Diversification of Aquaculture in the Sub-Saharan Region—The Obscure Snakehead

Abstract

The sub-Saharan region shows fast growth in aquaculture, but current production is dominated by two species, the African catfish and tilapia. In order to support the expansion of the industry and ensure food resilience of the sector, diversification is desirable. Indeed, several candidates have been identified, among them the African snakehead Parachanna obscura. In contrast to the fast expansion of Asian snakehead farming, African aquaculture does not anticipate this trend. Still, looking at Asia, recent scientific literature provides impulses and solutions for the most pressing problems such as dry feed, cannibalism of juvenile stages and reproduction. In this review, we illustrate future research needs, integrating the recent progress in snakehead farming. Based on the recent progress in commercial diets in Clarias and protocols established for the reproduction of Channa species, an expansion of African snakehead farming seems feasible in the near future.

1. Introduction

Over the last two decades, aquaculture in the sub-Saharan region has grown 11% annually, nearly twice as fast as the global average [1]. This development is dominated by two species, the African catfish and the Nile tilapia. Despite this noticeable growth, the sub-Saharan region only accounts for 1% of the global aquaculture production [2]. A projected increase in demand for fish and seafood requires an increase in production of 5 mio t by 2030 and over 10 mio t by 2050 [3]. Indeed, it has been reported that the sub-Saharan region has many underutilized reservoirs and water bodies for pond and cage-based aquaculture [1]. From the perspective of disease management as well as the resilience of the sector, a focus on two species only (catfish and tilapia) is problematic, and further diversification should be envisioned to advocate food security. Pullin [4] stated that the best strategy for diversification would be focusing on the species already farmed in small quantities.

Among the candidates for diversification is the obscure snakehead Parachanna obscura [5]. Within the genus Parachanna, there are three species distributed in African rivers and lakes. P. obscura is the most widely spread, found in Lake Chad; the Nile; and the Congo, Zaire, Cross and Senegal basins [6,7,8]. In contrast, the genus Channa is distributed to southern and eastern Asia, with more than 40 species. In general, snakehead farming in Asia is far more advanced, and technology transfer may facilitate the expansion of snakehead farming in Africa. Therefore, in this review, we have integrated information derived from Channa species if appropriate.

Being an obligate air-breather [9,10,11], P. obscura is less vulnerable towards hypoxia. In general, it is a hardy, robust species that supports stressful conditions [12], living in swamps, creeks, rivers, lakes, lagoons, marshes, floodplains and rivers. In Channa striatus, for example, embryos—as the most sensitive developmental stage—tolerate a temperature range from 20 °C to 32 °C [13]. Also, snakeheads have been exposed to salinity increases of up to 15 ppm, and it seems that they are in fact euryhaline fishes that can survive some degree of marine flooding [14].

Obscure snakehead has a high commercial value due to its tasty flesh and is well accepted by local consumers [15]. Its fast growth and high prices represent great aquaculture potential for intensification [12,16,17]. Currently, aquaculture production is minor (<3000 t, Figure 1) and, together with capture fisheries, cannot meet the actual demand [15]. Indeed, there have been reports on drastically declining wild stocks recently [18,19]. Today, most fish are extensively farmed in earthen ponds since the fish prefers calm and muddy areas and often remain motionless on the ground as an ambush predator [8]. Snakeheads have also been farmed in rice field polyculture [20] and aquaponics [21]. In polyculture with tilapia, snakeheads are often used for controlling overpopulation [22,23,24]. Therefore, some system diversification is promising for the future (Figure 2). Indeed, recirculation aquaculture systems including aquaponics support much higher production densities and, at the same time, allow for better control over the rearing conditions, reducing stress and disease susceptibility. Also, such closed systems provide better control over pathogens. Aquaponics systems allow for the double use of water since water from the fish unit is directed to plant crops, thereby providing the nutrients needed for plant growth and reducing nutrient emissions to adjacent water bodies.

