Introduction
Chinese herbal medicines have been used in China for thousands of years and are generally recognized as being natural and safe [
1]. These herbs contain many effective ingredients, including polysaccharides, alkaloids, flavonoids, volatile oils, organic acids, and tannins, as well as nutrients such as amino acids, carbohydrates, minerals, and vitamins [
2]. Studies have shown that these effective ingredients can increase appetite [
3,
4], promote metabolism, accelerate the synthesis of proteins [
5,
6], increase enzyme activity [
7], improve immunity [
8,
9], increase disease resistance [
10], and enhance meat quality [
11,
12] (Fig. 1).
Chinese herbal medicines have long been applied as attractants, growth promoters [
13], antibacterial agents, and immunoprophylactic agents in aquaculture [
14], and are considered to be effective alternatives to antibiotics, chemicals, vaccines, and other synthetic compounds [
15]. In recent years, many studies have been conducted on the further application of Chinese medicinal herbs because of their advantages of being natural, innocuous, easy to prepare, and inexpensive, and because they have few side effects for either the fish or the environment [
16]. Chinese herbal medicines can be administered as a whole plant, as parts of a plant (leaf, root, or seed), or as extract compounds used either alone or in combination with other feed additives; they can be applied in the feed or via the immersion of fish in treated water [
17]. However, the chemical constituents of Chinese herbal medicines are very complex and their biological activities are not always consistent. Additional investigations are needed to reveal the chemical structures and functional mechanisms of Chinese herbal medicines [
18,
19]. This study expounds on research progress in the extraction and purification of the effective ingredients of Chinese herbal medicine, introduces the chemical structures of some of the main effective ingredients in Chinese herbal medicines, and reviews the effects of Chinese herbal medicines on fish culture.
Studies on the chemical constituents of classical Chinese hebal medicines
The chemical constituents of Chinese herbal medicines are complex, diverse, and found in trace amounts, so it is very difficult to analyze and identify the effective ingredients in Chinese herbal medicines. More and more Chinese herbal medicines have been studied in detail and their key effective ingredients have been identified with the development of new analytical technologies (Table 1).
In 2001, ultrasound technology was used to extract alkaloids, flavonoids, anthraquinone, and polysaccharides from Chinese medicinal herbs [
20]. In 2004, supercritical fluid chromatography became an effective method for analyzing large molecular ingredients such as phospholipids, triglycerides, and carotenoids. Supercritical fluid technology has the advantage of high selectivity, which allows it to precisely analyze the effective ingredients in Chinese medicinal herbs when combined with technologies such as gas chromatography, high-performance liquid chromatography (HPLC), and gas chromatography/mass spectrometry [
21]. In 2005, the development of capillary electrophoresis technology, along with the development of other analytical technologies, provided a method to separate the trace effective ingredients in Chinese herbal medicines [
22].
In 2006, molecular distillation technology was applied to extract the volatile oils of
Atractylodes macrocephala,
Allium sativum,
Angelica sinensis, and
Forsythia suspensa [
23]. In 2008, membrane extraction technology, high-speed counter-current chromatography separation technology, and molecular imprinting separation technology were used for herb chemical analysis [
24]. In 2009, researchers found that cellulase could improve the extraction rate of polysaccharides [
25]. Fourier-transform ion cyclotron resonance mass spectrometry/sequential mass spectrometry technology combined with HPLC was later used to obtain accurate high-quality data. An accurate mass obtained by high-resolution mass spectrometry, comprised of multistage mass spectrometry fragmentation information and mass spectrometry, was used to analyze and identify the ingredients of
Epimedium brevicornu [
26]. In 2012, ultraviolet spectrophotometry was used to identify and quantify the effective ingredients in Chinese medicinal herbs, and the concentrations of various polysaccharides, alkaloids, flavonoids, and volatile oils were determined [
27]. In 2013, researchers used imidazole-based ionic liquid as the extractant to obtain quercetin and kaempferol from
Cedrela sinensis and
Rosae sinensis using a microwave extraction method [
28].
In 2014, three principal components and the corresponding principal component equations were obtained by principal component analysis from 12 Chinese medicinal herbs. The results showed that principal component analysis could be used for a comprehensive evaluation of the quality of Chinese herbal medicines [
29]. In 2015, magnetite (Fe
3O
4) magnetic nanoparticles were used as an intermediate carrier for the separation of active agents extracted from
Scutellaria baicalensis and
Glycyrrhiza uralensis, thus establishing a new method for the separation and purification of Chinese medicinal herbs [
30].
