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ISSN 2006-9839 ©2011 Academic Journals
Full Length Research Paper
Reproductive biology and some observation on the
age, growth, and management of Tilapia zilli (Gerv,
1848) from Lake Timsah, Egypt
Waheed Farg-Alla Mahomoud1, Amal Mohamed Mahmoud Amin2, Kamal Fathey Elboray1
Amal Mohamed Ramadan and Magdy Mostafa Kamal Osman EL-Halfawy1*
1Fish Reproduction and Induced spawning Laboratory, Aquaculture Division, National Institute of Oceanography and
Fisheries, Suez, Egypt.
2Stock Assessment Laboratory, Fisheries Division, National Institute of Oceanography and Fisheries, Suez, Egypt.
Accepted 2 December, 2010
In the present investigation, some biological aspects of Tilapia zillii from Lake Timsah were studied. This was
carried out on 700 fish samples collected during the period from October, 2002 to September, 2003. Total
length of females ranged from 7 to 16 cm and males from 8 to 21 cm. Age determination based on length
frequency distributions showed that the life span are two and four years for females and males, respectively.
Estimates of growth parameters are widely used for detailed assessment and modeling of fish species and
fisheries. The length-weight relationship, age at length zero (to), growth coefficient (K), asymptotic length (L¥)
and rates of natural (M) and total mortality (Z) were estimated for T. zillii from Lake Timsah. Length-weight
relationships were calculated from individual fish length and weight records collected from sampling using gill
nets. Analyses were obtained by sampling direct from the fisher’s catches. Data were analyzed using the FISAT
software. T. zillii, the estimate of K was 0.320 and 0.680 year-1 for males and females, respectively, whilst L¥
_was approximately 22.05 and 17.85 cm, for males and females, respectively. T. zillii (males) had the lowest
value of K and the largest L¥ _than females. Natural mortality was the lowest for T. zillii (females) (M=0.464 and
0.38 year-1) and fishing mortality was highest for T. zillii (males) (F=2.656, 1.88 year-1). In addition, exploitation
rate was estimated at E=0.83 and length at first capture (Lc) was calculated at 11.30 cm for males while E=0.78
and (Lc) was estimated as 6.68 cm for females. The majority of fish caught of the main commercial species in
the Lake Timsah fishery were from the first age group, at a size far below the optimal length at capture. This
suggests growth overfishing for T. zillii in Lake Timsah.
Key words: Lake Timsah, Tilapia zillii, age and growth, mortality, gonadosomatic index, length at first maturity,
population structure.
INTRODUCTION
Though fishes are the most divers among the major vertebrate groups (Moyle and Cech, 1996), they are
faced with considerable threads which are generally associated with a combination factors summarized in
acronym HIPO (habitat destruction, introduced species, pollution and over exploitation) (Getahun and Stiassny, 1998). Tilapia is the common name applied to three genera of family Cichlidae (Sarotherodon; Oreochromis
and Tilapia) including about 70 species (Meyer, 2002). Tilapia zillii is widely distributed and has a very wide
common range. However, in Egypt, T. zillii is widely distributed, supports important commercial fisheries and
it is indigenous to Lake Timsah. Many authors studied the reproduction of T. zillii (Dudzie and Wangila, 1980;
Maclaren, 1981; Latif et al., 1986; Khallaf et al., 1986; El-Haweet, 1991; El-Shazly, 1993; Phillips, 1994; Coward and romage, 1999; Negassa and Getahun, 2003; El-Sawy, 2006; Akel and Moharram, 2007; Negassa and
Padanillay, 2008). T zillii is highly euryhaline that is, can tolerate a wide range of salinity (Bayoumi, 1969; El-Zarka
et al., 1970; Fryer and Iles, 1972; Chervenski and Horing,1973; Meyer, 2002). El-Sayed (2006) mentioned that
T.zillii; Oreochromis mosambicus and Oreochromisaureus are the most salinity-tolerant tilapia species, while
other tilapias are generally less euryhaline, and can grow,survive, and reproduce at 10 to 30‰, depending on the
species, size and sex. Optimum temperature tolerance of T. zillii was found to be between 28.8 and 31.4°C (Phillip
and Ruwet, 1982). Fishes belonging to family Cichlidae are moderate in size, living in shallow and vegetative areas (Eccles,
1992). T. zilli is one of the most valued fish in North Africa. It constitutes an important part of inland fish
production especially in the brackish lagoons of Morocco, Senegal River, Egypt and Libya (Teugles and Thys van
den Audenaerde, 1991). Age and growth determinations are important in studying longevity, age at first maturity,
catchable size and other life history problems in fishes (Lagler, 1966; Ricker, 1971; Lagler et al., 1977). Hadi
(2008), studied age and growth of T. zillii in Umhfein Lake (Libya). Age and growth determination of T. zilli from the
Egyptian waters were studied by Botros (1968), El-Zarka et al. (1970), Khallaf and El-Nenaei (1987), Latif et al.
