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Reproductive studies of striped mullet (Mugil cephalus) from high brackish tropical lagoon

Soyinka Olufemi Olukolajo

Department of Marine Sciences, University of Lagos, Nigeria.
osoyinka@unilag.edu.ng

Abstract

The present investigation was to examine the sex ratio, fecundity, egg diameter and gonadosomatic index of Mugil cephalus from a high brackish tropical lagoon. The two thousand, six hundred and sixty-one (2661) specimens of the striped grey mullet, M. cephalus. used for this study, were collected from Lagos Lagoon, a high brackish lagoon in south-west Nigeria for a period of 24 months. The size of the specimens used for this study ranged from 1.9 to 39.0 cm total length (standard length: 1.7 – 29.5 cm).

A ratio of 1:0.53 was obtained for the sex ratio and it was statistically significant (p<0.05), showing the dominance of males over females. Fecundity estimates varied from 636,000 – 1,520,000, which was positively correlated to the fish weight and length. Egg diameter averaged 410.00 40.70 μm, an indication of an early stage of gonad maturation of M. cephalus in the lagoon before final spawning occurred in open sea. The gonadosomatic index indicated that spawning activity occurred from December to May in the open waters. M. cephalus would be a good candidate for mariculture and needed to be exploited adequately in coastal communities.

Keywords: egg diameter, fecundity, gonadosomatic index, sex ratio


Introduction

Mullets (Mugilidae) are one of the commonest fishes in the coastal waters of tropical and subtropical countries of the world and about sixteen species have so far been identified in West Africa according to Fowler (1936), Cadenat (1954) and Blay (1995). These constitute an important proportion of the catches of commercial and subsistence fishermen in some countries in this area (Brulhet 1975; Payne 1976). General information on the biology of striped grey mullet has been well documented by Anderson (1958), Thomson (1963, 1966) and Chubb et al (1981) but limited information is available on the reproductive biology of wild populations (Anderson 1958; Greeley et al 1987 and Render et al 1995). A few reports are available on the biology of mullet species in the coastal waters of West Africa and were principally on the food and feeding habits (Fagade and Olaniyan 1973; Brulhet 1975; Blay 1995; Soyinka 2008). Ditty and Shaw (1996) reported that Mugil cephalus are isochronal spawners, with all oocytes reaching maturity at the same time. It sheds its eggs in batches (Silva and Silva 1981; Greeley et al 1987; Render et al 1995). A thorough knowledge of the fecundity of fish is essential for evaluating the commercial potentialities, stock study, life history study, practical culture and actual management of the fishery (Das 1977; Rhema et al (2002).Information on the reproductive biology of mullet species is essential to understanding the population dynamics and the recruitment pattern of the fish, to harness it for conservation and management. The present investigation examined the sex ratio, fecundity, egg diameter and the gonadosomatic index of M. cephalus from a high brackish lagoon in Nigeria, a tropical climate, West Africa.


Materials and methods

Description of study area

The Lagos Lagoon (Figure 1) which lies between longitudes 30 20' and 30 40' E and latitudes 6015' and 60 40' has an area of 208km2 and is the largest of the lagoon systems of the West African sub-region. It is the largest of the nine coastal lagoons of south-western Nigeria (others are Yewa, Badagry, Iyagbe, Ologe, Kuramo, Epe, Lekki and Mahin Lagoons; FAO 1969). It has supported decades of small scale fisheries which have shown sign of continuous decline (Solarin 1998). The lagoon characterized with seasonal fluctuation in salinity – high brackish water during the dry season (December – May), while freshwater condition exists in the rainy season (June – November) (Fagade and Olaniyan 1974; Kusemiju 1975 and Solarin 1998). The lagoon is surrounded by swamp forest and riparian forest consisting of mangrove vegetation: Rhizophora racemosa, R. harrisonii, Aerosticum aureum, Paspalum orbiculare, Langucularia sp and Avicennia germinans. The lagoon empties into the Atlantic Ocean via the Lagos Harbour (Ogunwenmo and Kusemiju 2004). The lagoon is shallow in depth and in most places a little more than 1.5 metres depth (Solarin and Kusemiju 2003).

Figure 1. Map of Lagos State showing the Lagos Lagoon, Nigeria
Field studies

Specimens of M. cephalus were collected from the Lagos Lagoon by fisher-folks who use cast nets as gears for fishing. The collections were made fortnightly between February 2004 and January 2006. Fish were preserved in ice-chest on the field, but later transferred into a deep freezer at temperature of -200C in the laboratory of Marine Research Laboratory, University of Lagos, for further analysis. The salinity of the lagoon was determined in-situ twice a month between 8.00 – 11.00 a.m for 24 months using a Refractometer (BIOMARINE, Aqua Fauna Model).

