Review on the progress in the role of herbal extracts in tilapia culture

Tilapia is one of the most important fish in freshwater aquaculture. However, increased intensification of aquaculture has led to many constraints including, poor growth, and poor health. This has made it hard for fish farmers to turn the biological benefits associated with intensive farming systems into economical gain. Moreover, the wide adoption use of chemotherapeutic drugs to maintain health and sex reversal hormones to control reproduction appears to be merely production-oriented, thus unsustainable. Hence, improvement of productions in aquaculture should also focus on environmentally friendly, and sustainable methods. Today, huge attention is focused on the use of medicinal herbal extracts to improve yield in aquaculture as alternatives to chemical agents. Accordingly, the aim of this work was to critically review the effectiveness of herbal extracts in tilapia farming, and further highlight gaps in the existing knowledge and the way forward in using medicinal herbal extracts in tilapia aquaculture. A wide range of medicinal herbal products were reported to enhance growth, appetite, immune responses, antioxidation, hepatoprotective activity, and control reproduction in tilapia species; attributed by their abundant bio-actives such as saponin, phenols, flavonoids and polysaccharides. However, the lack of sufficient knowledge on herbal extracts limits the cost-effective use of herbal extracts in tilapia aquaculture. Little knowledge still Ndakalimwe Naftal Gabriel ABOUT THE AUTHOR The author, Ndakalimwe Naftal Gabriel was born on the 25 August 1985 in Kwaza-Sul, Angola. He obtained his first degree (BSc.) in Environmental Biology, Molecular and Physiological Biology at the University of Namibia (UNAM) (2006–2009). In 2010, he joined the Namibian Ministry of Fisheries and Marine Resources, directorate of aquaculture as a fisheries research technician, Hardap workstation. While with the ministry, he went to purse his MSc-Aquaculture (aquaculture and fish nutrition) at Nanjing Agricultural University (NAU) in China (2013-2016). After his MSc, He joined UNAM as an aquaculture and fish nutrition lecturer at the Department of Fisheries and Aquatic Sciences. He is currently (2019) a final year PhD candidate at UNAM, researching on the potential effects of herbal extracts in freshwater fish. PUBLIC INTEREST STATEMENT In aquaculture, the maintenance of good health of fish is achieved by using pharmaceutical drugs. These drugs are unaffordable, unavailable to poor farmers in the rural areas, some pathogenic bacteria develop resistance (i.e. antibiotics), their residues end up in the final products, affecting humans’ health and polluting the environment. Thus, the use of these drugs is unsustainable. Alternatively, several medicinal herbs have been reported to possess the potential to reduce or eliminate the use of pharmaceutical drugs. Studies have reported beneficial growth, health, and disease resistance promoting effects of several herbs in aquaculture including tilapia culture. In tilapia culture, herbs such as aloe vera, garlic, ginger lemon oils, among others have been reported to promote their growth and health. However, before herbs are fully implemented in aquaculture studies are still needed to validate their allied growth, health, toxicity effects, as well as preparation methods, and optimum dosages, among others. Gabriel, Cogent Food & Agriculture (2019), 5: 1619651 https://doi.org/10.1080/23311932.2019.1619651 © 2019 The Author(s). This open access article is distributed under a Creative Commons Attribution (CC-BY) 4.0 license. Received: 07 March 2019 Accepted: 12 May 2019 First Published: 24 May 2019 *Corresponding author: Ndakalimwe Naftal Gabriel, Department of Fisheries and Aquatic Sciences, Sam Nujoma Campus, University of Namibia, Private Bag 462, Henties Bay, Namibia E-mail: ngabriel@unam.na; gnaphtal85@gmail.com Reviewing editor: Pedro González-Redondo, University of Seville, Seville, Spain Additional information is available at the end of the article

Review on the progress in the role of herbal extracts in tilapia culture Ndakalimwe Naftal Gabriel* Abstract: Tilapia is one of the most important fish in freshwater aquaculture. However, increased intensification of aquaculture has led to many constraints including, poor growth, and poor health. This has made it hard for fish farmers to turn the biological benefits associated with intensive farming systems into economical gain. Moreover, the wide adoption use of chemotherapeutic drugs to maintain health and sex reversal hormones to control reproduction appears to be merely production-oriented, thus unsustainable. Hence, improvement of productions in aquaculture should also focus on environmentally friendly, and sustainable methods. Today, huge attention is focused on the use of medicinal herbal extracts to improve yield in aquaculture as alternatives to chemical agents. Accordingly, the aim of this work was to critically review the effectiveness of herbal extracts in tilapia farming, and further highlight gaps in the existing knowledge and