Perspective Chapter: Species Diversity and Distribution of Catfishes and Their Current Contribution to Global Food Security

1.1 Diversity of Catfishes

Catfishes are a diverse group of ray-finned fish named for their prominent barbells, which resemble cat whiskers. Catfishes belong to the Phylum Chordata, class Actinopterygii, and of Order siluriforms. The group has about 40 families distributed around the globe. Despite their name, not all catfishes have prominent barbells, some members of the order siluriforms are defined by features of the skull and the swim bladder. They are most diverse in tropical South America, Africa, and Asia, but more than half of all catfish species are to be found in America. Catfishes are important biodiversity resources, playing important ecological roles in food chains in aquatic ecosystems. Similarly, they form important food fish for local communities and contribute to income generation and livelihoods (Table 1). According to [2] the most cultured among global catfish populations are the Amur catfish (Silurus arsotus), Channel catfish, Ictalurus punctatus), Stripped catfish, Pangasianodon hypophthalmus, and the African catfish (Clarias gariepinus). Africa has ten siluriform families [2, 3, 4], which include: Amphiliidae, Ariidae, Austroglanididae, Bagridae, Clariidae, Claroteidae, Malapteruridae, Mochokidae, Plotosidae, and Schilbeidae. Catfishes group into 32 families, distributed globally, with currently 3,407 valid species [5]. Like the spectacular haplochromine cichlids of the Great lakes of Africa, some groups form species flocks in the habitats where they occur. These include Bathyclarias, endemic in Lake Malawi [6], where they comprise up to 12 species [6].

According to [7], catfishes are distinguished by pairs of barbels, cylindrical body without scales, largemouth, pectoral, and dorsal fins that bear spines. For the clariid family, the key distinguishing feature is the suprabranchial organ formed by arborescent structures from the second and fourth-gill arches [8, 9, 10] and flat dorso-ventrally flattened skulls. Clariidae is naturally distributed in Asia, Syria, and Africa. In Africa, this family has 13 genera and 74 species. Amongst the species include the Heterobranchus species, Dinotopterus, Bathyclarias, Clarias gariepinus, Clarias anguillaris, and other nominal species of Clarias in Africa. The clariids are mostly found in the muddy bottoms where they graze on variety of food materials hence regarded as omnivores. Some clariids such as Xenoclarias and Dinotopterus are found in Lakes Tanganyika, Victoria, and Malawi, the upper Congo, the middle and the upper Zambezi, the Okavango, L. Ngami, the Chobe, the Cunene and the Quanza in South Africa, and other Great lakes of Africa. The Gymnallabes live in turbulent well-oxygenated rivers of the Congo, but they lack the respiratory accessory organs. The Bathyclarias species are also found in Lake Malawi [10, 11] (Table 2).

3.1 Factors contributing to a rise in farmed Vietnamese striped catfish in the Mekong

The production units were vertically integrated, with seed production, fry production, fry to fingerling, and grow-out units being distinct, in different locations and setups. This ensured specialized labor and effort (cluster of operations) increased efficiency of the sector and effective dissemination of technology to farmers [38]. Similarly, special attention was given to seed production and broodstock management at hatcheries. The major technique that revolutionized the sector was the development of artificial propagation of tra catfish seeds [42], and the dissemination of the technology to all hatcheries in the Mekong. This ensured the availability of adequate high-quality seeds for the enterprises, following the ban on collection of natural seed stocks in the Mekong. Apart from this, a large number of hatcheries were developed, ranging from small-scale or back yard to commercial scale or large establishments (0.2 to 15 ha) [43]. There were 93 hatcheries for tra catfish in the Mekong delta, producing up to 818.3 million hatchlings annually under optimal operations of 29 cycles of fry production annually [44]. Many hatcheries maintained a large number of broodstock (up to 29,200) so the frequency of reusing a brooder was very low [38]. This reduced the rate of inbreeding and genetic drift, two processes that often negatively impact quality of seeds or fry, and this consequently improved the quality of the seed. Many hatcheries also replace their brood stock frequently [43], with stocks from their own extensive farms, neighboring grow-out farms, or from natural sources [38, 43]. Continuous improvement of seed quality is undertaken, for instance through selective breeding program for the species that has been initiated [45]. Feeding regimes in nursery units are also quite improved, ranging from recommended practices of live feeds such as Moina, or other such natural feeds for larvae, to commercial diets for fingerlings. The feeding ration was 5 to 18% body weight at a frequency of 4 to 8 times daily [43]. Grow-out systems were mainly operated by small-scale farmers (72%) [46], comprising deep ponds averaging 2.0 to 6.0 m, with a water depth of 3.5–4.5 m, with a high stocking density of 18 to 125 fish m^–2 depending on seed availability and financial ability of farmers. Average yields ranged from 70.0 to 850 t ha⁻¹ crop⁻¹, although the pond area and productivity per unit area have been increasing since 2000 [46].

