Reproductive Traits in Different Genetically Improved Nile Tilapia (Oreochromis niloticus) Strains Raised in Brackish Water
Abstract
Inland aquaculture using underground brackish water is a promising method to alleviate pressure on freshwater resources. Nile tilapia stains utilized for commercial production can also tolerate low-salinity brackish water. Therefore, the goal of this study was to compare the reproductive performance of females and males of five genetically different Nile tilapia strains under brackish water cultivation. The GIFT, Big Nin, Mag Nin, Thai red, and Taiwanese red strains, with initial mean weights of 165.5 ± 18.3, 157.8 ± 26.1, 157.1 ± 27.6, 163.3 ± 26.3, and 175.9 ± 27.3 g (±standard deviation), respectively, were used. The fish were individually tagged and each strain was maintained in 8 m2 rectangular concert tanks for 120 days (two fish/m2). The fish were fed at 1.5% of their body weight twice a day with a commercial diet (42% crude protein) at 29.0 ± 0.5°C. The highest female body weight was obtained in the red color strains (Thai and Taiwanese strains) compared to Big Nin strain (p < 0.05) and it was not significantly different in other black tilapia strains. The results of the reproductive indices including spawning frequency and cycles, fertilization and hatching rates, and hepatosomatic index indicated no significant differences among the strains (p > 0.05). However, Mag Nin strain exhibited higher working and relative fecundity values than GIFT strain. A longer reproduction cycle was recorded in GIFT strain than Taiwanese red strain (p < 0.05). In terms of egg quality, no significant differences were obtained among the tilapia strains. A higher sperm motility was recorded in GIFT strain than Mag Nin strain, while other groups showed no significant differences. Overall, GIFT strain demonstrated superior values of sperm motility and reproduction cycles, while Mg Nin strain showed high working and relative fecundity values. The results of the reproductive performance of each genetically improved Nile tilapia strain will be discussed by fish farmers upon their specific environmental conditions and production goals.
1. Introduction
In many countries, limited freshwater supply has had severe effects on the expansion of aquaculture practices [1]. Also, salinization of freshwater resources often occurs in many arid and semiarid regions worldwide, making them saltier over time [2]. In this regard, brackish water aquaculture using saline groundwater is a profitable alternative venture and amplified by freshwater scarcity due to its cost-effectiveness, large volume of pristine resources, and the possibility of rearing some fish tolerate low-salinity brackish water, like tilapia [2, 3]. Furthermore, China, for example, is the largest producer of tilapia and is now focusing on brackish/saline water aquaculture instead of freshwater aquaculture [3]. Other countries such as Australia, Thailand, India, Ecuador, and the United States tried to implement similar resource management approaches [1].
Tilapia is recognized as one of the most cultivated finfish species in the global aquaculture industry due to its growth rate rapid growth rate, high fecundity, extensive dietary plasticity, ability to tolerate salinity, suboptimal environmental conditions and stressors, affordable, and high-quality source of protein [4, 5]. As a result of these high-value aquaculture traits, tilapia has been introduced to tropical and subtropical regions and emerged as a key seafood resource worldwide [6]. Therefore, the rapid progress of tilapia intensification requires to produce high-quality seeds and subsequent larvae [7]. Nile tilapia (Oreochromis niloticus) and its hybrids are the most famous cichlid species used in the aquaculture industry around the world due to their desirable growth and reproductive traits. Meanwhile, there are some Nile tilapia stains utilized for commercial tilapia production in the world including Thailand red, Taiwanese red, Malaysian red, GIFT, Mag Nin, Big Nin, Chitralada, Norway GenoMar supreme tilapia (GST), Akosombo, and Abbassa [8–11]. In this study, the reproductive performance of five red and black Nile tilapia strains (i.e., Thai red, Taiwanese red, GIFT, Mag Nin, and Big Nin) was studied due to the initial import of the fish to Iran for indoor aquaculture practices.
The reproductive performance of fish brooders is an eminent factor in flourishing tilapia farming. In addition, the production and maintenance of broodstocks account for a significant share of operation costs in aquaculture [12]. However, achieving reproductive success and producing high-quality seed in tilapia depends on various factors, including broodstock selection, nutrition, stocking density, gender ratio, and environmental conditions [11]. Therefore, tilapia farmers encounter a widespread of challenges that lead to reduce reproductive success such as early sexual maturation, low female fecundity and male vitality, asynchronous spawning, high larval deformity, and low survival rate [13–15]. Therefore, the utilization of tilapia brooders with superior reproductive traits can facilitate sustainable production of tilapia.
