THE UNIQUE COOPER CREEK CATFISH FROM CENTRAL AUSTRALIA

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Peter J. Unmack

Introduction

The Cooper Creek catfish is an undescribed species and genus in the family Plotosidae (eeltailed catfish). It possesses a number of unusual features compared to other plotosids including low fecundity, large eggs, good eyesight, and very aggressive behaviour towards other fish. It appears to have no close relatives and occurs only in the Cooper Creek system in Queensland and South Australia which is part of the internal Lake Eyre drainage. The first museum specimens were collected in 1918 and again in 1938 (Queensland Museum specimens), however, the species was not recognised as being different until the late 1960's when Hamar Midgley collected additional specimens (Lake 1971). Since then, it has been collected infrequently, few specimens exist in museum collections and this species has rarely been kept in aquaria. All comparisons made here between Cooper Creek catfish and other plotosids are based on my experiences with the following Australian species, Hyrtl's catfish (Neosilurus hyrtlii), black catfish (N. ater), Dalhousie catfish (Neosilurus sp), silver tandan (Porochilus argenteus), Rendahl's catfish (P. rendahli), and freshwater catfish (Tandanus tandanus). Thus, when I refer to plotosids I am referring to these species only.

Description

The Cooper Creek catfish grows to at least 600mm (2ft) and has a very blunt head and relatively small eyes compared to other plotosids. It's mouth is underslung with bony tooth plates capable of drawing blood from unsuspecting fingers. Colouration in the wild is typically creamy brown with dark speckles along the upper body and tail. Under aquarium conditions, colour varies considerably from a light blue grey to a darker shade of blue grey depending upon light intensity and mood of the fish. Speckling along the body and tail varies from slightly lighter than the background colouration to white against a dark background. Only two photographs have previously been published (Lake 1978, Steptoe 1988).

Habitat

Cooper Creek catfish is common, though not abundant throughout the Cooper Creek system. It is usually found in larger, more permanent waterholes with an earthen/clay substrate (very few central Australian waterholes can be considered strictly permanent). As far as I'm aware they are yet to be collected from very ephemeral waterholes, making it one of only two riverine species [the other being Welch's grunter (Bidyanus welchi)] which I have not recorded from ephemeral waterholes in central Australia. Water conditions vary considerably depending upon time of year and hydrological conditions. The water is almost invariably turbid with no visibility, although occasionally the river runs clear after major flooding, as occurred in the Barcoo River after massive floods in 1990. After initial turbid runoff, the river is fed by ephemeral springs which release clear water until their supply runs out, then the river's flow ceases and turbidity increases again. (Spring discharge is determined by the quantity of rainfall received; significant spring flow occurs only after severe rainfall events).

Conservation Status

They are not presently considered to be in any danger of extinction.

Collection and transport

I have captured Cooper Creek catfish by angling only. Specimens may be captured in a variety of nets (illegal without relevant fisheries permits), however, despite considerable sampling effort with small seine nets I have never captured them this way. I have transported them with no losses from Queensland to Victoria in drums of about 70 litres with two large fish per container at temperatures up to about 32^oC. No aeration was required unless the vehicle stopped moving for more than an hour or so.

Aquarium care

I have maintained Cooper Creek catfish in aquaria between 12-32^oC with preferred temperatures probably being around 24-28^oC and a pH range of 6.8 to 7.5, although they can tolerate more acidic conditions. In the wild, based on the stomach contents of a few fish, they appear to feed on snails. In captivity they have accepted small live fish, fillets of fish, shrimps, earthworms and artificial pellets. Unlike most plotosids, this species must be kept alone as they are very aggressive, both amongst themselves and with different species, usually killing other fish in a short time. They are also likely to attack siphon hoses, human appendages and anything else placed in the aquarium.