In view of stagnating/decreasing fishery landings, an increase in aquaculture production may reduce the pressure on wild stocks. Also, it will help in the diversification of the aquaculture industry, thereby providing new growth incentives and improving the overall resilience of the sector. In this context, the current knowledge on the species P. obscura is presented here. In addition, problems and solutions encountered in Asia during the intensification of snakehead farming are highlighted if relevant for the expansion of obscure snakeheads.

2. Growth Performance

P. obscura has an excellent growth performance and may reach a maximum length of over 50 cm and over 1 kg in the wild [25,26]. In polyculture with tilapia, such sizes have been achieved within 4–5 months. Therefore, obscure snakehead grows much faster than tilapia and at least comparable to catfish [16]. The viscero-somatic index is slightly higher in snakeheads than in Clarias [27].

3. Nutrition

Although considered an omnivore by some authors [28,29], P. obscura is rather a carnivore with a well-differentiated digestive tract [30,31]. Also, enzymes of carbohydrate digestion have been identified in P. obscura and Parachanna africans [32,33]. Still, high carbohydrate concentrations in the feed may impact functionality of the digestive tract and the liver [34]. Most interestingly, cellulase has been detected in the pyloric caeca in P. obscura but not in P. africans [33]. Nevertheless, Li et al. have shown that increasing starch levels did not increase growth performance in C. argus but increased survival and immunity at a recommended dose of 9% [35]. In the wild, the species feeds on insects, frog and fish eggs, fish, crustaceans and plant remains [28,29]. Feeding habits are continuously changing with increasing body size from invertebrates (insect and crustaceans) to vertebrates (frogs and fishes). Due to its large mouth opening, snakehead can easily consume a fish half its size [36].

Gonella [37] stated that snakehead as a top predator can only be farmed with fresh feed rather than pellets. Although, indeed, snakehead have been farmed with trash fish, particularly in Africa, China and Vietnam, most farming, at least in Asia, is actually based on pellets nowadays (pers. commun.). Typically, fish are fed pelleted diets at high protein contents (42.5–45%) [30,38]. In larvae, higher protein concentrations (55%) revealed the best growth [39]. Here, alternative protein sources such as earthworm, black soldier fly, cricket or chicken viscera meal have been used successfully [40,41,42]. Indeed, soy bean meal may replace 30% of fish meal as the main protein source, as shown in C. striata [43]. Higher protein is also recommended for broodstock females, at least in bloched snakehead C. maculata [44]. Therefore, snakehead seems to have very similar requirements to Clarias and could profit from commercial diets currently established in catfish farming.

With regard to lipid content, the best growth performance was observed at 7% (tested between 5 and 14%) [45,46], congruent with findings in C. striata [47]. In other studies, higher lipid contents were used, but it has been recommended to keep lipid content below 19% to avoid growth depression [48]. Higher fat contents of up to 18% may help increase reproductive performance [49]. Interestingly, in a feeding experiment on different fat ingredients (palm oil, distilled palm oil and fish oil control), comparable growth of plant-derived ingredients was observed [46]. At the same time, no major differences in essential fatty acids (DHA and, to some extent, EPA) were detected, suggesting that snakehead may be able to compensate deficiency by endogenous synthesis [50]. Therefore, the endogenous synthesis of essential fatty acids needs to be studied in P. obscura in the future. Typically, a gross energy of approximately 18–19 kJ/g is recommended [30]. The feed conversion ratio (FCR) is relatively good and has been reported at 0.74–1 in 10 g of fish [30]. Differences in FCR and SGR between live feed and pelleted feed identify the need for high-quality feed, which may be accomplished through commercial diets in Clarias farming since both fish have similar requirements [51]. Indeed, the high costs of fresh fish made the monoculture of obscure snakehead unacceptable in the past [22]. Bad feed conversion and growth performance reported on dry feed emphasize the need for quality diets in snakehead farming [22].

