external connective tissues from the surface of the ovaries and removing excess water with the help of blotting paper, each ovary was weighed to the nearest 0.01 g with the help of an electronic balance. Six cross sectional subsamples, each of 0.1 g were taken randomly from anterior, middle and posterior portion of each of the two (left and right) lobes of the ovaries. Sorting and counting of the immature and mature eggs of each portion of ovaries were performed using a “Sedgwick Rafter Counting Cell” under a binocular dissecting microscope.

Absolute and relative fecundity were calculated using the following formulae [14]:$$\begin{array}{c}\text{(2)}& \text{Absolute\hspace{0.17em}fecundity}=\frac{\text{No}.\text{of}\text{eggs}\text{in}\text{sub}\text{sample}}{\text{Weight}\text{of}\text{sub}\text{sample}}\times \text{Gonad\hspace{0.17em}weight},\text{Relative\hspace{0.17em}fecundity}=\frac{\text{Absolute}\text{fecundity}}{\text{Total}\text{length}\text{or}\text{weight}\text{of}\text{fish}}.\end{array}$$

To establish the mathematical relationships of fecundity with the total body length (TL), total body weight (TW), ovary length (OL), and ovary weight (OW), the values of regression co-efficient (b), intercepts (a), and correlation co-efficient (r) were calculated using computer based statistical program SPSS (version 16). To justify the correlation co-efficient t-test was performed.

3. Results

3.1. Sex Ratio

Out of 1166 samples of R. corsula 394 were male and 772 were female. Overall yearly sex ratio was found to be 1 : 1.96 (male: female), which was highly significant (χ² = 122.54, $P<0.01$) (Table 1). Monthly data also showed highly significant difference between male and female ratio apart from December 2016 (sex ratio: 1 : 1.30, χ² = 1.99, ($P>0.05$) and November 2017 (sex ratio 1 : 0.94, χ² = 0.09, $P>0.05$), when no significant differences existed between the male and female population of R. corsula (Table 1). The highest percentage of male (51.55%) was collected in November 2017 and the lowest (23.97%) in June 2017; on the contrary, the highest (76.03%) percentage of female was collected in June 2017 and the lowest (48.45%) in November 2017. The sex ratio was close to 1 : 1, the expected natural ratio, in the late spawning season (November 2017) and early prespawning season (December 2016) (Table 1 and Figure 2).

Table 1

Monthly and overall sex ratio and χ² values of Rhinomugil corsula in the Sitakunda coast of Bay of Bengal.

Month	No. of fish	Male		Female		Sex ratio		χ 2	$P$	Remark
		Number	%	Number	%	Male	Female
Dec.16	113	49	43.36	64	56.64	1	1.30	1.99	$P>0.05$	NS
Jan.17	80	26	32.50	54	67.50	1	2.08	9.8	$P<0.01$	${\mathrm{S}}^{\ast \ast }$
Feb.17	101	33	32.67	68	67.33	1	2.06	12.13	$P<0.01$	${\mathrm{S}}^{\ast \ast }$
Mar.17	80	30	37.50	50	62.50	1	1.67	5.0	$P<0.05$	${\mathrm{S}}^{\ast }$
Apr.17	89	29	32.58	60	67.42	1	2.07	10.8	$P<0.01$	${\mathrm{S}}^{\ast \ast }$
May.17	117	37	31.62	80	68.38	1	2.16	15.8	$P<0.01$	${\mathrm{S}}^{\ast \ast }$
Jun.17	121	29	23.97	92	76.03	1	3.17	32.8	$P<0.01$	${\mathrm{S}}^{\ast \ast }$
Jul.17	79	20	25.32	59	74.68	1	2.95	19.25	$P<0.01$	${\mathrm{S}}^{\ast \ast }$
Aug.17	93	25	26.88	68	73.12	1	2.72	19.88	$P<0.01$	${\mathrm{S}}^{\ast \ast }$
Sep.17	91	28	30.77	63	69.23	1	2.25	13.46	$P<0.01$	${\mathrm{S}}^{\ast \ast }$
Oct.17	105	38	36.19	67	63.81	1	1.76	8.01	$P<0.01$	${\mathrm{S}}^{\ast \ast }$
Nov.17	97	50	51.55	47	48.45	1	0.94	0.09	$P>0.05$	NS
Overall	1166	394	33.79	772	66.21	1	1.96	122.54	$P<0.01$	${\mathrm{S}}^{\ast \ast }$

P = probability, χ² = chi-square value, NS = nonsignificant, ${\mathrm{S}}^{\ast \ast }$ = significant at 0.01 level, ${\mathrm{S}}^{\ast }$ = significant at 0.05 level.