Genetic improvements can help tilapia hatchery operators guarantee high egg quantity and quality production by developing broodstock with efficient reproductive traits [16, 17]. Collaborations between international breeding companies and research institutions have accelerated the development and utilization of genetically improved resources in recent years [8, 18]. In this regard, five Nile tilapia (O. niloticus) strains including GIFT, Big Nin, Mag Nin, Thai red, and Taiwanese red have been commercially developed as famous candidates in tilapia farming with various purposes worldwide.
The importance of finding efficient broodstock stains to establish a base population and mitigate the detrimental effects of inbreeding in tilapia breeding programs has been previously reported [19]. Besides, significant variations in fecundity and spawning frequency have been reported among different Nile tilapia strains [13, 20]. For instance, Mair et al. [12] studied the reproductive parameters of genetically improved Nile tilapia strains including GIFT, Fish-gen, and Philippine-selected (Chitralada) stains. In another study, Silva et al. [21] reported a higher relative fecundity in Aqua-America Nile tilapia stain than GIFT and Aqua-America × GIFT strains. Rossato et al. [22] also recommended GIFT strain as an ideal tilapia strain for commercial reproduction compared to Thai red strain. To the best of our knowledge, this is the first time to compare reproductive traits among black and red Nile tilapia strains in brackish water.
Various tilapia strains have been introduced to the aquaculture industry around the world to increase animal protein production and food security. In Iran, some commercial strains of Nile tilapia have been formally introduced to overcome the challenges of low fecundity in the base tilapia population. However, no study has compared the reproductive traits of different tilapia strains to discern which strain performs better in brackish water. Hence, this study was conducted to compare the reproductive performance of five tilapia strains (Big Nin, Mag Nin, GIFT, Thai red, and Taiwanese red) and recommend the best strain(s) for the aquaculture industry in brackish water.
2. Material and Methods
2.1. Tilapia Strain Origin
This study was conducted at the hatchery of Nilabzist Company (Bafq, Yazd, Iran) and the laboratory experiments were performed at the National Research Centre of Saline-Waters Aquatics (Bafq, Yazd, Iran). In total, five different Nile tilapia strains, namely, GIFT, Big Nin, Mag Nin, Thai red, and Taiwanese red were imported from Thailand (Nam Sai Co., Thailand) in 2019 (Table 1). The brood fish were obtained from the introduced and they were kept in brackish water from birth for 6 months to reach an initial mean weight of 165.5 ± 18.3, 157.8 ± 26, 157.1 ± 27, 163.3 ± 26, and 175.9 ± 27 g for GIFT, Big Nin, Mag Nin, Thai red, and Taiwanese red strains, respectively.
| Female tilapia strains | Coloration | Origin | Main purpose |
|---|---|---|---|
|
Black | International Center for Living Aquatic Resources Management (ICLARM) based in Philippines | A strong fish and disease resistance exhibiting good growth up to a large size |
|
Reported to be developed from the GIFT strain and acquired by Nam Sai Farms from a hatchery in the Philippines | A faster-growing strain of Nile tilapia and meat yield is expected to be high | |
|
A crossing from a developed line of GIFT strain improved for immune system (high disease-resistance) and an improved Big Nin line (grows fast to a large size), Nam Sai Farms | A fast-growing strain with a short and thick body shape | |
|
Red | Thai National Aquatic Genetics Research Institute (NAGRI), Thailand | Fat and round body, fast growing |
|
A local hatchery imported a few fish from Taiwan in 1997. Nam Sai Farms spent 4 years improving the color of this strain before introducing it to the Thailand market in 2001 | Vivid red and relatively disease-resistant |
2.2. Tilapia Broodstock Production
The broodstock fish were acclimated to the new experimental conditions for 2 weeks to prepare for mating. The fish were fed until apparent satiety with specialized commercial pellets based on the nutritional requirements [14] for 4 months. Furthermore, the water quality variables were kept steady at the standard range of tilapia farming (29.0 ± 0.5 °C water temperature, 5 ± 1 mg/L dissolved oxygen, 7.6 ± 0.3 pH, 0.0002 ± 0.00 mg/L nitrite, and ammonia <0.006 mg/L) by an HQ30d Hack multiparameter meter (HACH, US) and a Lambda 25 UV/VIS spectrophotometer (PerkinElmer, Inc., USA). In this study, the source of water was underground saltwater (8.2 ± 0.1 g/L salinity).