Breeding biology

Virtually nothing is known of their breeding biology. No specimens less than 160mm (6½") are found in museum collections and I am yet to meet anyone who claims to have caught any smaller fish. Most plotosids are easily sexed by examining their urinogenital papilla (H. Midgley pers. comm.), and Cooper Creek catfish are no exception. Males posses a longer, thinner pointed urinogenital papilla, while that of the female's is somewhat shorter, broader and flatter (see fig. 1). The urinogenital papilla size of both sexes varies depending with age and time of year.

During December 1993, a breeding attempt was made by R. Lewis, P. Unmack, C. Brumley, and A. Moore. Two eggs were extracted from a female (460mm total length) using a catheter. They were cloudy, yellowish, non adhesive and slightly distorted in shape by the catheter. The eggs were not sufficiently developed to attempt to fertilise and no sperm could be extracted from the male; thus the experiment was terminated.

In comparison to published information on other Australian plotosids, Cooper Creek catfish have a much lower fecundity with significantly larger eggs, (see Table 1.).

Table 1. Comparison of available data on fecundity and egg size for various Australian plotosids

species	length (mm)	weight (grams)	no. of eggs	egg size (mm)	reference
Cooper Creek catfish	460	-	1000 §	3×4, 2.75×3.75	this paper
Dalhousie catfish	72-120	-	136-1197	1	Glover 1989
black catfish	348-440	-	14600-20400	2.6 ‡	Orr and Milward 1984
Hyrtl’s catfish	186-267	-	1600-15300	1-1.5 †	Orr and Milward 1984
freshwater catfish	470-550	1250-2000	18000-26000	2.5-2.9*	Lake 1967a, b
freshwater catfish	-	675-2275	2000-20600	2.28-3.05*	Davis 1977

* from running ripe fish ‡ after fertilisation § estimated by R. Lewis † estimated by R. Wager (pers. comm.)

A number of factors need investigation before we begin to understand the breeding biology of Cooper Creek catfish. This includes investigation of environmental factors that stimulate reproduction, whether they are egg scatterers or nest builders (no nests have been reported), time to hatching, size of fry, time until first feeding, and the size at which they mature.

I believe this species will spawn at temperatures around 26^oC based on the observation that the female's urinogenital papilla appears to be most extended and swollen around this temperature. Based on the low fecundity I believe spawning occurs annually, although good recruitment is probably linked to major flooding when fry food is more abundant. I suspect, given their low fecundity, they will either guard their eggs or mouth brood them. Based on the egg size, fry will be large and well developed at hatching.

General discussion

Due to the limited collection of this species, one is forced to speculate and infer from knowledge of other species, thus what is said in this section should not be taken as fact.

I believe the Cooper Creek catfish is a relict that does not exhibit the usual adaptations to desert conditions. They would appear to be better suited to an environment with stable or seasonal conditions and clearer water. Based on aquarium observations, the plotosids that I have kept have relatively poor eyesight. Species such as Hyrtl's catfish and silver tandan appear to find food by chance or smell, not eyesight. As food is dropped in the tank they stumble across it on the bottom of the tank. In addition to an excellent sense of smell (like most plotosids), Cooper Creek catfish have very good eyesight and will usually rise off the bottom to take food as it falls through the water. They are also more efficient at catching live fish and shrimp than other plotosids. This poses the question of why a fish needs good eyesight to live in waterholes so turbid that most of the time visibility is no greater than 2cm. Presumably its ancestors lived in a clear water environment where good eyesight is advantageous. Usually, species with smaller eyes have poorer eyesight relative to those with larger eyes. In contrast to this, the Cooper Creek catfish has the smallest eyes, but the best eyesight among plotosids.

Compared to other fish from the Australian arid zone, Cooper Creek catfish have the largest eggs and lowest fecundity per body weight to egg ratio. Low fecundity combined with large egg size is not usually associated with fish in desert rivers. It is more likely to be encountered in fishes living in stable environments with regular seasons.

