Species Description: Clarias batrachus, the walking catfish, has an elongate body that is broader at the head, tapering toward the tail. It is readily recognizable as a catfish with four pairs of barbels (whiskers) and fleshy, papillated lips. The teeth are villiform (small and bristle-like), occurring in patches on the jaw and palate (Jayaram, 1981). The eyes are small. The pectoral spines are large and robust and finely serrate along the margins. There is no dorsal spine. The dorsal fin is continuous and extends along the back two-thirds of the length of the body. The dorsal, caudal, and anal fins together form a near-continuous margin; the caudal fin is rounded and not eel-like though it is occasionally fused with the other fins (Talwar and Jhingran 1991). The complete spine/ray count is: Dorsal = 62-72; Anal = 45-58; Pectoral = I + 8-11 (Sen 1985).
Color is drab but variable among individuals: olive to dark brown or purple to black on the dorsal surface; pale to white on the ventral surface; and blue-green on the sides. The fins are grey-green and small white specks are present on the back half of the body (Page and Burr 1991). An albino variant occurs naturally and has been commercialized for the aquarium hobby trade.
Potentially Misidentified Species: In Florida, novices may confuse this species with the native Ariid marine hardhead catfish (Ariopsis felis) and gafftopsail catfish (Bagre marinus). However, the forked tail, adipose fin set anterior to the caudal peduncle, and the presence of a dorsal spine on the native species are among the many features that easily differentiate them from C. batrachus. Similar distinguishing features can be used to distinguish C. batrachus from resident freshwater Ictalurid catfish such as the brown bullhead (Ictalurus nebulosus) and channel catfish (I. punctatus).
Regional Occurrence: Walking catfish can be found in a variety of habitats, but they are most commonly encountered in muddy or swampy water of high turbidity (Courtenay et al. 1974, Hensley and Courtenay 1980, Talwar and Jhingran 1991).
C. batrachus, a southeastern Asian native species, is now established throughout most of Florida (Courtenay et al. 1991), although Shafland and Pestrak (1982) suggest that cold intolerance puts the northernmost limit of potential range expansion at approximately Gainsville.
In the US, walking catfish have been collected in several locations in California and Connecticut (Courtenay et al. 1984), from Georgia (Courtenay et al. 1991), Massachusetts (Hartel 1992), and Nevada (Deacon and Williams 1984).
IRL Distribution: Within the IRL watershed, collection records exist for C. batrachus from Brevard, Indian River, St. Lucie, and Palm Beach counties.
Age, Size, Lifespan: Walking catfish typically attain a standard length of 225-300 mm, although animals twice that size are encountered (Courtenay and Miley 1975; Hensley and Courtenay 1980).
Abundance: Small, isolated ponds are reported to be particularly vulnerable to walking catfish infestation. As early as 1970, researchers were reporting C. batrachus abundance in small Florida ponds in excess of 3,000 pounds per acre (Lachner 1970).
Reproduction: Individuals become sexually mature at approximately one year of age (Talwar and Jhingran 1991). Where populations are established, walking catfish exhibit rainy season mass migration and spawning events. Adhesive egg masses containing as many as 1,000 eggs are laid in nesting hollows prepared by the breeding pair. Egg masses are found on on aquatic vegetation or within other suitable nest sites. They are guarded by the males until they hatch (Courtenay et al. 1974, Hensley and Courtenay 1980). The female, leaving care of the eggs to the male, guards the area around the nest.
Embryology: Embryonic development within the egg is rapid. Embryos hatch out in approximately 30 hours at 25°C. For the first two days after hatching, parents still remain by the nest to protect the fry. At this stage, the fry are egg-sac larvae that do not yet feed, but instead live off of energy reserves stored in the yolk sac for the first two to three days after hatching (Rao et al. 1995). When the free-swimming young have consumed the remaining yolk reserves, they begin to forage for themselves.
