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"Out with the old, in with the new". 

The 'Old World' catfish, Silurus.glanis; it's evolutionary success story and unrivalled feeding strategy.

Guy Bridgeman

The European Catfish is a worthy adversary of any specimen hunter, growing up to a recorded 674lbs in the Dnepr river, Russia (Berg, 1932), with a report of a 500lbs specimen from the Dnipro river, Ukraine (1995), both from commercial fishing vessels, it is no surprise that it is has given birth to a number of successful sport-fisheries in Spain, France, Germany, Italy and (to a lesser extent) eastern Europe. Like any quarry, it helps to understand how it behaves and what ambient conditions motivate it to feed, how and when it finds it's food, and what it's preferred food might be at any particular time. Of course, fishing's greatest lure is the magic of not being able to 'see' your quarry, and this means that nothing is in black and white, but being able to predict the behaviour of your target can only add to the feeling of excitement as you wait by your rods (and praying a monster is lurking near your baited hook!). 

Although I'm a keen angler myself, my own specialism is in the scientific side of things and my research and other interests have taken away the time that I used to have for fishing, so I shall concentrate on interpreting the most relevant information into useful facts to allow you to do what you're probably more qualified to do than me; that is, catch catfish! The term 'Old World' refers to the Asian origins of this fish and it's close relatives; which live as far east as Japan (Silurus.lithophilus) and are identical in every detail, but due to the climate are only active for two months of the year and correspondingly only grow to 60cm in length. In Japan, the catfish ('namazu') is highly respected, and legend says that a giant catfish used to swim under the surface of the earth and if it was angry it would thrash it's tail, causing earthquakes and creating tidal waves that would drown anyone unfortunate enough to be in it's path! 

The distribution of S.glanis reaches into countries south of the Aral Sea, such as Turkmenistan, (in the drainage catchments of the north western foothills of the Himalayas), and as far north as Sweden and Estonia (where it's populations are in decline due to global 'cooling' and is protected in both countries). The Swedish population is of great scientific value as it has been isolated from mainland Europe for thousands of years (current record is 163lbs). It is believed to have crossed the Black, Caspian and Aral Seas in the evolutionary growth of its distribution and ascended the large rivers that drained central and Eastern Europe. The greatest abundance of 'cats' in these rivers is in their lower reaches and delta basins due to the more stable temperature regimes and rich supply of prey organisms. 

It is believed the fish first introduced into the river Seille(twenty 8oz fish) in 1968, a tributary of the river Soane, France, have now populated the river Rhone to the west! of the river Soane by travelling down stream and across the Mediterranean Sea! This bears provenance to its evolutionary success and a ring of truth to the Japanese myth. What this fish can't escape is it's dependence on temperature, where a carp will digest food in water above 5 degrees Celsius the European catfish needs temperatures from 10 degrees, hence the shorter season. The strongly increasing activity of its digestive enzymes and their stability at higher temperatures is also an evolutionary indicator of its tropical ancestry.

Knowing what the temperature is of the water you're fishing is important because over 30% of its annual food ration may be consumed in the 8 weeks preceding spawning. Ovulation (development of the ova/spawn) is a continual process and may remain in a state of stasis if a cold season occurs. When water temperatures reach 14 degrees Celsius (spawning occurs over 18 degrees), the final stage of gametegenesis begins, and it is at this stage when the greatest weight gains are made. It's a shame that this period happens to be in our close season, but it gives us Brits something else to moan about I suppose! Following spawning stresses, and with increasing temperatures, 'cats' feed steadily until temperatures turn around August (associated to the shortening days) and like many other species, a late feeding binge occurs.

The feeding objective for these fish is not to get injured for a number of reasons; they are blind (the eyes lack the sclerotic membrane), they have no scales to prevent tissue damage and their sensitive barbules are the equivalent to our hands. To this end, a 'strike' on a prey organism will in the majority of cases be what the 'cat' perceives as the rear of its victim. Many natural historians made comparisons to the marine 'angler' fish, Lophius.piscatorius (Yarell 1836, Lacepede 1836, Demoll-Maier 1941), although none of the authors were alive when it was found in 1987 (Reutter,K) that the upper sections of the maxillary (upper jaw) barbules have highly advanced light sensitive cells that are able to determine shades of grey. These barbules, it has been determined, can not only 'fish' while the S.glanis lays dormant in the sediment, but can also be used to reveal densities of fry at the surf! ace and the position of prey just prior to being engulfed. The cells also tell the 'cats' when the weather is dull, reducing risk to predators such as Herons or Cormorants. So apart from your thermometer, take a light meter too!

The jaws have rows of small, inward facing cardiform teeth that act like sandpaper, creating friction to hold larger prey, which lead to the buccal cavity (back of the mouth to you and me) that has a toughened palate and tongue for crushing. The pharyngeal teeth (molars to you and!) at the entrance to the oesophagus have masticatory moliform pads similar to the common Bream, an inference of the more omnivorous nature of its menu. The oesophagus and stomach are lined with strong collagen fibres and muscles to manipulate prey, although the stomach is proportionately smaller than is expected of a predator. Likewise the intestine is long (50% longer than the common Carp) and has the dual purpose of allowing greater nutritional uptake of essential amino acids and control of oxygen consumption allowing it to feed even in extremely low concentrations of dissolved oxygen. This is an evolutionary adaptation that ensures its survival when fish around it are dying. The reason for great weight gains is very likely linked to the extended length of the intestine (besides it's sedentary nature), and an inference can be made that proportionately small food items are the preferred food source of this fish, which tallies with it's strategy of reducing risk to itself.