2.3. Breeding Conditions
After the acclimatization period, the brooders were separately tagged and housed in rectangular concrete tanks (8 m2) equipped with an air stone per tank as breeding tanks. The broodstocks were fed at 1.5% of their body weight in two daily portions (08:00 am and 05:00 pm) with a commercial extruded diet containing 42% crude protein, 8% moisture, 13% lipid, and 5% ash. The water temperature was maintained at 29.0 ± 0.5°C using a central water temperature system. Each tank had a flow-through system (10 L/min water flow rate) and the natural photoperiod regimes were followed during the trial (12-h light to 12-h dark cycles).
In the hatchery, the input water was initially stored and aerated in a 2 m3 tank before being pumped through a 5-μm disc filter to enter the incubators. The hatched larvae were then transferred to small handmade hapa nets with specific labels inside a container (50 cm × 220 cm × 20 cm) for the nursery stage.
2.4. Breeding Program
For each strain, a total of 12 mature fish at a 2:1 sex ratio (eight females and four males) were randomly selected, individually weighed, tagged, and transferred to the breeding tanks (a density of two fish/m3). The average initial weights for females and males in all strains were 163.9 ± 3.9 and 184.6 ± 4.2 g, respectively. The females were tagged using a Biotech tagging device (USA) and released into their respective tanks. In this study, each tagged fish was represented as one replication (n = 8).
The females were evaluated twice a week when the tilapia began to spawn. If eggs were presented in the mouth, the tag was read and then the eggs were removed. The eggs were counted by capturing images using MediaBang Paint software (Version 2.0; Copyright 2013). A sample of 30 eggs was also taken to assess the fertilization rate using 15% acetic acid, measuring the small and large diameters of the eggs with a caliper. The samples were then transferred to the laboratory for further analysis.
Furthermore, a total of 100 eggs for each tilapia strain were counted and stored in 1.5-L plastic jar incubators. The hatched larvae were counted and transferred to specific hapa nets to evaluate the quality of the produced larvae and fry.
2.5. Reproductive Traits
2.6. Egg Quality
A total of 10 randomly selected eggs from each strain were measured in terms of their large and small diameters using a caliper and their weights using a sensitive digital balance (AND, Japan) with an accuracy of 0.001 g. To measure the proximate composition of the eggs, the remaining labeled eggs from the incubator were dried at 60°C for 24 h. After measuring the moisture content, they were homogenized and stored in a freezer at −20°C analyses. The crude protein (Kjeldahl method), fat (Soxhlet set), fiber (Fibertech set), and ash (muffle furnace set) contents were measured based on the standard methods described in AOAC (2005).
2.7. Sperm Quality
To determine sperm quality (motility and concentration), all males from different strains were anesthetized with clove powder at a concentration of 200 mg/L [23]. Subsequently, sperm was collected using a microsampler. The samples were diluted in distilled water at a rate of 5 µL of semen per 1000 mL of normal saline water (0.85% NaCl) and the number of spermatozoa was counted using a hemocytometer.
To evaluate sperm motility, a volume of 5 µL of semen was diluted in 1000 mL of normal saline water at 28 ± 0.2°C and a drop of the resulting suspension was immediately observed under an optical microscope at ×400 magnifications [24].
2.8. Larvae Quality
In this study, artificial incubation of tilapia eggs occurred for 2–3 days and yolk sac absorption took place over 3–4 days at 29°C. The larvae absorbed their yolk sacs in the incubator, and then, swim-up fry were collected in separate hapa nets. The growth of larvae from each strain was evaluated individually in the hapa nets for 14 days. Finally, the survival rate was calculated by counting the number of larvae.
2.9. Statistical Analysis
The data were reported as mean ± standard deviation. The present study was performed in a completely randomized design. All statistical calculations were performed using SPSS version 21 and Microsoft Office Excel. The Shapiro–Wilk’s and Levene’s tests were used to assess the normality and homogeneity of the data, respectively. After confirming the normal distribution of the data and the homogeneity of variances, one-way analysis of variance (ANOVA) followed by Tukey’s post hoc comparisons at a 95% confidence level (p < 0.05).
