AFC#4 Hunters of the Murky Depths

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Hunters of the Murky Depths

By Anthony Williams

Some years ago, I was witness to a strange phenomenon which gives credence to the intelligence of one of our ugliest fish. Maramba, pong, barbel and catfish - names which conjure images of smelly, slimy belching scavengers. It is true that while few anglers revel catching a pong, their powerful hard-fighting nature make them a worthy adversary for even the toughest of tackle, or the most experienced of anglers.

Documented cases of catfish hunting in packs, working together to drive smaller fish into the shallows, have been common knowledge for decades. While we accept this behavior amongst more "intelligent" creatures, the thought of this "dumb" scavenger being capable of such intelligence is almost unbelievable. Reading about it, and actually witnessing it are two very different things.

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Mohomed Ali with his 11.18lb bass, caught at Darwendale on a Zoom frog in the late afternoon.

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Whilst at the Bangala Ranch before it became a government re-distributed wasteland, we visited the hunting camp established on the ranch. The camp is set on a small weir of about three or four acres, more of a pond providing a tropical setting for the chalets which ramble through the granite boulders. After dinner every evening, we gathered around the blazing camp fire, whiling away the evening until the cold seeped through the warmth of the glowing fire.

Directly below us, the waters of the weir lapped up against the platform on which the camp fire is sited. From the darkness beyond, loud splashes emanated from the water below, drowning out most other night sounds. My interest piqued, I found it hard to concentrate on conversation. Theories of feeding bass to hunting crocs (there were two small crocs in the weir) seemed likely given the volume of noise.

Panicked skittering noises (as small fish skimmed across the surface) were punctuated with loud splashing sounds similar to that made when the water is slapped with a plank, or cricket bat. The sounds would begin slowly, building to a crescendo... then stop, only to begin again in another part of the weir. I could stand it no longer, and armed with a Maglight went to investigate.

It took several attempts to pin-point the culprits. As a frenzy erupted just below us, I shone the torch into the murky water. There, with eyes shining were the sinister silhouettes of a dozen or more catfish, strung out in a semi-circle moving toward the shoreline. Small fish, trapped by the slowly approaching wall of hungry mouths, would come up against the shallow water. With no escape, and obviously aware that they could not get through the closing net, they took to the surface, tale walking (more like running) three or four metres before diving to their escape.

Homing in on the "walking" fish, the catfish seemed able to anticipate their line of travel, and the loud splashing noises were made as the large, flat, bony head rose from the depths to snatch a passing fish from the surface. It was a most chilling attack to witness.

Moving away from the cool water's edge, where only pinpricks of starlight reflected while eyes shone from the depths, and in spite of the leaping flames from the fire, shivers rose the hair on my neck. In my mind's eye, I could see those dark, menacing and persistent hunting machines moving through the murk. With human emotion, I felt a pang of sympathy for those hunted fish. Without a final retreat, they would spend the entire night running and hiding, running and hiding, until dawn brought a new game, and a new set of rules.

From the lodge a couple of hundred metres away, the din continued through the night. In the dying moments of consciousness before I drifted off to sleep, I felt a new sense of respect awakening for a creature I had previously given little regard.

It has been many years since this personal observation, and with the dawn of the Millennium new research into both catfish and other “nocturnal” species like Cornish jack has been undertaking, revealing the extent of their ability to communicate. What follows, are a couple of reports penned by John Minshull on this behavior. It is interesting stuff, and will force us simple anglers to re-think the way we approach these species.

Also included here, are a couple of videos showing catfish hunting prey. One is in the Okavango, which is more of a feeding frenzy which occurs annually and is spurred by a sudden abundance of food (small fish) as they re-emerge into the river proper after the annual flooding onto the plains.

The other video (below) - if one ignores the rather childish story boards insertions which accompany it - is very interesting as it is similar to the experience detailed above. Watch as the animals come to drink, and the catfish detect the movement at the water’s edge or on the water’s surface, and launch an attack, obviously believing it to be a small fish or at least something to eat.

Pack Hunting by Sharptooth Catfish in the Okavango Swamp

By John L. Minshull Msc TRE, Environmental Ecologist

Many anglers have only a hazy idea of the work that aquatic biologists actually do in their lives. Getting to know all the species of fish and giving illustrated talks at club meetings, is only a small part of the workload that is driven by an insatiable urge to understand the behaviour and interactions between individuals forming the many aquatic communities found in rivers and lakes.