[figure(s) omitted; refer to PDF]

3.2. Maturity

The gonads of 1166 R. corsula (where 394 males and 772 females) were examined externally to determine the numbers of fishes in different stages of their reproductive life. Of the male fishes, three stages of gonads could be distinguished externally—immature, mature and spent—of which, 69% (272) were immature, 27% (107) were mature and 4% (15) were spent (Figure 3). Of the female fish, five distinct stages of gonads could be distinguished externally—immature, maturing, mature, ripe and spent—of which 72% (559) were immature, 9% (73) maturing, 11% (82) mature, 5% (39) ripe, and 3% (19) were spent (Figure 3).

[figure(s) omitted; refer to PDF]

According to length frequency distribution, all male fishes below 10 cm length possessed immature gonad. The smallest male fish with mature gonad was found in 10–12 cm length class and the biggest fish with immature gonad was found in 20–22 cm length class, whereas fishes bigger than those, 22 cm to bigger, all possessed mature gonads. Males with spent gonads were found between 14 and 24 cm length classes (Figure 4).

[figure(s) omitted; refer to PDF]

The smallest female fish with mature ovary was found in 12–14 cm length class. Smaller than this length, all fishes possessed either immature or maturing ovaries. The biggest fish with immature ovary occurred in 26–28 length class. Bigger than 24 cm almost all fishes possessed overies beyond maturing stages. From 12–14 cm length and above, females with spent ovaries occurred in all length classes to some extent but female above 22 cm length possessed maximum spent ovaries (Figure 5).

[figure(s) omitted; refer to PDF]

3.3. Maturity Stages of Male (Testes) and Female (Ovary) Gonads

3.3.1. Male

Immature: Testes very small, 1.2–3 cm in length and weight 0.01–0.9 g, and filiform and transparent. Mature: Testes large in size with good weight (3–9 cm and 0.9–1.2 g); yellowish in colour. Milt came out when pressure was applied on the abdomen. Spent: Testes reduced in size and weight, flaccid, and reddish in colour.

3.3.2. Female

Immature: Ovaries thin, thread-like, translucent, colourless; with weight between 0.01 and 0.9 g. The ova were microscopic in size, irregular in shape, transparent with central nucleus. No yolk was present. Range of ova-diameter was 0.001 mm to 0.199 mm. Maturing: Ovaries thick, still thread-like, rounded, transparent, colourless eggs were seen with naked eyes; extended up to the one-third of the abdominal cavity; weight 0.99 to 3.8 g. Ova nearly spherical in size, partly opaque due to deposition of yolk at the central portion. Range of ova-diameter was 0.2 to 0.499 mm. Mature: Ovaries large, rounded, little finger-like, opaque, and yellow in colour. Eggs were visible to naked eyes; filled most of the abdominal cavity; weight 4.2 to 15.6 g. Ova more or less spherical in shape and nucleus was invisible. Range of ova-diameter was 0.5 to 0.699 mm. Ripe: Ovaries very large, rounded, larger than thumb, yellow in colour. Eggs were clearly visible to naked eyes, filled the whole abdominal cavity; weight 5.2 to 20.99 g. Large eggs were extruded by gentle pressing on the abdomen. Nucleus of ova was not visible as the cytoplasm was filled with yolk. Range of ova-diameter was between 0.7 and 0.899 mm. Spent: Ovaries very much reduced and shrunken, flaccid, reddish in colour, had a large number of blood vessels on its wall; range of weight was 0.1 to 0.99 g.

3.4. Length at First Sexual Maturity

Length at which 50% of male and female R. corsula attained maturity was calculated by adjusted proportion method as suggested by King [15]. Male became sexually mature at 16 cm (Figure 6) whereas female became sexually mature at 21.5 cm length (Figure 7).

[figure(s) omitted; refer to PDF]

3.5. Gonado-Somatic Index (GSI) and Gonadal Length Index (GLI)

The mean GSI values of both male and female reached their peak (male: 0.541 ± 0.145, female: 8.034 ± 6.107), once in a year, in September 2017 and gradually decreased in the subsequent months, whereas, the lowest mean values (male: 0.194 ± 0.158, female: 0.420 ± 0.449) of GSI were found in December 2016, for both male and female fish, which gradually increased to reach their maximum values in September 2017 (Figure 8).

