greater Zandvlei Estuary Nature Reserve
The African Catfish: Clarias gariepinus
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Compiled by: Cassandra Sheasby Cassandra.Sheasby@capetown.gov.za
This catfish is indigenous to the northern parts of the continent, which has been translocated into many river systems outside of its natural range.
Photograph: Sheasby, 2009
Terms of reference:
Alien: a species which has been translocated beyond its natural home range
Names and Classification:
Scientific name: Clarias gariepinus
In regions of Namibia this fish is known as Omahepala (Ehepala) (Owambo, northern Namibia), Hogo and Njoho (Ntjogho) (Kwangali and Gciriku, Kavango respectively), Ndombe and Unsoho (BaLozi and BaSubia, Caprivi respectively). (Okeyo et al, 2004)
Phylum: Chordata – chordates
The African catfish is a dominant freshwater fish. It can grow to between 1.4 and 2m long and can weigh anything from 8kgs to 59kgs. The South African angling record is 35 kgs, however a 58.9kg specimen was caught in the Vaal River. Its body colouration varies from olive green, to brown and black with the flanks often uniform grey to olive-yellow with dark slate or greenish brown back. Underparts are pale olive to white and are mottled irregularly with dark brownish green, or uniformly silvery olive.
It is a heavy boned, flat headed fish with premaxilla and lower joa pointed teeth arranged in several rows and four pairs of long trailing sensory organs known as ‘barbels’ around its mouth giving it a similar appearance to a cat, hence the name catfish. It has a high number of gill rakers varying from 24 to 110, the number increasing with the size of the fish.
The African Catfish Clarias gariepinus.
The body is elongated with long, low dorsal and anal fins and a smoothly rounded tail fin. The skin is leathery and has no scales. It has a small but powerful pectoral fin set immediately in front of the anal fin which has a serrated spine.
Having a bi-loded swim bladder which is connected to the oesophagus via a narrow phneumatic duct all making the catfish negatively buoyant. The swim bladder is reduced to compensate for this buoyancy. Air is retained in the suprabranchial chamber when a vertical stationary position is required but the air is expelled when the fish need to plunge down suddenly to avoid predation (De Moor & Bruton, 1988; Bruton, 1977;
The African catfish is the most widely distributed fish in Africa. Its native range extends from South Africa through central, west and north Africa into the Middle East and eastern Europe. It is indigenous to many rivers from the Nile to the Orange River. In southern Africa it occurs in the Limpopo, Orange-Vaal, Okavango and Cunene River systems. It has been found at altitudes of 1400m in northern Kwazulu Natal and up to 500m in southern Kwazulu Natal (De Moor & Bruton, 1988).
It is an invasive alien species to the Western Cape and can now be found in many river systems in the Southern, Eastern and Western Cape. It escaped into the Eerste River system from the Jonkershoek Fish Hatchery. Anglers and farmers illegally translocated this catfish into many dams and river systems in the hope of “improved fishing”. There are confirmed reports of this catfish in the Kuils River, Berg River, Breede River, Theewaterskloof dam. They are believed to have been introduced into Zandvlei Estuary, Zeekoevlei and Rondevlei on the Cape Flats by conservation authorities in the 1960’s. They have been caught at Zoarvlei wetlands. They have also been reported in the Olifants River below its confluence with the Doring River and possibly in the Gourits River.
Prior to 1988 it was collected in the Bongolo dam (Swart Kei River, Kei system) and this translocation history is unknown.
It was recorded in the Tyume River (Keiskamma system) in 1985. The translocation is assumed to have been escape from the experimental farm at Fort Hare University into the river during flood conditions. In 2001 they were recorded in the Kouga, Swartkops, Sundays, Fish, Keiskamma, Buffalo, Nahoon and Mtata rivers. There are unconfirmed reports from the Kwelera and Gonubie Rivers.
An unconfirmed report of these catfish was compiled in 1988 in the Nuwejaars Dam (Bushmans River system, Alicedale).
