Globally, plants are used in several domains such as health, food, and construction. In Gabon as in other regions of the world, some of these plants are widely exploited not only for their medicinal purposes, but also for fishing and hunting (Tag et al., 2005; Yumnam and Tripathi, 2013).

Relationships between humans and plants have existed for several decades (Chikamai et al., 2009). In terms of health, nearly 80% of populations depend on traditional medicine. Naturally, in Africa, the use of plants for  therapeutic   purposes   is   part   of   the  culture  and tradition for these populations (Koudokpon et al., 2017). Ghourri et al. (2012) provided a list of 131 medicinal species used to treat diseases that affect certain body systems (respiratory, urinary, genital, digestive), like cancer and diabetes. These medicinal properties of plants are also indicated in the Pocket Guide to Phytotherapy by Caroline Gayet (2013), revealing that plants could act on the nervous system and on other systems (digestive, respiratory, immune, hormonal and urinary). In Gabon, 117 plants used by pygmies in pharmacopoeia  and    traditional medicine   have  been recorded (Kwenzi et al., 2009). In general, literature reveals that there would be more than 200,000 plant species in the tropical countries of Africa with medicinal properties out of the 300,000 listed on the whole planet (Sofowora, 1993). In terms of number and medicinal properties, plants are invaluable resources for the pharmaceutical industry (Jiofack et al., 2010). In Congo, nearly 1500 traditional medicines are made from plants by the traditional healers in this country (Nkeoua and Boundzanga, 1999). Despite the satisfactory results of plant medication (Jiofack et al., 2010), it is nonetheless true that some of these plants are sometimes the cause of inexplicable poisonings observed in many individuals. This means that medicinal plants in addition to their therapeutic aspect can also be toxic. In Cameroon, for example, a study indicated that drugs derived from plants are not very toxic compared to synthetic drugs (Dibong et al., 2011).

Highly toxic plants such as Carica papaya and Pentaclethra macrophylla, used in fishing practices, have been sources of medicines in the traditional pharmacopoeia (N'Guessan et al., 2009; Djego et al., 2011). Carica papaya, a native to tropical America and grown in several other tropical regions, is used to treat diseases such as malaria and cancer (Nnanga Nga et al., 2016). Pentaclethra macrophylla is a medium-sized tree found along rivers and people use it in indigenous medicine as an anti-dysenteric in enema, and as an antipsoric in frictions (Nnamani et al., 2020). It seems these two ichthyotoxic plants, like other plants used in traditional fishing (Tephrosia Vogelii, Erythrophleum ivorense, Tetrapleura tetraptera, etc.), are medicinal and do not affect human organisms, warm-blooded animals (Amakoe, 2011), but are very toxic to cold-blooded animals.

It should be noted that the permanent ingestion of fish obtained from these plants could lead to serious long-term poisoning in humans (Kimpouni et al., 2011). It has been reported that the ingestion of a small quantities of the prepared parts of some of these plants (seeds, leaves, roots, fruits or sap) would cause lesions (internal or external), to humans and animals (Kabemba, 2015).

Literature provides a considerable list of plants involved in traditional fishing (Kimpouni et al., 2011). However, experiments conducted to clarify the ethnobotany and ethno-pharmacology of these plants in Gabon are rare. A recent study carried out in three provinces of Gabon (Haut-Ogooué, Ogooué lolo and Nyanga) reported some 15 plants frequently used in fishing by the populations of these different localities (Mouele et al., 2021), and that could probably serve as phytomedicines. Little is known on the use of these plants in pharmacopoeia and traditional medicine.