Growth in the production of P. hypophthalmus was also boosted by export-oriented markets, with the USA being the main market in the 1990s. Increasing imports of tra catfish to the USA were boosted by the admission of Vietnam to the Asia Pacific Economic Consortium (APEC) block, and the dropping of tariffs on raw seafood [47]. The increased imports of tra catfish affected markets of the local ictalurid catfish and occasioned trade restrictions via new laws against tra catfish and renegotiation of bilateral agreements against the species [48]. These restrictions were worsened by negative internet and media campaigns against striped catfish industry of Vietnam [49]. However, with the explosion in the production of striped catfish in the 2000s, export market diversified, to about 136 countries between 2002 to 2008 [38], with the European Union being the major market [38]. This phenomenal growth occurred despite trade restrictions by the USA due to negative publicity about Pangasius. Expanding export markets was due to deliberate efforts by Vietnamese authorities to promote production while exploring alternative markets. Some of these efforts included: certification of groups of small-scale operators. Certification is a procedure by which a certification body gives written or equivalent assurance that a product, process, or service conforms to specified requirements and is carried out by competent and accredited body [38] (Figure 3).

Certification is important as it ensures that food quality and safety are achieved with respect to internationally traded food commodities and that the production systems comply to accepted norms and are socially responsible. Similarly, certification of a product ensures that farming systems have minimal environmental impact and that the production systems are sustainable. Certification also assures markets (buyers, retailers, and consumers) that the fisheries products are safe to consume and originate from aquaculture farms or capture fisheries adopting responsible management practices [38].

Certification of tra catfish farmers, therefore, assured the international market of the safety and quality of the product, and the confidence associated with this spurred demand for the product. Related to marketing of the product, the Vietnamese Government adopted negotiation with organizations such as the WWF, to address the biased negative publicity against tra catfish, and the WWF removed the species from its red list of endangered species. Similarly, Government encouraged farmers to adopt BMPs, cluster management approaches, and compliance to responsible farming practices to deter future problems. Due to the very marginal profits made across the tra catfish value chain, improved processing operations increased efficiency of processing, so that a relatively small amount of fish was required to produce 1 kg of processed fish. Production was also sustained by increased turnover or high production by farmers, exploiting the benefits of economy or scale of production [38]. This was further boosted by authorities implementing the guaranteed price scheme at each link of the value chain, to cushion operators against large fluctuations in prices [50]. Generally, there was deliberate action to maintain a suitable quality of the product, with desirable nutritional profile, as well as low content of pollutants, such as mercury, organochlorine pesticides, and polychlorinated biphenyls, consistent with the European rapid alert notification system [38]. Such actions included reduced use of veterinary drugs and other restricted or banned chemicals in the value chain of the fish by farmers and other operators, in compliance with food quality and food safety regulations.

Despite the rapid explosion of tra catfish culture in the Mekong, the industry has had very minimal environmental impacts on the Mekong ecosystem [38]. This is because only 2% of the waters of the Mekong pass through the ponds, and therefore the impact of aquaculture on the water quality of the river is minimal since sedimentation, mineralization, and infiltration occur in the ponds [51]. Further limitation of environmental impacts is achieved through the use of sludge from ponds as fertilizer for rice paddies, which helps to achieve environmental and economic sustainability for the farmers [52]. Similarly, hatchery-reared stocks of tra catfish have not impacted natural populations of the species and related fishes in the Mekong, through hybridization from escapees [53].