Importantly, obscure snakeheads can be weaned on dry food as early as at 0.9 g [30]. As in other species, weaning on dry feed should be realized as early as possible to avoid rejection of the feed. In C. striata, weaning is most effective 17–25 days after hatching, replacing 10% of live feed with formulated feed every 3 days [52,53]. In other Channa species, up to 40 d post hatch has been recommended [52]. Using formulated feed, despite reduced growth rates, costs were reduced by approximately 30% [54]. An economic analysis in Vietnam and Cambodia reported that formulated diets were too expensive for low- and medium-productivity farms [55]. The availability of affordable feed formulated preferably from agricultural by-products is a major obstacle for the expansion of aquaculture in Africa [51]. In Asia, food-born aflatoxin is a major concern for the industry, and it has been suggested to supplement feed with alpha-lipoic acid to ameliorate toxicity [56,57].

4. Reproduction

In the wild, obscure snakeheads spawn over the entire year [58], although some studies reported a main breeding season around seasonal flooding. The average fecundity of females is relatively low, ranging between 1700 and 4000 eggs [58], which is low compared to Asian species such as C. striata with 3000–12,000 eggs or C. punctatus with 11,000–28,000 eggs [59]. Fecundity may even decrease in captivity [17]. The gonadosomatic index in females ranges between 1 and 2.8% and is <<1% in males [58]. The eggs are relatively small, at 0.88 to 1.11 mm. The presence of multiple ovarian stages within a population indicates spawning readiness throughout the year. This may allow year-round or at least multiple spawning. The development of the gonad is comparable to that of other teleosts. The developmental stages of gonad development have been described in C. punctatus [60] and C. aurantimaculata [61]. A major bottleneck in seed production in snakeheads is the control of maturation in captivity and a relatively narrow spawning window [62]. Macrophytes may stimulate faster maturation (20–30% water spread) [62].

There is a sexual dimorphism at first maturity, with males reaching first maturity at approximately 23.5 cm and females at approximately 17.5 cm [63], but sex-specific growth has been questioned by some authors [28]. It has been reported that fish can be sexed by the anatomy of the genital papilla, though difficult to recognize [63]. More studies are needed to clarify the issues relating to differences in growth performance between sexes and to also confirm the use of all-male stocks. In this context, C. maculata has been shown to be susceptible towards hormone-induced sex reversal using methyltestosterone [64]. All-male stocks (Channa argus, female x Channa maculata, male) were produced by hybridization of YY super-male C. maculata and normal XX female C. argus [65]. These all-male stocks revealed improved growth performance. In addition, in P. obscura, the condition factor of males has been reported to be twice as high as in females [63]. All this suggests the development of all-male stocks for aquaculture.

Presently, farming depends mainly on very limited natural reproduction for seed production. In contrast, hormone therapy is widely used in Asian farming. Indeed, hormone-based, controlled reproduction has not been explored so far in P. obscura but has been achieved in C. striata and hybrid snakeheads [66]. Suitable (500–800 g) spawners are collected three to four months prior to the peak breeding season and transferred to concrete tanks with floating macro vegetation (1 kg/m³, sex ratio 1:1, 26–30 °C, 7.5–8.0 pH, 4–7 mg/L dissolved oxygen). In some cases, implants have been used to mature spawners more effectively and synchronously [66]. Using carp pituitary extract or LHRHa (Glp-His-Trp-Ser-Tyr-Ser-tBu-Leu-Arg-Pro-NHEt) at 80 mg/kg and 0.8 µg/kg, it was possible to induce final maturation and spawn C. striata [67]. Still, this dose is rather high, and pituitary extracts are often injected at 30–35 mg/kg. For males, dosage is usually 75% of that for females [66]. Latency is usually 12–24 h.

Eggs are transferred to a hatching tank to avoid cannibalism. They are incubated at mild flow influx. To control cannibalism of the fish, fry have to be graded regularly and fed ad libitum [68].