3. Results
3.1. Reproduction Indices
The reproductive parameters and larvae productivity in different genetically improved Nile tilapia strains kept in brackish water are presented in Table 2. The average initial weight of females among different tilapia strains had no significant differences (p > 0.05). However, the female red color strains (Taiwanese red and Thai red) showed higher final weight than the black color strains (GIFT, Mag Nin, and Big Nin) with a significant difference with Big Nin at the end of the trial (p < 0.05).
| Parameters | Different tilapia strains | ||||
|---|---|---|---|---|---|
| G | B | M | Th | Ta | |
| Female initial weight (g) | 165.5 ± 18.3a | 157.8 ± 26.1a | 157.1 ± 27.6a | 163.3 ± 26.3a | 175.9 ± 27.3a |
| Female final weight (g) | 270.8 ± 37.8ab | 233.4 ± 27.3b | 265.8 ± 37.1ab | 300.8 ± 51.5a | 319.1 ± 43.2a |
| Working fecundity (number of eggs) | 1057 ± 302b | 1428 ± 383ab | 1638 ± 332a | 1630 ± 550ab | 1525 ± 270ab |
| Relative fecundity (number of eggs/kg) | 5512 ± 1552b | 7130 ± 1382ab | 8271 ± 2208a | 8010 ± 2165ab | 7363 ± 1475ab |
| Reproduction cycles (number) | 7.9 ± 0.9a | 7.4 ± 1.0ab | 6.4 ± 0.7ab | 6.3 ± 0.8ab | 5.9 ± 1.2b |
| Reproduction frequency (day) | 14.3 ± 1.2a | 14.3 ± 1.0a | 14.8 ± 2.2a | 14.4 ± 2.5a | 15.0 ± 3.4a |
| Fertilization (%) | 83.5 ± 15.7a | 94.8 ± 5.2a | 95.4 ± 3.5a | 89.6 ± 7.1a | 94.3 ± 8.3a |
| Hatchability (%) | 78.1 ± 21.8a | 87.9 ± 15.5a | 96.3 ± 2.5a | 76.3 ± 23.1a | 91.8 ± 23.0a |
| Swimming fry survival rate (%) | 83.8 ± 22.9a | 88.4 ± 9.5a | 89.7 ± 12.9a | 72.0 ± 20.3a | 71.7 ± 17.3a |
| Larvae survival rate (%) | 87.8 ± 13.1a | 82.3 ± 14.3a | 88.5 ± 14.6a | 59.3 ± 27.4a | 55.5 ± 16.3a |
| Fry weight (mg) | 88.0 ± 0.0a | 88.2 ± 0.0a | 291.8 ± 0.5a | 202.0 ± 0.2a | 157.3 ± 0.1a |
| Female GSI (%) | 2.1 ± 0.5ab | 1.3 ± 0.1b | 1.8 ± 0.1a | 1.4 ± 0.1b | 1.4 ± 0.1b |
| Female HSI (%) | 2.3 ± 0.1a | 2.3 ± 0.5a | 2.7 ± 0.7a | 2.2 ± 0.6a | 2.1 ± 0.2a |
- Note: Different superscript letters in each row show that means are significantly different (p < 0.05).
- Abbreviations: GSI, gonadosomatic index; HIS, hepatosomatic index.
There were no significant differences in the reproduction frequency, fertilization, and hatchability values among the groups (p > 0.05; Table 2). The reproduction cycles in Taiwanese red strain exhibited a significantly low value (5.9 ± 1.2) compared to GIFT strain (7.9 ± 0.9; p < 0.05; Table 2). The levels of working fecundity and relative fecundity showed significant differences between GIFT and Mag Nin strains with higher values in Mag Nin strain (1638 ± 332 number of eggs and 8271 ± 2208 number of eggs/kg, respectively; p < 0.05; Table 2).
In terms of larvae and fry quality, the larval and swimming fry survival rates and fry weight did not differ significantly among the tilapia strains (p > 0.05; Table 2).
The HSI showed no significant differences among all tilapia strains (p > 0.05). However, Mag Nin strain showed a significantly high GSI (1.8% ± 0.1%) compared to other groups (p < 0.05), except for GIFT strain (2.1% ± 0.5%).