(1989), Faltas (1995) and El-Kashef (2002), but in Nigerian waters which were studied by Basu and Kalu
(1999), and from the French waters which were studied by Panfili and Tomas (2001). Due to its economic
importance and its common presence at the sample site, T, zilli was selected for investigation. The purpose of this
study is thus twofold: To assess the impact of the marine habitat in Lake Timsah on the reproductive biology of T.
zillii. This study included the reproductive aspects (sex ratio, gonadosomatic index and gonad maturation) in
addition to condition factor and it was carried out to determine the fundamental growth parameters required
for assessing and managing T. zillii stock at Lake Timsah.
MATERIALS AND METHODS
Materials for this study were obtained from 700 individuals ranging in size from 7 to 21 cm. Monthly samples of T. zillii were collected
during the period from October 2002 to September 2003 by the gill net from Lake Timsah which lies on Suez Canal (Figure 1).
Fish total lengths were measured to the nearest centimeter and weighed to the nearest gram. The sexes and maturity stage were
determined on sub-samples by dissection (Legendre and Ecoutin,1989). Gonads were weighed to the nearest milligram and the
gonado-somatic indexes of both sexes separately were determined as: GSI=GW·100/BW
where GW=gonad weight and BW=body weight (de VIaming et al.,1982).
Methods to estimate the length-weight relationship of fishes are described by Pauly (1983). The length-weight relationship (LWR)
was estimated by using the equation: W = aL b where W = weight in grams, L = total length in centimeters, a is a scaling constant and b the allometric growth parameter.Condition factor was estimated according to the formula: K=W * 100/ L^3 where W=gutted body weight (gm) and L=total length in (cm). The FISAT software (Gayalino et al., 1997) was used with the length-frequency distributions (to) estimate von Bertalanffy growth
parameters, (K) (growth coefficient) and (L¥__ (asymptotic length). The growth performance index _ described in Pauly and Munro
(1984) was calculated using the relationship: _’=logK+2·log10L¥. Total mortality (Z) was estimated from length converted catch
curves and natural mortality (M) which caused by all other factors except fishing, was estimated from the empirical relationship of
Pauly (1980), as following: log10M=-0.065-0.287·log10L¥+0.604·log10K+0.513·logT using a mean annual environmental temperature of 25°C. The
estimate of F was obtained by subtracting M from Z. then exploitation ratio was obtained as: E=F/Z. The length at first capture Lc was estimated by Pauly (1984a, b). Relative yield per recruit (Y/R)_ and relative biomass per recruit (B/R)_ were estimated using Beverton and Holt’s (1966) model as
follows: (Y/R)_ =E U M/K [1 –(3U/1 + m) +(3U2/1 +2M) -(U3/1 +3m).(B/R)_ = (Y/R)__ /F
RESULTS AND DISCUSSION
Length- weight relationship
The length-weight relationship is helpful for estimating the weight of a fish of a given length and can be used in studies of gonad development, rate of feeding, metamorphosis,maturity and condition (Le Cren, 1951). The constants of the length weight relationship were determined separately for each sex and for combined sexes (Figure 2) the results were given by the following equation: W=0.0185* L3.0055 (r2 = 0.973) (for males).W=0.0207* L2..975
(r2 = 0.921) (for females). W=0.0207* L2.9667 (r2= 0.956) (for combined sexes). The previous data indicated high degree of correlation among both measures. A perusal of length-weight relationship, worked for T. zillii from, Lake Timsh showed
the value of "b" for male and female and unsexed fish are very close to 3, that is, reflecting isometry, with highly
significant values of the coefficient of determination (r2). Our results are in agreement with Mehanna (2004) who
recorded exponent (b) 3.088 of T. zillii in Wadi EL-Raiyan Lakes. The b value was 2.9 in Lake Qaurun recorded by
Mosaad (1990). Abd-Allah et al. (2000) and Sholloof (2009) recorded the same value of (b) =2.69 in Lake
Qaurun and in Edko Lake. The differences in these values may be due to the difference in ecological condition.