[figure(s) omitted; refer to PDF]

The mean GLI values (male: 25.75 ± 4.022, female: 27.28 ± 7.264) also reached their peak once in a year, for both male and female, and which was also found in September 2017; then gradually decreased towards their lower values in the subsequent months. But lowest values of this index were found in two different months, for male in November: 18.29 ± 2.756) and for female in January: 18.48 ± 2.9 (Figure 9).

[figure(s) omitted; refer to PDF]

3.6. Progression of Intraovarian Oocytes towards Maturity

The lowest (0.075 ± 0.034 mm) and the highest (0.651 ± 0.069 mm) mean values of ova-diameter were observed in December 2016 and September 2017, respectively (Figure 10). On average, there was a tendency of a gradual increase of the mean ova-diameter from December 2016 until June 2017, after that there was a sharp increase towards the peak in September 2017, and then a sharp decline up to November 2017 (Figure 10). The eggs were categorized into four groups based on their size, namely, immature (0.001–0.199 mm), maturing (0.2–0.499 mm), mature (0.5–0.699 mm), and ripe (0.7–0.899 mm) (Figure 11). The ova-diameter of R. corsula ranged from 0.01 mm to 0.81 mm (Figure 12). Large number of smallest eggs (0.01 mm) were found in January-February, 2017, whereas the largest eggs (0.599–0.81 mm) were found in June–November 2017 (Figure 12).

[figure(s) omitted; refer to PDF]

Immature eggs were found throughout the year except for being totally absent in July 2017, and a few in August 2017 and September 2017 (Figures 11 and 12). They appeared in a small proportion in October 2017 and gradually increased during November–March (Figures 11 and 12). Maturing eggs were first observed in February, 2017, their proportion were maximum during the months of February–May 2017, then gradually decreased and none were found in September 2017, as well as in December 2016 and January 2017, but were found in smaller proportion in October 2017 and November 2017 (Figures 11 and 12). Mature eggs were first appeared in April 2017 and increased gradually (April–August), and maximum quantity was found in September and October 2017 (Figures 11 and 12). Ripe eggs were found during June 2017 to November 2017. From June 2017, the proportion of this type of eggs increased gradually and found its maximum proportion in September 2017 then declined and found at least to some extent up to November 2017 (Figures 11 and 12).

In the first two months (December 2016 and January 2017) of the study year, only immature eggs were found. In the next two months (February 2017 and March 2017), only immature and maturing eggs were found (Figures 11 and 12). From July 2017 to September 2017, no immature eggs were found. Whereas, in September 2017, only mature and ripe eggs were found, i.e., no immature or maturing eggs were observed in this month (Figures 11 and 12). In October 2017 and November 2017, all the four types of eggs were observed but immature eggs increased in proportion gradually (Figures 11 and 12). The size frequency distribution of mean ova diameter during different months and progression of ova towards maturity indicated that yearly the fish was a single breeder with prolonged duration (Figure 12).

3.7. Fecundity

Fecundity varied from 31488 to 251880 eggs/fish with an average of 129622.2 ± 63351.54; fecundity factor varied between 1099.09 and 2808.23 eggs/g of body weight with an average of 1742.47 ± 365.69 (Table 2). No fecund fishes were found in December 2016, January 2017, and February 2017. Higher mean fecundity was observed in May 2017 and July 2017 during breeding season (Figure 13). Relationships between fecundity (F) and total body length (TL), total body weight (TW), ovary length (OL), and ovary weight (OW) are presented in Table 3, Supplementary Figures 1–4. Highly positive significant correlations ($P<0.01$) were found between Fecundity and all four body parameters (TL, TW, OL, and OW) (Table 3). Regression lines were also drawn for each relationship (Supplementary Figures 1–4).

Table 2

Total body length, total weight, gonad length, gonad weight, fecundity, and fecundity factor of 38 mature corsula.