As shown in Figure 1, there was a negative correlation between reproduction cycles and final weights (Pearson’s correlation coefficient = −0.241, p = 0.158). The interaction between working fecundity and reproduction cycles in different Nile tilapia strains is illustrated in Figure 2. The results showed that the working fecundity was decreased by increasing the reproduction cycles in the tilapia strains (Pearson’s correlation coefficient = −0.708, p = 0.181; Figure 2).
The reproduction cycles of the tilapia strains kept in brackish water for 4 months are illustrated in Figure 3. The number of capable females from different strains varied over the experimental period. A peak of the reproductive cycle occurred after every 2 weeks in all female breeders. GIFT strain had a higher number of female breeders above the specified threshold for 50% of the spawning capable females in the population (half or more of the fish spawned six times). However, Thai red strain showed that only once did more than half of their population spawn during the study.
3.2. Egg and Sperm Quality
The gonadal quality indices in different genetically improved Nile tilapia strains kept in brackish water are summarized in Table 3. The results showed that none of the parameters related to egg quality (i.e., egg number, total weight, large diameter, and short diameter) indices and proximate composition (i.e., egg crude protein, fat, and ash) values showed significant differences among different tilapia strains (p > 0.05).
| Parameters | Different tilapia strains | ||||
|---|---|---|---|---|---|
| G | B | M | Th | Ta | |
| Egg number (number of eggs/g) | 223.0 ± 14.8a | 256.7 ± 33.9a | 259.1 ± 39.6a | 271.0 ± 61.9a | 251.4 ± 28.8a |
| Egg weight (mg) | 4.5 ± 0.3a | 4.0 ± 0.6a | 3.9 ± 0.5a | 3.9 ± 1.0a | 4.0 ± 0.5a |
| Large egg diameter (mm) | 2.3 ± 0.1a | 2.3 ± 0.1a | 2.2 ± 0.1a | 2.2 ± 0.2a | 2.2 ± 0.1a |
| Short egg diameter (mm) | 1.7 ± 0.1a | 1.7 ± 0.1a | 1.6 ± 0.1a | 1.6 ± 0.1a | 1.6 ± 0.1a |
| Egg crude protein (% in dry weight) | 55.4 ± 0.9a | 58.7 ± 3.0a | 56.9 ± 0.2a | 57.3 ± 1.5a | 56.6 ± 1.4a |
| Egg fat (% in dry weight) | 29.5 ± 2.5a | 29.7 ± 0.5a | 26.4 ± 1.1a | 26.4 ± 2.2a | 27.3 ± 1.2a |
| Egg ash (% in dry weight) | 12.3 ± 1.2a | 9.0 ± 2.3a | 8.3 ± 1.2a | 11.8 ± 0.6a | 9.0 ± 3.7a |
| Spermatozoa number (million/ml) | 7.4 ± 2.8a | 8.7 ± 3.6a | 6.1 ± 1.1a | 7.3. ±1.6a | 10.3 ± 1.1a |
| Sperm motility (minute) | 39.1 ± 8.7a | 30.5 ± 9.6ab | 13.9 ± 5.0b | 27.7 ± 8.4ab | 29.1 ± 0.4ab |
- Note: Different superscript letters in each row show that means are significantly different (p < 0.05).
The semen analysis in different genetically improved Nile tilapia strains kept in brackish water is displayed in Table 3. Although the spermatozoa number did not exhibit significant differences among all strains (p > 0.05), higher sperm motility was obtained in GIFT strain (39.1 ± 8.7 min) than Mag Nin strain (13.9 ± 5.0 min; p < 0.05). However, other tilapia strains did not show significant differences in sperm motility (p > 0.05).
4. Discussion
Tilapia is the second-most farmed species in the world today and the global aquaculture production is expected to reach more than 6 million tonnes by 2030. China, Indonesia, India, Vietnam, and Bangladesh are the largest producers of tilapia in the world and one of the crucial reasons for their leadership could be the use of improved strains with superior growth and reproductive performances such as GIFT, GST, genetically enhanced tilapia 2000 (GET 2000), the freshwater aquaculture center selected tilapia (FaST), Akosombo, and Abbassa [25, 26]. Therefore, it is necessary to evaluate different strains of tilapia under different environmental conditions such as different levels of water salinity to finally introduce the best strains to salinity-prone regions [27, 28]. Sustainable aquaculture practices require the introduction of ideal tilapia strains and this study demonstrates the strengths of each tilapia strain to meet the specific goals of the tilapia farmers using underground brackish water. In the present study, the reproductive performances of five tilapia strains that were originally produced by Nam Sai Co. (Thailand), one of the internationally famous tilapia producers in the world, have been compared.