It is amazing how much information lies in fish anatomy for instance, and in gut contents, state of male and female gonads and length and age of each fish. Once species are determined, a daily or weekly trapping and gill-netting program is set up and all possible data extracted from every fish caught for a year or two. Thus fairly detailed information on each species life-style emerges that directs you into various other scenarios to be investigated. Publishing the results brings other scientists into the picture from other Universities, often from other countries. The behaviour of fish is to me the most fascinating study, out of which yet more aspects arise, dangling like carrots in front of the proverbial donkey. We can never stop learning from Nature which envelopes us; we are very much part of it.

The story of catfish pack hunting will, I hope, illustrate to you the driving force behind scientists to find out more and more and finally, to turn a hypothesis into a scientific theory that will stand the test of time; for this, it must be replicable by other scientists the world over. This is what Creationists refuse to understand.

In 1979 the JLB Smith Institute (now the South African Institute of Aquatic Biodiversity) at Rhodes University conducted the first of many expeditions to Okavango swamp to capture and identify fishes. In 1983 Glenn Merron from the University of Michigan was appointed as Okavango Research Officer and continued sampling and observing fish behaviour in the delta. There he witnessed the pack-hunting of Clarias gariepinus, the Sharptooth catfish, described by Dr Mike Bruton in 1979 at Lake Sibayi in Zululand. Intrigued, he captured 363 catfishes by gill-nets during pack-hunting runs and removed their gut contents; later identified by him back in the Grahamstown laboratories. He found that 67% were male bulldogs only, Marcusenius altisambesi and 26% the Churchills, Petrocephalus catastoma. Both of them belong to the weakly electric mormyrid group of fishes that German and USA scientists had shown able to communicate with each other using varying pulses of high frequency electricity from organs in their tails. This species specific “language” appears to be their evolutionary speciality. They apparently have no need of good eyesight as their eyes are small and they are very active at night. Glenn published these results in 1993, hypothesizing that catfish had selected weakly electric mormyrids as prey.

Dr Bernd Kramer of Regensburg University in Germany was intrigued by this data, thus joined SAIAB expeditions to Okavango Swamp and Upper Zambezi River, to record electric pulses produced by various mormyrids in the region, storing them on CD’s. Back in Germany he arranged for young C. gariepinus, bred at Utrecht University, Netherlands, to be sent to him. Live bulldogs that are the most abundant Okavango mormyrids, and Churchill came from southern Africa. Susanne Hanika performed the laboratory experiments to find out if Sharptooth catfish could sense mormyrid presence by their electric discharges, using both live fish and the CD’s.

The results were at first paradoxical, catfish surprisingly being unable to sense the high frequency pulses over 2500Hz, generated by the mormyrids. This problem was solved when they discovered a marked sexual dimorphism in the EOD’s (electric organ discharge) of male bulldogs on becoming sexually mature, with no other mormyrid species exhibiting this. Young adolescent male bulldogs of 12.6cm length, showed an almost 10% increase in the strength and duration of their DC component, producing an EOD of low frequency below 2500Hz that was detected by the catfish (perhaps this is analogous to the traumatic voice-breaking phase in teenage boys as their testes drop and they start raucously running after pretty girls?).

And what was the average length of Glenn’s large sample of male bulldogs from catfish stomachs?: 12.5cm! This clearly confirms the hypothesis that Sharptooth catfish specifically electro-locate adolescent male bulldogs after they become sexually mature.

Unfortunately for the bulldogs in the Okavango swamps, the dry winter months cause shallow water levels to drop and floating papyrus beds become grounded, causing vast numbers of small fish to leave cover and take to well oxygenated river channels where they are open to all predators in the delta. Catfishes appear to be the most efficient as thousands congregate, hunting in packs, forcing and enclosing shoals of small fish against walls of papyrus; electro-locating and swallowing mostly bulldogs.

One of Glenn’s catfishes had eight bulldogs in its stomach. Numerous other predators line the papyrus alongside the frenzied melee that moves slowly down the channel, taking their pick of little fish. Pack-hunting is a very efficient way of feeding; far better than hunting on your own and catfishes capitalize on this bonanza by being one of the first species to breed in the swamp when the Okavango floods arrive.

This research is typical of the work carried out by Universities, Museums and Conservation Departments/Organisations, utilizing keen young students who wish to obtain MSc or PhD degrees that can carry them around the world into varying ecological positions. Such investigations open up new research projects too, eg the Clariid catfishes of Asia and Africa and other swamp-dwelling fish may be tested to reveal if electro-location is more prevalent than originally thought. Indeed, it may be far more common throughout the world of fish.

Pack Hunting by Cornish Jack

By John L. Minshull Msc TRE, Environmental Ecologist

Lake Malawi, with crystal clear water, offers unique opportunities for the study of many species of fish using underwater scuba. A main predator has been found to be the strange Mormyrid fish, Mormyrops anguilloides, the Cornish jack, also found in Lake Kariba. It is only 50 years ago that Mormyrid fishes were first recognized as weakly electrical fish by the scientific world, although anglers had received surprise shocks from large bottlenose or Cornish jack.