Temperature: The native habitat of walking catfish is tropical southeast Asia. Intolerance to cold temperature is range limiting (Shafland and Pestrak 1982). Behavioral avoidance of environmental extremes during cold/dry seasons involves burrowing into pond and river banks to enter a dormancy that is interrupted with the arrival of spring rains (Courtenay et al. 1974). Deep water may also serve as a thermal refugium during cold snaps (Courtenay 1970).
Winter thermal kills have been documented in walking catfish from as far south as Florida's Broward County (Courtenay 1970).
Salinity: C. batrachus is euryhaline across its native range, inhabiting fresh and brackish water as well as muddy marshes (Sen 1985). The species thrives in estuarine waters up to 18 ppt salinity (Courtenay et al. 1970).
Trophic Mode: Walking catfish are benthic (bottom-dwelling) fish that have have been characterized as voracious opportunistic consumers. Most feeding activity occurs at night and their dietary range includes aquatic insects, insect larvae, small fish, fish eggs and larvae, and occasional plant material (Courtenay 1970, Courtenay et al. 1974). Periods of drought may lead to the aggregation of many individuals within isolated pools where they usually rapidly consume available prey items (Courtenay et al. 1974).
When individuals are dormant within mud burrows they typically survive for several months without feeding (Courtenay 1970).
Associated Species: Florida native species interactions with walking catfish are typically non-beneficial. C. batrachus will outcompete or directly consume several co-occurring native species.
Invasion History: Walking catfish are native to southeastern Asia, and the natural range of the species includes Sri Lanka, eastern India, Pakistan, Bangladesh, Burma, Malaysia, Singapore, Philippines, Borneo, Java, and Thailand (Axelrod et al. 1971, Jayaram 1981 Sen 1985, Talwar and Jhingran 1991).
They were originally imported to the U.S. in the 1960s as an aquarium industry candidate species. Unlike many other accidental introductions to the state of Florida, the release of C. batrachus is partly documented. The initial escape of this species, in the mid-1960s, was either from a Broward County, Florida fish farm or from a truck transporting broodstock. The animals that escaped were derived from stock originally imported from Thailand (Courtenay et al. 1974, Courtenay et al. 1984).
A second, independent introduction in 1967-1968 has been implicated in the establishment of a Hillsborough County C. batrachus population (Courtenay et al. 1974, Courtenay 1978). In this case, animals were reportedly intentionally released by fish farmers after the state banned the possession of walking catfish.
Since these initial release events, walking catfish have rapidly expanded their range within Florida. A combination of traits such as high fecundity, the ability to cross land barriers to move between isolated water bodies, and the ability to avoid desiccation during dry seasons make this fish a capable and pioneering invader. The ability of this species to utilize networks of man-made drainage canals has further facilitated its spread in Florida and elsewhere (Nico 2005).
Populations of walking catfish became established in Florida within just a couple years of the initial introduction; by 1968 the species was already established in three counties (Courtenay et al. 1984). Within a decade of initial establishment, the Florida range of walking catfish would expand to nearly 20 counties, and by 1984 the range included all counties within the IRL watershed except Volusia.
Potential to Compete With Natives: Early on, the invasive potential of C. batrachus was recognized, and Lachner (1970) suggested that the walking catfish Florida introduction was at that time potentially the most harmful introduction to date in the country. That author harvested as much as 3,000 lbs of walking catfish per acre from isolated Florida small ponds.
The walking catfish is a threat to the native fish populations in Florida and the broader Gulf of Mexico region. The impacts from this opportunist feeder are probably most pronounced in small, isolated wetland ponds where walking catfish quickly consume or outcompete other resident populations to become the dominant species in the pond (Shafland 1996). Resident centrarchids (freshwater sunfish) and native catfish species appear particularly susceptible to impacts from this invader.
Baber and Babbitt (2003) note that C. batrachus can also negatively impact native amphibian populations by preying on tadpoles. The ability of walking catfish to exploit isolated, ephemeral water bodies allows them to access tadpole prey stocks that other fish cannot reach.
Axelrod H.R., Emmens C.W., Sculthorpe D., Winkler W.V., and N. Pronek. 1971. Exotic Tropical Fishes. TFH Publications, Inc. Jersey City, NJ.