There is concern over the genetic integrity of the Namibian native range as it is believed that specimens from the Orange River system were introduced into farm dams in this region and may have escaped forming genetically contaminated strains of the species. The invasive alien organisms in South West Africa/Namibia scientific report classified Clarias gariepinus as an indigenous species to the Cuvelai system which could spread to the Cunene River via the artificial Omblanta Canal. It was also listed as a potential invader of the Okavango system via the Eastern National Water Carrier (Schrader, 1985). It is possible that these fish have in the last 24 yaers been able to move and establish themselves in these river systems.
They have also been recorded in the Karoo and parts of the Transkei.
(De Moor & Bruton, 1988; Cambray, 2003;
In the 1950’s it was introduced into Indo-China by the French. It was widely distributed in south and south-east Asia for fish farming. The introduced fish is a hybrid between Clarias gariepinus and Clarius macrocephalus.
Invasive alien fish species were introduced into Bangladesh purposefully in the hope of supporting the needs of the growing population. The most devastating fish introduced include the African Catfish which was introduced from Thailand between 1953 and 1990. 266 species of inland fish are found in Bangladesh as well as 442 marine fish species. These species are now threatened due to the appetite and explosive nature of invasive species including the African Catfish. There are also knock on effects including the decline in vulture populations due to the collection of all carcasses to feed to these fish in captive cultures (Barua et al, 2000).
Between May 2002 and February 2004, studies were undertaken in Brazil to ascertain the extent of the invasion by African Catfish. All individuals that were caught were found to be larger than the original literature had recorded. The first records of the African Catfish were in Sao Francisco, Parana, Doce river basin and the estuary of laguna dos Patos. Between 2006 and 2007 they were recorded in the riacho Sossego, rio Doce basin, rio Guaragacu basin and lagoa Encantat, rio Almada basin. Cases of reduced numbers of native fish species and local extinctions have recently been reported in the Brazilian inland waters and the introduction of the African Catfish is believed to be one of the causal factors (Rocha, 2008). The interest in the ecological effects of this Catfish were sparked by fishermen in Brazil who reported catching African Catfish while they also noticed a decline in indigenous fish stocks. The potential problems created by this species are seen in the ones created by the walking catfish, Clarias batrachus which has rapidly colonized the southern parts of USA invading aquaculture farms to prey on fish stocks. It is regarded as a threat to the indigenous species and is listed in the Global Invasive Species Database as one of the 100 worst invasive species (Vitule et al, 2005).
Clarias batrachus, a related species (indigenous to south and south-east Asia, was introduced into Florida USA in the 1960’s. It is now widespread throughout the state. This species is known as the walking or Thai catfish.
Internationally they have been found in Israel, Turkey, Lebanon, Syria, Asia (Phillipines to Java) and America.
Methods of moving:
Due to the nature of this fish and its ability to stay alive out of water, it makes it one of the easiest fish to translocate. An example of angler aided movement occurred at Grahamstown municipal dam (Kowie River system). He was apprehended but his punishment was to do a few hours of community service by helping to Rotenone the dam (a substance used for the eradication of all gill breathing fish in closed water bodies). The costs of the Rotenone were not included in the charges.
Aquaculture, hatcheries and farm dams:
Due to articles in the public arena about the advantages of farming African Catfish, many aquaculture and fish farmers turned to stocking the Catfish. Due to its great ability to move overland and breathe air, the escape of this species from one water body to another is almost certain (Cambray, 2003).
Habitat and Habits:
Being a freshwater fish it is often found in rivers, dams, weirs, lakes, swamps, muddy waters, floodplains and other water bodies. They can be found at depths between 4 and 80m.
It is able to bury itself in the river bed when there is a decrease in water or drought is occurring. They have been known to stay in muddy ground of ponds gulping air directly using their accessory breathing organ instead of their gills. They are unlikely to survive in ground that has dried completely.
During intra-specific aggressive interactions, this fish was found to produce electric organ discharges that were monophasic, head-positive and lasting from 5 to 260ms. (De Moor & Bruton, 1988; www.zipcodezoo.com/Animals/C/Clarias_gariepinus/
Clarias gariepinus is a non-guarding, substrate spawner, which awaits optimal conditions before spawning commences.