Laboratory procedures

Thawing of the specimens was done upon removal from the freezer before being examined. Total length (TL) and standard length (SL) of the specimens were measured to the nearest 0.1 centimeter using measuring board. The total weight of the fish was determined to the nearest tenth of a gram on a ‘Sartorious’ top loading balance (Model 1106) and triple beam balance for larger specimens.

Sex determination

The fish were grouped into females, males and juvenile or immature fish after dissecting the fish and observing the genital papillae visually. The female has a pair of bulging or sac-like ovaries; male had a pair of thread-like tubes with whitish milt, while the gonad of immature fish could not be differentiated with the naked eye.

Fecundity

Ripe ovaries of matured females were removed carefully from the abdominal region of the fish, the weight determined on the balance, before preserving in 10% formalin or 4% Gilson’s fluid. The eggs were separated from the ovarian tissue into petri dish. Two methods were used for the enumeration. By gravimetric method, the number of eggs in a known weight was used to estimate the total number in the dry sample. By volumetric method, the number of eggs in 1ml using ‘Gilson Pipetman’ egg counting device (model: D-82-13103) were counted under a microscope and used to estimate the total number in the remaining volume. The relationship between the fecundity, length and weight was expressed as:

Y = a + bX

Where: Y = fecundity estimate; X = standard length (cm)/total weight (g); a = regression constant; b = regression coefficient

Egg diameter

The eggs were placed on a slide, observed under a graduated eye-piece of a light microscope and the average diameter recorded in micrometers (μm).

Gonadosomatic index (GSI)

This indicates gonadal development and maturity of fish. This was estimated thus:

G.S.I = Weight of the gonad x 100

Weight of the fish (Parameswarn et al 1974; McDonough et al 2003)


Results

The results of the sex ratio, fecundity, egg diameter and gonadosomatic index of M. cephalus are presented below:

Sex ratio

From the specimens of M. cephalus collected from the Lagos Lagoon, 426 were males while 224 females, giving a sex ratio of 1: 0.53 (male/female). The calculated Chi-square (χ2) test showed that the males were significantly (p < 0.05) more than the females in the Lagoon.

The monthly sex ratio values (Figure 2.) showed that the males were more than the females throughout the period of study except in the dry season of February 2004 and December 2004. The occurrences were nearly the same in the dry season of April 2004. Statistically, Chi-square (χ2 ) values of sex ratio were significantly (p < 0.05) different in the months of February and July 2004, April - October 2005.

The relationship between sex ratio and salinity fluctuations (which is the critical physico-chemical parameter affecting distribution of species in the lagoon) is shown in Figure 3. Salinity values ranged from 0.0 to 23.5‰ (mean: 9.8 9.13‰). From the result, immature specimens were predominant in the low brackish to fresh water condition of the lagoon; males were significantly more than females in most months, while the population of females was more abundant during dry season than the wet season.

 
Figure 2. Monthly variation in sex ratio in M Cephalus from Lagos Lagoon (Feruary 2004- January 2006


 
Figure 3. Monthly salinity / sex relationship of M. cephalus in Lagos Lagoon (February 2004 – January 2006)
Fecundity of M. cephalus

Only (28) fecund specimens from the lagoon were examined for fecundity. The size ranged from 16.5 – 19.2cm standard length (total length, 22.0 – 24.9cm) and weighed between 90.0 – 146 g. Estimated fecundity varied from 636,000 – 1,520,000 eggs (average fecundity estimate: 832,000 156,000 eggs). The Log fecundity – Log standard length and Log fecundity – Log weight relationships are shown in Figs 4 and 5 respectively.

A slightly high correlation existed between fecundity and the standard length (r = 0.54) and the weight (r = 0.51) of the fish.

The linear regression equations were:

a) Log Fecundity = 3.78 + 1.70 Log Standard length

(n = 28, r = 0.54).

b) Log Fecundity = 4.85 + 0.52 Log Total weight

(n = 28, r = 0.51)

Egg diameter of M. cephalus

The diameter of eggs of mature females in the Lagos Lagoon ranged from 36 0– 460 μm (0.36 – 0.46 mm) as shown in Table 1. The least total length of female fish with ripe gonad was 22.0 cm (standard length: 16.5cm).

Table 1. Diameter of eggs in mature females of M. cephalus from Lagos Lagoon.

Size range/total length (cm)

Number of eggs

Egg diameter

22.0 – 24.9 (SL: 16.5 – 19.2cm)

636,000 – 1,520,000

360 – 460 μm

(mean = 17.8 0.66)

(mean = 832,000 156,000)

(mean = 410 40.7 μm)

Gonadosomatic index

The Gonadosomatic Index (GSI) of the 28 specimens ranged from 6.45 – 10.0% (mean value: 8.16 0.97. This was an indication that M. cephalus on the average used 8.16% of its body weight for egg production.