3.2 The Channel catfish, Ictalurus punctatus (Rafinesque, 1818)

The channel catfish (Ictalurus punctatus) is a member of the family Ictaluridae in the order Siluriformes. The native range of the Channel catfish or the North American Channel catfish, Ictalurus punctatus, is the southern Canadian Prairie Provinces south to the Gulf States, west to the Rocky Mountains, and east to the Appalachian Mountains [54]. Further, it extends to St. Lawrence River and its tributaries from southern Quebec through to Ontario including the Ottawa River and its tributaries, all the Great Lakes except Lake Superior, in southwestern Ontario and the southern part of Manitoba (Figure 4). This extends to Susquehanna River and the Florida Peninsular. However, the fish has been introduced in Georgia, North Carolina and South Carolina, and in most of the USA, including the Pacific and Atlantic drainages [54]. It is also widely introduced in Europe, Japan, and China (Figure 4). Its large size and excellent taste make it a popular target of anglers. Its high fecundity, tolerance to extreme environmental conditions, and resistance to diseases make it an excellent species for commercial cultivation.

The congener, Blue catfish, I. furcatus, and the hybrid between I. punctatus * I. furcatus, are also important in culture [56], and the three contribute to more than 60% of total aquaculture production in the USA, especially in the southern states of Mississippi, Louisiana, Arkansas, and Alabama [57]. The US catfish industry peaked in 2003, but declined later, in part due to competition from the Vietnamese catfish imports.

Sizeable fingerlings are stocked in fertilized ponds. During growth, fish of bigger size are intermittently harvested for market, and an equal number of fingerlings is added to ponds to replace the harvested fish. Therefore, multiple-batch production is often adopted, although fish grow faster in single-batch stocking [58]. So a pond often has fish of different sizes, and cropping for market is frequently carried out, to supply the market steadily, and also control flooding the market with fish and negatively affect fish prices [55]. This is maintained for several years, without draining ponds. Initial feeding ration is about 50% body weight, which is reduced gradually, to 3–4% body weight. However, water quality in ponds is monitored frequently, and aeration of water is carried out, to ensure the fish under high stocking density do not suffer stress, disease outbreaks, and mortality. Apart from ponds, fishes are also reared in cages, raceways, and tanks (Figure 5).

3.3 The Culture of Channel Catfish in China

The Channel catfish, I. punctatus was introduced to China from the USA in 1984, and successful artificial propagation was achieved in 1997, to support seed production for commercial culture. Rapid growth in the culture of the species in the country was recorded, due to several factors. Domestic consumption of the species reached 130,000 tons in 2013, of which 120,000 tones were fresh fish while 10,000 tones were processed [59]. The culture of the species has led to the growth of associate industries, strengthening synergies in the sector. For instance, due to the high demand for Channel catfish in many provinces of China, companies such as the Gaobazhou Aquatic Products processing company, promote value addition to the fish products, boosting sales, while simultaneously creating employment opportunities for locals [60] (Figure 6 and Table 4).

3.4 Seed production to support the expansion of Channel catfish culture in China

Following the importation of I. punctatus to China in the 1980s, artificial production was successfully initiated and perfected, assuring the sector of sufficient seed production. However, production of adequate amounts of quality seed was affected by degradation of germplasm quality, inbreeding, variation of germplasm, reduced growth rates of fry, and increased incidence of diseases [60]. In order to address these challenges, Government authorities established the National Channel Catfish improved variety bases in Sichuan, Hubei, and Anhui, to undertake breeding schemes for improved varieties of I. punctatus for culture [60]. The result of these efforts was the development of improved strain of catfish named the Jiang Feng No. 1 [60], whose growth rate is 22.1 to 25.3% greater than that of any other strain [60]. Later, 2 improved lines were developed by screening different populations for economic gain and growth performance, using the Best Linear Unbiased Prediction (BLUP) method. This led to the development of two hybrid lines with high growth rates, resistance to diseases, and survival. Further research on this focuses on identifying populations of high genetic variation and quantitative trait locus (QTL) of economic interest, to develop superior strains for use by farmers [63]. In line with these, fish seed breeding centers, together with other hatcheries for farmers adopted best management practices, especially focusing on managing quality of broodstock. Most hatcheries maintain a large number of brood fish, ranging from 30,000 to 50,000. This is necessary because a large sample size from which breeding pairs of fish are drawn maintains high genetic variation that positively impacts seed quality. Due to these improvements, seed production for Channel catfish farmers reached 800,000,000 to 1 billion, and rose to 1.5 billion fry in 2013 [60], helping to spur a vibrant culture industry for the species.