Breeding of this species occurs in very shallow, weedy waters normally after heavy rains and usually once the fish have migrated upstream. It normally takes place after heavy rains, often within 2 to 3 days of the new moon of last quarter. Courtship, spawning and egg deposition take place at night with the peak spawning times between 20h00 and 02h30. Tiny fertilized eggs hatch out within 24 to 36 hours of being attached to plants and debris in the water. The larvae swim after 50 hours and begin to feed at 80 hours. The fry live in inshore vegetated zones.
The average number of eggs produced is 45 000 eggs for a 2kg fish. Very large catfish (larger than 1.2m) in Hardap Dam Namibia may produce over 800 000 eggs, while Leeupan catfish between 60 and 90cm contained between 293 000 and 446 000 eggs.
The medium, minimum population doubling time is between 1.4 and 4.4 years. This catfish can walk short distances to new breeding areas or through very shallow damp pathways. African catfish reach maturity at the end of their first year of age when they are approximately 20 to 25cm in length (De Moor & Bruton, 1988; Bruton, 1977; www.zipcodezoo.com/Animals/C/Clarias_gariepinus/
Feeding and Behaviour:
These fish are voracious predator and eat almost anything. Literature includes insects, crabs, plankton, snails, fish, young birds, amphibians, reptiles, rotting flesh, plants and fruit in the diet. It is normally an individual bottom feeder, however they are known to be extrememly adaptable to conditions and feed in groups at the water surface. They migrate in the upper Okavango Delta where they prey on momyrid fish.
They hunt socially, swimming in formations on the water surface or in a claw like formation to the shore. The pack herd cichlid prey towards the shallows where they are easily caught by these fish who use their pectoral spines to ‘walk’ out of the water to engage prey.
The mouth is wide, subterminal, traverse and capable of opening extremely wide for engulfing prey items or sucking in large amounts of water which is flushed through the gills for filter feeding. Once the prey is in the mouth, the jaws snap closed and the broad bands of sharp teeth on both the upper and lower jaws prevent the prey from wriggling free. The prey is swallowed whole. The oesophagus is short, muscular and dilatable. It opens into a distended stomach typical of creatures capable of carnivory. The four types of feeding described by Bruton, 1977 are:
a) Individual foraging (general solitary searching for food through the water column)
Catfish are opportunistic feeders and will take any fish species which is abundant. They respond quickly to newly available food sources and will change their feeding patterns to match organisms freely available. Young fish feed mostly on small invertebrates in shallow inshore areas.
Factors which restrict the food niche of these fish are interspecific competition; predation pressure, constant low supply levels of food and harsh physic-chemical conditions. Interspecific competition and predation pressure are buffered by the large body size of the catfish, the shape and protection of the head, pectoral spines and piscivorous habits. The versatility of their physical adaptations enable them to survive almost all conditions (De Moor & Bruton, 1988; Bruton, 1977).
In 2004, a study was undertaken by Linden Rhoda at Zandvlei Estuary Nature Reserve to ascertain what these fish were eating. The results showed that the most dominant food sources utilized by these fish were crabs (two species), bloodworms, earthworms, fish and frogs. Less dominant food types consumed included plant material, prawns and snails. This will more than likely change with the available food and during breeding season, may be dominated by birds eggs and chicks as well as tadpoles or young frogs.
They tend to remain in the deeper waters during the day and move to the shallows at night.
This species is a nutritious food source in Africa and there is contention between various sources on its viability as an aquaculture species. It can be housed in higher densities than most fish species, is a hardy, quick growing and tasty fish. Market research and trials have shown in some areas that this species is not economically viable for aquaculture due to their high food intake requirements (Marr, Pers Comm., 2009). It is marketed fresh and frozen in Africa and is eaten boiled, fried and baked by members of the community. (De Moor & Bruton, 1988; www.zipcodezoo.com/Animals/C/Clarias_gariepinus/). Fourie states that between 65 and 100 tons of fish can be produced annually. In the Vaalhurts Dam, Clarias gariepinus is an endemic species and would therefore be a more suitable species for aquaculture in a catchment where the species is not alien. There are various methods of marketing the fish, however no formal established market exists in South Africa (Fourie, 2006).
In areas of the Western Cape, reports have reached conservation authorities of recreational fishermen having their lives threatened by informal settlement dwellers unless they can produce sale stock of these fish. These people are willing to pay up to R60 for a fair sized fish. The selling price in 2008 was R5 per kilo (Pers. Comm., van Der Westhuizen, 2008).