the way forward in using medicinal herbal extracts in tilapia aquaculture. A wide range of medicinal herbal products were reported to enhance growth, appetite, immune responses, antioxidation, hepatoprotective activity, and control reproduction in tilapia species; attributed by their abundant bio-actives such as saponin, phenols, flavonoids and polysaccharides. However, the lack of sufficient knowledge on herbal extracts limits the cost-effective use of herbal extracts in tilapia aquaculture. Little knowledge still Ndakalimwe Naftal Gabriel ABOUT THE AUTHOR The author, Ndakalimwe Naftal Gabriel was born on the 25 August 1985 in Kwaza-Sul, Angola. He obtained his first degree (BSc.) in Environmental Biology, Molecular and Physiological Biology at the University of Namibia (UNAM) (2006)(2007)(2008)(2009). In 2010, he joined the Namibian Ministry of Fisheries and Marine Resources, directorate of aquaculture as a fisheries research technician, Hardap workstation. While with the ministry, he went to purse his MSc-Aquaculture (aquaculture and fish nutrition) at Nanjing Agricultural University (NAU) in China (2013China ( -2016. After his MSc, He joined UNAM as an aquaculture and fish nutrition lecturer at the Department of Fisheries and Aquatic Sciences. He is currently (2019) afinal year PhD candidate at UNAM, researching on the potential effects of herbal extracts in freshwater fish.

PUBLIC INTEREST STATEMENT
In aquaculture, the maintenance of good health of fish is achieved by using pharmaceutical drugs. These drugs are unaffordable, unavailable to poor farmers in the rural areas, some pathogenic bacteria develop resistance (i.e. antibiotics), their residues end up in the final products, affecting humans' health and polluting the environment. Thus, the use of these drugs is unsustainable. Alternatively, several medicinal herbs have been reported to possess the potential to reduce or eliminate the use of pharmaceutical drugs. Studies have reported beneficial growth, health, and disease resistance promoting effects of several herbs in aquaculture including tilapia culture. In tilapia culture, herbs such as aloe vera, garlic, ginger lemon oils, among others have been reported to promote their growth and health. However, before herbs are fully implemented in aquaculture studies are still needed to validate their allied growth, health, toxicity effects, as well as preparation methods, and optimum dosages, among others. exists on herbal extract concentration, types of extracts, and their potential negative impacts on fish, consumers, and environment, just to mention but a few. More researches are therefore deemed necessary to optimize the use of herbal extracts in tilapia culture.

Introduction
Globally, aquaculture (marine and freshwater) remains one of the fast growing food-producing sectors, with the highest potential to assist marine and inland capture fisheries meet global demand for aquatic food (FAO, 2016). In freshwater aquaculture, tilapias (til ah pe ah) are reported to be the second most important group of farmed fin fish just after carps (Waite et al., 2014). An impressive global production performance by aquaculture is a result of wide adoption of intensive production systems, which are associated with higher productivity per unit area as a result of higher stocking density (Basha et al., 2013). However, higher stocking density in intensive farming systems coupled with other general aquaculture activities such as fish handling, transportation and harvesting may be stressful to fish. This may consequently lead to a number of conditions including poor metabolism capacity (Santos, Schrama, Mamauag, Rombout, & Verreth, 2010), poor meat quality (Jittinandana et al., 2003), increased susceptibility to diseases (Wu et al., 2013a), and in extreme cases lead to deaths (Mckenzie et al., 2012). All these constraints have made it hard for fish farmers to convert the benefits of higher production yield associated with intensive production systems into economical gains. Thus, aquaculture is yet to reach its maximum potential.
In an effort for fish farmers to economically benefit from intensive farming systems, they started using synthetic antibiotics and other chemotherapeutic drugs to maintain good health of farmed fish. The adoption of these drugs in aquaculture appears to be only profit-driven and unsustainable, as they cause several other constraints such as fish pathogen drug resistance, immunosuppression, environmental pollution, and accumulation of chemical residues, which can be potentially hazardous to the public health (Bulfon, Volpatti, & Galeotti, 2013;WHO, 2006). Thus, many nations across the globe such as the United States, European Union (Bulfon et al., 2013), and Asian countries (Ji et al., 2007) have strict demand for aquatic products free from chemical/drugs. Today, the need of replacing antibiotics and other synthetic chemicals with dietary supplements or ingredients that is capable of strengthening fish health, and enhance their growth, feed utilization ability, and ultimately ensure safety and good quality products from aquaculture are becoming increasingly vital.