Cooper Creek catfish, based on its general appearance (morphology) and what little else is known of it, doesn't appear to have any close relatives, at least not at the generic level of classification. Where have its relatives gone? What has it evolved from? Perhaps it is the last in a plotosid evolutionary branch that was once widespread, or alternatively one that evolved in a particular area and never became widespread. Only details from the fossil record and examination of plotosid osteology or genetics may answer this. Irrespective of all that, Cooper Creek catfish would have most likely been widespread in the Lake Eyre drainage during recent wetter phases in central Australia's history such as when Lake Dieri is thought to have existed. Lake Dieri is the ancestral Lake Eyre which is thought to have existed sometime in the last 20-45 000 years before present when central Australia experienced higher rainfall. During this period, Lake Dieri was probably larger than the present day Lake Eyre and contained a permanent body of freshwater (Dulhunty 1982, 1983) [There is some dispute as to the timing, size and existence of precursors to Lake Eyre, see Stevens (1991).] Cooper Creek catfish were probably one of the higher predators in the system (with the advantage of good eyesight), along with yellowbelly/golden perch (Macquaria ambigua) and possibly other species no longer found in central Australia. I believe increasing aridity eliminated Cooper Creek catfish from most of their range and they were only able to persist in Cooper Creek because it has more permanent waterholes than other central Australian rivers. If the hypothesised reduction in range in central Australia with increasing aridity is correct, and aridity increases in the future, Cooper Creek catfish will probably join the dinosaurs in extinction.

I predict that detailed research into the biology and taxonomy of Australia's largely unknown plotosids will increase the number of unique and interesting features recognised in the Cooper Creek catfish.

Conclusions

The Cooper Creek catfish is not for the faint hearted. It has a voracious appetite, an aggressive manner, and requires a large tank. Despite this, its rewarding to keep and specimens become very responsive towards their owners.

Acknowledgments

Thanks to R. Lewis, C. Brumley, and A. Moore for their assistance in trying to breed this species. Thanks also to those who commented on earlier drafts.

References

Davis, T. L. O. 1977. Reproductive biology of the freshwater catfish, Tandanus tandanus Mitchell, in the Gwydir River, Australia II. Gonadal cycle and fecundity. Australian Journal of Freshwater and Marine Research. 28: 159-169.

Dulhunty, J. A. 1982. Holocene sedimentary environments in Lake Eyre, South Australia. Journal of the Geological Society of Australia. 29: 437-442.

Dulhunty, J. A. 1983. Lake Dieri and its Pleistocene environment of sedimentation, South Australia. Journal and Proceedings, Royal Society of New South Wales. 116: 11-15.

Glover, C. J. M. 1989. Fishes. In Natural History of Dalhousie Springs. Ed. Zeidler, W. & Ponder, W. F. South Australian Museum, Adelaide. pp 89-112.

Lake, J. S. 1967a. Rearing experiments with five species of Australian freshwater fishes I. Inducement to spawning. Australian Journal of Freshwater and Marine Research. 18: 137-153.

Lake, J. S. 1967b. Rearing experiments with five species of Australian freshwater fishes II. Morphogenesis and ontogeny. Australian Journal of Freshwater and Marine Research. 18: 155-173.

Lake, J. S. 1971. Freshwater fishes and rivers in Australia. Nelson, Melbourne. 61pp.

Lake, J. S. 1978. Freshwater fishes of Australia. An Illustrated Field Guide Nelson, Melbourne. 160pp.

Orr, T. M. and Milward, N. E. 1984. Reproduction and development of Neosilurus ater (Perugia) and Neosilurus hyrtlii Steindachner (Teleostei: Plotosidae) in a tropical Queensland stream. Australian Journal of Freshwater and Marine Research. 35: 187-195.

Steptoe, W. 1988. Expedition to Min Min country. Modern Fishing. December 1988.

Stevens, B. P. J. 1991. Some aligned claypans in the Strzelecki dunefield, central Australia. Australian Journal of Earth Sciences. 38: 485-495.