Effects of Chinese herbal medicines on growth and feed utilization in fish
Some Chinese medicinal herbs can improve the palatability of feed by stimulating the taste buds of cultured fish, and therefore promote feed intake. For example,
Syzygium aromaticum and
Pericarpium Citri Reticulatae can significantly promote the food intake of the common carp,
Cyprinus carpio, when added to a basic diet in a proportion of 0.2% [
31]. Goldfish,
Carassius auratus, are distinctly attracted by
Astragalus membranaceus,
Dioscorea opposita,
Crataegus pinnatifida,
Lycium chinense,
Pericarpium Citri Reticulatae, and
Cortex Phellodendri. Amur sturgeon,
Acipenser schrenckii, are attracted by feed containing berberine and kaempferol [
16]. In addition, 0.4% (w/w) water extractions of
Pericarpium Citri Reticulatae,
Crataegus pinnatifida,
Syzygium aromaticum,
Eucommia ulmoides, and
Schisandra chinensis play important roles in promoting feed consumption in koi,
Cyprinus carpio [
32].
Some researchers consider that the free amino acids, nucleotides, and alkaloids in preserved
Pericarpium Citri Reticulatae may be the cause of its attractiveness to fish [
14]. Most scholars believe that Chinese herbal medicines have a unique smell and taste (i.e.,
Cinnamomum tamala,
Angelica sinensis,
Crataegus pinnatifida,
Illicium verum Hook F, etc.), which play an important role in stimulating feed intake [
33]. In addition, Chinese herbal medicines can stimulate the secretion of digestive juices and promote intestinal motility, which may also explain the increased feed intake from their use [
13].
Studies show that Chinese herbal medicines can improve the growth of fish because of the effects of their bioactive substances that can promote metabolism, enhance the synthesis of protein, and activate digestive enzymes. A 10.4%–32.3% growth rate increase over the control group was found in
Sciaenops ocellatus as a result of feeding
Scutellaria baicalensis and
Poria cocos by 2% proportion in total in diet [
34]. Another study showed that the lignin, baicalin, polysaccharides, and berberine in some Chinese herbal medicines (i.e.,
Forsythia suspensa,
Coptis chinensis, and
Poria cocos) can act as growth-promoting agents in the Pacific white shrimp,
Litopenaeus vannamei [
35]. The growth rate of the ricefield eel,
Monopterus albus, was remarkably increased by adding
Poriacocos to the feed [
36].
Codonopsis pilosula and
Glycyrrhiza uralensis significantly improved growth and protein utilization by 33.6% in catfish,
Silurus asotus [
37]. A mixture of
Scutellaria baicalensis,
Astragalus membranaceus,Angelicasinensis,
Codonopsis pilosula,Lonicera japonica,
Acanthopanax senticosus, and dried
Pericarpium Citri Reticulatae in feed can significantly improve the growth of darkbarbel catfish,
Pelteobagrus vachelli, with 1% being the optimum inclusion rate found in the experimentally [
38]. Feeding grass carp,
Ctenopharyngodon idellus, a mixture of allicin and
Pericarpium Citri Reticulatae was also shown to result in a 31% faster growth rate at a dosage of 0.3% (w/w).
Eucommia ulmoides leaf may also improve the growth of grass carp [
39]. Another study showed that
Houttuynia cordata,
Lonicera japonica,
Glycyrrhiza uralensis, and
Poria cocos can improve the growth of grass carp—a finding that was attributed to an increase in lysozyme activity [
12].
Crataegus pinnatifida,
Eucommia ulmoides,
Cuminum cyminum, and
Cinnamomum cassia improve the growth of the ricefield eel,
Monopterus albus, because they improve the digestibility of nutrients by increasing the activity of protease by 61.33% [
7]. Improving intestinal function and promoting nutritional metabolism are important roles in Chinese herbal medicines [
40]. Supplementing the diet of tilapia,
Oreochromis mossambicus, with 1000–1500 mg·kg
−1 of
Astragalus membranaceus polysaccharide can improve the length of the intestinal villus, crypt depth, and thickness of muscular and quantity of intestinal mucous cells, as well asincreasing the number of intraepithelial lymphocytes [
41]. Adding dried
Pericarpium Citri Reticulatae,
Astragalus membranaceus, and
Angelica sinensis at 0.5% (w/w) to feed can promote intestinal villus growth and improve the liver cytoplasmic density of the Chinese perch,
Siniperca chuatsi [
42]. Adding 1 g·kg
−1 of
Eriobotrya japonica polysaccharide to the diet can significantly increase the intestinal villous length and muscle layer thickness of the digestive tract in goldfish [
5]. Adding a 2% proportion of
Crataegus pinnatifida to the feed of crucian carp was observed to increase the apparent digestibility of protein by 10.2% and the intestinal protease activity by 25.68% [
6].