Over the past two decades, there have been an increase in the number of research with common conclusions that indeed medicinal herbal extracts have the potential to eradicate the use of synthetic chemicals such as antibiotics and other chemotherapeutic drugs in aquaculture.

Some medicinal herbs investigated
Medicinal herbs are as old as civilization and throughout history they have been used as popular folk medicine because of their broad-spectrum medicinal properties. The attempt to adopt them in aquaculture is a novel development that got huge attention virtually in every part of the globe, with Asia having the most researched herbs (Bulfon et al., 2013). Furthermore, among aquaculture species, effects of herbal extracts are widely researched in tilapia species . A wide range of medicinal herbs used in tilapia aquaculture is shown in Table 1. These herbs could be used as a whole or parts (i.e. leaves, flowers, roots, seeds, or barks) in a crude form or as extracts/compounds from whole plant or parts (Table 1). For instance, crude extract in the form of powder from Aloe vera (Gabriel, Qiang, He, Yu, et al., 2015), and Camellia sinensis (Abdel-Tawwab, Ahmad, Seden, & Sakr, 2010) was incorporated in feed to investigate their effects on growth and resistance of Nile tilapia, Oreochromis niloticus against pathogenic bacteria, respectively. Similarly, dietary blackberry syrup was studied for growth, antioxidation, immune response, and resistance against Pleiomonas shigelloides in O. niloticus (Yilmaz, 2019a). Meanwhile, essential oil extracted from Citrus sinensis , and Citrus limon peels (Baba et al., 2016) were investigated for growth, health parameters, and resistance against Streptococcus iniae, and Edwardsiella tarda in Oreochromis mossambicus, respectively. In the same fish, dietary Pimenta dioca powder was investigated for haemato-immunological and serum biochemical responses (Güllü et al., 2016;Yılmaz, Acar, Kesbiç, Gültepe, & Ergün, 2015), and dietary red pepper for growth and pigmentation (Yilmaz et al., 2013a). Other compounds such as saponins extracted from Trigonella foenumgraecum (Stadtlander et al., 2013) were mixed with feed to study their sex ratio and gonadal histology effects in O. niloticus, while polysaccharides from Astragalus membranaceus (Abuelsaad, 2014;Ardó et al., 2008) were incorporated with feed to investigate their effects on O. niloticus immune responses. Moreover, herbal extracted Caffeic acid was also reported in O. niloticus for antioxidation, immunological and liver gene expression responses, and resistance against Aeromonas veronii (Yilmaz, 2019b).

Administration of herbal extracts
In general, herbal extracts in aquaculture may be delivered in three ways including oral, injection and immersion administration method. The selection of these methods is mainly dictated by the purpose of administration, size and type of the fish species, types of extract, and farming system as explained in . In addition, injection and immersion methods may be effective in delivering herbal extracts (Harikrishnan, Balasundaram, & Heo, 2010); however, they may not be feasible at all levels of aquaculture operation as they tend to be labor intensive, stressful to fish, and require highly skilled personnels. Today, oral administration tend to be the widely used method in aquaculture including tilapia farming, especially for fish growth and health parameters improvement (Tables 2 and 3). However, immersion as a method of extract delivery may also be useful in tilapia farming particularly for the purpose of sex reversal (Mukherjee, Ghosal, & Chakraborty, 2015a), which uses fish larvae that are not fully developed to consume artificial feed. It is therefore clear that more research are still deemed necessary to homogenize herbal extract administration methods for different purposes (i.e. fish growth, health, disease resistance, meat quality, sex reversal), types of extracts, fish species, and fish size.