Historically, fishing from plants is an ancient practice. Bishop et al. (1982) and Heizer (1953) listed some of these plants in several regions globally. Furthermore, Kerharo et al. (1960) retraced the history  of  these plants and other authors described fishing sessions using fish-toxic plants. Plant species belonging to families such as Annonaceae, Papilionaceae, Rubiaceae, Myrsinaceae, Mimosaceae, Gnetaceae and Passifloraceae have been cited for this purpose (Kimpouni et al., 2011; Moyon and Arunkumar, 2018). Likewise, species of the Fabaceae family are known to be widely used as fish poisoning agents. This group of plants are used in the form of raw pastes, and are said to kill fish in streams and ponds very quickly (Tag et al., 2015). In Africa, there are more than 325 species of poisonous plants which are commonly used to capture fish (Neuwinger, 2004). In Gabon, knowledge on the plants used both for fishing, pharmacopoeia and traditional medicine is in the hands of very few individuals, and is thought to be disappearing disappearing. It is noticed from literature that the transmission of this knowledge is currently in danger since it is not always assured. No specific research has been conducted on this subject regarding ichthyotoxic plants even though there is a need to preserve and enhance this knowledge in Gabonese populations. Consequently, ethnobotanical and ethno-pharmacological research on ichthyotoxic plants is essential to safeguard the knowledge acquired by the indigenous populations of the various localities of Gabon.

The aim of the study is to conduct an inventory on the ichthyotoxic plants used in the pharmacopoeia and traditional medicine to collect as much information as possible on their parts used, the methods of preparation and the diseases they treat. This will contribute not only to the assessment of the potential risks for the health of the individuals who exploit these different plants, but also for a better understanding of the toxic and medicinal plants of Gabon.

Study localities

This study was conducted in several localities of the seven provinces (Haut-Ogooué, Nyanga, Ngounié, Ogooué-Ivindo, Ogooué-Lolo, Moyen-Ogooué and Woleu-Ntem) of Gabon (Figure 1). Indeed, Gabon extends over a total surface area of ??267,667 km². It is a country bordered to the North-West by Equatorial Guinea, to the North by Cameroon, to the North-East and to the South by Congo and to the West by the Atlantic Ocean (Figure 1) (Nzigou et al., 2021). The country has an equatorial climate-type, characterized by four seasons: two rainy seasons (from mid-September to mid-January and from mid-February to mid- June) and two distinct dry seasons (from mid-June to mid-September, and from mid-January to mid-February) (Tamura and Akomo-Okoue, 2021).

Gabon has a maritime region of 800 km with coasts, lagoon environments, estuaries, and deltas which are concentrated along the coast (Ekouala, 2013). Fishing from fish-toxic plants is non-existent in these environments, because the populations of the coast rather use other types of methods to fish such as the net which is adapted for this kind of marshy areas. However, the rest of the country is covered with forests, that is, 88% of the national territory (Poulsen et al., 2021). Fishing from poisonous plants exists in this part of the country. The presence of many rivers and the seasonal variability are suitable for fishing in these forest environments. Our survey only focused on forest areas.

Ethnobotanical and ethno-pharmacological data collection

The data were collected in the period from January to May 2021. The ethnobotanical and pharmacological study focused on interviews of men and women whose ages were between 28 and 68. There were 48 women and 52 men. The method used was the administration of questionnaire to collect information from the target populations. The main language of communication was French. For every study location, people with knowledge on the use of plants for fishing were the first to be interviewed. After collecting all the necessary information and harvesting the corresponding plant sample, traditional healers were then consulted. In total, 100 people chosen at random were questioned about ichthyotoxic plants and their importance in traditional medicine. For this, 37 people agreed to give information on poisonous plants and 63 traditional healers who agreed to talk on the importance of each plant species in traditional medicine. In both cases, the parts of the plants investigated, the protocol used and the type of diseases they treat were noted (Table 1). The determination of the scientific names was carried out at the herbarium of the Department of Biology of the University of Science and Technology of Masuku. The frequency of citation (FC) of each of the species collected in this study was calculated according to the formula by Orsot (2016) and Yolidje et al. (2020):

Where:

n: the number of people who mentioned the species,

N: the total number of people questioned.

To compare the toxic effects of each plant, statistical tests (Wilcoxon test) were applied using the R statistical software of version R.3.4.3. In case of multiple comparisons, the P-values were adjusted by the Holm's method.