Impact and biodiversity threat:
The invasive alien species have been identified as agents of loss of native biodiversity. Alterations of ecological communities by these aliens change the functioning and overall health of the ecology. The cost of controlling these also takes funding away from conserving our native species in nature reserves (Barua, 2000).
The generalized feeding habits and mobility make this an extremely efficient predator posing a threat to indigenous fish, amphibian and invertebrate populations outside of its native range. Several countries have reported adverse ecological impacts after the introduction of this species. Trade in this species has been restricted in Germany.
In 1986 it was proposed that this species could potentially threaten Sandelia bainsii in the Tyume River and Sandelia bainsii and Babus pallidus in the Great Fish River. African Catfish were first recorded here in 1985 but since then have spread down the confluence with the Keiskamma River. Where Sandelia bainsii were originally found, there are now only juvenile African Catfish. Mayekiso has stated that the introduction of the Catfish into this river system has lead to the decreased abundance or even total disappearance of other fish species. The Catfish are thought to have been introduced from the ponds at the University of Fort Hare (Cambray, 2003).
Studies done in 1972 in central Africa suggest that this species have profound negative ecological effects on aquatic insect communities. Insect diversity in the Coleoptera order can be reduced by up to 78% and Hemiptera by 66% by this predator (De Moor & Bruton, 1988; Bruton & van As., 1986; www.zipcodezoo.com/Animals/C/Clarias_gariepinus/).
Twenty parasites were recorded in or on this catfish including one which is alien to South Africa. The translocations of this fish will facilitate the spread of these parasites and could lead to local extinctions of indigenous fish species due to their lack of resistance to these new parasites.
The presence of this species in the southern Cape Coast threatens the redfin minnows Pseudobarbus spp. This area also include 3 endangered, 3 vulnerable and 7 rare species which face localized extinctions by the predation and competition of the African catfish.
“A real threat exists in the Cape ichthyofaunal region due to the widespread introductions of trout and bass in the upper and lower reaches of rivers and the increasing colonization of sharptooth catfish Clarias gariepinus in the middle reaches” (Bruton & van As., 1986). Pseudobarbus asper (Smallscale redfin minnow) inhabits the Gamtoos River system and is a main channel species. It relies on the main channel pools during drought periods to survive. When a fish geneticist E Swartz went to sample these fish, he found very few but found large numbers of African Catfish. This has already led to the decline of the species and could ultimately spell localized extinction (Cambray, 2003).
In 2004, an ecological survey of alien fishes in the Guaraguacu river basin, southern Brazil, a Clarias gariepinus specimen was recovered with a native frog Leptodactylus ocellatus measuring 85mm total length lodged in its esophagus. The catfish has been reported as a severe ecological problem species. Native fishes to Brazil have been recorded in its diet and there is concern that it could modify the biotic balances of the native fish and invertebrate communities. Impacts of alien fishes on amphibian populations have been recorded previously in temperate regions and the record of this amphibian being preyed on points to the potential high impact that this catfish will have on ecological communities in aquatic environments. Maximum adult size is a biological variable denoting invasion success of non-native fishes in Iberian watersheds.
In 2008 a study was undertaken on capture methods of this catfish and using amphibian bait was the only successful way of catching these catfish. This may be an feeding adaptation in the Brazilian waterways (Vitule et al., 2008).
JA Cambray’s paper on the need for research and monitoring of this species in South Africa was written in 2003, outlining the reasons that greater research is necessary to survey the impacts these catfish are having on their prey items and the ecology of the water bodies they now inhabit.
Control and Law Enforcement:
Various factors make this species very difficult to control: omnivorous diet, direct airbreathing, ability to walk on land, burrowing capabilities and ability to hide in vegetation.
Fish farms breeding these fish for commercial purposes have to be secured with fencing buried at least 30 cm to prevent escape. Farming of this species in catchments outside of their natural distribution range should be prohibited and more stringent laws, by laws and law enforcement efforts should be implemented including higher fines (in excess of R50 000) to act as a deterrent for the translocation of this species.
Trained law enforcers should be better equipped to prosecute and fine people who are caught illegally transporting this species as well as other invasive fish such as Bass, Carp and Tilapia (Cambray, 2003).