Hematological indices
In vertebrates including fish, blood is the most frequently examined tissue in efforts to establish their health status or physiological status. Accordingly, health status such as oxygen carrying capacity has been directly determined by reference to main hematological indices including red blood cell (RBC), hemoglobin concentration (Hb), percentage of blood volume consisting of red cells, and hematocrit (Hct) (Houston, 1997). Secondary indices, sometimes called Wintrobe indices (Urrechaga, Izquierdo, & Escanero, 2014) can be derived from these indices for the classification of anemia condition such as mean corpuscular volume (MCV) = (Hct x 10/RBC), mean corpuscular hemoglobin (MCH) = (Hb x 10/RBC), and mean corpuscular hemoglobin concentration (MCHC) = (Hb x 100/Hct) as demonstrated in (Gabriel et al., 2015a). Furthermore, other hematological indices such as white blood cell (WBC), and a number of their differential counts (i.e. leucocyte counts such as lymphocytes, neutrophils, eosinophils, monocytes, and basophils) have been assessed to take account of innate immune status of animals especially during stressful conditions (Roberts & Rodger, 1978). In addition, neutrophils to lymphocytes ratio has been proven to be a useful tool to estimate stress level in vertebrates (Van Rijn & Reina, 2010) Several studies in aquaculture have adequately demonstrated that numerous medicinal plants have the capacity to enhance some of the aforementioned hematological parameters. In tilapia culture for instance, A. vera supplemented diet was reported to have significantly increased RBC, Hct, Hb, WBC, and some differential leukocyte counts of O. niloticus (GIFT-strain) before and after Streptococcus iniae challenge . Similarly, administration of Rosmarinus officinalis, Trigonella foenum graecum, Thymus vulgaris administration in O. mossambicus (Gültepe et al., 2014), Camellia sinensis in O. niloticus (Abdel-Tawwab et al., 2010), Citrus sinensis in O. mossambicus , Citrus limon in O. mossambicus (Baba et al., 2016), ginseng in O. niloticus (Goda, 2008), Ulva clathrata in O. niloticus (Quezada-Rodríguez & Fajer-Ávila, 2016), Cynodon dactylon, Aegle marmelos, Withania somnifera, Zingiber officinal (Immanuel et al., 2009), Lonicera japonica and Ganoderma lucidum (Yin, Ardó, Jeney, Xu, & Jeney, 2008), P. dioca (Güllü et al., 2016; reportedly improved some hematological parameters. The observed improvement of hematological indices signify the ability of herbal extracts to stimulate erythropoiesis, thus increasing the capability of oxygen transport and strengthening of defense mechanisms against physiological stress. This is suggested to have been attributed by their rich nutritional properties especially polysaccharides, essential vitamins (i.e. riboflavin, thiamine, folic acid), and nonessential amino acids, which are primarily required for hemoglobin synthesis (Latona, Oyeleke, & Olayiwola, 2012;Hamman, 2008).
Furthermore, some medicinal herbal extracts have been reported to have no effects on hematological parameters in tilapia species. For instance, Cinnamomum camphora, Euphorbia hirta, Azadirachta indica, or Carica papaya crude extracts were reported to impose no effects on RBC, Hb, MCV, MCHC, Hct, and WBC of O. niloticus (Kareem et al., 2016). It appears that, the capacity or effects of a medicinal herb on hematological indices can only be well established if an animal is exposed to some stressful conditions as demonstrated in , of which several studies did not perform (Zaki et al., 2012;Shalaby et al., 2006;Quezada-Rodríguez & Fajer-Ávila, 2016;Prasad & Priyanka, 2011;Park & Choi, 2014;Elkamel & Mosaad, 2012). Moreover, several studies in tilapia culture (Baba et al., 2016;Gabriel, Qiang, He, Yu, et al., 2015a;Goda, 2008), and in other fish species such as Labeo rohita (Andrews, Sahu, Pal, Mukherjee, & Kumar, 2011), Cyprinus carpio (Pakravan, Hajimoradloo, & Ghorbani, 2011), and Oncorhynchus mykiss (Haghighi, Rohani, Samadi, & Tavoli, 2014) reported that herbal extracts presented lower hematological indices and to a certain extent cause anemia in fish (Gabriel et al., 2015a), especially at higher dosage. The causes and mechanisms of this are barely known. However, they are assumed to do this by interfering with erythropoiesis, hemosynthesis, and osmoregulatory functions or increasing erythrocyte destruction in hematopoietic organs (Jenkins, Smith, Rajanna, & Rokkam, 2003). Therefore, optimization of herbal extracts based not only on growth, and feed utilization parameters but also on blood parameters such hematology is essential for aquaculture.
Furthermore, A. vera crude extracts were reported to have enhanced antioxidant enzymes (CAT, SOD, GSH-Px), reduced stress (lower level of glucose and cortisol) (Gabriel et al., 2015a) and improved hepatoprotective activity (lower level of AST) (Gabriel et al., 2015b) in GIFTtilapia, pre and post Streptococcus iniae challenge. The same results were obtained by Metwally (2009) in O. niloticus after been fed a diet supplemented with A. sativum extracts, however without these animals being challenged. In the same line, unchallenged animals (O. niloticus) fed diet incorporated with A. sativum acetone extracts presented improved hepatoprotective activity (Shalaby et al., 2006). Meanwhile, acetone extracts from C. dactylon, A. marmelos, W. somnifera, and Z. officinale were reported to have reduced stress in O. mossambicus (Immanuel et al., 2009).