3.5 The culture of African Catfish, Clarias gariepinus (Burchell, 1822)

While the different families of catfishes are widely distributed in natural aquatic ecosystems in Africa and are landed from fisheries by local fishermen, the African Catfish, Clarias gariepinus is the most commonly exploited species. There are 58 species of Clarias, of which 33 are in Africa, while 25 are distributed in Asia [21, 64]. In Africa, apart from C. gariepinus, C. anguillaris and Heterobrachus longifilis are also common in aquaculture, although these 2 are of restricted distribution [64], mainly in the Nile and West Africa. Other non-Clarias clariid catfish genera in Africa include Bathyclarias (in Lake Malawi) and Gymnallabes typus (in the lower Niger River and Cross River basin of Nigeria and Cameroon).

A systematic revision of 120 nominal species of Clarias by Teugels reduced these to only 32 valid species [65]. In a related study, Teugels [66] documented 74 species of African clariid catfishes within 12 genera. The description of two species of Xenoclarias as Clarias eupogon (Norman, 1928) and Xenoclarias holobranchus (Greenwood, 1958) was changed to Xenoclarias eupogon (Gee, 1975) using more samples, as a monotypic genus. Similarly, three clariid genera, Clarias, Clariallabes, and Heterobranchus occur in Southern Africa [67], with Clarias having 6 species, while Clariallabes and Heterobranchus have a single species each [67]. In Southern Africa, a total of six Clarias species exist: Clarias gariepinus, C. liocephalus, C. cavernicola, C. theodorae, C. stappersii, and C. ngamensis [67]. On the other hand, six Clarias species inhabit Central Africa: C. gariepinus, C. platycephalus, C. camerunensis, C. gabonensis, C. buthupogon, and C. angolensis [68]. In lower Guinea, a total of 13 Clarias species are documented, including C. gariepinus, C. anguillaris, C. agboyiensis, C. gabonensis, C. buthupogon, C. angolensis, C. pachynema, C. submarginatus, C. platycephalus, C. maclaroni, C. ajensis, C. longior, and C. camerunensis [69]. Similarly, several Clarias species exist in Lake Victoria and its influent rivers in East Africa. Apart from C. gariepinus, several species including C. liocephalus (Boulenger 1898), C. alluaudi (Boulenger 1898), C. theodorae (Weber 1897), C. werneri (Boulenger 1906), and Xenoclarias also abound [70].

In Asia, 3 Asian clariid catfish species are used in aquaculture: Clarias batrachus (Linnaeus, 1758) in India [71], C. macrocephalus in South-East Asia [72], and C. fuscus (Lacep’de, 1803) in Taiwan and Hawaii [73]. Clarias batrachus also inhabits Southern Thailand in Malay Peninsula, Mekong, and Chaophraya River basins. Clarias magur (Hamilton, 1822) is also present in the Malay Peninsula and India, while C. nieuhofii (Valenciennes, 1840) inhabits South East Asia, successfully tried as an aquaculture species [74].

While these descriptions may not be quite accurate and require urgent revisions, they illustrate the diversity of Clarias, which is often unrecognized and undescribed, yet extremely important as biodiversity for local economies, and food resources for local communities. Similarly, some of these clariid species could be useful aquaculture candidates, to help overcome some of the challenges of C. gariepinus as a farmed species. From the diversity of clariid catfishes, it is also clear that there are two groups: the large and small-sized Clarias species. The large Clarias is represented by C. gariepinus, while the small group includes most other species across different regions of Africa. Clarias gariepinus has been widely translocated, at least in 35 countries, mainly for culture [64]. These translocations often ignore existing laws that ban transfer of fish across drainage basins [75] and pose risk to existing native stocks or species, due to hybridization. Examples of hybridization between different Clarias species include C. anguillaris*C. gariepinus in Nigeria [76] and a hybrid of C. macrocephalus (better taste and culinary attributes) *C. gariepinus (faster growth and higher resistance to diseases). This hybrid is the main fish stocked for culture, because of faster growth and higher resistance to diseases, with its product valued at over US$100 million in Thailand [77]. The introduction of C. batrachus and C. macrocephalus from Thailand to China, the Philippines, Taiwan, and Hong Kong led to the loss of endemic cyprinids from Lake Lanao, Mindanao due to predation and the displacement of native C. macrocephalus from Luzon [64]. Introgression of alleles from this hybrid into the genome of natural population of C. macrocephalus devalues the natural genetic resource of C. macrocephalus [78] and also causes decline in the natural stocks of C. batrachus (Figure 7) [79].