Effect of Chinese herbal medicines on immunity and disease resistance in fish
Many Chinese medicinal herbs act as antibacterial and bactericidal agents, which can strengthen immunity and enhance the disease resistance of an animal [
43,
44]. Lysozyme, alkaline phosphatase, serum protein, and superoxide dismutase are important non-specific immunological indexes of fish; these indicators can be used to study the effects of Chinese herbal medicines on immunity. Effective ingredients in Chinese herbal medicines such as polysaccharides, saponins, alkaloids, anthracene, essential oils, and organic acids were found to dramatically improve these immunological indexes in fish. It was also found that Chinese herbal medicines can enhance the body’s immune response to antigens [
45,
46].
The spleen is a peripheral immune organ, and is the place where immune cells proliferate in response to an antigen. The development of the immune organs is closely related to the immune status of the body [
47]. For fish, the body surface mucosal layer is an important barrier that aids resistance to pathogens, thus not only protecting the body from being eroded by parasites, fungi, bacteria, and other harmful organisms, but also resulting in skin with a normal permeability [
48].
Astragalus membranaceus contains many effective ingredients that affect the humoral and cellular immunity of fish, and improve lymphokine-activated immunity and the activity of immunological regulatory factors [
49].
Codonopsis pilosula has tonic and anti-inflammatory effects that can moderate the stress response of the tilapia,
Oreochromis mossambicus, to their environment and strengthen their disease resistance [
50].
Angelica sinensis has beneficial effects in the regulation of blood rheology as well as providing anti-inflammatory, anti-tumor, and antibacterial immunity; oral administration of a dietary dosage of 50 mg·kg
−1 of
Angelica sinensis polysaccharide can increase blood leukocytes and leukocyte phagocytosis in the orange-spotted grouper,
Epinephelus coioides [
51].
Five species of Chinese medicinal herbs—
Forsythia suspensa,
Polyporus umbellatus,Scutellaria baicalensis,
Poria cocos, and
Coptis chinensis—were fed to the red drum,
Sciaenops ocellatus, and were found to increase the leucocyte phagocytic percentage and the activity of lysozyme by 5.89% and 138.7 IU·mL
−1, respectively, after 28 days of feeding [
52]. Chinese herbal medicine mixtures containing
Achyranthes bidentata,
Isatis indigotica,
Glycyrrhiza uralensis, dried
Pericarpium Citri Reticulatae,
Cinnamomum cassia,
Hordeum vulgare, and
MassaMedicata Fermentata were found to enhance lysozyme and antiproteinase activities and to increase the nitroblue tetrazolium (NBT)-positive cells of turbot,
Scophthalmus maximus, when 2% (w/w) herb powder was added to the basal feed [
53]. Yellow catfish,
Pelteobagrus fulvidraco, juveniles were fed herbal mixtures of
Scutellaria baicalensis,
Poria cocos,
Isatis tinctoria,
Astragalus membranaceus,
Rheum officinale,Glycyrrhiza uralensis, and
Lonicera acuminate, and significant increases in growth, phagocytic activity of leukocytes, and lysozyme activity were observed at a 1% (w/w) addition to the diet [
54]. Another study showed that a Chinese herbal medicine mixture containing
Ephedra sinica,
Sophora flavescens,
Scutellaria baicalensis, and
Rhus chinensis Mill. could effectively enhance the phagocytic activity of leukocytes and the lysozyme activity of the yellow catfish at a concentration of 400–800 mg·kg
−1 [
55]. The phagocytic activity of leukocytes, number of NBT-positive cells in the blood, and activity of superoxide dismutase, catalase, and lysozyme in the serum of tilapia
, Oreochromis mossambica, were dramatically improved when the fish were fed a mixture of
Astragalus membranaceus,
Codonopsis pilosula,
Atractylodes macrocephala,
Rheum palmatum,
Isatis tinctoria,
Scutellaria baicalensis, and
Chrysanthemum indicum [
51]. A mixture of
Acanthopanax senticosus, Lycium chinese,
Lonicera japonica,and
Astragalus membranaceus was fed to mirror carp,
Cyprinus carpio var.
specularis, juveniles, which increased the number of leukocytes, enhanced phagocytosis in the reticuloendothelial cells, and promoted the production of interferons, antibodies, and lymphocytes [
56]. In cultured perch, the activities of superoxide dismutase, lysozyme, and catalase in the serum, spleen, liver, and kidney increased significantly when the fish were fed a mixture of Chinese herbal medicines at 1% and 2% proportion in feed [
57]. In addition, some Chinese herbal medicines act to excite the adrenal cortical pituitary and induce hormone-like or gonadotropin-like effects in fish [
45].