The enhancement of antioxidant and hepatoprotective activity in fish by herbal extract is fairly understood, and often times is correlated with certain phytochemicals. Phytochemicals such as phenol/polyphenols (gallic acid, tannins, and ellagic acid), enzymes (SOD, CAT, GSH-Px), vitamins (C, E, and carotenoids), flavonoids (flavones, isoflavone, flavone, anthocynins and catechins) (Gupta & Sharma, 2014) present in herbal extracts are known to support the inhibition or suppression of oxidation process (Kenari, Mohsenzadeh, & Amiri, 2014). They can inhibit or suppress oxidation process by increasing the amount and activity of the body's natural antioxidant enzymes such as liver CAT, SOD, and glucose-6-phosphate dehydrogenase (Rajasekaran, Ravi, Sivagnanam, & Subramanian, 2006) or by increasing the bioavailability of vitamin E and C (important antioxidant agents) (Vinson, Al Kharrat, & Andreoli, 2005). Furthermore, antioxidation and hepatoprotective properties of herbal extracts cannot be emphasized; however, their optimization in aquaculture requires more research including quantification of extracts' beneficial phytochemicals (i.e. flavonoids and phenols) and their correlation to different immune biochemical indices. The extend of possible negative impacts of these phytochemicals on fish species, environment, and consumers also needs to be established, as they are known to be toxics at higher dosages (Taiwo, Olukunle, Ozor, & Oyejobi, 2005;Gabriel et al., 2015a).

Herbal extract as reproduction controlling agent in tilapia culture
In intensive production systems, tilapias are associated with precocious maturity and prolific breeding behaviors, which affects their production parameters and ultimately economic return (Budd, Banh, Domingos, & Jerry, 2015). Farming with monosex tilapia population is therefore highly recommended. Today, there are several methods used to produce monosex tilapia population including manual separation of sex, hybridization, genetic manipulation and sex reversal hormone (Gabriel et al., 2015c). The later is acknowledged as the most effective and widely used method responsible with mass production of male tilapias in commercial production systems (El-Greisy & El-Gamal, 2012). Hence, the current global tilapia production success owes it to this method. While hormonal sex reversal method is believed to offer practical and economical solution to control reproduction in tilapia, this method is associated with a number of shortcomings including potential health risk to workers, consumers and environment (Phelps & Popma, 2000). Thus, more studies on the safety of sex reversal hormones in fish are needed, and exploration of affordable, environmental and appropriate technology is deemed necessary.
Early studies have revealed that several medicinal herbs contain phytochemicals (phytoestrogen or phytoandrogens) that are structurally similar to steroid hormones, i.e. 17-β estradiol (E2) in animals (Glazier & Bowman, 2001). Recently, these phytochemicals have been reported to induce masculinization, feminization or impair fertility in tilapia species (Gabriel et al., 2017;Jegede, 2010). For instance, saponin extracts from Quillaja saponins (QS) (Francis, Levavi-Sivan, Avitan, & Becker, 2002), fenugreek (Trigonella foenum-graecum) and soapbark tree (Quillaja saponaria) (Stadtlander et al., 2008), and Tribulus terrestris (Omitoyin, Ajani, & Sadiq, 2013), reportedly produced a significant number of males Nile tilapia when were incorporated in their diet, and high percentage of males was recorded at high concentrations. Masculinization effects of saponin extracts on tilapia larvae may be explained by the fact that saponin is able to elevate testosterone hormone production (Ganzera, Bedir, & Khan, 2001) and as a result, plants that contain saponin compounds, especially Tribulus terrestris have been used to treat impotence in human (Akram et al., 2011). Similarly, masculinization effects were also reported in Nile tilapia fed dietary Aloe vera (Gabriel et al., 2017), Mucuna pruriens (Mukherjee, Ghosal, & Chakraborty, 2015b), Butea superb (Kiriyakit, 2014), and in other tilapia species such O. mossambicus fed dietary moringa and paw paw crude extracts, respectively (Ampofo-Yeboah, 2013). In addition, a study by (El-Sayed, Abdel-Aziz, & Abdel-Ghani, 2012) revealed that soybean meal significantly reduced the number of males in Nile tilapia culture, and further advised tilapia farmers to avoid using soybean as a source of protein during sex reversal. Considering these findings, herbal extracts therefore present potent potential to eradicate the use of synthetic sex reversal hormones in tilapia culture. Because, compared to sex reversal hormones, herbal extracts are believed to be easily accessible, simple to apply, and may be safe to the environment and human as they tend to be more biodegradable .