Discussion

From the sex ratio of M. cephalus collected from the Lagos Lagoon, the males were significantly (p < 0.05) more than the females. This indicated that probably, more male M. cephalus occur in the Lagos Lagoon than the females. McDonough et al (2005) recorded a male to female ratio of 2:1 until the fish were larger than 32.5cm TL. The ratio was 1:3.8 for fish greater than 32.5cm TL. The ratio of 2:1 in South Carolina estuary almost agreed with the present study from the Lagos Lagoon. Mature females with ripening and ripe gonads were collected often in the dry season (December – May) with peak between late January and March, when salinity is high in the Lagos Lagoon. Fagade and Olaniyan (1974) reported the availability of the fish during brackish (0.5 to 28.0‰) condition of Lagos Lagoon.

 
Figure 4. Log standard length / Log fecundity relationship of M. cephalus from Lagos Lagoon.


 
Figure 5. Log weight / Log fecundity relationship of M. cephalus from Lagos Lagoon.

Variations in the monthly sex ratio of M. cephalus from the lagoon indicated that the males were more than the females except in February 2004 and December 2004 which fall within the dry season. In Figure 3, an increase in number of female fish caught in the dry season (high salinity) than the wet season (low salinity) was reflected. This coincided with the period females with ripe gonads were found in the collection. Male specimens, with sexually differentiated gonads also had a slightly similar seasonal distribution pattern, but the immature ones were more available at both seasons. This indicated that salinity does influence gonad maturation in the striped mullet. McDonough et al (2005) reported that striped mullet are sexually undifferentiated for the first 12 months, began differentiation at 13 months, and are 90% fully differentiated by 15 – 19 months of age and 22.5cm TL. The abundance of 1 and 2 years old striped mullet in South Carolina indicated that immature fish dominated the estuarine population. This agreed with the present study.

From the present study, an average fecundity estimate of 832,000 eggs (~ 0.83 million eggs) was recorded for fish ranging in standard length from 22.0 – 24.9cm and weight 90.0 – 146.0g. Fecundity was averagely correlated with standard length and body weight. Silva and Silva (1981) recorded a fecundity of 0.45 to 4.2 million in M. cephalus ranging in length from 32 – 56 cm and weight from 0.7 – 2.2 kg; fecundity was significantly correlated to body length, body weight and gonad weight. Collins (1985) estimated fecundity of striped mullet at 0.5 – 2.0 million eggs per female, depending on the size of the female. Ditty and Shaw (1996) reported that M. cephalus are isochronal spawners, with all oocytes reaching maturity at the same time. It sheds its eggs in batches (Silva and Silva 1981; Greeley et al 1987; Render et al 1995). The minimum spawning size of females is between 31 and 34 cm (Ditty and Shaw 1996), while Silva and Silva (1981) recorded female maturity at 31.5cm and males at 34.0 cm. McDonough et al (2005) recorded male spawning size at 24.8 – 30.0 cm, while the spawning size for the female was 29.1 – 40.0 cm. This implied that the specimens in the present investigation were still at the early stage of gonad maturation and would not spawn possibly until they embark on the open ocean spawning migration where the gonads reach final maturation. Size at maturity has been found to range widely from 23.0 cm standard length (Thomson 1963; Greeley et al 1987) to 41.0 cm SL (Thomson 1963; Chubb et al 1981) for two- and three-year-old fish. The slightly earlier lower end of the maturation size of fish recorded in the present study could probably be as a result of the tropical climate of the Lagos Lagoon as compared with the waters where those earlier findings were reported.

Slight variation in egg sizes was reflected in the results of the measurement of the diameter of the ripe eggs (360 – 460 μm) of mature females of M. cephalus from the lagoon. McDonough et al (2003) recorded oocyte diameter of 463 to 682 μm (mean size, 596 μm). Rottmann et al (1991) reported an approximate mature egg diameter of 0.60 – 0.80 mm for Mugil cephalus. The slight variation of the lower end of the egg diameter in the present study is probably because of the size of fish used and the possibility of tropical or warm water fishes maturing earlier than temperate or cold water fishes of same species.

The GSI value of 8.16 recorded in this study fell within the range of 7.70 – 27.7 reported by McDonough et al (2003) for fecund specimens in South Carolina estuary. It was clear that the gonad weight had a considerable influence on total body weight in fecund specimens (McDonough et al 2003). The value of the GSI levels calculated in fish during the dry season (December – May) with peak between late January and March, showed that reproductive activity was probably within that period. GSI increases with the maturation of fish but decline abruptly thereafter (Rhema et al 2002). McDonough et al (2003) reported that striped mullet that recruit into South Carolina estuaries spawn from October through April. The information provided in the present study would be helpful in the adequate management of the M. cephalus stock in the Lagos Lagoon.


Conclusion


Acknowledgements

The author immensely appreciates the conscious efforts of Prof. K. Kusemiju in supervising the study and the constructive criticisms; the Department of Marine Sciences, Department of Cell Biology and Genetics and Department of Biochemistry of the University of Lagos for equipment used for this study.


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Received 9 December 2016; Accepted 11 July 2017; Published 1 August 2017

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