	Month	TL	TW	GL (cm)	GW (g)	E/0.01 g	Fecundity	F factor
1	March	19.7	76.36	6	10.44	125	130500	1709.01
2	March	19.8	76.88	7	9.53	130	123890	1611.47
3	April	14.1	27.24	4.9	4.46	127	56642	2079.37
4	April	14.6	26.65	3.8	4.5	125	56250	2110.69
5	April	16.8	49.5	3.9	5.5	130	71500	1444.44
6	May	22	105.44	7	14.57	132	192324	1824.01
7	May	22.4	113.36	8	15.6	135	210600	1857.8
8	May	24	137.12	8.9	14.44	138	199272	1453.27
9	June	15.7	35.98	6.2	8.42	120	101040	2808.23
10	June	13.8	26.24	3.8	2.46	128	31488	1200
11	June	14.5	26.02	3.5	3.85	130	50050	1923.52
12	June	22.6	122.5	8.1	16.8	125	210000	1714.29
13	June	24.2	142.89	8.2	17.5	125	218750	1530.9
14	Juiy	15.7	36.27	3.4	4.98	120	59760	1647.64
15	July	19.3	77.61	6.9	16.66	120	199920	2575.96
16	July	23.1	125.2	8	17.9	127	227330	1815.73
17	July	25.1	154.32	8.4	19.92	120	239040	1548.99
18	July	25.8	168.1	8	15.1	130	196300	1167.76
19	July	26.3	177.47	8.4	20.99	120	251880	1419.28
20	August	13.2	21.86	4.2	3.2	130	41600	1903.02
21	August	19.7	71.15	6.5	12.4	125	155000	2178.5
22	August	18.8	69.96	5.5	7.88	127	100076	1430.47
23	September	18.1	62.98	5.3	7.5	125	93750	1488.57
24	September	17.8	62.41	5.8	7.98	123	98154	1572.73
25	September	17.5	58.76	5.5	6.57	125	82125	1397.63
26	September	17.1	51.25	5.6	8.43	120	101160	1973.85
27	September	17.2	54.5	5.4	6.4	127	81280	1491.38
28	September	17.7	61.82	5.9	9.85	122	120170	1943.87
29	September	17.7	61.81	6	8.95	125	111875	1809.98
30	September	19.5	78	7	11.55	128	147840	1895.38
31	September	16.9	53.43	5.5	8.26	123	101598	1901.52
32	October	17.6	50.13	5	7.63	128	97664	1948.21
33	October	17.5	57.87	5.5	10.84	120	130080	2247.8
34	October	20	80.21	6	11.59	125	144875	1806.2
35	November	16.3	48.3	3.7	4.18	127	53086	1099.09
36	November	19.2	71.52	5.7	10.5	122	128100	1791.11
37	November	22	130.59	7	18.9	120	226800	1736.73
38	November	18.8	72.59	4.5	6.71	125	83875	1155.46
39	December	0	0	0	0	0	0	0
40	January	0	0	0	0	0	0	0
41	February	0	0	0	0	0	0	0
						AV ± SD	129622.263351.5	1742.47365.69

[figure(s) omitted; refer to PDF]

Table 3

Relationships between fecundity (F) and four body parameters (TBL, TBW, OL, and OW).

Parameters	Relationship	r value	t value	df	Significance	Figures
F vs TL	F = 29.85TL2.815	0.921	15.10	37	$P<0.01$	Supplementary Figure 1
F vs TW	F = 2546.83TW0.905	0.928	14.91	37	$P<0.01$	Supplementary Figure 2
F vs OL	F = 4355.12OL1.857	0.928	14.94	37	$P<0.01$	Supplementary Figure 3
F vs OW	F = 12823.31OW0.99	0.998	95.39	37	$P<0.01$	Supplementary Figure 4

4. Discussion

Sex ratio study is an important aspect of Fish biology as it constitutes fundamental information required for the estimation of the potential for successful reproduction and stock assessment in fish populations both in natural and in artificial breeding ground [1]. In this study, a disproportionate occurrence of two sexes, female dominance over male, was observed throughout the year (significant difference) except in November 2017 when male numbers were more than females—which was early prespawning and late (or post) spawning season of the fish—but the difference was insignificant, however, in December 2016 although female numbers were more over male but the difference was also insignificant (Table 1). Almost throughout the year dominance of female over male has been a deviation of sex ratio from the expected ratio of 1 : 1. Koutrakis, Amin et al., and Hasanen et al. [6, 8, 18] reported the same phenomenon for other species of Mugil from different parts of the world. Deviation of the hypothetical expected ratio of 1 : 1 of male and female, in favour of female dominance, could be governed by various factors such as growth rate and food availability. Nikolsky [14] reported that when food is abundant, female fishes predominate, with the situation inverting when food is limited. Females require better condition than males, suffering in their development when environmental condition deteriorates [14]. The present Mullet samples were collected from the mixed shoal of mullet from the coast of the Bay of Bengal which is a mangrove habitat—naturally very rich in food resources for aquatic life—might have supported the females for better development.