Persicaria hydropiper,
Rheum palmatum, and
Diospyros ebenum have strong inhibitory effects on
Aeromonas hydrophila, which may cause hematosepsis on fish [
58].
Allium sativum and
Rheum palmatum have therapeutic effects on the skin diseases of bighead carp,
Aristichthys nobilis, and silver carp,
Hypophthalmichthys molitrix.Euphorbia humifusa and
Azadirachta indica have significant effects on gill disease in bighead carp and common carp, while
Polygonum orientale and
Acalypha australis have obvious effects on mouth disease in silver carp and bighead carp [
59]. Bathing fish with extract of
Zingiber officinale,
Capsicum annuum, and
Artemisia carvifolia at concentrations of 5 mg·L
−1, 2.5 mg·L
−1, and 1 mg·L
−1, respectively, had dramatic effects on
Ichthyophthirius disease; the insecticidal rate reached 100% in 24 h [
60]. Azadirachtin has a positive correlation with the killing effect of
Trichodina; an insecticide rate of more than 90% was obtained at a concentration of 0.8 mg·L
−1 [
10].
Sophora flavescens,
Melia azedarach,
Nicotiana tabacum, and
Stemona japonica were found to effectively prevent
Trichodina and other diseases [
46]. A concentration of 2.75 g·L
−1 of
Azadirachta indica and 5.67 g·L
−1 of
Sophora flavescens was found to be optimal for killing
Chilodonella cyprinid, with no insects surviving after 48 h [
61].
Effects of Chinese herbal medicines on meat quality of cultured fish
Chinese herbal medicines can also be used to improve meat quality through improvements in the taste or color of fish flesh. The flavor of fish flesh is affected by its chemical composition. Amino acids, unsaturated fatty acids, aldehydes, and phenolics in muscle are important components that affect the taste of fish.
Allium sativum,
Eucommia ulmoides, and
Hippophae rhamnoides are usually used as flavor agents; a total content of 60 mg·kg
−1 in the diet can increase crude protein and flavor amino acids percentages in muscle as compared with the control group [
62].
Feeding a 0.5% (w/w) mixture of
Poria cocos,
Paeonia lactiflora,
Herba Houttuyniae, and
Rheum offcinale has been shown to increase the crude protein and fatty acid content in the muscle of the Amur sturgeon,
Acipenser schrenckii [
11].
Astragalus membranaceus,
Angelica sinensis,
Flos lonicerae, and
Isatis indigotica have been shown to significantly increase the inosinic acid concentration in the muscle of the Nile tilapia,
Tilapia nilotica,while improving its flavor [
63]. A mixture of
Scutellaria baicalensis,
Poria cocos,
Isatis indigotica,
Astragalus membranaceus,
Rheum palmatum,
Glycyrrhiza uralensis,
Lonicera japonica, and
Eucommia ulmoides added to the feed of common carp in 1% (w/w) proportion was found to increase the concentration of flavor by producing amino acids such as glutamate and aspartate in muscle [
64].
Problems in the application of Chinese herbal medicines in aquaculture
At present, there are some problems with the wide application of Chinese herbal medicines in aquaculture. Firstly, because the effective ingredients in Chinese medicinal herbs are numerous, it is difficult to identify all the effective ingredients. The bioavailability of effective ingredients in plants is affected by the stage of growth and by the location where the herbal plants are grown; therefore, it is difficult to determine the accurate utility dosage of Chinese medicinal herbs. As a result, further research is required on how to control the quality of Chinese medicinal herbs [
65,
66]. Secondly, difficulties arise in applying Chinese herbal medicines to feed or water under commercial conditions. It is difficult to determine how to refine herbal products and cheaply extract the effective ingredients, especially when there are few effective ingredients present in Chinese medicinal herbs [
67]. Thirdly, very few studies have been conducted to determine the functional mechanisms of Chinese herbal medicines. Until recently, research on Chinese medicinal herbs has mainly focused on fish performance, leaving the mechanism of action unclear for most herbs. Furthermore, traditional Chinese medical theory has mainly been developed with respect to humans, and thus cannot explain the true mechanisms of herb activity on aquatic animals, because fish have different mechanisms for the metabolism and immunization of these products [
46].
In conclusion, existing studies have clarified the positive role of Chinese herbal medicines in growth, immunity, and disease resistance in aquaculture. Inadequate research on the functional mechanisms and on the differences in the quality of herbal plants causes uncertainty in their application. In the future, studies on Chinese herbal medicine should mainly focus on how to cheaply refine and extract the effective ingredients in classical Chinese medicinal herbs. Studies on the functional mechanisms of these effective ingredients should also be conducted.
2017 THE AUTHORS. Published by Elsevier LTD on behalf of the Chinese Academy of Engineering and Higher Education Press Limited Company. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).