The Weberian apparatus (ottolith bones forming the inner ear), lateral line, taste cells and olfactory organ (inside the four sinuses on the front of the head) are all well developed. The taste cells are located all over the body, but in their highest concentrations around the head, on the barbules and on the caudal peduncle (the 'wrist' of the tail) allowing a greater field of 'smell'. As 'Andrew Black' so eloquently described in his article (sensory systems), the olfactory organ and taste cells enable 'cats' to determine minute concentrations of molecules excreted by all fish to maintain their osmotic balance with their surroundings. 

As you are aware, freshwaters are in a constant state of flux (coloured when it rains) and if mineral salts ingested are not expelled fish will expand like sponges, and then die (not a pleasant way to go). This ability to 'smell' so well is important for a blind fish as it enables it to determine what type of food is where, how near it is and most likely, its state of health (dead, I'm in trouble or can't catch me!). It would seem obvious that they should choose a fish exhibiting signs of stress as such a victim will be weaker (and less perceptive to their surroundings). By following these chemical pathways and their concentration gradients, allied with the vibrations picked-up by their lateral line and Weberian apparatus, 'cats' can soon be at close quarters with their potential meal. If their interest is in a live organism (even one excreting stress hormones), smell or taste will not tell them how large it (possibly what type), and they need to know this to protect themselves and proceed safely with their intentions of diner (not forgetting that they are blind).

The greatest weapon in their armoury has got to be their electrical powers. The European catfish has evolutionary and physiological links to the weakly electrical fishes of Africa and South America (the 'Knife' fishes, which includes the Electric Eel) who share the very small dorsal fin and tapering, long, slim bodies of S.glanis. The South American Knife fishes have a very strong resemblance to S.glanis, especially the Electric Eel. This kind of research is still in its infancy but with recent developments in computing technology a new insight is beginning to appear. It is known that S.glanis has a well developed organ in it's brain called the 'valvuli cerebelli' (as does the Electric Catfish), which it uses to orientate itself, locate prey and control muscle tone. The later is the reason these fish fight so hard for their relative size. S.glanis also has electro receptive properties (small pit organs) that are sensitive to temperature and to the small electric fields that all living things emit (known as quasi-constant electric fields), even you. 

I was surprised to learn that electric fields emitted by the human brain are strong enough to damage microelectronics; hence the 'protective' silicon chips. These weak electric signals emitted by prospective prey can't be hidden and give S.glanis a picture of the size, position and path of it's intended meal. Emmitions by S.glanis itself are bounced off objects and received on its large domed head, (all crucial in establishing a 'soft' target and minimising risk to itself). The small dorsal fin had me stumped for years, as it obviously serves no purpose in hydrodynamic terms, until I came across some research that revealed it to be a 'radar'! (Used for electro reception). This 'specialised' part of their anatomy is also shared by the Knife fishes, which tend to live in very coloured water (high in suspended solids).

Habitat is dependant on the environment you may be fishing as these fish are crepuscular (shy to you and me) and find comfort in dark places, but always with the best compromise to being near a suitable food source. Where bank-side cover exists (fallen trees etc), near to deep water, in soft mud (but not if the water is clear or shallow) or if suitable substrate or bank-side cover is not available, always the deeper part of the water. The most stable environment (flow or temperature) has the greater attraction. Molluscs (snails, mussels, leeches) and other invertebrates are never shunned if in plentiful supply and require little energy to capture (these really are the get-fat quick fish), as 'surprise' rather than 'pursuit' is what they have chosen to evolve as. Whatever the need, these fish find the easiest way to meet it for the least expenditure of energy, I can relate to that! As the fish reach maturity ( 3ft+/20lbs+), there comes an onset of territorial behaviour leading up to spawning, and larger fish defend 'chosen' feeding ranges vigorously against rivals in preparation for courtship. 

Mating is a challenging and demanding time for both the male and the female; the feeding spell in the weeks leading up to this provides the energy required for the female to ovulate hundreds of thousands of eggs (as many as 50,000/kg body weight in large fish) and the male to build a nest (of debris and branches in an 'excavated' site), wrestle rivals, tussle with the female (to get her to spawn in the nest) and finally 'nurse' and protect the fry to first feed (old world meets new!). In limiting environments (still-waters) larger fish will (like Pike) protect food sources by cannibalising young rivals, thus ensuring their survival. How nature defines this is one of life's wonders, but I guess the rules apply to the fact that some people drive Ferraris, and I don't!

Further Details available from Guy at silurusglanis@europe.com

 

 

   

(C) Baintonfisheries.co.uk, 29 May, 2014 . All rights reserved, no reproduction without prior permission

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