The condition factor (k)
The value of the condition factor have been used to measure various ecological and biological factor such as
degree of fatness, gonad development and the suitability of the environment with regard to the feeding condition
(Mac Gregoer, 1959). The mean values of condition factor for the total length between 8 and 21 for male and 7 to 16 for female of T. zillii ranged between 1.6603 to 2.0190 and 1.6354 to 2.1340 for males and females, respectively (Figure 3). It
can be seen that the K values for males data increased at 16 cm (2.2019) then decreased gradually, followed by an
increase up to 19 cm (2.0930). The values again declined at 21 cm, (1.894) while females recorded a higher value
of (K) at 10 cm. (2.134) followed by decreased then increased at 16 cm (2.005). This may be attributed to
sexual maturation and active spawning of the larger fish (El-Agami, 1988; Hadi, 2008; Shalloof 2009). Hadi (2008)
estimated that (K) value of T. zillii in Umhfein Lake about (2.195), Anene (2005) found that K value was 4.3 in Imo
State, while Shallof (2009) recorded K value 2.01 in Lake Qarun; this difference is due to the different ecological
conditions.
Sex ratio
From Table 1, it appears that the over all sex- ratio (M / F) was 1:0.9 which in agreement with other localities: in Lake Mariut 1:0.91 (El- Shazly, 1993); in Lake Edku 1:0.91 (Phillips, 1994) and 1:0.97 (El-Sawy, 2006). Also, it was more or less in agreement with Abu Qir Bay, 1:1.05 (Akel and Moharram, 2007). On the other hand, Fryer and Iles (1972) pointed out that in the African lakes, it is common in the cichlid populations that males dominate because they generally exhibited more growth than females.
Morphology of the gonad
Macroscopic structure of female and male gonads of T. zillii collected from Lake Timsah during the period (October, 2002 – December, 2003) is summarized as follow:
Stage I (Immature or virgin and resting adult)
Ovaries very small, thin, thread like pale in colour, occupying a small part of the body cavity. Testes is thin, slender ranslucent and pale in colour. Both gonads are invisible to the naked eye
Stage II (Early maturing)
Ovaries become slightly larger and increase in weight and volume with minute opaque whitish eggs occupied about half of the body cavity. Testes become enlarge, flat, increase in weight and volume, and creamy white in colour. Both gonads are readily seen without any aid.
Stage III (Developing)
Ovaries distended occupied, about 2/3 of abdominal cavity with large pale yellow eggs. Testes enlarge, increase in weight and volume, light pinkish and thicker in size and look more vascular. Blood capillaries become conspicuous.
Stage IV (Developed /pre spawning)
Ovary becomes more enlarged occupying almost entire body cavity, with large number of big, turgid, spherical, translucent, swollen green ripe ova. Testes become soft swollen milky white and increase in weight and volume. Blood capillaries are prominent. Roe to milt run with slight pressure.
Stage V (Spawning)
Ovary walls become thin almost transparent. Ripe eggs are visible through the ovarian wall and some ripe eggs are present in the oviduct. Testes become flabby, thin and dull white in colour.
Stage VI (Spent)
Shrunken gonad has loose walls. Ovaries are flaccid, sac like and reduced in volume. Ovary contains ripe unspawned darkened eggs and a large number of small ova. Testes become flaccid empty with evidence of hemorrhaging (bleeding).
Percentage of gonadal maturity stages were as follows: In females 15.5% of the total fish were in developing stage (III); 35% were in developed stage (IV) and 49.5% were in spawning stage (IV). Therefore, 84.5% of total fish were in the reproductive process. In males 22.4% of the total fish were in developing stage (III); 13.2% were in developed stage (IV) and 64.4% were in spawning stage (V). So, 77.6% of the total fish were in reproductive process.
Gonadosomatic Index (G.S.I.)
Monthly variations in G.S.I. revealed that both sexes followed nearly the same trend. In females, several peaks of G.S.I. values were observed during January, April, June and August (Figure 4). This means that females could breed more than once in the season and the period from January to August represented the spawning (breeding) period of T. zillii in this region. It is also clear that females acquired higher values of GSI than males. The present study agreement with Maclaren (1981) who mentioned that cichlids exhibited prolonged spawning season in Lake Manzalah extended from April to September. Negassa and Getahun (2003) declared that T. zillii in Lake Zwai in Ethiopia) breeds all the year round with peak activities between April and September. This observation was more or less similar to those in other localities: In Lake Borollus (El- Haweet, 1991); Lake Mariut (El-Shazly, 1993) and Lake Edku (Phillips, 1994). Also, Negasssa and Padanilly (2008) stated that Indeed, fish with well-developed gonads and mature eggs in both species were noted almost throughout the year. GSI
values and percentages of mature fish indicated that breeding in both species was year-round with its peak during April to September for T. zillii and February to August for O. niloticus.