5. Health

Knowledge of diseases and disease outbreaks in P. obscura is limited, mostly because aquaculture is still in its infancy. There are a couple of reports on intestinal parasites such as trematodes, cestodes and nematodes [69,70,71]. Nevertheless, potential pathogens that are ubiquitously distributed or may present a serious concern will be discussed with reference to Channa spp. Also, recent findings in disease therapy and prevention will be presented.

Recently, lactic acid bacteria (LAB) have been isolated from Channa striatus and subsequently used as probiotics, inhibiting Aeromonas hydrophilia in vitro and in a challenge test [72]. Similarly, LAB isolated from C. argus revealed the inhibition of A. veronii and higher survival [73]. The application of probiotics may therefore represent a promising approach against bacterial diseases that caused major losses in C. micropeltes [74]. In addition, vaccination against A. hydrophilia recently increased survival from 27% to 88% in the vaccination group [75]. Edwardsiella tarda was reported as the causative agent of mass mortalities in Channa spp, which lead to tremendous economic losses in snakehead hybrid farming [76].

Epizootic ulcerative syndrome (EUS) caused by the oomycete fungus Aphanomyces invadans resulted in substantial losses in C. argus farming [77]. A. invadans is an obligate pathogen affecting wild and farmed freshwater fishes alike since it was first reported in 1971 [78]. It occurs seasonally and may be worse in some years. It causes severe lesions and deep dark ulcers. EUS has been reported in P. obscura [78], but massive mortalities are so far not documented. Nevertheless, it is very probable that EUS is a threat to snakehead culture in Africa. Several antimicrobial agents have been applied successfully: Salar-bec, a vitamin premix, and Ergosan, an alginate that inhibits germination and subsequent growth of cysts effectively [77]. Often, EUS has been linked to poor water quality, and liming of water is often effective [79]. In contrast to P. obscura, tilapia is resistant to EUS. Routine disinfection of eggs should be used in critical areas to prevent transmission. Ensuring no leakage of water from EUS-infected areas into fish ponds is a normal practice that easily prevents the spread of EUS into farms.

Snakehead vesiculovirus (SHVV) is among the most harmful viral infections to the aquaculture industry in China, but promising results have been reported on vaccination [80]. Recently, SHVV infection of Mandarin fish has been reported [81], suggesting that SHVV is able to infect even distantly related species. Channa species present a major risk of disease transmission. Consequently, imports of Asian snakeheads to Africa need to be avoided to prevent the introduction of diseases.

6. Water Parameters

P. obscura is a potamodromous species, primarily inhabiting diverse freshwater ecosystems such as creeks within swamps, ponds, streams, rivers, lakes, marshes and floodplains [82]. The obscure snakehead is a strictly tropical fish species that thrives within a water temperature range of 24 to 30 °C. Overall, a pH of 5.5–8.0 is suitable for rearing, as reported in the larvae and fry of C. lucius [83]. Also, snakehead species are relatively tolerant against high ammonia and nitrite concentrations [84,85,86]. In aquaculture, the recommended limits for key water quality parameters are set at 0.002 mg/L for NH₃, 1.0 mg/L for NH₄+, 0.01 mg/L for NO₂− and 0.1 mg/L for NO₃− [12]. However, P. obscura has shown adaptability to water conditions that fall outside of these permissible ranges. High-temperature tolerance and independence from dissolved oxygen due to air breathing turn snakehead less vulnerable towards climate change scenarios than, for example, tilapia.

7. Fisheries

The total recorded fishery landings of the African snakehead in its range countries are insufficiently documented. Nonetheless, FishStat [2] data highlight that captures are reported from four West African nations. Among them, Nigeria (annual capture between 8377–8798 t) stands out as the leading producer, followed by Gabon (annual capture between 1340 and 1415 t), Benin (annual capture between 753 and 845 t), and lastly Côte d’Ivoire (annual capture production of approximately 19 t). In these countries, P. obscura is a key species in inland fisheries, predominantly caught in reservoirs and rivers.