Ethnobotanical characteristics of fish poisonous plants

From the ethnobotanical study, women (70%) knew more about the plants used for fishing than men (30%) (Figure 2A). The results suggest that the use of ichthyotoxic plants is a common practice in the age groups ranging from 28 to 68 years old with a predominance among individuals between 58 and 68 years (37%) followed by those from 48 to 58 years (28%), 38 to 48 years (22%) and 28 to 38 years (13%) (Figure 2B).

The present study identified 701 plants belonging to 16 botanical families, and the majority in terms of number of species were Fabaceae (41%), Rubiaceae (13%), Rutaceae (15%). Species such as T. vogelii, Fillaeopsis discophora, Cylicodiscus gabunensis, Brenania brieyi and  Zanthoxylum  gilletii  were cited several times by the respondents (Figure 3). The majority of plants identified were trees (60%), followed by shrubs (18%) and small proportions of lianas (12%) and grasses (10%) (Figure 4A). The parts of the plants used during fishing were barks, leaves, fruits, sap and even the whole plant in the case of certain lianas and herbs. It was noticed that the bark (53%) is the most used plant part followed by the leaves (30%). A small proportion of fruits (11%), saps (3%) and whole plants (3%) were recorded (Figure 4B).

In addition, comparing the toxic effects of each plant part revealed that for all the trees studied, the fruits took less time to intoxicate fish than the bark (Wilcoxon test 1, W = 100, Padj = 0, 009567) (Figure 5A). In the case of shrubs, the leaves took less time than the saps (Wilcoxon test 3, W = 89.5, Padj =  0.00302) (Figure  5B).  However,  no significant difference was obtained between the leaves and fruits of the shrubs (Wilcoxon test 1, W = 62, Padj = 0.3834) (Figure 5B). Similarly, the time taken to kill the fish did not differ significantly between the use of sap and fruit (Wilcoxon test 2, W = 25.5, Padj = 0.06566) (Figure 5B).

Ethnobotanical results showed that most plants are used in association, and the different modes of their preparation were crushing, crumpling, and cutting. Grinding (74%) was the most used technique, even though some people conducted crumpling (15%) and cutting (11%) (Figure 6).

Ethno-pharmacological characteristics of ichthyotoxic plants

For  the  use  of  ichthyotoxic  plants  in  traditional medicine, the results obtained showed high usage by men (65%) than women (35%) (Figure 7A). It was also found that several methods were used for drug preparation to facilitate their administration. Decoction, maceration, powder, grating, roasting and expression were the main methods of drug preparations used in the study area. However, decoction (43%) was the most used preparation approach (Figure 7B).

In general, this ethno-pharmacological study has made it possible to distinguish 14 recipes based on medicinal plants which are likely to treat several diseases. The ichthyotoxic species considered as herbal medicines and the frequency of their usage, the different recipes, their mode of preparation and administration are recorded in Table 1. Figures 8, 9 and 10 show three  fish  poisonous  plants  collected during this study.

The ethnobotanical studies conducted in several localities in Gabon made it possible to identify several morphological types of plants involved in fishing practices, that could be associated to the group of into rivers to intoxicate the fish, numb it, and then capture it very easily. It is a known fishing practice in almost all parts of the world (Kimpouni et al., 2011). It has been reported in other African countries such as Republic Democratic of Congo (Koto-te-Nyiwa et al., 2018) and Nigeria (Akpa, 2010). In addition, it is important to recall that the drunken behavior of fish is due to the phytochemicals of these ichthyotoxic plants. The chemical composition of many ichthyotoxic plants has already been the subject of several studies, and the capture it very easily. It is a known fishing practice in almost all parts of the world (Kimpouni et al., 2011). It has been reported in other African countries such as Republic Democratic  of  Congo  (Koto-te-Nyiwa  et  al., 2018)  and Nigeria (Akpa, 2010). In addition, it is important to recall that the drunken behavior of fish is due to the phytochemicals of these ichthyotoxic plants. The chemical composition of many ichthyotoxic plants has already been the subject of several studies, and the main active compounds identified were rotenone, saponins and tephrosin (Mouele et al., 2021). Recently, (Mouele et al., 2021) reported in a phytochemical study the presence of certain secondary metabolites which could be strongly involved in the plant poison. On the other hand, the difference observed could be linked to the high experience of men in the field of indigenous medicine. In Gabon, women are more specialized in the field of artisanal fishing, while men are destined for hunting.

