Home

 Articles

 News

 Find Us

 Rules

 Join Us

 Gallery

"Good cop, bad cop".

Silurus.glanis, 'the Police fish'; the law and order it lays down in the food- chain and why it's deployed in commercial fisheries to prevent 'trouble'.

Guy Bridgeman

Archaeological evidence shows that the European catfish has been in mans diet since pre-dating modern civilisation and it has formed part of the catch in commercial nets and fish traps for several hundred years. In Hungary it has been farmed extensively in poly-culture systems (with other species) for nearly 300 years and semi-intensively in monoculture (on it's own) for 135 years. Intensive aquaculture growing S.glanis in tank systems has shown that these fish can tolerate incredible crowding stresses (up to 290lbs per 100cm cubed!), but the energy requirement of this developing technology makes it unviable for most markets. 

Financially successful operations at present are those that use thermal springs or cooling water from nuclear power stations such as Thiange in Belgium (I bet they don't put that on the label!). Opportunities may exist in Egypt and Saudi Arabia where heat and solar power are not problems, (as I know that the Egyptians are dabbling with American Channel catfish). As S.glanis is predatory in nature it can only be grown in tanks with high flow rates as juveniles, and under a constant feeding regime to minimise the inevitable casualties of cannibalism, although if I lived in such circumstance I think I would probably try to eat my neighbour too, if only for a little more space! The net result for the new 'market' economies of Eastern Europe is that fish now go to the highest bidder, and catfish command high prices due to the primitive methods of supply being unable to meet growing demands.

In Eastern Europe most people catch their own catfish from rivers and lakes (UN-FAO, 2001) and this accounts for about 15% of national consumption in most countries, but in some it can be more than 50% (farmed catfish contribute only a couple percent to national figures). Carp account for 90% of farmed production on average and are grown in open ponds with little mechanical input, enabling them to be sold as a cheap and tasty source of protein. To fish natural waters, permits must be bought, and of course 'you' then have to actually catch the fish. For the fine taste of catfish, this is a motivation (they are the east European equivalent of our Salmon), but for carp you just pays'ya money! Almost all marine produce leaves the country for more affluent societies. This is where S.glanis (supplemental crop) comes into its own, where it is grown along side Carp (primary crop) and often with Tench, Perches, Breams and to a lesser extent, Pikeperch (secondary crops). Carp are easy to produce (they'd breed in a toilet if you left them to it!) and their fry are plentiful and hardy, whereas catfish fry are delicate and require an input of labour, specialised diets and financial energy, all limiting factors to their greater supply. 

Inoculations of catfish are made as follows (approximately); if the Cyprinids (carps) stocked average 2lb, then add 100x 5g catfish/hectare; if the Cyprinids average 4lb, then add 30x 100g catfish/ha; if the Cyprinids average more than 4lbs, then add 10x 1lb catfish.

S.glanis is the predator of choice in these culture systems and is known as the 'police' fish, that is, 'a control fish'. When the initial stocking takes place there are casualties from stress or damage and during the growing season weaker, deformed and sick examples add to the casualties. The nature of S.glanis is to take advantage of these unutilised food sources that would otherwise consume available oxygen or compete for food. By eating dead or diseased fish, catfish reduce the likelihood of pathogens multiplying. This also reduces the BOD (biological oxygen demand) in the water that bacteria would otherwise consume in the decomposition of the dead fish (or animal), in a chemical process know as 'oxidation' or to you and me 'respiration'. Once all these available food sources have been utilised, the catfish will then pick out the weaker examples of the fish that are left. This keeps the fish moving, which stimulates their muscles and in turn their appetites, and most importantly circulates the water with the oxygen that it contains. 

Recent pioneering work in the USA has shown the importance of this water movement; much of the oxygen present comes into solution by diffusion from the atmosphere (known as the partial pressure), but a significant amount is a by-product of photosynthetic algal 'Carbon-fixing' (the splitting of carbon dioxide/CO2 molecules). The algae, it has been found, only require one 'hit' from a photon of light (a particle of light from the sun) every ten seconds, which means in shallow (but densely stocked) carp ponds this induced 'movement' of fish allows saturation of dissolved oxygen at all depths in the pond. This of course, is essential for respiration and the metabolism of food, and increases productivity in the water.

This presence results in far greater yields of healthier, and genetically stronger fish that have been 'naturally' selected by the best quality controller you can get for no pay, S.glanis on studies on natural populations it has been shown that where S.glanis establishes itself there is a proportional increase of its food species. But be warned, if this fish is introduced and overstocked into small waters, the only result can be one or two big and skinny catfish. I recently learned that in Belgium, where S.glanis has been extinct for nearly 150 years due to pollution and over-fishing (as is sadly the case now for much of the natural distribution of this fish), there are plans afoot to reintroduce it to improve the aquatic capacity for diversity of fauna. When fingerling Cyprinids are stocked for culture there are many more hungry mouths ready to gobble them up in the form of carnivorous insect larvae, which, (by virtue of their slow movement), make great snacks for the young Silurids that I'm sure you will have stocked! If you've managed to follow all of this (are you still awake?), then you will understand how you can apply this feeding strategy of S.glanis to wild populations of fish stocks and how such huge fish managed to evolve over 71 million years, let alone 71 years! 

These fish are on a trophic level of the food chain that has parity to the Lions of the African savannas, at the top. They are an evolutionary success story because they 'managed' their success, and only nature knows the secret of taking as much as possible and yet somehow giving back more than it took, where did we go wrong?

Further Details available from Guy at silurusglanis@europe.com