Common Name: Walking catfish (clarius catfish, freshwater catfish, thai hito, thailand catfish, alimudan, hito, hitong batukan, ikan keling, ito, kawatsi, keli, klarievyi som, konnamonni, leleh, magur, mah-gur, wagur, manguri, mangur, marpoo, musi, halimeena, pla duk dam, nga-khoo, paltat, trey andaing roueng, wanderwels)
Scientific Name: Clarias batrachus
Identification: Walking catfish, which are scale-less, are typically a uniform shade of gray or gray-brown with many small white spots along their sides. The head is flat and wide and the body tapers to the tail. The eyes are very small and the mouth is broad with fleshy lips and numerous small pointed teeth in large bands on both the upper and lower jaw. There are four pairs of barbels, one pair each of maxillary and nasal barbels and two pairs of mandibal barbels. The fish has a lengthy dorsal and anal fin that each terminate in a lobe near the caudal fin. The pectoral fins, one on each side, have rigid spine-like elements. To move outside of water, the fish uses these "spines" and flexes its body back and forth to "walk". The walking catfish is easy to distinguish from many of the other North American catfish because it doesn't have an adipose fin.
In addition to the brown or gray-brown coloring noted above, albinos and calico morphs are also possible. However, these are uncommon in the wild. For example, in Florida the fish that escaped were albinos but today the albino is rare and descendants have generally reverted to the dominant, dark coloring.
The fish reach reproductive maturity at one year and grow up to 24 inches in their native range. However, in Florida they rarely exceed 14 inches.
Walking catfish possess a large accessory breathing organ which enables them to breath atmospheric oxygen. They are well known for their ability to "walk" on land for long distances, especially during or after rainfall.
Original Distribution: The walking catfish are a widely distributed species found across Southern Asia including Pakistan, Eastern India, Sri Lanka, Bangladesh, Myanmar, Thailand, Malaysia, Indonesia, Singapore, Borneo, Laos and the Philipines. It's hard to determine, though, to what extent this distribution comprises the native range. In Southeast Asian this fish in valued for food and it's probable that human activity is responsible for the presence of this species in parts of its current range.
Current Distribution: In addition to the locations listed above, the walking catfish has been found in the US. Specimens have been collected in multiple locations across California, the All American Canal in Arizona, widely separated bodies of water in Connecticut, the Flint River in Georgia, a lake in Massachusetts and a spring in Nevada. They are found across southern Florida. The only established, wild population is in Florida.
Outside of Florida: Tropical fish dealers in the US imported the walking catfish to be sold as pets. The walking catfish that were found in the US outside of Florida most likely came from aquarium releases (intentional or accidental).
Florida: In the early 1960's, the walking catfish was imported to Florida from Thailand for the aquarium trade. The first introductions apparently happened in the mid 60's when adult fish, imported to be brood stock, escaped from Penagra Aquarium in Broward County and/or from a truck transporting brood fish between Dade and Broward counties. In 1967, the state of Florida banned the importation and possession of walking catfish. However, this led to another release of the fish into the wild. Fish farmers in Tampa Bay who possessed the fish purposefully released them so that they would not be found in violation of the new law.
In 1968, this species was only found in three south Florida counties. However, by 1978, the walking catfish had spread to 20 counties in the southern half of the peninsula. The fish accomplished this migration by using the many hundreds of miles of interconnected canals across south Florida and by moving over land, typically during rainy nights. By the mid 1970's, the walking catfish was established in Everglades National Park and in Big Cypress National Preserve.
Besides quickly colonizing a large area, walking catfish are extremely abundant in certain areas of south Florida (as great as 3,000 pounds of walking catfish per acre).
Walking catfish are sensitive to cold temperatures (lethal temperature is 9.4 - 12.8 degrees Celsius). During cold, dry months, they burrow into the sides of streams and ponds where they remain dormant until the spring rains. Even with this precaution, though, the populations suffer periodic die-offs when the temperature dips below freezing. This sensitivity has limited their migration north into colder areas. However, concern remains that they could find refuge in the warm springs in north central Florida and survive short cold spells by burrowing in the mud.
Mode(s) of Introduction: Since the 1960's (and possibly before) walking catfish have been imported into the US to be sold as pets. Once in the US, they either escape from their environment or are purposefully let go. On the internet, there are anecdotal stories of walking catfish owners who have lost the fish because they literally walk away. Today, the US government requires a federal permit to own one of these fish but there are still pet stores advertising them for sale.
Reason(s) Why it has Become Established: Walking catfish are hardy fish which can thrive where many other fish struggle to survive. In addition to lakes and rivers, they can be found in brackish waters or warm, stagnant, often hypoxic waters such as muddy ponds, canals, ditches, swamps and flooded prairies. They can remain dormant through periods of drought and go several months without eating. When they do eat, they consume a wide variety of prey.
In addition, walking catfish have high fecundity and the males guard the eggs and free-swimming young, giving them a better chance of survival than the native, non-protected young of other species.
Ecological Role: Walking catfish are voracious, opportunistic feeders who are mainly active at night. They consume a wide variety of prey including eggs and larvae of other fishes, small fishes, a number of invertebrates including crustaceans and insects and sometimes plant materials. In densely populated drying pools, these fish become even more indiscriminate and quickly consume most other species present.
Walking catfish of all ages and size fall victim to a wide variety of predators including other fish, reptiles, birds and mammals. They are also killed by cars when migrating en masse across streets from one body of water to another.
Benefit(s): The walking catfish can survive extended periods out of water. In its native areas, this makes it an attractive food fish which easily can be sold and traded live. The species is fished by subsistence fisherman as well as managed at commercial farming operations.
Threat(s): Walking catfish have been know to invade aquaculture farms and eat large amounts of fish stock. Fish farmers in Florida have had to put up fences or build levees to keep them out. An additional threat to catfish fisheries, specifically, is the fact that wild walking catfish carry the disease enteric septicemia (ESC) caused by the bacterium Edwadsiella ictaluri. Wild walking catfish could infect farmed catfish with the disease.
In Florida, the total impact on native species is unknown. We do know, though, that walking catfish are extremely pervasive across southern Florida and many scientist consider the introduction of the walking catfish into the area as one of the most harmful introduction in North America. Walking catfish are especially devastating in small wetland pools during the dry season where they can quickly become the dominant species. The species that appear to be most affected are native centarchids and catfishes.
The walking catfish is a tropical fish and, if introduced into other warm areas of the US, the spread of the fish could mirror what happened in Florida. Southern Texas and Hawaii are examples of two US areas that could be vulnerable.
Control Level Diagnosis: I rank this threat as medium priority. In Florida, the population is established and it's northern migration is slowed by freezing temperatures. However, it is crucial to keep this fish out of other areas where it could flourish.
Control Method: Numerous countries have "blacklisted" the walking catfish. The United States has classified all members of the family Clariidae as injurious wildlife, illegal to possess without a federal permit. It is important to keep this fish contained because, once out in the wild, the population growth could be explosive in areas where there is a mild climate. In addition, the walking catfish is very hardy. It can survive months without food, and live in water that other fish would find intolerable. Poisoning it would be very difficult because it could walk elsewhere to avoid the poisons.