Our results are similar to those of other authors from Cameroon (Ngoule et al., 2015) and Ivory Coast (N'guessan et al., 2011), with 60.41 and 87% of men reported to have a high knowledge on the use and properties of medicinal plants. It appears that knowledge of the use of plants is acquired through long experience (Ghourri et al., 2012). Our study suggests that men whose ages varied between 58 and 68 years had high knowledge  on  ichthyotoxic  plants  compared  to  all  the other age groups of the study population. This agrees with the results of the study of Ghourri et al. (2012) which revealed that older people had high knowledge on medicinal plants than their younger counterparts. Similarly, Gbékley et al. (2015) indicated that the practice of traditional medicine was the prerogative of middle-aged men.

This ethnobotanical and ethno-pharmacological study shows that Gabon has a great floristic diversity of interest in terms of medicinal plants for the treatment of several diseases. Some plants whose proportions are shown in Figure 3 have been studied severally (Kwenzi et al., 2009; Kimpouni et  al.,  2011).  Some  diseases  such  as hemorrhoids, malaria, sinusitis, chickenpox and many more have been cited, and the plants used to treat them are documented. In addition to these pathologies, the « night gunshot » has been identified as a socio-cultural disease. This condition of witchcraft origin (disease due to bewitchment) could sometimes come up against the treatment options of modern medicine (Missounga et al., 2018).

The present study showed that barks (53%) and leaves (30%) were the most frequently used parts of plants and the decoction preparation (43%) was the mode of preparation often practiced by traditional healers. These results are consistent with  those  of Gnagne et al. (2017) and Kpabi et al. (2020) who indicated that decoction is the galenic form mostly used by traditional healers in the Department of Zouénoula in Ivory Coast and the prefecture of Doufelgou in Northern Togo. Moreover, the study of Fah et al. (2013) already indicated that plant medicines were essentially prepared by  decoction.  Most of the plants were used both for fishing and in traditional medicine. For instance, the fruits of Raphia sp and the leaves of Manihot esculenta were combined to increase toxicity, and the leaves of Carica papaya and T. vogelii were combined to asphyxiate fish faster (Mouele et al., 2021).

In view of the frequency of citations from respondents, it appears that T. vogelii (61.90%) and T. tetraptera (30.15%) seem to be the well-known ichthyotoxic plant species in the populations of the different localities studied. Data from literature show that these are plant species frequently used in fishing and in traditional medicine in several countries around the world (Dougnon et al., 2015; Mouele et al., 2021).

The present study demonstrates that not all plant parts have the same degree of toxicity. Indeed, from the results of this study, fruits seemed to be more effective than the peels when used as fish poison. However, it is known that the bark is the site for the production and even the storage of secondary metabolites responsible for the biological properties of the plant (Vandi et al., 2016). Therefore, the barks of plants must have had a much more intense biological activity than the fruits. On the other hand, a study revealed that fruits were widely requested by people for medicinal preparation (Sereme et al., 2008).  

The present ethnobotanical and ethno - pharmacological study carried out in several localities of the seven provinces of Gabon revealed 16 botanical families involved in both fishing and traditional medicine. These are a few trees, shrubs, lianas and herbaceous plants found in most forests and savannahs of Gabon. The different parts of these plants can be used in the form of ashes or powder, or in decoction, and even in maceration to cure several diseases. Of all these diseases, we cite the ‘night gunshot’ of witchcraft origin, which modern medicine sometimes comes up against. Other diseases such as hemorrhoids, malaria, sinusitis and chickenpox have been reported. Overall, this study made it possible to collect the necessary data that could allow for the assessment of the risks of intoxication that people who consume fish caught from ichthyotoxic plants could be exposed to. Also, this is a pilot study which could permit the understanding of the Gabonese medicinal flora, and which could probably serve as a database for subsequent research in pharmacology.

The authors have not declared any conflict of interests.