In Côte d’Ivoire, for example, P. obscura is one of the main fish species harvested from major reservoirs such as Buyo [87], Ayamé [70,88], Taabo [89] and Kosso. It is also an important catch in principal rivers, which collectively form the backbone of the country’s continental fisheries production. Furthermore, P. obscura has been documented as being among species captured in the Niger River, the largest river basin in West Africa [90,91], which flows through several countries, including Cameroon, Nigeria, Benin, Niger, Mali and Guinea.

A recent market analysis reveals that P. obscura holds significant economic value, with a price reaching up to EUR 3 per kilogram, underscoring its importance and high demand among local communities [18]. That study highlighted the motivation of the local communities for expanding snakehead farming.

8. Processing

There are many processes involved in food spoilage, from rigomortis to chemical variations. The deterioration of n-3 fatty acids reduces shelf-life and makes fish more susceptible than other meat sources. Traditionally, snakehead was either sold fresh, dried or salted. Still, there is an increasing trend observed in the marketing of frozen snakehead. Here, major investments in chilling facilities are needed. In Asia, the northern snakehead C. argus is sold as fish soup, which is rich in lysine, proteins and the trace element Zn [92]. Also, snakehead provides an excellent surimi.

Feeding ascorbic acid, green tea leaves, guava or sodium citrate increases the shelf-life of chilled fish without impacting their sensory quality [93,94]. Such strategies may become interesting since traditional methods of preservation such as smoking are continuously replaced by freezing [94]. Exotically, aerosols are promoted for accelerated wound healing [95,96]. Similarly, skin mucus extract has been used due to its antimicrobial effect for the preservation of fish fillets [97].

9. Further Tasks

Hybrids have been produced and, in Asia, hybrid snakehead (C. argus x C. maculata) is the most frequently farmed snakehead in the market due to its higher survival, better stress resistance and superior growth performance [98]. Nevertheless, no Asian species should be imported to Africa to avoid infectious risks, as outlined earlier. If biosecurity is assured, focusing on African species, hybrids are an option for aquaculture as well. It is however important to emphasize that snakehead in general are highly invasive species [99,100,101]. A focus on farming P. obscura will reduce the actual risk.

10. A Global Perspective on Snakehead Farming

Over the recent decade, snakehead farming emerged in eight regions in China, namely Shandong, Jiangxi, Guandong, Zhejiang, Hubei, Anhui, Jiangsu and Hunan. Here, the production focus is on two hybrids, C. argus x C. maculata and C. maculata x C. argus [102]. Both hybrids exhibit a significant heterosis effect and grow better. In southern China, the promotion of extruded feeds substantially reduced the deterioration in water quality and improved sustainability of farming [102]. With 553,196 t, China is the biggest producer worldwide (Figure 1).

In Indonesia, C. striata is the most popular species among the ten endemic snakehead species [103]. It is mainly farmed in Kalimantan, Sumatra and Java. Both countries, Indonesia and particularly China, represent a nucleus to ongoing expansions to Thailand, Malaysia and Myanmar [74,104].

In Africa, due to a lack of experience, snakehead farming is still in its infancy. The production focus, though small, is clearly on Nigeria. Due to its popularity over its natural distribution, there is high potential for snakehead farming in Western Africa.

11. Conclusions

Snakehead P. obscura is a promising candidate for the expansion of aquaculture in the sub-Saharan region, well accepted by the local markets. Nevertheless, there are some obstacles remaining. Most importantly, regular grading and quality feed are necessary to discourage cannibalism. Quality feed currently developed for Nigerian catfish farming may be used in snakeheads due to very similar nutritional requirements. As seen in snakehead farming in Asia, reproduction can be controlled by hormone treatment, but pioneering work is required to establish species-specific protocols in P. obscura. As in most species, diseases represent a risk that needs to be controlled by proper hygiene management. The import of Asian snakeheads should be avoided to prevent the introduction of pathogens, in particular viruses.