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Prey Detection In Silurus Glanis

By Andrew J. Black

Catfish are one of the most sensitive fish to chemical stimuli, ie. chemicals dissolved in the water. It has been discovered that catfish can detect certain amino acids and other substances from very long distances. The use of chemoreception to detect food has the advantage that chemical stimuli can be used in conditions of poor visibility, but because such stimuli are persistent (stay in the general area of release, for a long time, except in flowing water eg. rivers) they may give information as to where potential prey has been rather than where it is at present. The chemoreceptor systems used in prey detection consist of the olfactory (smell) and gustatory (taste) systems, with the olfactory system being more sensitive, and able to pick up chemical changes some distance away. The gustatory system is a more close range sense.

Smell and taste in fish operate in close conjunction with each other. Both respond to similar stimuli, chemicals diffusing through the water.

Smell (Olfaction):
Predator species such as the catfish respond to a large range of chemicals emanating from prey, and are attracted to amino acids and other organic compounds, such as betaine, nucleotides and nucleosides. There is also evidence that water born ATP (adenosine triphosphate) can stimulate feeding behaviour. Basically ATP is a chemical produced in a cell, that is used in the process of respiration, ATP transfers energy from energy-rich substances (sugars in the blood, obtained from digested food) to energy-requiring processes of living cells, is found universally in biological material. ATP is a highly unstable compound that is liable to displacement or change ie. it is readily broken down into other chemicals (ADP and AMP). Therefore water-borne ATP would indicate to the fish, the presence of freshly damaged tissue, or an injured fish (easy meal).

But ADP and AMP in the water would indicate degrading fish, or rotting food. Catfish can detect relative concentrations of chemical stimuli, this enables the fish to find the source of an odour in still water by swimming up the concentration gradient. In flowing water finding the source of an odour is easier, as the fish just has to swim upstream against the current until the source of the stimulus is found.

The catfish has two pairs of nostrils on the dorsal surface of its head. Water enters into the nostril, and it passes over the olfactory epithelium which locates the chemical stimuli and passes the information down the olfactory nerve to the brain where the information is "analysed", and a response is made, ie. is the chemical sensed related to food? if so, follow, and find the source of the chemical.

One must realise that in water, as in air, there is an ever changing number of chemical signals, some of which may be stimulatory to the fish, eg the chemical releases of food, others may be non-stimulatory, eg rotting substances, pollution, etc.

The catfish is monitoring these stimuli all the time, subconsciously, it is only when the brain relates to a specific chemical stimulus that an action is made. As humans we are continuously smelling our environment (air) but we are not aware that we are doing so until a smell is recognised, eg. we may find ourselves walking down a street, (subconsciously analysing chemical signals "smells"), when we are confronted by the appetising smell of fish and chips. Our brain recognises the smell, and tells us that there must be fish and chips nearby, the concentration of the "smell" will tell us how close we are to the source.

Taste (gustation):
The primary organ of the gustatory system in the taste bud. Unlike humans, which have taste buds only on the tongue, catfish have them on their barbules, and also in and around their mouth. The external taste buds act in harmony with the olfactory system (distance receptors). Once satisfied that detection of food chemicals is positive one, then the quality of the food is analysed by the taste buds. Having taste buds on the outside of the mouth and barbules, enables the catfish to actually taste the food, before consuming it. This is of great benefit especially in low light conditions (when the catfish normally feeds), where the chemical signals to the olfaction system might indicate their is food in the area, but the quality of the food may be poor, this is picked up by the external taste buds. It should be noted that each species of fish, will have a slightly different smell, taste. Some may be more to the fish's liking then others, eg. a perch might have a smell/taste that has significance to that particular attacking catfish, ie. in the past it might have been hurt by a perch's spines, and so may break off the attack. This is, of course, assuming that catfish is capable of remembering!.

Being a piscivore (fish eater) the catfish is more likely to respond to meaty/fishy tastes/odours than herbaceous ones. One must be careful when artificially increasing the smell/taste of a bait, whether with "natural" amino acids, lipids, etc. or other "artificial" chemicals, such as those found in bottled flavours, as it is possible to temporarily overload the catfish's chemoreceptors. The consequence of this being that the fish might not be able to find your bait, as "flavour pollution" might be so strong that every thing in the specific area (of the bait) will smell like potential food. You will know if you have over loaded your bait, if you only start getting takes after your bait has been in the water for a long term.

Catfish are also sensitive to excretions given out by fish, ie. urine and excrement, and various other substances. A fish that is stressed will excrete more then a "happy" non-stressed fish. Stressed fish are normally injured, weak or diseased fish, and consequently easy prey. It can be said that a live bait kept in a keep net or similar will excrete a lot in the keepnet and surrounding water, so when eventually use as bait, will be not as effective. It can therefore be said that for maximum attraction the live baits should be caught when needed, so the bait excretes in the vicinity where it is fished. Sound reception:

Catfish can also detect potential prey by sound reception. There are two types of sound reception:
1) Particle displacement - vibrations caused by the back and forth movement of water molecules.
2) sound pressure - sound waves.

Catfish can locate prey by "homing" in on the particle displacement caused by injured fish. Particle displacement is sensed by the lateral line. The lateral line is outlined by pores, which let in water and link the outside environment with a subcutaneous canal, which is situated underneath this line of pores. Inside this canal are many neuromasts. Neuromasts are a basic sensory cell found in the lateral line and inner ear. Inside these neuromasts are a number of hair cells, which are free to move within the gelatinous cupula.

Pressure/movement is sensed by the hair cells moving, ie. if movement vibrations are coming from the right of the fish, the hair cells down the right side of the fish, will move more than the cells on the left side of the fish, so the fish knows which direction the vibrations are coming from. When the fish has "homed" in on these vibrations, and it is significantly close to the source, vibrations will also be felt along its barbules. It should be noted that the disturbance given out by a injured fish and the disturbance giving out by a lively fish/bait may be completely different, which might be why when using a fresh live bait, results are better after a few hours of "working", when the fish is tired, and its movements are more erratic.

For the final attack the catfish will use all its combined senses to locate the prey, and catch it.