Length at first sexual maturity is essential for fisheries management as it can help the control of over-exploitation and conservation of a fish species by ensuring minimum mesh size of fishing nets and minimum legal size of catchable fish [15]. The chance of mature individual fish to spawn at least for once can be ensured if we know the length at first sexual maturity of a fish and hence can maintain a suitable spawning stock [8].

No previous report was found on the length at first sexual maturity of R. corsula from the coastal water of Bangladesh. Length at sexual maturity of other species of mullets (Mugils) showed a considerable variation in different parts of the world. Gannam et al. [9] reported it to be 13.2 cm for male and 13.5 cm for female of Liza carinata (Valenciennes, 1836). De Oliviera et al. [19] reported 264 mm and 240 mm for male and female of Mugil curema (Valenciennes, 1836). Male and female Liza ramada (Risso, 1827) got matured at 14.0 cm and 16.0 cm in length, respectively, as reported by El-Halfawy et al. [20]. Amin et al. [8] reported smaller size—11.9 cm for both male and female—for Mullus surmuletus (Linnaeus, 1758). Hasanen et al. [18] reported 20 cm, for both male and female Liza aurata (Risso, 1810). However, the present observations, to some extent, agreed with Hasanen et al. [18] and El-Halfawy et al. [20].

In the present study, male attained sexual maturity in smaller lengths than of female. Similar observations were also made by Koutrakis [6], De Oliviera et al. [19], and El-Halfawy [20] for other mugil species.

Both male and female individuals showed weight changes in the gonad during the two gameto-genetic stages, prespawning and spawning. During the prespawning period, December to May, a gradual increase in the GSI and GLI was observed which was followed by a sharp increase during the spawning reaching its peak in September then a gradual decline in the later part of the spawning season. These are all indications of well-marked seasonal changes in gonad condition. Higher values of GSI and GLI, and a high percentage of mature specimens from June to October coincide with higher light, temperature, and rainfall during this period. Similarly, the recovering gonads remained quiescent from November to January this might probably be due to lower values of light and temperature as well as rainfall. More or less similar observations were recorded by Fatima and Khan [12] for R. corsula during winter from Yamuna River in India, and also by Anderson [21], Avanesov [22], and Wijeyaratne and Costa [23] for different fishes including mullets.

Fatima [24] deduced that all ovarian eggs of similar size probably spawn together by some fish species, whereas a continuous gradient in oocyte size may have several groups of eggs mature and shed periodically during the breeding season. In the present study, during the spawning season (June to November, more precisely the first quarter of November) distinct batches of mature ova was observed in higher frequency than those of immature eggs; an observation which was coupled with the unimodal frequency distribution of eggs during the spawning season, where the modal class always belonged to mature eggs class. These observations confirmed that R. corsula spawns only once a year covering a longer duration (June to the first quarter of November). Different authors [6, 9] also investigated on the reproductive biology of mullets and found single spawning with longer duration which agrees with the present observation. Koutrakis [6] reported the spawning season of a mullet, Liza saliens (Risso, 1810), to be from June to October which also agrees with the present observations.

The spawning season of R. corsula, in the present study, was found to be from June to November. Of these six months, in November, mature or ripe eggs were found only in the fish that were collected only in the first quarter of the month; hence, mature eggs did not form the modal class in this month (Figure 12). However, from June to October mature eggs form the modal class (Figure 12) but during August and September the modal class of the mature eggs was the biggest, hence marked as peak spawning period (Figure 12). The total duration of spawning season (June to November) slightly differs from that observed by Fatima and Khan [12] (July to October), but peak spawning period, in both studies, was same–August and September. This might be due to different geographical locations and the onset of the breeding season in different months.

The findings of the breeding season in the present study closely resembles the findings of Qasim and Qayyum [25] (July to September) and Fatima and Khan [12] (July to October), but earlier studies reported the breeding season in June to July [10] and March to April [26]. It might be due to different geographical locations and the onset of monsoon.

The diameter of ripe ova ranged from 0.7 to 0.81 mm in the present study (Figure 12). Fatima and Khan [12] recorded the diameter of the ripe eggs of R. corsula ranging between 0.61 and 0.773 mm (620 to 773 µ), whereas Pakrasi and Alikunhi [27] and Qasim and Qayyum [25] reported the diameter of ripe eggs ranging between 0.75–0.86 mm and 0.6–0.75 mm, respectively. In all these studies the spawning of R. corsula was recorded to be restricted to a definite but longer time and the individuals spawn only once during the breeding season. Although mature eggs were observed from April, the majority of the mature and ripe eggs were found between June and November. So, it is reasonable to infer that R. corsula spawns when a huge amount of flood water feeds into the coast of the Bay of Bengal through all the rivers of Bangladesh.