The final weight of female breeders in the red color strains (Taiwanese red and Thai red) was significantly higher than Big Nin and numerically higher than other black tilapia strains. It appears that the red tilapia strains assign their energy more towards somatic growth instead of reproductive potentials, since there was a negative correlation between final weight and reproduction cycles among different tilapia strains. The increase in tilapia female weight along with a fewer reproductive event has been reported in Chitralada Nile tilapia strain [13]. Similar results were obtained in different female tilapia strains from Ghanaian, Senegal, Singapore, Taiwan, and Thailand breeders by Bolivar et al. [20]. In other studies, no significant differences were reported in the female final weight of some tilapia strains (Fishgen, GIFT, and Chitralada) from the Philippines [12] and GIFT × AquaAmerica strain [21]. These studies also indicated that reproductive traits were not influenced by the different tilapia strains. A similar significant difference has been reported between Ghanaian female tilapia breeders and the tilapia strains from Egypt, Senegal, Israel, Singapore, Taiwan, and Thailand [20].
Tilapia has asynchronous ovarian development and, therefore, it produces a small number of eggs in each reproduction cycle [14]. In this context, high fecundity is a valuable economic trait for the fish farmer. Among the investigated strains, there was no significant difference in the working and relative fecundities across different strains, except for GIFT, which showed the lowest values. In agreement with our results, Mair et al. [12] noted the lowest fecundity in GIFT strain compared to other tilapia strains including Fishgen, GIFT, and Chitralada from the Philippines. Silva et al. [21] also demonstrated that the GIFT strain had a low fecundity, but when hybridized with the AquaAmerica strain, it was significantly improved. Although GIFT strain may have a lower fecundity rate, other advantages such as higher growth and survival rates, make this strain a good candidate for aquaculture practices as reported by Moses et al. [8]. The differences in working fecundity among different tilapia strains may be due to their varying responses to environmental conditions [29]. For instance, although the environmental conditions were the same in this study, but GIFT strain was genetically improved for higher thermal conditions and exhibited lower working fecundity in low water temperatures [28]. Therefore, maybe a higher water temperature (more than 29.0°C) is required to reach maximum reproductive performances in GIFT strain, however, further investigation is needed.
In the study of five Nile tilapia populations from Egypt and Sagana, the results indicated that the relative fecundity of the Nile tilapia strains ranged from 3100 to 3900 eggs per kg of body weight [19]. However, the relative fecundity of different tilapia strains was reported around 8300 and 11,900 eggs/kg [19, 30]. These results confirmed the obtained results in this study that the lowest relative fecundity was found in GIFT strain (5512 eggs/kg) and the highest value was observed in Mag Nin strain (8270 eggs/kg).
The fecundity results in the present study indicated the acceptable performance of five tilapia strains introduced to Iran to make tilapia breeding centers more efficient. Because the previously introduced strains imported from Indonesia (black and red color tilapia) had a lower reproductive performance with 2770 egg/kg relative fecundity [31].
In this study, the spawning frequency did not exhibit a significant difference among the different strains ranging between 14 and 15 days. This finding was also reported by Almeida et al. [30], who observed the spawning frequencies of 14–21 days for different tilapia strains including Supreme, Premium Aquabel, and Chitralada. According to Yoshida et al. [32] spawning frequency is heritable and has less variation due to environmental conditions in various tilapia strains. Therefore, based on similar environmental conditions and the heritability of spawning frequency, studying the genetic similarity within experimental tilapia strains is recommended for future investigations. The lower spawning frequency of these strains compared to what was reported by Almeida et al. [30] makes these strains more suitable and attractive for recommendation to tilapia breeding centers as they produce more eggs in each reproduction cycle. Based on the results, tilapia hatcheries can replace the spawned brood fish every 2 weeks to rest and consequently increase the total number of eggs harvested during each reproduction cycle. Better reproductive performance of rested tilapia brooders than not rested ones has been supported by the findings of Abou-Zied [33] and Addo et al. [34].