Furthermore, as mentioned early in this paper, another way herbal extracts could control reproduction in tilapia is by impairing fertility through gonads (testes and ovaries) destruction. A study by Jegede and Fagbenro (2008) reported swollen spermatids nuclei, increased interstitial cells and focal necrosis in testes; and hydropic degeneration, ruptured follicles, granulomatous inflammation in the interstitium and necrosis ovaries when neem (Azadirachta indica) leaves were incorporated in Tilapia zilli basal diet at 2.0 gkg −1 . Similar findings were reported in O. niloticus fed Carica papaya (Abdelhak et al., 2013), Hibiscus rosa-sinensis (Jegede, 2010), and Aloe vera (Jegede, 2011) as well as in O. mossambicus fed dietary Carica papaya and Moringa oleifera respectively (Ampofo-Yeboah, 2013). This is an indication that indeed herbal extracts may be used to control reproduction in tilapias by delaying maturing, and producing monosex population as mentioned earlier. However, optimization of herbal extracts as sex reversal and fertility controlling agents in tilapia requires extensive research.

Gaps in the existing knowledge and way forward
Constraints associated with the wide adoption of intensive aquaculture farming systems such as increased susceptibility to diseases and poor meat quality among others, can not be overemphasized. And the consequent use of chemotherapeutic drugs (i.e. antibiotics and sex reversal hormones) to improve production appears to be more production-oriented, while ignoring the potential impacts these chemicals might have on the environment, humans, and animals. The findings of this review paper give an impression that there are several medicinal herbs that indeed possess ability to enhance growth, feed utilization, immune response (both innate and specific), antioxidation, hepatoprotective activity, resistance against disease, and control reproduction in tilapia culture. However, the pace of implementation of this innovation is slow, despite numerous advantages associated with it, as mentioned earlier.
Effective application of herbal extract in tilapia culture and aquaculture in general is limited in many ways. These limitations include lack of sufficient knowledge on effective herbal extraction methods for different purposes/functions (i.e. growth, immune, antioxidation, sex reversal among others), types of effective extracts (i.e. raw, aqueous, methanolic, ethanolic), optimum dosage for different purpose, potential herbal extracts harmful effects in fish, environment, and consumers, mode of actions, effect on fish meat quality, effects of herbal extracts between different physiological process (i.e. growth optimum herbal extract dosage on fish reproduction. For example, saponin compound found in most medicinal plants that is believed to be a promising tilapia masculinizing agent Francis et al., 2002;Omitoyin et al., 2013;Stadtlander et al., 2008), and anti-inflammatory properties (Zhang et al., 1990), it also believed to act as feed deterrent, reducing feed intake in fish, and consequently growth (Dongmeza et al., 2006). Method of extraction and types of extracts are among the factors that indeed determine the beneficial properties and efficacy of herbal extracts on the health of cultured fish. For instance, fish fed with diet containing methanol extracted moringa leaf have been reported to present a better feed intake and growth performance than those fed with diet containing raw moringa leaf meal extract (Afuang et al., 2003). In addition to the limitations, effects of herbal extracts in fish at different life stages (i.e. fries, fingerlings, juveniles, preadults and adults) are unknown, and another is lack of quantitative estimation of beneficial phytochemicals (i.e. phenols & flavonoids) of herbal extraction and correlations to their effects in fish. Therefore, the need to research more and standardize every aspect around the use of herbal extracts in tilapia culture is overwhelming. Moreover, exploration and implementation of herbs in tilapia culture and aquaculture in general has to be done in a sustainable manner, as adopting them in aquaculture may double the pressure already exerted by agricultural sectors and humans.
In conclusion, this review reveals that there are numerous medicinal plants with potential to enhance growth, several physiological parameters, control early maturity and prolific breeding in tilapia aquaculture. However, the lack of sufficient knowledge on herbal extracts limits the costeffective use of herbal extracts in tilapia aquaculture. Therefore, more research is required to further validate herbal extracts with their allied growth-promoting effects, immune-stimulating effects, sex reversal effect, toxicity effects, extraction methods, and extract concentration, among others.

Funding
The author received no direct funding for this research.