Section 51 states that no person shall kill, damage or willfully disturb any fish or any spawn of fish in inland waters except in the case of a fish being killed or injured while being caught.
Section 57 states that no person shall without a permit transport live fish of the following species: Carp, Bluegill sunfish, trout, black bass, banded tilapia or any exotic freshwater fauna.
Section 58 states that no person shall without a permit import any living fish or fry thereof into the Province.
Nature Conservation staff getting ready for a night’s barbel removal
The fishing and boating by-laws have started being revised by a reserve manager and will eventually change for the whole City and be gazetted to replace the various ones across the metropole. Zandvlei’s by-law will now include no alien species to be released once caught and must be killed and removed from the water/land. All indigenous fish species are to be released after the bag limit amount has been removed.
Adequate signage will o need to be erected around the water body to ensure compliance of the new by-laws.
The future and solutions to this problem:
With global warming raising the temperatures of the waters, it spells out faster reproduction and higher growth rate for this species giving it an even better competitive edge against indigenous fish species. The larval stage of this fish species grows 89% per day at 22.1◦; 150% per day at 25◦ and 183% per day at 28.1◦ (Wood & McDonald, 1997).
Two main ways to combat the problem are:
The way forward would be to survey where the animals are currently found, which species they will have the most impact on and introduce management plans for these catchments first. Ensure that there is control of translocation through catchment connections by installing filtration grids in joining canals and pipes to prevent the movement of fish eggs, juveniles and adults (Bruton & van As., 1986)
The most comprehensive and only true effective way of ensuring eradication in catchments is to treat the entire catchment and find a selective method of removal which can remove the catfish in a small period of time (Marr, Pers. Comm., 2009).
There is great room for improvement in the translocation of species through imports.
Research, surveys and monitoring:
In order to begin understanding the scale of the problem, scientific surveys should be undertaken in the river systems inhabited by the African Catfish. Fish treks that are currenly done are only focused on some water bodies with no focus on the invasive alien fish species caught during these fish treks.
During spawning season, the numbers of African Catfish seen spawning should be recorded and the numbers of fish removed from water bodies must be entered into the biodiversity database on City reserves. More extensive studies with more than 50 individuals should be undertaken at different water bodies
Educating the public and the user groups:
A large scale environmental education drive is necessary to get the message out there and to slowly change the minds of anglers about the illegal introductions of this species. The translocation of species like this African Catfish causes long term irreparable damage to the biodiversity of our rivers and open water bodies. Poster 1 has been designed as an informative poster for people like those who have good general knowledge of biology or environmental science such as friends groups or anglers.
An A4 double sided foldable brochure was designed from poster 1 that could be distributed to residents around water bodies, anglers, schools, visitors, tourists and students studying natural sciences. With the Mallard Duck eradication undertaken by City of Cape Town, a brochure drop to the residents is generally well received and many people have a more scientific background to why eradication programmes are necessary. For tourists, local communities and local governments a brochure and poster combo will be best.
Talks and articles in angling television shows and magazines are a good first step in asking the angling fraternity to help conserve our indigenous fish species.
For school children a poster (see poster 2) and interactive play will be the best way to reach the children and show them the impact of the invasive potential of this catfish.
Media coverage in the local municipality newspapers and newsletters of user groups such as the Zandvlei Trust Newsletter and the Marina da Gama newsletter will also reach a large number of the community.
Engineers who are involved in linking of river systems and the impacts of changing river courses also need to be well educated in aquatic ecology prior to undertaking these projects and Nature Conservation bodies should have stronger influence in the final decisions (Cambray, 2003).
Cassy Sheasby and Morne Carstens measuring one of three African catfish removed from Zoarvlei wetlands during a fish trek in 2005. No other fish were recorded in this water body.
Many thanks to Sean Marr of UCT Freshwater Research Unit for access to his references and knowledge on the species.
Many thanks to Johan van der Westhuizen for his company’s insight into the commercial market of Catfish and Carp.
Many thanks to Linden Rhoda for making his previous research on this species undertaken at Zandvlei Nature Reserve available to me for this assignment.
Thanks to Koos Retief for photographing the barbel removal at Zoarvlei in 2005.
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