Length at first maturity
Length at first maturity (Lm50) is the length at which 50% of the fish have reached maturity. Table 2 summarized the previous study and the smallest mature male, and female observed in the sample. Data showed that smallest mature male observed during the present study was 8.4 cm total length whereas the smallest mature female was 7.5 cm total length.. This agrees with Akel and Moharram (2007) in Abu Qir Bay. This suggests that females reach their first sexual maturity before males (8.7
cm, females and 9.7 cm, males) and this is in agreement with Lake Manzalah 8.6 cm (females) and 10.1 cm (males) (El– Shalloof, 1991). Both sexes reached first sexual maturity at the same length 7 cm in Lake Borollus (El–Haweet, 1991). In contrast to these findings, males reached first sexual maturity at smaller lengths than females: At 9 cm, TL for females and 8 cm TL for males in Lake Edku (Phillips, 1994). These differences in length at first sexual maturity may be attributed to differences in genetical and environmental conditions such as food supply, population density and changes in temperature and salinity (Bardackci and Tanyoloc, 1990; Unlo and Balci, 1993). From the aforementioned results, the length of males at which 50% of the fish have reached maturity was fluctuated between 7 to 9 cm) while the corresponding length for females fluctuated from 7 to 10.1 cm)
Age
A total of 700 T. zillii specimens were examined. Total length of females ranged from 7 to 16 cm and males from 8 to 21 cm. Age was determined based on length frequency studies using Bhattacharya’s method (1967). Figure 5 showed that the life span is two and four years for females and males, respectively. These variation between males and females due to the ifferent recorded maximum length.
Growth
The growth in length of T. zillii is assumed to be best described by the von Bertalanffy Growth Formula (VBGF): Lt=L_ (1-e-K·(t-to)) where, Lt is the mean total length (in cm) of the fish at age t; L¥ is the mean asymptotic total length (cm); K is a growth constant (year-1); t is the age of the fish; and t0 is the theoretical “age” of the fish at zero length.
Theoretical growth in length
The von Bertalanffy growth equations for growth in length for T. zillii were estimated as follow: Lt = 22.05 (1-e-0.320 ( t+1.4128) ) for males Lt = 17.83 (1-e-0.680 ( t+0.304 ) ) for females
Theoretical growth in weight
The von Bertalanffy growth equations for growth in weight for T. zillii were predicated as follows: Wt = 201.74 (1-e-0.320 ( t+1.4128) ) 3.0055 for males Wt = 109.54 (1-e-0.680 ( t+0.304 ) ) 2.975 for females
Growth performance index (_)
Growth performance index (_) had been used since it is the best index for expressing the fish growth (Moreau et
al., 1986). It was found that the growth performance of T. zillii is 2.19 for males and 2.34 for females it was nearly the same for both sexes, indicating that differences in growth by sex are very small.
Mortality
Total mortality (Z) can often be estimated from a length converted catch curve The calculated (Z) = 2.656 for males is higher than females (Z) = 1.88 for T. zilliilin Lake Timsah. Figures 6 and 7 The recorded natural mortality was 0.464 and 0.38 for
males and females, respectively. This result may be due to the high levels of pollution in Lake Timsah (Elhalfawy et al., 2006). The corresponding estimates of the fishing mortality rate were 2.192 for males and 1.50 for females.
Exploitation rate
The exploited rate (E) was calculated as equal to the fraction of death caused by fishing (E=F/Z); the estimation values were (E=0.83) for males and 0.78 for females. According to Gulland (1971), the present estimated values of exploitation rate of T. zillii refer to an overexploitation in this species in the Lake Timsah. The high rates of exploitation indicate that this is due to highlevels of fishing mortality. This agrees with the numbers and sizes of fish caught calculated for the lake fishery The initial indication is that there is a high mortality of the early year groups. The majority of the fish caught were below the optimal length at capture. This suggests growth overfishing. It must raise the length at first capture to management T. zillii in Lake Timsah. Figures 8 and 9
Relative yield-per-recruit, biomass-per-recruit and biological reference point
Y’/R and B’/R were determined as functions of L50/L¥ and M/K, respectively (Figure 10 and 11). It is clear that the relative yield per recruit increase with the increase in exploitation rate. These results indicated that the current exploitation level of the T. zillii in Lake Timah (E=0.83 males and 0.78 for females) is higher than that associated with the optimum Y’/R which take place at an exploitation rate of (E0.1 =0.616 for males and 0.513 for females) and it must be reduced by about 26 and 34% for
males and females, respectively to achieve the optimum yield per recruit.
CONCLUSION
T. zillii must be protected in Lake Timsah till their third year of life, when they have a good marketable size, this
can be attained by: 1. Increase in the length at first capture by selecting the optimum mesh size which release small fish and allow each fish to produced eggs at least once in its life. 2. Reduce the fishing mortality by decreasing the number
of boats and the number of fishing trap.