Baber M.J. and K.J Babbitt. 2003. The relative impacts of native and introduced predatory fish on a temporary wetland tadpoles assemblage. Oecologia 136:289-295.
Courtenay, W.R., Jr., Sahlman H.F., Miley W.W., II, and D.J. Herrema. 1974. Exotic fishes in fresh and brackish waters of Florida. Biological Conservation 6:292-302.
Courtenay W.R. Jr. and W.W. Miley, II. 1975. Range expansion and Environmental impress of the introduced walking catfish in the United States. Environmental Coservation 2:145-148.
Courtenay W.R., Jr. 1978. Additional range expansion in Florida of the introduced walking catfish. Environmental Conservation 5:273-275.
Courtenay W.R., Jr., Hensley D.A., Taylor J.N., and J.A. McCann. 1984. Distribution of exotic fishes in the continental United States. Pages 41-77 in Courtenay W.R. , Jr., and J.R. Stauffer, Jr. (eds.). Distribution, biology and management of exotic fishes. Johns Hopkins University Press, Baltimore, MD.
Courtenay W.R., Jr. and J.R. Stauffer, Jr. 1990. The introduced fish problem and the aquarium fish industry. Journal of the World Aquaculture Society 21:145-159.
Courtenay, W.R., Jr., Jennings D.P., and J.D. Williams. 1991. Appendix 2, exotic fishes. Pages 97-107 in Robins C.R., Bailey R.M., Bond C.E., Brokker J.R., Lachner E.A., Lea R.N., and W.B. Scott (eds.). Common and scientific names of fishes from the U.S. and Canada. Special Publication 20, American Fisheries Society, Bethesda, MD.
Deacon J.E. and J.E. Williams. 1984. Annotated list of the fishes of Nevada. Proceedings of the Biological Society of Washington 97:103-118.
Hartel K.E. 1992. Non-native fishes known from Massachusetts freshwaters. Occasional Reports of the Museum of Comparative Zoology, Harvard University, Fish Department. September 1992:1-9.
Hensley D.A. and W.R. Courtenay, Jr. 1980. Clarias batrachus (Linnaeus) Walking Catfish. Page 475 In Lee D.S., Gilbert C.R., Hocutt C.H., Jenkins R.E., McAllister D.E., And J.R. Stauffer, Jr. Atlas Of North American Freshwater Fishes. North Carolina Biological Survey Publication #1980-12. North Carolina State Museum Of Natural History. 854 p.
Jayaram K.C. 1981. The Freshwater Fishes Of India, Pakistan, Bangladesh, Burma, and Sri Lanka- A Handbook. Zoological Survey Of India, Calcutta. 475 p.
Lachner E.A., Robins C.R., and W.R. Courtenay, Jr. 1970. Exotic fishes and other aquatic organisms introduced into North America. Smithsonian Contributions to Zoology 59:1-29.
Page L.M. and B.M. Burr. 1991. A field guide to freshwater fishes of North America north of Mexico. The Peterson Field Guide Series, volume 42. Houghton Mifflin Company, Boston, MA.
Nico L. 2005. Clarias batrachus Nonindigenous Aquatic Species Database, Gainesville, FL.
Rao G.R.M., Singh S.K. and A.K. Sahu,., 1995. Fry and fingerling production of Clarias batrachus (Linnaeus). Asian J. Zool. Sci. 4:7-10.
Sen T.K. 1985. The Fish Fauna Of Assam And Neighbouring North-eastern States Of India. Records Of The Zoological Survey Of India, Miscellaneous Publication, Occasional Paper No. 64. Calcutta. 217 p.
Shafland P.L. and J.M. Pestrak. 1982. Lower lethal temperatures for fourteen non-native fishes in Florida. Environmental Biology of Fishes 7:139-156.
Shafland P.L. 1996. Exotic Fishes of Florida-1994. Reviews in Fisheries Science 4:101-122.
Talwar P.K. and A.G. Jhingran, (eds.). 1992. Inland fishes of India and adjacent countries. A.A. Balkema, Rotterdam, The Netherlands.