The success and duration of the breeding season, in many freshwater fishes as well as in R. corsula, appears to be directly dependent on the monsoon, as evident in the above-mentioned findings. In most of the upstream rivers flood water recedes during October and November but is still very much available in the downstream rivers as well as in the coast of the Bay of Bengal. As flood water seems to be directly related to the spawning activity, specimens collected from the coast of the Bay of Bengal were found to be spawning till the first quarter of November. This, availability of flood water, might be the reason of the variation observed by different authors of the breeding season as mentioned earlier.

Fecundity is the number of eggs released by a female individual fish during a single spawning season, which measures the reproductive capacity of a species that predicts the reproductive potential of population of fish. Fecundity calculation of fish assumes that all the ovarian eggs of similar size of a species, within an ovary, spawn together [28, 29]. As individual fish may spawn more than once during a single breeding season [25], subsequent spawning would possibly contain fewer ova. Therefore, to make the estimate more reliable, only mature and ripe ovaries that showed no evidence of previous spawning were taken for fecundity estimation. During the study period, it was observed that ovaries of the same size of fishes contained different numbers of eggs. This may be due to the variations in environmental conditions and food intake by the individual. Oren [30] reported that the fecundity of females varies greatly according to species, size, age, region, and period.

The mean fecundity range of R. corsula was 31488–251880. The findings of the present study agree with Suganan and Vinci [11] on R. corsula (63000–260500), but disagree with those of Akter et al. [31] (9506–16113) and Sultana et al. [4] (8924–82642). The present investigation also agrees with the findings of Amin et al. [8] (19423–83097) for another mugil species. The fecundity of mullet increases with the increase in total length [30]. The relationships between fecundity, fish weight [30], and gonad weight were linear [31].

For understanding the mean fecundity of the size composition of spawning stock, knowledge of the relationship between the size of a fish and fecundity is essential. The relationship of different body parameters with fecundity of R. corsula was calculated due to this practical utility. Kesteven [32] stated that the gonad of an organism maintains size relationships with various body parameters, whereas Clark [33] suggested that the fecundity of a fish increases in proportion to the square of its length. However, Simpson [34] argued with such a conception because egg production in any ovary is not a surface phenomenon and that the germinal epithelium is highly folded to fill the volume of the ovary. Simpson [34] concluded that the number of eggs is related to the volume and, consequently, to the cube of the length. Many researchers (Pillay [35] and Sarojini [36]) also suggested that fecundity of fish is related to the length of fish by a factor closer to the cube of length and is directly proportional to the weight of fish.

The exponential value of the relationship between the fecundity and the total body length of fish in the present study is close to three (2.815) (Supplementary Figure 1), which indicates that fecundity increases at a rate to the cube of the length which agrees with the suggestion made by Simpson [34], Pillay [35], and Sarojini [36] that is why a curvilinear regression line is produced (Supplementary Figure 1). The relationship between fecundity and total body weight forms a straight line as the exponential is close to one (0.905) (Supplementary Figure 2), which totally follows the theoretical assumption and agrees with the findings of several workers [31, 37, 38]. The relationship between fecundity and ovary length forms a curvilinear line as the exponential is close to 2 (1.857) (Supplementary Figure 3). More or similar observation made by Bagenal [37].

Hickling [39] stated that the production of eggs is the dominant function of the ovary; a close correlation expected between the weight of the ovary and the number of eggs produced. In the present investigation, the relationship between fecundity is closely correlated to ovary weight (r = 0.998) and form a linear regression line as the exponential value of the regression equation is very close to one (0.99) (Supplementary Figure 4). This observation agrees with those of Bagenal [37] and Varghese [38].

5. Conclusion

Corsula Mullet should be conserved by regulating fishing close season during June to September, the peak spawning time of this species in the Sitakunda coast of Bangladesh. To ensure first recruitment of this valuable fish resource, R. corsula, the length at first sexual maturity of male (16 cm) and female (21.5 cm) corsula should be given opportunity for breeding.

Acknowledgments

We would like to show our great attitude to Syeda Khirun Nesa for helping during the data collection. We would also like to express our great acknowledgement to late Dr. Aftab Hossian, who helped us during data collection and analysis, and for his forever support.

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