In this study, the fertilization and hatching percentages did not significantly change among the tilapia strains. Similarly, no significant difference in the fertilization percentage was reported between GIFT and AquaAmerica × GIFT strains [21]. On the other hand, the higher hatching percentage has been confirmed by Rossato et al. [22]. These contradictions in the reported findings may be due to different age and environmental conditions, as Osure and Phelps [19] demonstrated various spawning success ranged from 36.9 to 81.2 in Nile tilapia strains from different regions including Ivory Coast, Egypt, Sagana, and Lake Victoria.
The highest GSI was observed in Mag Nin strain, likely attributed to the difference in their fecundities and reproduction cycles. Komolafe and Arawomo [29] suggested that different maturation stages and yolk deposition in tilapia eggs are the main reasons for differences in GSI, similar to what was mentioned for Mag Nin strain and other strains in this study.
The fertilization success and subsequent hatching and larval survival rates are directly influenced by the quality of eggs and sperm [35, 36]. The egg composition can reflect the quality of the oocyte [37], however, different tilapia strains showed no significant differences. In general, environmental conditions, nutrition status, disease, stress, husbandry, and handling can significantly affect egg quality in fish [38].
In the current study, the values of large and small egg diameters and egg weight did not show a significant difference among different strains and ranged at 2.2–2.3 mm, 1.6–1.7 mm, and 0.038–0.045 g, respectively. In agreement with this result, large egg diameters ranging between 2.47 and 2.58 mm were reported for Nile tilapia strains in Nigeria and Indonesia, respectively [29, 31]. It has been suggested that these differences may be related to the size and rearing conditions of the broods [38].
In the assessment of sperm quality, no significant difference was observed among the different treatments with the number of spermatozoa ranging from 6.1 to 10.3 million/mL of semen. Also, the total duration of sperm activity varied from 13.9 to 39 min, while a higher value was reported in GIFT than Big Nin. To have successful fertilization in Nile tilapia, 7.1 million/mL of semen and 18 min have been reported for the spermatozoa number and sperm motility, respectively [36].
5. Conclusion
Environmental conditions and specific production goals (e.g., female body weight, vigor offspring production, maximum egg production, and market demands) should be considered to introduce the ideal tilapia strains in each region before commercialization to maximize the hatchery efficiency and profitability. In this study, the highest female body weight was obtained in the red tilapia (Thai and Taiwanese strains) compared to Big Nin strain raised in brackish water. The spawning frequency and cycles, fertilization and hatching rates, and hepatosomatic index did not significantly change among the strains. However, Mag Nin strain exhibited higher working and relative fecundity values than GIFT strain. In terms of egg quality, no significant differences were obtained among the tilapia strains. A higher sperm motility was recorded in GIFT strain than Mag Nin strain, while other groups showed no significant differences. GIFT strain demonstrated superior values of sperm motility and reproduction cycles, while Mg Nin strain showed high working and relative fecundity values. Although GIFT strain showed considerably lower fecundity, it can be recommended to tilapia farmers because of higher reproduction cycles that compensate for lower working fecundity. A better reproductive performance may be achieved in all strains by resting the spawned brooders at 2-week intervals. It is highly recommended to find out the genetic mechanisms behind the differences in reproductive performance among the tilapia strains for future studies.
Ethics Statement
This work received ethical approval from the Iranian Fisheries Science Research Institute (IFSRI) (Registered Number: 2-89-12-008-00073) on April 21, 2021.
Conflicts of Interest
The authors declare no conflicts of interest.
Author Contributions
The experiment was designed by M. Mohammadi, M. Alizadeh, M. Bahmani, and H. Sarsangi Aliabad. Sampling was carried out by M. Mohammadi and A. Ghaedi. Laboratory analysis was conducted by M. Mohammadi, A. Gharaei., and A. Nabi. The statistical analysis was carried out by S. P. H. Shekarabi and M. Akhavan-Bahabadi. The manuscript was written by all authors with an edition by S. P. H. Shekarabi.
Funding
This work was supported by Iranian Fisheries Science Research Institute.
Acknowledgments
The authors express their gratitude and appreciation to Nilabzist Company and Yazd Reproductive Sciences Institute for their kind cooperation during the study.
Open Research
Data Availability Statement
The data are available upon request from the authors.
