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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).
II. HABITAT AND DISTRIBUTION
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
(GSMFC).
III. LIFE HISTORY AND POPULATION BIOLOGY
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 vulberable 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.
IV. PHYSICAL TOLERANCES
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).
Dissolved oxygen:
The common name "walking catfish" comes from this species'
impressive ability to "walk" on land, traveling between ponds when a
home pond dries up or after a heavy rainfall (Courtenay et al.
1974, Hensley and Courtenay 1980, Liem 1987).
They walk on their robust pectoral spines, flexing their body to
effect awkward movement on land.
Several physiological adaptations allow the species to leave the
water for extended periods. These include a greatly reduced gas
bladder and gills that are structurally stiffened to prevent
collapse on land. The gills also exhibit highly vascularized
arborescent (tree-like) organs that act as accessory breathing
structures aiding respiration on land and in stagnant waters.
V. COMMUNITY ECOLOGY
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.
VI. INVASION INFORMATION
Invasion History:
Walking catfish are native to southeastern Asia, and the natural
range of the species includes Sri Lanka, eastern India, Pakistan,
Bangladesh, Burma, Ceylon, Malaya, 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
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 Gulf States Marine Fisheries
Commission calls it "one of the most notorious and harmful
non-indigenous species" in the ecosystem.
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.
Possible Economic Consequences of Invasion:
The actual ecological and economic impact of C. batrachus
introduction in Florida is still largely unknown. One specific
example of an observed economic impacts is the cost associated with
barrier fences. Florida fish farmers have had to install such fences to keep
walking catfish out of their ponds (Courtenay and Stauffer 1990,
Nico 2005).
Although they are reared as a food crop within much of their native
range, particularly in India (Sen, 1985), walking
catfish are not at this time valued as a food or sport fish in
Florida
The walking catfish has been nominated as among 100 of the
"World's Worst Invaders" by the Invasive Species
Specialist Group (ISSG).
VII.
REFERENCES
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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.
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impress of the introduced walking catfish in the United States. Environmental
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India. Records Of The Zoological Survey Of India, Miscellaneous Publication,
Occasional Paper No. 64. Calcutta. 217 p.
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non-native fishes in Florida. Environmental Biology of Fishes 7:139-156.
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Talwar P.K. and A.G. Jhingran, (eds.). 1992. Inland fishes of India and
adjacent countries. A.A. Balkema, Rotterdam, The Netherlands.
Report by:
J. Masterson, Smithsonian Marine Station
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