3.6 Challenges facing the culture of Clarias gariepinus in Africa

Although it is well recognized as a suitable fish species for aquaculture to meet food security, income generation, and livelihoods for local communities in Africa, annual cultured production of the species is low. FAO estimates that average production for farmed clariid catfishes was over.

300,000 tons, which was valued at nearly US$400 million in 2006 [77]. Total production of C. gariepinus, the main farmed clariid catfish species, averages 200,000 tons in 2014 [41] (Figure 8). Diverse efforts to increase average production in many countries of Africa have not yielded much fruit, despite ample supplies of water, land, labor, and technical know-how, as well as diverse populations of natural germplasm (Figure 7).

The main challenge stifling the culture of C. gariepinus is poor survival of fry or larvae [81]. This reduces the quantity of fry produced by farmers at hatcheries, negatively impacting seed availability and supply for commercial aquaculture ventures. In East Africa, Clarias fingerlings have taken on an even bigger role and importance, being used as live baits for catching Nile perch and Lates niloticus in the Lake Victoria long line fishery activities [82]. It is reported that a total of 3 million Clarias live baits are required daily by L. niloticus fishermen on Lake Victoria [83], a demand that creates enormous opportunities for Clarias farmers and hatchery operators to generate income and livelihoods. Despite this, the supply of fingerlings from hatcheries in East Africa is very low, forcing bait traders to exploit natural populations for fingerlings. Harvesting fingerlings from natural populations is not environmentally sustainable, with intermittent supply as availability is rain-dependent. This practice also exacerbates overexploitation of natural fish stocks, contrary to the need to conserve natural fisheries resources, especially for the Lake Victoria basin, whose endemic fish species already declined [84] due to many factors [82, 85]. Apart from endangering the health of fishermen who collect the fingerlings from the wild, this practice does not yield adequate numbers of fingerlings required quickly. It, therefore, involves a waste of time, and transport costs as fishermen have to venture into different parts of the lake, swamp, or river and spend many hours scouring different sites for catfish fingerlings. For sites within Lake Victoria, this task is especially complex and risky, due to dense mats of the water hyacinth, Eichhornia crassipes [86] that cover large areas of the lake.

In order to avoid these challenges and support the conservation of natural fisheries resources while simultaneously helping hatchery operators to generate income and livelihoods, artificial propagation of Clarias at hatcheries has been proposed and encouraged [85, 87, 88, 89]. However, despite the fact that artificial propagation of Clarias at hatcheries is well perfected, practiced, and documented, high mortality of Clarias fry at hatcheries still persists. In some hatcheries, mortality of more than 80% of hatchlings occurs, severely curtailing efforts of increasing seed availability for farmers. In some cases, the whole batch of incubated eggs fails to hatch, due to contamination with bacteria and fungi [90] in hatchery and egg incubation facilities. Over the last several decades, research efforts have concentrated on addressing poor survival of Clarias fry or fingerlings at hatcheries. These research efforts included studies on improving quality of diets for larval catfish [81, 91, 92, 93], expanding the range of Clarias production systems [94], control of bacteria and fungi in egg incubation units [95], as well as fry nursery units [96], and improving the choice of source population of brood stock used at hatcheries for propagation [88, 89], as well as management of elite lines of brood stock for seed production [86]. In this regard, hatchery managers and farmers need to understand that the use of poor quality broodstock of mixed ancestry or suffering low genetic variability yields seeds of poor quality [89] because of outbreeding depression that compromises the fitness of offspring [97]. Despite this suite of studies aimed at addressing the challenge of poor survival of Clarias fry or fingerlings, the problem is far from being comprehensively addressed, in order to achieve sufficient seed availability and supply. There is need to understudy seed production and supply in the farmed catfish industry of Asia, especially the Vietnamese striped catfish, P. hypophthalmus and the Chinese Channel catfish, I. punctatus. We suggest that future efforts in addressing seed quality and quantity in C. gariepinus aquaculture will have to focus on using improved strains. Techniques of improvement, such as selective breeding and genomic selection will need to be employed, and research in these fields need to be strengthened.