It was thought that shocks deterred predators and small impulses used for communication and sensing prey. In the 70’s we observed bottlenoses swimming in tandem, nose to tail as if one fish, in a very large tank at Lake Kariba Fisheries Research Institute, suggesting they could indeed keep in contact in the dark. To my knowledge this was not published. In the 1960’s the Germans and Americans began to experiment with small Mormyrids from West Africa and they became important in medical biology in unravelling the intricacies of electric fish brains, nerves and “organic batteries”, all of which are important in understanding how the human brain functions too; (remember, that all vertebrates started off as fish and evolved into amphibians, reptiles, birds and mammals).

We now know that amongst the estimated 400 odd Mormyrids of Africa, most of which are in the Congo Basin and West Africa, certain groups have evolved a so-called split brain which deciphers electric signals from other fish of the same or different species to an astonishing degree. They have a sort of dot-dash Morse code, a crude “language” so to speak; capable of identifying individuals, sexes, age groups and different species. Recordings have been taken and sound like fat sizzling in a frying pan, varying in timing and intensity. Fishes with an ordinary Mormyrid brain produce a limited range of impulses. The whole issue is very complicated, with many types of differing electric cells in the West African and Congo fish. Our bottlenose and Cornish jacks appear to be split-brains with a fairly advanced language.

The organic battery consists of lateral muscle cells that form plates, stacked in piles, each connected to another by nerves and forming a series. Each cell/plate is called an electrocyte and the organ is situated in the tail end of the body. The electric organ discharge (EOD) is triggered by the brain and can be affected by hormones in the blood.

M. Arnegard and B. Carlson, scientists from America, researched fish behaviour in Lake Malawi and observed fish at night. The Mbuna (cichlid bream) inhabit the rocks and are very numerous, feeding on algae, tiny crustaceans and insects. The Cornish jack was found to actively hunt these fish in the dark. By day they holed up in a cave out of the sun and were inactive. Activity began with sunset with the start of EOD’s. By detecting distortions in their own electric field Mormyrids can detect objects, their size, distance and electric field. When swimming alone at night a Cornish jack will send out irregular pulses ranging from 40 – 300 millisecs.

However, when they hunt in a pack, they keep in touch with each other by coordinated bursts of EOD’s at 40 – 70 msecs. As they sense prey they slow down and creep up to within a few centimetres, then start probing at 18 – 20 msecs to estimate prey size. Prey fish appear unaware of them. If size is right, then within a few seconds a strike ensues. If a miss occurs then fleeing prey fish may be caught by another pack-member. Pack hunting yielded more prey per individual than hunting alone. The pack follows a circuitous route around the reef, occasionally returning briefly to their lying up hole that can be 20 metres away. Pack fish often leave the group on catching a fish but soon after return easily to the same pack, even if it has moved away 15 metres. They consistently hunt together in the same packs of 1-10 fish at night. The packs retain their members even though they mingle with other packs after identifying each other before moving on. This shows clearly that individual recognition maintains group integrity. It is based on individual variations of EOD’s that measured from 0.7 to 2.4 msecs.

This interesting behaviour is similar to hunting dogs or to wolves, whose cohesion relies on sight, sound and smell and this appears to be the first ever recorded pack hunting in fish, albeit electric pulses and sound are the senses used. We can expect a lot more research done on Lake Malawi to elucidate the finer points of Mormyrid communication. For example, in breeding behaviour; are they monogamous or catholic in choice of mates, do they guard areas where eggs are laid? Do bottlenose exhibit more specialized behaviour due to their electro-communication? They do follow each other nose to tail like circus elephants, as previously mentioned. Is this group cohesion in both species due to family connections or not? This will entail both genetic and behavioural observations.

Connor Lashbrook (6 years old) with his 18lb Cornish Jack, caught just off Antelope Island at half term at the end of 2012.
Liesl Grundling of Bulawayo caught this 85cm Cornish Jack at D Camp Nyakasanga at 8am on September 6th, 2014.
Alec Mammoth with his Cornish Jack in Kariba.

Reference ; Arnegard, ME & Carlson, BA 2005. Electric Organ discharge in Hunting by a Mormyrid Fish. Proc. R. Soc. B 237 : 1305 – 1314.

Hunters of the Murky Depths

Connor Lashbrook (6 years old) with his 18lb Cornish Jack, caught just off Antelope Island at half term at the end of 2012.

Liesl Grundling of Bulawayo caught this 85cm Cornish Jack at D Camp Nyakasanga at 8am on September 6th, 2014.

Alec Mammoth with his Cornish Jack in Kariba.